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2010 - Current
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Ryan E.M., Tartakovsky A.M., Recknagle K.P., Khaleel M.A., Amon C.H., Pore-Scale Modeling of the Reactive Transport of Chromium in the Cathode of a Solid Oxide Fuel Cell, Journal of Power Sources 196(1): 287-300, 2011.
[Abstract] We present a pore-scale model of a solid oxide fuel cell (SOFC) cathode. Volatile chromium species are known to migrate from the current collector of the SOFC into the cathode where over time they decrease the voltage output of the fuel cell. A pore-scale model is used to investigate the reactive transport of chromium species in the cathode and to study the driving forces of chromium poisoning. A multi-scale modeling approach is proposed which uses a cell level model of the cathode, air channel and current collector to determine the boundary conditions for a pore-scale model of a section of the cathode. The pore-scale model uses a discrete representation of the cathode to explicitly model the surface reactions of oxygen and chromium with the cathode material. The pore-scale model is used to study the reaction mechanisms of chromium by considering the effects of reaction rates, diffusion coefficients, chromium vaporization, and oxygen consumption on chromium's deposition in the cathode. The study shows that chromium poisoning is most significantly affected by the chromium reaction rates in the cathode and that the reaction rates are a function of the local current density in the cathode.
Nabovati A., Sellan D.P., Amon C.H., On the Lattice Boltzmann Method for Phonon Transport, Journal of Computational Physics 230(15): 5864-5876, 2011.
[Abstract] The lattice Boltzmann method is a discrete representation of the Boltzmann transport equation that has been employed for modeling transport of particles of different nature. In the present work, we describe the lattice Boltzmann methodology and implementation techniques for the phonon transport modeling in crystalline materials. We show that some phonon physical properties e.g., mean free path and group velocity, should be corrected to their effective values for one- and two-dimensional simulations, if one uses the isotropic approximation. We find that use of the D2Q9 lattice for phonon transport leads to erroneous results in transient ballistic simulations, and the D2Q7 lattice should be employed for two-dimensional simulations. Furthermore, we show that at the ballistic regime, the effect of direction discretization becomes apparent in two dimensions, regardless of the lattice used. Numerical methodology, lattice structure, and implementation of initial and different boundary conditions for the D2Q7 lattice are discussed in detail.
McGaughey A.J.H., Sellan D.P., Landry E.S., Amon C.H., Size-Dependent Model for Thin Film Thermal Conductivity, Proceedings of the ASME/JSME 8th Thermal Engineering Joint Conference 2011 : T30011-T30011-5, 2011.
[Abstract] We present a closed-form classical model for the size dependence of thin film thermal conductivity. The model predictions are compared to Stillinger-Weber silicon thin film thermal conductivities (in-plane and cross-plane directions) calculated using phonon properties obtained from lattice dynamics calculations. By including the frequency dependence of the phonon-phonon relaxation times, the model is able to capture the approach to the bulk thermal conductivity better than models based on a single relaxation time.
Ryan E.M., Tartakovsky A.M., Amon C.H., Pore-scale modeling of competitive adsorption in porous media, Journal of Contaminant Hydrology 120-121(C): 56-78, 2011.
[Abstract] In this paper we present a smoothed particle hydrodynamics (SPH) pore-scale multicomponent reactive transport model with competitive adsorption. SPH is a Lagrangian, particle based modeling method which uses the particles as interpolation points to discretize and solve flow and transport equations. The theory and details of the SPH pore-scale model are presented along with a novel method for handling surface reactions, the continuum surface reaction (CSR) model. The numerical accuracy of the CSR model is validated with analytical and finite difference solutions, and the effects of spatial and temporal resolution on the accuracy of the model are also discussed. The pore-scale model is used to study competitive adsorption for different Damköhler and Peclet numbers in a binary system where a plume of species B is introduced into a system which initially contains species A. The pore-scale model results are compared with a Darcy-scale model to investigate the accuracy of a Darcy-scale reactive transport model for a wide range of Damköhler and Peclet numbers. The comparison shows that the Darcy model over estimates the mass fraction of aqueous and adsorbed species B and underestimates the mass fractions of species A. The Darcy-scale model also predicts faster transport of species A and B through the system than the pore-scale model. The overestimation of the advective velocity and the extent of reactions by the Darcy-scale model are due to incomplete pore-scale mixing. As the degree of the solute mixing decreases with increasing Peclet and Damköhler numbers, so does the accuracy of the Darcy-scale model.
Goicochea J.V., Madrid M., Amon C.H., Thermal Properties for Bulk Silicon Based on the Determination of Relaxation Time Using Molecular Dynamics, Journal of Heat Transfer 132(1): 1-11, 2010.
[Abstract] Molecular dynamics simulations are performed to estimate acoustical and optical phonon relaxation times, dispersion relations, group velocities, and specific heat of silicon needed to solve the Boltzmann transport equation (BTE) at 300 K and 1000 K. The relaxation times are calculated from the temporal decay of the autocorrelation function of the fluctuation of total energy of each normal mode in the [100] family of directions, where the total energy of each mode is obtained from the normal mode decomposition of the motion of the silicon atoms over a period of time. Additionally, silicon dispersion relations are directly determined from the equipartition theorem obtained from the normal mode decomposition. The impact of the anharmonic nature of the potential energy function on the thermal expansion of the crystal is determined by computing the lattice parameter at the cited temperatures using a NPT (i.e., constant number of atoms, pressure, and temperature) ensemble, and are compared with experimental values reported in the literature and with those computed analytically using the quasiharmonic approximation. The dependence of the relaxation times with respect to the frequency is identified with two functions that follow the functional form of the relaxation time expressions reported in the literature. From these functions a simplified version of relaxation times for each normal mode is extracted. Properties, such as group and phase velocities, thermal conductivity, and mean free path, needed to further develop a methodology for the thermal analysis of electronic devices (i.e., from nano- to macroscales) are determined once the relaxation times and dispersion relations are obtained. The thermal properties are validated by comparing the BTE-based thermal conductivity against the predictions obtained from the Green-Kubo method. It is found that the relaxation times closely resemble the ones obtained from perturbation theory at high temperatures; the contribution to the thermal conductivity of the transverse acoustic, longitudinal acoustic, and longitudinal optical modes being approximately 30%, 60%, and 10%, respectively, and the contribution of the transverse optical mode negligible.
Le Corre J.-M., Yao S.-C., Amon C.H., Two-Phase Flow Regimes and Mechanisms of Critical Heat Flux Under Subcooled Flow Boiling Conditions, Nuclear Engineering and Design 240(2): 245-251, 2010.
[Abstract] A literature review of critical heat flux (CHF) experimental visualizations under subcooled flow boiling conditions was performed and systematically analyzed. Three major types of CHF flow regimes were identified (bubbly, vapor clot and slug flow regime) and a CHF flow regime map was developed, based on a dimensional analysis of the phenomena and available experimental information. It was found that for similar geometric characteristics and pressure, a Weber number (We)/thermodynamic quality (x) map can be used to predict the CHF flow regime. Based on the experimental observations and the review of the available CHF mechanistic models under subcooled flow boiling conditions, hypothetical CHF mechanisms were selected for each CHF flow regime, all based on a concept of wall dry spot overheating, rewetting prevention and subsequent dry spot spreading. Even though the selected concept has not received much attention (in term or theoretical developments and applications) as compared to other more popular DNB models, its basis have often been cited by experimental investigators and is considered by the authors as the "most-likely" mechanism based on the literature review and analysis performed in this work. The selected modeling concept has the potential to span the CHF conditions from highly subcooled bubbly flow to early stage of annular flow and has been numerically implemented and validated in bubbly flow and coupled with one- and three-dimensional (CFD) two-phase flow codes, in a companion paper. [Le Corre J.M., Yao S.C., Amon C.H., in this issue. A mechanistic model of critical heat flux under subcooled flow boiling conditions for application to one and three-dimensional computer codes. Nucl. Eng. Des.].
Le Corre J.M., Yao S.C., Amon C.H., A Mechanistic Model of Critical Heat Flux Under Subcooled Flow Boiling Conditions for Applications to One and Three-Dimensional Computer Codes, Nuclear Engineering and Design 240(2): 235-244, 2010.
[Abstract] Based on a review of visual observations at or near critical heat flux (CHF) under subcooled flow boiling conditions and consideration of CHF triggering mechanisms, presented in a companion paper [Le Corre J.M., Yao S.C., Amon C.H., 2010. Two-phase flow regimes and mechanisms of critical heat flux under subcooled flow boiling conditions. Nucl. Eng. Des.], a model using a two-dimensional transient thermal analysis of the heater undergoing nucleation was developed to mechanistically predict CHF in the case of a bubbly flow regime. The model simulates the spatial and temporal heater temperature variations during nucleation at the wall, accounting for the stochastic nature of the boiling phenomena. It is postulated that a high local wall superheat occurring underneath a nucleating bubble at the time of bubble departure can prevent wall rewetting at CHF (Leidenfrost effect). The model has also the potential to evaluate the post-DNB heater temperature up to the point of heater melting. Validation of the proposed model was performed using detailed measured wall boiling parameters near CHF, thereby bypassing most needed constitutive relations. It was found that under limiting nucleation conditions; a peak wall temperature at the time of bubble departure can be reached at CHF preventing wall cooling by quenching. The simulations show that the resulting dry patch can survive the surrounding quenching events, preventing further nucleation and leading to a fast heater temperature increase. The model was applied at CHF conditions in simple geometry coupled with one-dimensional and three-dimensional (CFD) codes. It was found that, within the range where CHF occurs under bubbly flow conditions (as defined in Le Corre et al., 2010), the local wall superheat underneath nucleating bubbles is predicted to reach the Leidenfrost temperature. However, a better knowledge of statistical variations in wall boiling parameters would be necessary to correctly capture the CHF trends with mass flux (or Weber number).
Turney J.E., McGaughey A.J.H., Amon C.H., In-Plane Phonon Transport in Thin Films, Journal of Applied Physics 107(2), 2010.
[Abstract] The in-plane phonon thermal conductivities of argon and silicon thin films are predicted from the Boltzmann transport equation under the relaxation time approximation. We model the thin films using bulk phonon properties obtained from harmonic and anharmonic lattice dynamics calculations. The input required for the lattice dynamics calculations is obtained from interatomic potentials: Lennard-Jones for argon and Stillinger-Weber for silicon. The effect of the boundaries is included by considering only phonons with wavelengths that fit within the film and adjusting the relaxation times to account for mode-dependent, diffuse boundary scattering. Our model does not rely on the isotropic approximation or any fitting parameters. For argon films thicker than 4.3 nm and silicon films thicker than 17.4 nm, the use of bulk phonon properties is found to be appropriate and the predicted reduction in the in-plane thermal conductivity is in good agreement with results obtained from molecular dynamics simulation and experiment. We include the effects of boundary scattering without employing the Matthiessen rule. We find that the Matthiessen rule yields thermal conductivity predictions that are at most 12% lower than our more accurate results. Our results show that the average of the bulk phonon mean free path is an inadequate metric to use when modeling the thermal conductivity reduction in thin films.
Nain A.S., Filiz S., Burak Ozdoganlar O., Sitti M., Amon C.H., Aligned Deposition and Model Characterization of Micron and Submicron Poly (methyl methacrylate) Fiber Cantilevers, Review of Scientific Instruments 81(1), 2010.
[Abstract] Polymeric micro-/nanofibers are finding increasing use as sensors for novel applications. Here, we demonstrate the ability to deposit an array of poly(methyl methacyrlate) fibers with micron and submicron diameters in aligned configurations on customized piezoelectric shakers. Using lateral motion of an atomic force microscope tip, fibers are broken to obtain fiber cantilevers of high aspect ratio (length/diameter>20). The resonant frequencies of fabricated microfiber cantilevers are experimentally measured using a laser Doppler vibrometer. An average Young's modulus of 3.5 GPa and quality factor of 20 were estimated from the experimentally obtained frequency responses.
Karajgikar S., Agonafer D., Ghose K., Sammakia B., Amon C.H., Refai-Ahmed G., Multi-Objective Optimization to Improve Both Thermal and Device Performance of a Nonuniformly Powered Micro-Architecture, Journal of Electronic Packaging 132(2): 210081-210088, 2010.
[Abstract] Integration of different functional components such as level two (L2) cache memory, high-speed I/O interfaces, and memory controller has enhanced microprocessor performance. In this architecture, certain functional units on the microprocessor dissipate a significant fraction of the total power while other functional units dissipate little or no power. This highly nonuniform power distribution results in a large temperature gradient with localized hot spots that may have detrimental effects on computer performance, product reliability, and yield. Moving the functional units may reduce the junction temperature but can affect performance by a factor as much as 30%. In this paper, a multi-objective optimization is performed to minimize the junction temperature without significantly altering the computer performance. The analysis was performed for 90 nm Pentium IV Northwood architecture operating at 3 GHz clock speed. Each functional unit on the die has a specific role, so functional units with similar roles were grouped together. Thus, the actual Pentium IV die was divided into four groups (front end, execution cores, bus and L2, and out-of-order engine). Repositioning constraints were determined using circuit delay models of major functional units in a micro-architectural simulator. Thus, depending on the scenario, relocating functional units can result in virtually no performance loss (less than 2% is assumed to be minimal and is reported as 0%) to as much as 30% performance loss. From the results, the minimum and the maximum temperatures were 56.6°C and 62.2°C. This ΔT corresponds to thermal design power of 60.2 W. For microprocessors with higher thermal design power (115 W) and operating at higher clock speed, higher ΔT can be realized. Based on this paper's analysis, the optimized scenario resulted in a junction temperature of 56.67°C at the cost of a 14% performance loss.
Goicochea J.V., Madrid M., Amon C.H., Hierarchical Modeling of Heat Transfer in Silicon-Based Electronic Devices, Journal of Heat Transfer 132(10): 1-11, 2010.
[Abstract] A hierarchical model of heat transfer for the thermal analysis of electronic devices is presented. The integration of participating scales (from nanoscale to macroscales) is achieved by (i) estimating the input parameters and thermal properties to solve the Boltzmann transport equation (BTE) for phonons using molecular dynamics (MD), including phonon relaxation times, dispersion relations, group velocities, and specific heat, (ii) applying quantum corrections to the MD results to make them suitable for the solution of BTE, and (iii) numerically solving the BTE in space and time subject to different boundary and initial conditions. We apply our hierarchical model to estimate the silicon out-of-plane thermal conductivity and the thermal response of an silicon on insulator (SOI) device subject to Joule heating. We have found that relative phonon contribution to the overall conductivity changes as the dimension of the domain is reduced as a result of phonon confinement. The observed reduction in the thermal conductivity is produced by the progressive transition of modes in the diffusive regime (as in the bulk) to transitional and ballistic regimes as the film thickness is decreased. In addition, we have found that relaxation time expressions for optical phonons are important to describe the transient response of SOI devices and that the characteristic transport regimes, determined with Holland and Klemens phonon models, differ significantly.
Sellan D.P., Landry E.S., Turney J.E., McGaughey A.J.H., Amon C.H., Size Effects in Molecular Dynamics Thermal Conductivity Predictions, Physical Review B - Condensed Matter and Materials Physics 81(21), 2010.
[Abstract] We predict the bulk thermal conductivity of Lennard-Jones argon and Stillinger-Weber silicon using the Green-Kubo (GK) and direct methods in classical molecular dynamics simulations. While system-size-independent thermal conductivities can be obtained with less than 1000 atoms for both materials using the GK method, the linear extrapolation procedure must be applied to direct method results for multiple system sizes. We find that applying the linear extrapolation procedure in a manner consistent with previous researchers can lead to an underprediction of the GK thermal conductivity (e.g., by a factor of 2.5 for Stillinger-Weber silicon at a temperature of 500 K). To understand this discrepancy, we perform lattice dynamics calculations to predict phonon properties and from these, length-dependent thermal conductivities. From these results, we find that the linear extrapolation procedure is only accurate when the minimum system size used in the direct method simulations is comparable to the largest mean-free paths of the phonons that dominate the thermal transport. This condition has not typically been satisfied in previous works. To aid in future studies, we present a simple metric for determining if the system sizes used in direct method simulations are sufficiently large so that the linear extrapolation procedure can accurately predict the bulk thermal conductivity.
Thomas J.A., Turney J.E., Iutzi R.M., Amon C.H., McGaughey A.J.H., Predicting Phonon Dispersion Relations and Lifetimes from the Spectral Energy Density, Physical Review B - Condensed Matter and Materials Physics 81(8), 2010.
[Abstract] We derive and validate a technique for predicting phonon dispersion relations and lifetimes from the atomic velocities in a crystal using the spectral energy density. This procedure, applied here to carbon nanotubes, incorporates the full anharmonicity of the atomic interactions into the lifetime and frequency predictions. It can also account for nonperiodic interactions between phonons and nonbonded molecules near the solid surface. We validate the technique using phonon properties obtained from anharmonic lattice dynamics calculations and thermal conductivities obtained from nonequilibrium molecular-dynamics simulation.
Sellan D.P., Turney J.E., McGaughey A.J.H., Amon C.H., Cross-Plane Phonon Transport in Thin Films, Journal of Applied Physics 108(11), 2010.
[Abstract] We predict the cross-plane phonon thermal conductivity of Stillinger-Weber silicon thin films as thin as 17.4 nm using the lattice Boltzmann method. The thin films are modeled using bulk phonon properties obtained from harmonic and anharmonic lattice dynamics calculations. We use this approach, which considers all of the phonons in the first Brillouin-zone, to assess the suitability of common assumptions. Specifically, we assess the validity of: (i) neglecting the contributions of optical modes, (ii) the isotropic approximation, (iii) assuming an averaged bulk mean-free path, and (iv) the Matthiessen rule. Because the frequency-dependent contributions to thermal conductivity change as the film thickness is reduced, assumptions that are valid for bulk are not necessarily valid for thin films.
Karajgikar S., Sammakia B., Agonafer D., Amon C.H., Ghose K., Refai-Ahmed G., Effect of Relocation of Functional Units of a Non-Uniformly Powered Microprocessor on Thermal and Device Clock Performance, ASME International Mechanical Engineering Congress and Exposition, Proceedings 5: 179-184, 2010.
[Abstract] Integration of different functional components such as level two (L2) cache memory, high-speed I/O interfaces, memory controller, etc. has enhanced microprocessor performance. In this architecture, certain functional units on the microprocessor dissipate a significant fraction of the total power while other functional units dissipate little or no power. This highly non-uniform power distribution results in a large temperature gradient with localized hot spots that may have detrimental effects on computer performance, product reliability, and yield. Moving the functional units may reduce the junction temperature but can also affect performance by as much as 30%. In this paper, multi-objective optimization is performed to minimize the junction temperature without significantly altering the computer performance. From the results, the minimum and the maximum temperature was 56.6°C and 62.2°C with a corresponding penalty on the performance of 14% and 0% respectively. The numerical analysis was performed for 90 nm Pentium IV Northwood architecture at 3 GHz clock speed.
Ryan E.M., Tartakovsky A.M., Amon C.H., A Novel Method for Modeling Neumann and Robin Boundary Conditions in Smoothed Particle Hydrodynamics, Computer Physics Communications 181(12): 2008-2023, 2010.
[Abstract] We present a novel smoothed particle hydrodynamics (SPH) method for diffusion equations subject to Neumann and Robin boundary conditions. The Neumann and Robin boundary conditions are common to many physical problems (such as heat/mass transfer), and can prove challenging to implement in numerical methods when the boundary geometry is complex. The new method presented here is based on the approximation of the sharp boundary with a diffuse interface and allows an efficient implementation of the Neumann and Robin boundary conditions in the SPH method. The paper discusses the details of the method and the criteria for the width of the diffuse interface. The method is used to simulate diffusion and reactions in a domain bounded by two concentric circles and reactive flow between two parallel plates and its accuracy is demonstrated through comparison with analytical and finite difference solutions. To further illustrate the capabilities of the model, a reactive flow in a porous medium was simulated and good convergence properties of the model are demonstrated.
Sellan D.P., Turney J.E., McGaughey, A.J.H., Amon C.H., Phonon Transport in Thin Films: A Lattice Dynamics/Boltzmann Transport Equation Study, Proceedings of the 2010 14th International Heat Transfer Conference : 393-402, 2010.
[Abstract] The cross-plane and in-plane phonon thermal conductivities of Stillinger-Weber (SW) silicon thin films are predicted using the Boltzmann transport equation under the relaxation time approximation. We model the thin films using bulk phonon properties obtained from harmonic and anharmonic lattice dynamics calculations. The cross-plane and in-plane thermal conductivities are reduced from the corresponding bulk value. This reduction is more severe for the cross-plane direction than for the in-plane direction. For the in-plane direction, we find that the predicted reduction in thermal conductivity gives a good lower bound to available experimental results. Including the effects of boundary scattering using the Matthiessen rule, which assumes that scattering mechanisms are independent, yields thermal conductivity predictions that are at most 12% lower than our more accurate results. Neglecting optical phonon modes, while valid for bulk systems, introduces 22.5% error when modeling thin films. Using phonon properties along the [001] direction (i.e., the isotropic approximation) yields bulk predictions that are 15% lower than that when all of the phonon modes are considered. For thin films, this deviation increases to 25%. Our results show that a single bulk phonon mean free path is an inadequate metric for predicting the thermal conductivity reduction in thin films.
Sellan D.P., Amon C.H., Landry E.S., Turney J.E., McGaughey, A.J.H., Size Effects in Green-Kubo and Direct Method Molecular Dynamics Predictions of Thermal Conductivity, International Mechanical Engineering Congress & Exposition, IMECE2010-38841, Vancouver, BC, 2010.
Nabovati A., Sellan D.P., Amon C.H., Ballistic Heat Transfer Modeling in Semiconductor Electronic Devices: A Modified Fourier-Based Approach, International Mechanical Engineering Congress & Exposition, IMECE2010-38810, Vancouver, BC, 2010.
Nabovati A., Sellan D.P., Amon C.H., Considerations on the Numerical Implementation of the Lattice Boltzmann Method for Phonon Transport, International Mechanical Engineering Congress & Exposition, IMECE2010-40750, Vancouver, BC, 2010.
Tse L., Shek T.L.T., Nabovati A., Amon C.H., Computational Fluid Dynamics Modeling of Redundant Stent Graft Configurations in Endovascular Aneurysm Repair, International Mechanical Engineering Congress & Exposition IMECE2010-39941 Vancouver, BC, 2010.
Goicochea J.V., Michel B., Amon C.H., Molecular Dynamics Simulations of Oblique Phonon Scattering at Semiconductor Interfaces, 3rd International Conference on Thermal Issues in Emerging Technologies Theory and Applications : 111-116, 2010.
[Abstract] Equilibrium molecular dynamics simulations are used to determine the transmission probability of oblique phonons scattering on flat and rough surfaces. The transmission is determined from the total energy change of the materials comprising the interface. We consider semiconductor films of silicon (Si) and germanium (Ge) as interfacing materials. A symmetric sawtooth (triangular) structure of varying height (similar to that analyzed in Appl. Phys. Lett., 93(8), 2008) is used to introduce surface roughness. We have found that the transmission is a strong function of the phonon incident angle, frequency, mass ratio of the comprising semiconductors and roughness height. An interesting behavior in the transmission probability is observed with the introduction of controlled surface roughness. Low frequency phonons can have transmission values higher than those predicted in the acoustic limit. Conversely, they decrease significantly for high frequency phonons. Maximum and minimum values in the transmission probability are found for surface roughness of 4.34 nm height.
Romero D., Amon C.H., Finger S., Improving Multi-response Metamodels with Upper/Lower Bound Information using Multi-stage Non-stationary Covariance Functions, ASME 2010 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference 1: 611-621, 2010.
[Abstract] Metamodels have been proposed in the literature to reduce the time and resources devoted to design space exploration, to learn about design trade-offs, and to find the best solution to the design problem in the context of simulation-based design and optimization. In previous work in engineering design based on multiple performance criteria, we have proposed the use of Multi-response Bayesian Surrogate Models (MR-BSM) to model several response variables simultaneously, instead of modeling them independently. By doing so, it is expected that the correlation among the response variables can be used to achieve better models with smaller data sets. In this work, we extend the capabilities of MR-BSM by developing a multistage formulation with non-stationary covariance functions. This formulation for multi-response metamodeling in successive stages of experimental design, data acquisition and model fitting, enables the integration of different sources of information about system responses, with different levels of accuracy, into a single, global model of the system. The feasibility of the proposed formulation is demonstrated with an example in which two test functions are jointly approximated in two stages. In addition, we demonstrate the potential of the methodology to take advantage of a priori information, expressed as upper and lower bounds on the responses, to improve the accuracy of the metamodels. Results show that the use of bound information can result in order-of-magnitude improvements in metamodel accuracy.
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2005 - 2009
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Martufi G., Di Martino E.S., Amon C.H., Muluk S.C., Finol E.A., Three-dimensional Geometrical Characterization of Abdominal Aortic Aneurysms: Image-based Wall Thickness Distribution, Journal of Biomechanical Engineering 131(6), 2009.
[Abstract] The clinical assessment of abdominal aortic aneurysm (AAA) rupture risk is based on the quantification of AAA size by measuring its maximum diameter from computed tomography (CT) images and estimating the expansion rate of the aneurysm sac over time. Recent findings have shown that geometrical shape and size, as well as local wall thickness may be related to this risk; thus, reliable noninvasive image-based methods to evaluate AAA geometry have a potential to become valuable clinical tools. Utilizing existing CT data, the three-dimensional geometry of nine unruptured human AAAs was reconstructed and characterized quantitatively. We propose and evaluate a series of 1D size, 2D shape, 3D size, 3D shape, and second-order curvature-based indices to quantify AAA geometry, as well as the geometry of a size-matched idealized fusiform aneurysm and a patient-specific normal abdominal aorta used as controls. The wall thickness estimation algorithm, validated in our previous work, is tested against discrete point measurements taken from a cadaver tissue model, yielding an average relative difference in AAA wall thickness of 7.8%. It is unlikely that any one of the proposed geometrical indices alone would be a reliable index of rupture risk or a threshold for elective repair. Rather, the complete geometry and a positive correlation of a set of indices should be considered to assess the potential for rupture. With this quantitative parameter assessment, future research can be directed toward statistical analyses correlating the numerical values of these parameters with the risk of aneurysm rupture or intervention (surgical or endovascular). While this work does not provide direct insight into the possible clinical use of the geometric parameters, we believe it provides the foundation necessary for future efforts in that direction.
Turney J.E., Landry E.S., McGaughey A.J.H., Amon C.H., Predicting Phonon Properties and Thermal Conductivity from Anharmonic Lattice Dynamics Calculations and Molecular Dynamics Simulations, Physical Review B - Condensed Matter and Materials Physics 79(6), 2009.
[Abstract] Two methods for predicting phonon frequencies and relaxation times are presented. The first is based on quasiharmonic and anharmonic lattice dynamics calculations, and the second is based on a combination of quasiharmonic lattice dynamics calculations and molecular dynamics simulations. These phonon properties are then used with the Boltzmann transport equation under the relaxation-time approximation to predict the lattice thermal conductivity. The validity of the low-temperature assumptions made in the lattice dynamics framework are assessed by comparing to thermal conductivities predicted by the Green-Kubo and direct molecular dynamics methods for a test system of Lennard-Jones argon. The predictions of all four methods are in agreement at low temperature (20 K). At temperatures of 40 K (half the Debye temperature of Lennard-Jones argon) and below, the thermal-conductivity predictions from the two methods that use lattice dynamics calculations are within about 30% of those made using the more accurate Green-Kubo and direct molecular dynamics methods. The thermal-conductivity predictions using the lattice dynamics techniques become inaccurate at high temperature (above 40 K) due to the approximations inherent in the lattice dynamics framework. We apply the results to assess the validity of (i) the isotropic approximation in modeling thermal transport and (ii) the common assertion that low-frequency phonons dominate thermal transport. Lastly, we suggest approximations that can be made within the lattice dynamics framework that allow the thermal conductivity of Lennard-Jones argon to be estimated using two orders of magnitude less computing effort than the Green-Kubo or direct molecular dynamics methods.
Nain A.S., Sitti M., Jacobson A., Kowalewski T., Amon C.H., Dry Spinning based Spinneret based Tunable Engineered Parameters (STEP) Technique for Controlled and Aligned Deposition of Polymeric Nanofibers, Macromolecular Rapid Communications 30(16): 1406-1412, 2009.
[Abstract] Polymeric nanofibers are finding increasing number of applications and hold the potential to revolutionize diverse fields such as tissue engineering, smart textiles, sensors, and actuators. Aligning and producing high aspect ratio fiber arrays (length/diameter>2 000) in the submicron and nanoscale diameters has been challenging due to fragility of polymeric materials, thus making it difficult to deposit them as one dimensional structures functionally interfaced with other systems. Here, we present a pseudo dry spinning technique which allows precise control on fiber diameters and further allows deposition of fiber arrays in aligned configurations. Control on fiber diameters ranging from 50-500nm and having lengths of several millimeters is achieved by altering the polymeric solution concentration. In the dilute and semi-dilute unentangled concentration domain droplets or beaded fibers are observed to form. Smooth uniform diameter fibers are observed to form at the onset of semi-dilute entangled concentration regime. For a given molecular weight, the increase in fiber diameter with increasing solution concentration is attributed to both the increase in the entanglement density and the decrease in the radius of gyration of solvated polymer molecules. Using this technique polymeric fiber arrays in single and multiple layers are demonstrated which can be used towards developing strong textiles, biological scaffolds, and sensor networks.
Turney J.E., McGaughey A.J.H., Amon C.H., Assessing the Applicability of Quantum Corrections to Classical Thermal Conductivity Predictions, Physical Review B - Condensed Matter and Materials Physics 79(22), 2009.
[Abstract] The validity of the commonly used quantum corrections for mapping a classically predicted thermal conductivity onto a corresponding quantum value are assessed by self-consistently predicting the classical and quantum thermal conductivities of a crystalline silicon system via lattice-dynamics calculations. Applying the quantum corrections to the classical predictions, with or without the zero-point energy, does not bring them into better agreement with the quantum predictions compared to the uncorrected classical values above temperatures of 200 K. By examining the mode dependence of the phonon properties, we demonstrate that thermal conductivity cannot be quantum corrected on a system level. We explore the source of the differences in the quantum and classical phonon relaxation times on a mode-by-mode basis.
Nain A.S., Weiss L., Campbell P., Amon C.H., Depositing Aligned Micro/Nanofibers in Single and Multiple Layers for Tissue Engineering, BMES 2009, Pittsburgh, PA, 2009.
Phillippi J.A., Nain A.S., Eskay M.A., Kubala A.A., Campbell P.G., Amon C.H., Gleason T., An in vitro 3-Dimensional Fibrous Scaffold to Study ECM Homeostasis in Patients with Bicuspid Aortic Valve, BMES 2009, Pittsburgh, PA, 2009.
Rojas-Solorzano L.R., Anna S.L., Bradeddine B., Amon C.H., Modeling and Simulation of a Rollerball Microfluidic Device, Proceedings of the 7th International Conference on Nanochannels, Microchannels, and Minichannels 2009, ICNMM2009 (PART A): 705-716, 2009.
[Abstract] The fluid delivery process through a rollerball device is investigated by means of physical modeling and numerical simulations. The microfluidic device is intended to deliver liquid above a substrate interacting with the surrounding air. While the fluid is delivered, air entrainment occurs through the capillary gap, creating a two-phase liquid-gas mixture whose composition and properties affect significantly the quality of the continuous fluid deposition. For the numerical solution of the 2D two-phase flow governing equations, the finite volume-based finite element method is used with 2nd order time-space schemes for the fully coupled system of equations. The quality of the liquid micro-volume delivery proves to be largely affected by both the speed of the roller and fluid properties. It is found that only under very low speed and some fluid properties, it is possible to guarantee a gas free liquid deposition. Envisioning the potential use of this convenient and popular device in the deployment of microfluid layers or substances at very small quantities with controlled quality, it is apparent the need for handling and channeling out the air entrainment without perturbing the liquid quality.
Ryan E.M., Tartakovsky A.M., Amon C.H., A Pore Scale Reactive Transport Model of a Solid Oxide Fuel Cell Cathode, 20th International Symposium on Transport Phenomena, Vancouver, BC, 2009.
Turney J.E., McGaughey A.J.H., Amon C.H., Critically assessing the application of quantum corrections to classical thermal conductivity predictions, Proceedings of the ASME Summer Heat Transfer Conference 2009, HT2009 2: 139-143, 2009.
[Abstract] Quantum corrections can be used to map the thermal conductivity predicted in a classical framework [e.g., a molecular dynamics (MD) simulation] to a corresponding value in a quantum system. This procedure is accomplished by equating the total energies and energy fluxes of the classical and quantum systems. The validity of these corrections is questionable because they are introduced in an ad hoc manner and are not derived from first principles. In this work, the validity of these quantum corrections is examined by comparing the thermal conductivity of Stillinger-Weber silicon calculated using a full quantum mechanical treatment to a quantum-corrected value predicted from a classical framework between temperatures of 10 K and 1000 K. The quantum and classical predictions are obtained using anharmonic lattice dynamics calculations. We find discrepancies between the quantum-corrected predictions and the quantum predictions obtained directly. We investigate the causes of these discrepancies and from our findings, conclude that quantum thermal conductivities cannot be predicted by applying simple corrections to the values obtained from a purely classical framework.
Sellan D.P., Amon C.H., Increasing Efficiencies of Thermoelectric Devices through Angular Interface Geometries, Proceedings of the ASME 3rd International Conference on Energy Sustainability 2009, ES2009 1: 781-789, 2009.
[Abstract] The phonon Boltzmann transport equation model is used to evaluate the reduction of out-of-plane thermal conductivity and subsequent increase in thermoelectric figure of merit when an angular interface is patterned between a germanium thin-film and silicon substrate. According to the acoustic mismatch model, the angular structure reduces the out-of-plane thermal conductivity by spatially redistributing phonons traveling in the out-of-plane direction. Simulation results demonstrate a 43% reduction in out-of-plane thermal conductivity when operating in the fully ballistic regime. This decrease in phononic thermal conductivity would result in an increase of intrinsic thermoelectric efficiency by a factor of 1.75.
Marin V.E., Romero D.A., Rincon J.A., Amon C.H., Finger S., A Comparison of Metamodel-Assisted Pre-Screening Criteria for Multi-Objective Genetic Algorithms, ASME 2009 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference , 2009.
[Abstract] Over the last few years, research activity in approximation (e.g. metamodels) and optimization (e.g. genetic algorithms) methods has improved upon current practices in engineering design and optimization of complex systems with respect to multiple performance metrics, by reducing the number of evaluations of the system's model that are needed to obtain the set of non-dominated solutions to a given multi-objetive optimal design problem. To this end, several authors have proposed to enhance Multi-Objective Genetic Algorithms (MOGAs) with metamodel-based pre-screening criteria (PSC), so that only those solutions that have the most potential to improve the current approximation of the Pareto Front are evaluated with the (costly) system model. The main goals of this work are to compare the performance of several PSC with an array of test functions taken from the literature, and to study the potential effect on their effectiveness and efficiency of using multi-response metamodels, instead of building independent, individual metamodels for each objective function, as has been done in previous work. Our preliminary results show that no single PSC is observed to be superior overall, though the Minimum of Minimum Distances and Expected Improvement criteria outperformed other PSC in most cases. Results also show that the use of multi-response metamodels improved both the effectiveness and efficiency of PSC and the quality of solution at the end of the optimization in 50% to 60% of test cases.
Nain A.S., Campbell P.G., Amon C.H., Cellular Dynamics in the Vicinity of Topological Constraints in Micro/Nanofiber Scaffolds, 2009 ASME International Mechanical Engineering Congress and Exposition, IMECE2009, Orlando, FL, 2009.
Nain A.S., Amon C.H., Polymeric Micro/Nanofiber Arrays: Manufacturing and Applications, 2009 ASME International Mechanical Engineering Congress and Exposition, IMECE2009, Orlando, FL, 2009.
Goicochea J.V., Madrid M., Amon C.H., Effects of Quantum Corrections and Isotope Scattering on Silicon Thermal Properties, 15th International Workshop on Thermal Investigations of ICs and Systems, THERMINIC 2009 : 197-202, 2009.
[Abstract] A quantum correction procedure is proposed to correct silicon thermal properties estimated with molecular dynamics (MD). The procedure considers the energy quantization per mode basis and the anharmonic nature of the potential energy function (including the thermal expansion of the crystal) and is applied to reported thermal properties of silicon estimated with MD in ref. [11], such as temperature, specific heat and thermal conductivity. The procedure facilitates the use of these properties as input to faster numerical methods, such as those based on the Boltzmann transport equation under the single relaxation time approximation. In addition, the effect of isotope scattering is included in reported values of phonon-phonon relaxation times. The effects of the correction procedure and the scattering with isotopes are analyzed in terms of the change of phonon specific heat, mean free path and thermal conductivity. We have found that the application of quantum corrections yields a significant reduction in the contribution of high-frequency modes to the overall thermal conductivity. This contribution is further reduced by the inclusion of isotope scattering. At 220 K, the total contribution of optical modes reduces from 12.3 % (before quantum corrections) to 5.8 %; and to 2 % when the isotope scattering is also considered. The quantum corrections and the inclusion of isotope scattering are found to bring the estimated thermal conductivity into close agreement with experimental values. The relative contributions of the acoustic and optical modes after quantum corrections agrees very well with recently reported ab initio results.2009.
Nain A.S., Miller E., Sitti M., Campbell P., Amon C.H., Fabrication of Single and Multi-layer Fibrous Biomaterial Scaffolds for Tissue Engineering, ASME International Mechanical Engineering Congress and Exposition, Proceedings 2: 231-238, 2009.
[Abstract] For regenerative medicine applications, we need to expand our understanding of the mechanisms by which nature assembles and functionalizes specialized complex tissues to form a complete organism. The first step towards this goal involves understanding the underlying complex mechanisms of highly organized behavior spanning not only diverse scientific fields, but also nano, micro and macro length-scales. For example, an engineered fibrous biomaterial scaffold possessing the hierarchal spatial properties of a native extracellular matrix (ECM) can serve as a building block upon which living cells are seeded for repair or regeneration. The hierarchical nature of ECM along with the inherent topological constraints of fiber diameter, fiber spacing, multi-layer configurations provide different pathways for living cells to adapt and conform to the surrounding environment. Our previously developed Spinneret based Tunable Engineered Parameters (STEP) technique to deposit biomaterial scaffolds in aligned configurations has been used for the first time to deposit single and multi-layer biological scaffolds of fibrinogen. Fibrinogen is a very well established tissue engineering scaffold material, as it improves cellular interactions and allows scaffold remodeling compared to synthetic polymers. Current state-of-the-art fiber deposition techniques lack the ability to fabricate scaffolds of desired fiber dimensions and orientations and in this study we present fabrication and aligned deposition of fibrinogen fiber arrays with diameters ranging from sub-200 nm to sub-microns and several millimeters in length. The fabricated scaffolds are then cultured with pluripotent mouse C2C12 cells for seven days and cells on the scaffolds are observed to elongate resembling myotube morphology along the fiber axis, spread along intersecting layers and fuse into bundles at the macroscale. Additionally, we demonstrate the ability to deposit poly (lactic-co-glycolic acid) (PLGA), Polystyrene (PS) biomaterial scaffolds of different diameters to investigate the effects of topological variations on cellular adhesion, proliferation and migration. Previous studies have indicated cells making right angle transitions upon encountering perpendicular double layer fibers and cellular motion is thwarted in the vicinity of diverging fibers. Current ongoing studies are aimed at determining the effects of fiber diameter and fiber spacing on mouse C2C12 cellular adhesion and migration, which are envisioned to aid in the design of future scaffolds for tissue engineering possessing appropriate material and geometrical properties.
Turney J.E., McGaughey A.J.H., Amon C.H., Thin Film Thermal Conductivity by Anharmonic Lattice Dynamics Calculations, ASME International Mechanical Engineering Congress and Exposition, Proceedings 13(PART B): 1195-1197, 2009.
[Abstract] Lattice dynamics calculations are used to investigate thermal transport in argon thin films with thicknesses ranging between one and ten nanometers. Quasi-harmonic lattice dynamics calculations are used to find the frequencies and mode shapes of non-interacting phonons. This information is then used as input for anharmonic lattice dynamics calculations, whereby we compute the frequency shift and lifetime of each phonon mode due to interactions with other phonons. The phonon frequencies, group velocities, and lifetimes determined with the lattice dynamics techniques are then used to compute the in-plane thermal conductivity of the thin films as a function of film thickness. The thermal conductivities predicted by the lattice dynamics methods are compared to predictions from molecular dynamics simulations. Differences in the phonon characteristics in thin films compared to bulk crystals are examined by comparing the contribution to the thermal conductivity as a function of frequency.
Nain A.S., Sirti M., Amon C.H., Polymeric Micro/Nanofiber Manufacturing and Mechanical Characterization, ASME International Mechanical Engineering Congress and Exposition, Proceedings 13(PART A): 295-303, 2009.
[Abstract] Polymeric nanofibers are finding increasing number of applications and hold the potential to revolutionize diverse fields such as tissue engineering, smart textiles, sensors, and actuators. Aligning and producing long smooth, uniform and defect-free fibers with control on fiber dimensions at the submicron and nanoscale has been challenging due to fragility of polymeric materials. Besides fabrication, the other challenge lies in the ability to characterize these fibers for mechanical properties, as they are widely believed to have improved properties than bulk due to minimization of defects. In this study we present an overall strategy for fabrication and mechanical characterization of polymeric fibers with diameters ranging from sub-50 nm to sub-microns. In the proposed fabrication strategy, polymeric solution is continuously pumped through a glass micropipette which is collected in the form of aligned fiber arrays on a rotating substrate. Polymer molecular weight and polymer solution concentration play dominant roles in controlling the fiber dimensions, which can be used to deposit fibers of different diameters in the same layer or successively built up multi-layer structures. Using this approach, we demonstrate single and multi-layer architectures of several polymeric systems such as Polystyrene (PS), Poly(methyl methacrylate) (PMMA), Poly lactic acid (PLA), and poly(lactic-co-glycolic acid) (PLGA). Further, we demonstrate the ability to manufacture PMMA fixed-free boundary condition cantilevers by breaking the fixed-fixed boundary condition PMMA fibers using Atomic Force Microscope (AFM) in the lateral mode. An integrated approach for mechanical characterization of polymeric fibers is developed. In this approach, the fibers are first deposited on commercially available Transmission Electron Microscopy (TEM) grids in aligned configurations and are mapped for accurate locations under the TEM. Subsequently, the fibers are carefully placed under the AFM and mechanically characterized for flexural modulus using lateral force microscopy (LFM). Finally, accurate fiber dimensions are determined under the Scanning Electron Microscope (SEM). The unique advantage of this approach lies in the ability to deposit a large number of fibers with tunable diameters in aligned configurations with fixed-fixed boundary conditions and requires no external manipulation. Finally, we present a novel methodology to study the resonance characteristics of fixed-fixed boundary condition suspended fibers using a commercially available Laser Doppler Vibrometer (LDV) for sensor applications. The methods developed in this study will greatly aid in increasing our fundamental knowledge of polymeric materials at reduced lengthscales and allow integration of these one-dimensional building blocks in bottom-up assembly environments.
Turney J.E., McGaughey A.J.H., Amon C.H., Argon Thermal Conductivity by Anharmonic Lattice Dynamics Calculations, 2008 Proceedings of the ASME Summer Heat Transfer Conference, HT 2008 1: 345-347, 2009.
[Abstract] Lattice dynamics calculations are used to investigate thermal transport in the face-centered cubic Lennard-Jones (LJ) argon crystal between temperatures of 20 and 80 K. First, quasi-harmonic lattice dynamics calculations are used to find the frequencies and mode shapes of non-interacting phonons [1]. This information is then used as input for anharmonic lattice dynamics calculations. Anharmonic lattice dynamics is a means of computing the frequency shift and lifetime of each phonon mode due to interactions with other phonons [2]. The phonon frequencies, group velocities, and lifetimes, determined with the lattice dynamics methods, are then used to compute the thermal conductivity. The thermal conductivities predicted by the lattice dynamics methods are compared to predictions from molecular dynamics simulations. The two methods are found to agree well at low temperature but diverge at higher temperatures (i.e., near the melting point). The properties of individual phonon modes are used to identify the modes that dominate thermal transport.
Romero D.A., Amon C.H., Finger S., A Study of Covariance Functions for Multi-Response Metamodeling for Simulation-based Design and Optimization, 2008 Proceedings of the ASME International Design Engineering Technical Conferences and Computers and Information in Engineering Conference, DETC 2008 1(PART B): 883-893, 2009.
[Abstract] The optimal design of complex systems in engineering requires the availability of mathematical models of system's behavior as a function of a set of design variables; such models allow the designer to find the best solution to the design problem. However, system models (e.g. CFD analysis, physical prototypes) are usually time-consuming and expensive to evaluate, and thus unsuited for systematic use during design. Approximate models, or metamodels, of system behavior based on a limited set of data allow significant savings by reducing the resources devoted to modeling during the design process. In our work in engineering design based on multiple performance criteria, we propose the use of Multi-response Bayesian Surrogate Models (MRBSM) to model several aspects of system behavior jointly, instead of modeling each individually. By doing so, it is expected that the observed correlation among the response variables can be used to achieve better models with smaller data sets. In this work, we study the approximation capabilities of several covariance functions needed for multi-response metamodeling with MRBSM, performing a simulation study in which we compare MRBSM based on different covariance functions against metamodels built individually for each response. Our preliminary results indicate that MRBSM outperforms individual metamodels in 46% to 67% of the test cases, though the relative performance of the studied covariance functions is highly dependent on the sampling scheme used and the actual correlation among the observed response values.
Escobar R.A., Amon C.H., Thin Film Phonon Heat Conduction by the Dispersion Lattice Boltzmann Method, Journal of Heat Transfer 130(9), 2008.
[Abstract] Numerical simulations of time-dependent thermal energy transport in semiconductor thin films are performed using the lattice Boltzmann method applied to phonon transport. The discrete lattice Boltzmann Method is derived from the continuous Boltzmann transport equation assuming nonlinear frequency-dependent phonon dispersion for acoustic and optical phonons. Results indicate that the heat conduction in silicon thin films displays a transition from dififusive to ballistic energy transport as the characteristic length of the system becomes comparable to the phonon mean free path and that the thermal energy transport process is characterized by the propagation of multiple superimposed phonon waves. The methodology is used to characterize the time-dependent temperature profiles inside films of decreasing thickness. Thickness-dependent thermal conductivity values are computed based on steady-state temperature distributions obtained from the numerical models. It is found that reducing feature size into the subcontinuum regime decreases thermal conductivity when compared to bulk values, at a higher rate than what was displayed by the Debye-based gray lattice Boltzmann method.
Nain A.S., Phillippi J.A., Sitti M., MacKrell J., Campbell P.G., Amon C.H., Control of cell behavior by aligned micro/nanofibrous biomaterial scaffolds fabricated by spinneret-based tunable engineered parameters (STEP) technique, Small 4(8): 1153-1159, 2008.
[Abstract] Single and multilayered micro/nanofibrous scaffolds of different biomaterials were fabricated with tunable fiber spacing in aligned configurations. Seeded mouse C2C12 progenitor cells on the scaffolds resulted in cellular elongation resembling myotube morphology along the fiber axis, spreading of cells along orthogonal layers and fusing of cells into bundle at the macroscale. cells were observed to make right angle transitions along orthogonal layers and stop or reverse direction of motion upon encountering diverging fibers. Spinneret-based tunable engineered parameters (STEP) offers scalability to fabricate biomaterial-based scaffolds from micro/ nanoelectromechanical systems based implantable devices to tissue engineering macroscales. This study could lead to the creation of putative biomimetic scaffolds to control cell proliferation, migration and differentiation for tissue engineering applications.
Trivic D.N., Amon C.H., Modeling the 3-D radiation of anistropically scattering media by two different numerical methods, International Journal of Heat and Mass Transfer 51(11-Dec): 2711-2732, 2008.
[Abstract] An original model and code for 3-D radiation of anisotropically scattering gray media is developed where radiative transfer equation (RTE) is solved by finite volume method (FVM) and scattering phase function (SPF) is defined by Mie Equations (ME). To the authors' best knowledge this methodology was not developed before. Missing the benchmark, another new 3-D model and code, which solve the same problems, based on a combination of zone method (ZM) and Monte Carlo method (MC), as a solution of RTE, is developed. Here SPF is also calculated by Mie Equations. The conception ZM + MC is numerically expensive and is used and recommended only as a benchmark. The 3-D rectangular enclosure and the spherical geometry of particles are considered. The both models are applied: (i) to an isotropic and to four anisotropic scattering cases previously used in literature for 2-D cases and (ii) to solid particles of several various coals and of a fly ash. The agreement between the predictions obtained by these two different numerical methods for coals and ash is very good. The effects of scattering albedo and of wall reflectivity on the radiative heat flux are presented. It was found that the developed 3-D model, where FVM was coupled with ME, is reliable and accurate. The methodology is also suitable for extension towards: (i) mixture of non-gray gases with particles and (ii) incorporation in computational fluid dynamics.
Ryan E., Amon C.H., Modeling the Species Transport and Reactions in an SOFC Cathode using Smoothed Particle Hydrodynamics, Proceedings of the 6th International Conference on Fuel Cell Science, Engineering, and Technology : 283-289, 2008.
[Abstract] A computer model for species mass transport within the cathode of a solid oxide fuel cell (SOFC) is being developed based on the smoothed particle hydrodynamics method. Smoothed particle hydrodynamics is a grid-free modeling technique which uses a Lagrangian framework to model fluid dynamics systems as a discrete system of particles using a smoothing approximation. The method is able to handle complex geometries and complex physical and chemical phenomena with relative ease compared to grid based methods. The model is being developed to investigate chromium poisoning in the cathode of solid oxide fuel cells by examining the transport and reactions of chromium species in the cathode. The model includes ordinary and Knudsen diffusion within the cathode, which have both been shown to be significant mass transport processes in solid oxide fuel cells, and the adsorption reactions on the surface of the cathode. Chromium is thought to adsorb to the surface of the cathode reducing the number of adsorption sites for oxygen. The pore scale nature of smoothed particle hydrodynamics eliminates the need for effective properties of the porous structure to be used in the diffusion model. Previous solid oxide fuel cell electrode models based on effective diffusion models such as the dusty gas model rely on the bulk properties of the porous medium such as porosity and tortuosity to describe the complex microstructure of the electrodes. Bulk properties add unwanted uncertainty and error into the model, by using the smoothed particle hydrodynamics method some of these uncertainties can be removed from the model. The paper reviews past work on chromium poisoning, discusses the applicability of smoothed particle hydrodynamics for modeling the solid oxide fuel cell cathode and describes the model in detail.
Goicochea J.V., Madrid M., Amon C.H., Hierarchical Modeling of Heat Transfer in Silicon-based Electronic Devices, 2008 11th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, I-THERM : 1006-1017, 2008.
[Abstract] Heat transfer modeling in electronic devices has gained importance over the last decade in the design of better performing devices. The trend towards miniaturization of these devices has led to components that operate in the micro and nano-meter and in the micro and pico-second ranges. When the characteristic dimensions of the electronic components are comparable to or smaller than the mean free path of the energy carriers (in this case phonons), the thermal conductivity, which affects their performance and reliability, reduces due to the scattering of the energy carriers with the boundaries. Several modeling approaches have been proposed in the literature to describe sub-continuum heat transport; however, the hierarchical modeling of heat transfer in electronic devices has been limited. This has precluded, at the industry level, the analysis of how changes at sub-continuum level impact the overall performance and reliability of these devices. There are numerous devices and applications whose design, performance and reliability are suitable for optimization if a hierarchical model was available. In this work, we present a hierarchical model capable of integrating the scales involved in the thermal analysis of electronic components. The integration of participating scales is achieved in three steps. First, we use molecular dynamics simulations to estimate the thermal properties (i.e. phonon relaxation times, dispersion relations and group velocities, among others), required to solve the Boltzmann transport equation (BTE). Then we apply quantum corrections (QCs) to the MD results to make them suitable for BTE, and lastly, we solve the BTE on various domains, subject to different boundary and initial conditions. Our hierarchical model is applied to silicon-based devices.
Wesner J.W., Amon C.H., Bigrigg M.W., Subrahmanian E., Westerberg A.W., and Filipski K.J., Student Team Formation and Assignment in a Multidisciplinary Engineering Design Projects Course, International Journal of Engineering Education, Vol. 23(3), pp. 517-626, 2007.
Nain A.S., Gupta A., Amon C.H., Sitti M., Dry spinning polymeric nano/microfiber arrays using glass micropipettes with controlled porosities and fiber diameters, 2007 7th IEEE International Conference on Nanotechnology - IEEE-NANO 2007, Proceedings : 728-732, 2007.
[Abstract] We present a method for dry spinning polymeric nano/microfiber arrays. In this technique polymer solution is continuously ejected from a stationary glass micropipette and the fibers are deposited as continuous arrays in parallel and complex geometrical configurations on a rotating substrate mounted on to a translation stage. As the polymer solution exits the glass micropipette, ambient air is used to evaporate the solvent, thus solidifying the fiber which is then deposited on the rotating substrate. For a given polymer, altering the processing and material parameters allows depositing fiber arrays with highly tunable porosities and uniform fiber diameters. The fiber array porosity is observed to decrease with increasing angular velocity of the rotating substrate at a constant translational stage velocity. Fiber array breaking strength experiments as a function of porosity show higher loads required to break low porosity arrays, which is critical in designing stronger materials. Additionally, single and double layered biological scaffolds fabricated using this technique are seeded with mouse C2C12 cells and cellular dynamics of adhesion, migration and proliferation is investigated.
Smith B., Amon C.H., Simultaneous Electrothermal Test Method for Pyroelectric Microsensors, Journal of Electronic Packaging 129(4): 504-511, 2007.
[Abstract] Pyroelectric film materials, including polyvinylidene fluoride (PVDF) and its copolymers (e.g., P(VDF/trifluoroethylene)), are attractive candidates for low-cost infrared detection and imaging applications due to their compatibility with complementary metal-oxide semiconductor processing and inexpensive packaging requirements compared to semiconductor-based detectors. The pyroelectric coefficient (p) describes the material's electric response to a change in sensor temperature and is the main contributor to the sensitivity and detectivity of the system. However, this value can vary greatly with film fabrication and poling processes, and its measurement is often highly coupled to the material's thermal diffusivity. This paper describes a new approach to film characterization that combines the popular "3-omega" technique for thermal characterization with a modified version of the laser intensity modulation method for determining the film's pyroelectric coefficient. The new method is capable of simultaneously measuring film conductivity, diffusivity, and pyroelectric coefficient. It could increase the accuracy of the pyroelectric measurements by providing in situ thermal data to the electrical model instead of relying on published values or thermal measurements of a different sample. We also present a fabrication process that can be used to pole and measure a variety of pyroelectric materials and a mathematical framework to study the thermal phenomena of the setup. The thermal model is used to highlight the methodology's sensitivity to uncertainties in the geometric and material property values of the layers surrounding the pyroelectric film.
Escobar R.A., Amon C.H., Influence of Phonon Dispersion on Transient Thermal Response of Silicon-on-Insulator Transistors Under Self-Heating Conditions, Journal of Heat Transfer 129(7): 790-797, 2007.
[Abstract] Lattice Boltzmann method (LBM) simulations of phonon transport are performed in one-dimesional (ID) and 2D computational models of a silicon-on-insulator transistor, in order to investigate its transient thermal response under Joule heating conditions, which cause a nonequilibrium region of high temperature known as a hotspot. Predictions from Fourier diffusion are compared to those from a gray LBM based on the Debye assumption, and from a dispersion LBM which incorporates nonlinear dispersion for all phonon branches, including explicit treatment of optical phonons without simplifying assumptions. The simulations cover the effects of hotspot size and heat pulse duration, considering a frequency-dependent heat source term. Results indicate that, for both models, a transition from a Fourier diffusion regime to a ballistic phonon transport regime occurs as the hotspot size is decreased to tens of nanometers. The transition is characterized by the appearance of boundary effects, as well as by the propagation of thermal energy in the form of multiple, superimposed phonon waves. Additionally, hotspot peak temperature levels predicted by the dispersion LBM are found to be higher than those from Fourier diffusion predictions, displaying a nonlinear relation to hotspot size, for a given, fixed, domain size.
Nain A.S., Chung F., Rule M., Jadlowiec J.A., Campbell P.G., Amon C.H., Sitti M., Microrobotically Fabricated Biological Scaffolds for Tissue Engineering, Proceedings - IEEE International Conference on Robotics and Automation : 1918-1923, 2007.
[Abstract] A microrobotic method for fabricating multilayered poly (lactic acid) (PLA) biological scaffolds using micropipettes for tissue engineering applications is presented. Biological scaffolds are fabricated over several different substrates by drawing and solidification of a viscous liquid polymer solution pumped continuously through a glass micropipette. The proposed method produces highly aligned, multilayered, crisscrossed fiber scaffolds with user specified pore sizes and diameters in the range from 1 to 10 micrometer. Attachment, proliferation and differentiation of C2C12 mouse pluripotential cells seeded on individual, parallel, and intersecting fibers is successfully demonstrated. The proposed robotic methodology consistently provides parameterized biological scaffolds to aid studies in tissue engineering and to develop novel MEMS, filtration and controlled drug delivery devices.
Smith B., Amon C.H., Compact Thermal and System Modeling of Chip-Scale Pyroelectric Infrared Imager, 2007 Proceedings of the ASME InterPack Conference, IPACK 2007 2: 757-766, 2007.
[Abstract] The performance of pyroelectric infrared detectors is directly related to the ability of the sensor material to retain infrared energy (heat) incident from the source and to react fast to changing heat loads. This leads to a complicated, three dimensional, transient thermal models when many detectors are assembled into an infrared focal plane array (IRFPA) for thermal imaging. Adjacent pixels and the underlying substrate conduct heat away from the sensor material and add thermal mass to the system. This paper describes efforts and drawbacks in deriving a system model to capture thermal phenomena in a candidate IRFPA. Of particular interest is the tradeoff between cumbersome finite element models (long solve time, complicated meshes) and a reduced-size RC network circuit model that is simple to solve and integrate with the electrical design but may not capture the full thermal behavior of the system adequately. The thermal models are cast in terms of the operating principles of pyroelectric devices to describe a full electricalthermal system model that adapts existing literature in the field to the specific system described in this work.
Smith B., Amon C.H., Experimental Measurement of Multiple Thermal Properties by Error Minimization, 2007 Proceedings of the ASME/JSME Thermal Engineering Summer Heat Transfer Conference - HT 2007 2: 283-291, 2007.
[Abstract] Common thin film thermometry techniques are usually based on transient heat diffusion within a sample and its surroundings and are therefore sensitive to the film's thermal conductivity (k) and heat capacity (C). This presents a problem of under-constraint in the numerical fitting models when both k and C of a given film are unknown. A number of approaches and assumptions have been studied to eliminate this dual dependence or estimate C analytically. However, they often amount to little more than fitting parameters, experimental assumptions, and rough estimates for many composite and polymer films that are emerging in the microelectronics and MEMS industries. The effect that the uncertainty in one property has on the prediction of the other is discussed in the framework of the polymer film PVDF used in many microsensor and actuator applications. An error surface analysis is used to describe the link between assumption and prediction for thermoreflectance and temperature phase measurement techniques. A methodology is presented that combines the results of two thermal tests through an error minimization algorithm to solve for both k and C with no analytical assumptions or approximations. This approach is demonstrated with an experimental test case, validated with synthesized data, and generalized to any system variable and a multitude of thin film thermometry variable or thin film thermometry technique.
Turney J.E., McGaughey A.J.H., Amon C.H., Effects of Confinement and Surface Reconstruction on the Lattice Dynamics and Thermal Transport Properties of Thin Films, 2007 Proceedings of the ASME/JSME Thermal Engineering Summer Heat Transfer Conference - HT 2007 2: 789-797, 2007.
[Abstract] Phonon transport in argon and silicon thin films is examined using harmonic lattice dynamics theory and the Lennard-Jones and Stillinger-Weber potentials. Film thicknesses ranging from 0.8 to 33.5 nm for argon and 0.4 to 8.6 nm for silicon are examined at a temperature of 0 K. Both reconstructed films and films built using the bulk unit cell are considered. Phonon dispersion curves for the in-plane direction and the density of states are computed from lattice dynamics and compared to predictions for a bulk system. The results from the lattice dynamics calculations are used to predict the thermal conductivities of the bulk and thin film structures.
Escobar R.A., Amon C.H., Guzman A.M., Thin Film Phonon Heat Conduction by the Dispersion Lattice Boltzmann Method, 2007 Proceedings of the ASME/JSME Thermal Engineering Summer Heat Transfer Conference - HT 2007 3: 257-266, 2007.
[Abstract] Numerical simulations of time-dependent thermal energy transport in semiconductor thin films are performed using the Lattice Boltzmann Method applied to phonon transport. The discrete Lattice Boltzmann Method is derived from the continuous Boltzmann transport equation assuming nonlinear, frequency-dependent phonon dispersion for acoustic and optical phonons. Results indicate that the heat conduction in silicon thin films displays a transition from diffusive to ballistic energy transport as the characteristic length of the system becomes comparable to the phonon mean free path, and that the thermal energy transport process is characterized by the propagation of multiple, superimposed phonon waves. The methodology is used to characterize the time-dependent temperature profiles inside films of decreasing thickness. Thickness-dependent thermal conductivity values are computed based on steady-state temperature distributions obtained from the numerical models. It is found that reducing feature size into the subcontinuum regime decreases the thermal conductivity when compared to bulk values, at a higher rate than what was displayed by the Debye-based gray Lattice Boltzmann Method.
Gomes C.J., Madrid M., Goicochea J.V., Amon C.H., In-plane and Out-of-plane Thermal Conductivity of Silicon Thin Films Predicted by Molecular Dynamics, Journal of Heat Transfer 128(11): 1114-1121, 2006.
[Abstract] The thermal conductivity of silicon thin films is predicted in the directions parallel and perpendicular to the film surfaces (in-plane and out-of-plane, respectively) using equilibrium molecular dynamics, the Green-Kubo relation, and the Stillinger-Weber interatomic potential. Three different boundary conditions are considered along the film surfaces: frozen atoms, surface potential, and free boundaries. Film thicknesses range from 2 to 217 nm and temperatures from 300 to 1000 K. The relation between the bulk phonon mean free path (Λ) and the film thickness (ds) spans from the ballistic regime (Λ » ds) at 300 K to the diffusive, bulk-like regime (Λ « ds) at 1000 K. When the film is thin enough, the in-plane and out-of-plane thermal conductivity differ from each other and decrease with decreasing film thickness, as a consequence of the scattering of phonons with the film boundaries. The in-plane thermal conductivity follows the trend observed experimentally at 300 K. In the ballistic limit, in accordance with the kinetic and phonon radiative transfer theories, the predicted out-of-plane thermal conductivity varies linearly with the film thickness, and is temperature-independent for temperatures near or above the Debye's temperature.
Valencia A.A., Guzman A.M., Finol E.A., Amon C.H., Blood Flow Dynamics in Saccular Aneurysm Models of the Basilar Artery, Journal of Biomechanical Engineering 128(4): 516-526, 2006.
[Abstract] Blood flow dynamics under physiologically realistic pulsatile conditions plays an important role in the growth, rupture, and surgical treatment of intracranial aneurysms. The temporal and spatial variations of wall pressure and wall shear stress in the aneurysm are hypothesized to be correlated with its continuous expansion and eventual rupture. In addition, the assessment of the velocity field in the aneurysm dome and neck is important for the correct placement of endovascular coils. This paper describes the flow dynamics in two representative models of a terminal aneurysm of the basilar artery under Newtonian and non-Newtonian fluid assumptions, and compares their hemodynamics with that of a healthy basilar artery. Virtual aneurysm models are investigated numerically, with geometric features defined by β=0 deg and β=23.2 deg, where β is the tilt angle of the aneurysm dome with respect to the basilar artery. The intra-aneurysmal pulsatile flow shows complex ring vortex structures for β=0 deg and single recirculation regions for β=23.2 deg during both systole and diastole. The pressure and shear stress on the aneurysm wall exhibit large temporal and spatial variations for both models. When compared to a non-Newtonian fluid, the symmetric aneurysm model (β=0 deg) exhibits a more unstable Newtonian flow dynamics, although with a lower peak wall shear stress than the asymmetric model (β=23.2 deg). The non-Newtonian fluid assumption yields more stable flows than a Newtonian fluid, for the same inlet flow rate. Both fluid modeling assumptions, however, lead to asymmetric oscillatory flows inside the aneurysm dome.
Escobar R., Smith B., Amon C.H., Lattice Boltzmann modeling of subcontinuum energy transport in crystalline and amorphous microelectronic devices, Journal of Electronic Packaging 128(2): 115-124, 2006.
[Abstract] Numerical simulations of time-dependent energy transport in semiconductor thin films are performed using the lattice Boltzmann method applied to phonon transport. The discrete lattice Boltzmann method is derived from the continuous Boltzmann transport equation assuming first gray dispersion and then nonlinear, frequency-dependent phonon dispersion for acoustic and optical phonons. Results indicate that a transition from diffusive to ballistic energy transport is found as the characteristic length of the system becomes comparable to the phonon mean free path. The methodology is used in representative microelectronics applications covering both crystalline and amorphous materials including silicon thin films and nanoporous silica dielectrics. Size-dependent thermal conductivity values are also computed based on steady-state temperature distributions obtained from the numerical models. For each case, reducing feature size into the sub-continuum regime decreases the thermal conductivity when compared to bulk values. Overall, simulations that consider phonon dispersion yield results more consistent with experimental correlations.
Yao S.C., Tang X., Hsieh C.C., Alyousef Y., Vladimer M., Fedder G.K., Amon C.H., Micro-electro-mechanical Systems (MEMS)-based Micro-scale Direct Methanol Fuel Cell Development, Energy 31(5): 636-649, 2006.
[Abstract] This paper describes a high-power density, silicon-based micro-scale direct methanol fuel cell (DMFC), under development at Carnegie Mellon. Major issues in the DMFC design include the water management and energy-efficient micro fluidic sub-systems. The air flow and the methanol circulation are both at a natural draft, while a passive liquid-gas separator removes CO2 from the methanol chamber. An effective approach for maximizing the DMFC energy density, pumping the excess water back to the anode, is illustrated. The proposed DMFC contains several unique features: a silicon wafer with arrays of etched holes selectively coated with a non-wetting agent for collecting water at the cathode; a silicon membrane micro pump for pumping the collected water back to the anode; and a passive liquid-gas separator for CO2 removal. All of these silicon-based components are fabricated using micro-electro-mechanical systems (MEMS)-based processes on the same silicon wafer, so that interconnections are eliminated, and integration efforts as well as post-fabrication costs are both minimized. The resulting fuel cell has an overall size of one cubic inch, produces a net output of 10 mW, and has an energy density three to five times higher than that of current lithium-ion batteries.
Nain A.S., Amon C.H., Sitti M., Proximal Probes based Nanorobotic Fabrication Modeling and Characterization of Polymer Micro/Nanofibers, IEEE Transactions on Nanotechnology 5(5): 499-510, 2006.
[Abstract] This paper proposes a nanorobotic fiber fabrication method which uses proximal probes to draw polymer fibers down to few hundred nanometers in diameter and several hundred micrometers in length. Using proximal probes such as Atomic Force Microscope (AFM) and Scanning Tunneling Microscope (STM) or glass micropipettes, liquid polymers dissolved in a solvent are drawn. During drawing, the solvent evaporates in real-time which solidifies the fiber. Controlling the drawn fibers trajectory and solidification in three-dimensions (3-D), suspended fibers, fiber cantilevers, custom 3-D fibers, and fiber networks, are proposed to be fabricated. Poly(methyl methacrylate) (PMMA) polymer dissolved in chlorobenzene is used to form a variety of suspended polymer fibers with diameters from few microns to 200 nm. Fabrication of crossed and linear networks of fibers is also demonstrated. Viscoelastic modeling of polymer fiber drawing is realized using a finite element method to test the significance of the drawing speed and velocity profile on the extensional behavior of the drawn fiber. Since the mechanical properties of the drawn micro/nanofibers could vary from the bulk polymer material significantly, mechanical characterization of suspended fibers using an AFM and a Nanoindenter setup is proposed. Extending this technique to a variety of nonconductive and electroactive polymer fibers, many novel applications in micro/nanoscale sensors, actuators, fibrillar structures, and optical and electronic devices would become possible.
Ghai S.S., Kim W.T., Amon C.H., Jhon M.S., Transient Thermal Modeling of a Nanoscale Hot-spot in Multilayered Film, Journal of Applied Physics 99(8), 2006.
[Abstract] A subcontinuum based lattice Boltzmann method is used to accurately model the transient thermal response of a nanoscale hot spot in solids. We developed the numerical scheme for the hot spot in a thin uniform material and extended the approach to study the multilayered materials. We observed that subcontinuum effects of high temperature rise become more prominent as the size of the film reduces to the scale of carrier mean free path. The thermal transport through a double layer is also considered, both for constant temperature difference across the double layer and hot-spot generation in one of the layers, using the diffusive mismatch scattering model at the interface. A finite temperature jump is observed at the interface whose magnitude depends upon the dimensions and properties of the material on the either side of the interface. The insight into the nanoscale thermal modeling, acquired in this work via a relatively simple model, will be critical for the design and operation of complex data storage and electronic systems, dealing with subcontinuum systems.
Escobar R.A., Ghai S.S., Jhon M.S., Amon C.H., Multi-Length and Time Scale Thermal Transport Using the Lattice Boltzmann Method with Application to Electronics Cooling, International Journal of Heat and Mass Transfer 49(1-Feb): 97-107, 2006.
[Abstract] The lattice Boltzmann method (LBM) is used to investigate one-dimensional, multi-length and -time scale transient heat conduction in crystalline semiconductor solids, in which sub-continuum effects are important. The implementation of this method and its application to electronic devices are described. A silicon-on-insulator transistor subject to Joule heating conditions is used as a case study to illustrate the essence of the LBM. We compare our LBM results, for the diffusive to the ballistic transport regimes, with various hierarchical methodologies of heat transport such as the Fourier, Cattaneo, and ballistic-diffusive transport equations.
Narumanchi S.V.J., Murthy J.Y., Amon C.H., Boltzmann Transport Equation-based Thermal Modeling Approaches for Hotspots in Microelectronics, Heat and Mass Transfer/Waerme- und Stoffuebertragung 42(6): 478-491, 2006.
[Abstract] Fourier diffusion has been found to be inadequate for the prediction of heat conduction in modern microelectronics, where extreme miniaturization has led to feature sizes in the sub-micron range. Over the past decade, the phonon Boltzmann transport equation (BTE) in the relaxation time approximation has been employed to make thermal predictions in dielectrics and semiconductors at micro-scales and nano-scales. This paper presents a review of the BTE-based solution methods widely employed in the literature and recently developed by the authors. First, the solution approaches based on the gray formulation of the BTE are presented. The semi-gray approach, moments of the Boltzmann equation, the lattice Boltzmann approach, and the ballistic-diffusive approximation are also discussed. Models which incorporate greater details of phonon dispersion are also presented. Hotspot self-heating in sub-micron SOI transistors and transient electrostatic discharge in NMOS transistors are also examined. Results, which illustrate the differences between some of these models reveal the importance of developing models that incorporate substantial details of phonon physics. The impact of boundary conditions on thermal predictions is also investigated.
Amon C.H., Ghai S.S., Kim W.T., Jhon M.S., Modeling of Nanoscale Transport Phenomena: Application to Information Technology, Physica A: Statistical Mechanics and its Applications 362(1): 36-41, 2006.
[Abstract] We develop the fundamentals in rule-based mathematics and physics dealing with nanoscale transport processes and apply our formulations to the design of nano/information technology systems. We showcase the versatility of lattice Boltzmann method (LBM) as a tool to simulate apparently different physical systems and present the underlying similarity in the discrete rules required in different systems which helps in providing the insight into one system by using an analogous system. We use a unified framework to simulate nanoscale air bearing and heat transfer. LBM not only enhances the computational capability of a nanoscale confined gaseous system by calculating both velocity and pressure fields simultaneously but also simulates nanoscale energy transport in both magnetic and electronic material accurately.
Ghai S.S., Chung P.S, Kim W.T., Jhon M.S., Amon C.H., Thermal Modeling of a Multilayered Film via Taylor Series Expansion- and Least Squares-based-Lattice Boltzmann Method, IEEE Transactions on Magnetics 42(10): 2474-2476, 2006.
[Abstract] An enhanced lattice Boltzmann scheme, Taylor series expansion- and least squares-based-lattice Boltzmann method (TLLBM), is adopted to simulate transient thermal behavior in a thin multilayer. The meshless formulation of TLLBM not only handles the changes in material properties from one layer to another, but also facilitates the general geometry handling capabilities. The energy carrier interaction at the interface is governed via diffusive mismatch model (DMM). We simulated transient thermal behavior of a hot-spot in the dual-layer by incorporating a heat source in one of the layers. The sub-continuum effect of anisotropic thermal transport is presented where the heat preferentially flows laterally in the layer with higher conductivity as its thickness is reduced. The insight into the nanoscale thermal behavior acquired via a relatively simple model will be critical for the design and operation of complex data storage and electronic systems, where thermal transport plays an active and critical role
Nain A.S., Wong J.C., Amon C.H., Sitti M., Drawing suspended polymer micro-/nanofibers using glass micropipettes, Applied Physics Letters 89(18), 2006.
[Abstract] This letter proposes a method for fabricating suspended micro-/nanoscale polymer fibers continuously, in which polymeric micro-/nanofibers are formed by drawing and solidification of a viscous liquid polymer solution which is pumped through a glass micropipette. By controlling the drawing parameters, this method is demonstrated to form networks of suspended fibers having amorphous internal structure and uniform diameters from micrometers down to sub- 50-nm for different molecular weights of polystyrene dissolved in xylene.
Baekyoung S., Chepuri K., Dewan-Sandur, Yongje L., Agonafer D., Agonafer D., Amon C.H., Thermal Enhancement of Stacked Dies Using Thermal Vias, American Society of Mechanical Engineers, Heat Transfer Division : 287-293, 2006.
[Abstract] Following Moore's law, the number of transistors on a die continues to rise and has recently exceeded a billion on high end processors. In light of the convergence of technology, power requirements is becoming a serious concern even on low density interconnect systems such as cellular phones and personal digital assistants. Also, in order to minimize foot prints, the recent trend in packaging is stacking. The stacking, however, creates challenges in cooling and especially if one is to include logic in the stack. The primary heat flow path for stacking is through the substrate and as the number of stacks increase, the cooling problem is amplified. Thermal vias are emerging as a viable technology for transferring heat and in effect creating a thermal short circuit from individual die to the substrate. Some of the authors of this paper earlier reported on the reliability of stacked memory dies. In a subsequent paper the thermal reliability that included geometrical stacking architecture (rotating, spacer, ..) and the inclusion of both logic and memory dies was addressed. In this present paper, the heat transfer enhancement using silicon vias on various stacking schemes is discussed. The CAD models required for this study were developed in Pro/Engineer® Wildfire™ 2.0 and for the result simulation ANSYS® Workbench® 10.0 have been used. Packaging architectures that have been taken in to consideration are die, solder ball, substrate, mold cap and thermal vias.
Escobar R., Amon C.H., Transition to Ballistic Energy Transport in Silicon Thin Films, CIMENICS 2006, Isla Margarita, Venezuela, March 2006.
Amon C.H., Wesner J., Hoff R., Identifying and Implementing Projects for a Multidisciiplinary Engineering Design Projects Course at Carnegie Mellon, ASEE Annual Conference and Exposition, Conference Proceedings , 2006.
[Abstract] This paper describes the process of identifying, selecting, and implementing sponsored projects in a multidisciplinary Engineering Design Projects Course at Carnegie Mellon University. In order for the course to be most effective, the projects made available to student teams in a multidisciplinary projects course need to have several characteristics, including being "realistic" (i.e., needed by someone), having multidisciplinary aspects so that all team members can contribute, and allowing for significant results within a single semester. At the same time, each project must have the potential to produce useful results for the sponsor, while not requiring unreasonable contributions, and be priced appropriately. This paper shares the lessons learned as we have managed these, at times, conflicting issues in a course which has been successfully offered for twelve semesters and involves a mix of one-time and repeat sponsors from industry, government agencies, non-profits, and the university community.
Romero D.A., Amon C.H., Finger S., On Adaptive Sampling for Surrogate Models in Simulation-based Design and Optimization, CIMENICS 2006, Isla Margarita, Venezuela, March 2006.
Dewan-Sandur B.P., Kaisare A., Agonafer D., Agonafer D., Amon C.H., Pekin S., Dishongh T., Thermal Management of Die Stacking Architectures That Includes Memory and Logic Processor, Proceedings - Electronic Components and Technology Conference 2006: 1963-1968, 2006.
[Abstract] The convergence of computing and communications dictates building up rather than out. As consumers demand more functions in their hand-held devices, the need for more memory in a limited space is increasing, and integrating various functions into the same package is becoming more crucial. Over the past few years, die stacking has emerged as a powerful tool for satisfying these challenging Integrated Circuit (IC) packaging requirements. Previously, present authors reported on the thermal challenges of various die stacking architectures that included memory (volatile and non-volatile) only. In this paper, the focus is on stacking memory and the logic processor on the same substrate. In present technologies, logic processor and memory packages are located side-by-side on the board or they are packaged separately and then stacked on top of each other (Package-on-package [PoP]). Mixing memory and logic processor in the same stack has advantage and challenges, but requires the integration ability of economies-of-scale. Geometries needed were generated by using Pro/Engineer® Wildfire™ 2.0 as a Computer-Aided-Design (CAD) tool and were transferred to ANSYS® Workbench™ 10.0, where meshed analysis was conducted. Package architectures evaluated were rotated stack, staggered stack utilizing redistributed pads, and stacking with spacers, while all other parameters were held constant. The values of these parameters were determined to give a junction temperature of 100°C, which is an unacceptable value due to wafer level electromigration. A discussion is presented in what parameters need to be adjusted in order to meet the required thermal design specification. In that light, a list of solutions consisting of increasing the heat transfer co-efficient on top of the package, the use of underfill, improved thermal conductivity of the PCB, and the use of a copper heat spreader were evaluated. Results were evaluated in the light of market segment requirements.
Romero D.A., Amon C.H., Finger S., Adaptive Sampling for Surrogate Models in Simulation-based Design and Optimization, Proceedings of the IDETC/CIE 2006, Philadelphia, PA, 2006.
Goicochea J.V., Madrid M., Amon C.H., Phonon Relaxation Rates in Silicon Thin Films Determined by Molecular Dynamics, Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference 2006: 1185-1191, 2006.
[Abstract] Silicon thin films with nanometer dimensions are increasingly being used in the electronic and nanotechnology industries. At such small scales, the continuum assumption is no longer valid and the interactions of the energy carriers (phonons) with the boundaries affect the thermal conductivity of the films. For semiconductors and dielectric thin films, understanding phonon properties in the nanometer scale is important not only to predict their thermal transport behavior, but also to propose solutions to a broad range of thermally induced problems, such as self-heating, sub-continuum localized heating effects and thermally induced reliability. In this work, we estimate, by means of molecular dynamics, the phonon relaxation times in silicon thin films, in the out-of-plane direction, at different temperatures and thin film thicknesses. The relaxation times are determined from the temporal decay of the autocorrelation function of the energy components of the phonons allowed in the crystal. The results are compared with the relaxation times obtained from perturbation theory and Mathiessen's rule. Two major trends were observed, the relaxation rates for transversal acoustic modes are lower than those for the longitudinal acoustic mode for all thickness and temperatures studied, and the longitudinal acoustic modes do not follow the theoretical predictions.
Romero D.A., Amon C.H., Finger S., On adaptive sampling for single and multi-response Bayesian surrogate models, Proceedings of the ASME Design Engineering Technical Conference 2006: 393-404, 2006.
[Abstract] In order to reduce the time and resources devoted to design-space exploration during simulation-based design and optimization, the use of surrogate models, or metamodels, has been proposed in the literature. Key to the success of metamodeling efforts are the experimental design techniques used to generate the combinations of input variables at which the computer experiments are conducted. Several adaptive sampling techniques have been proposed to tailor the experimental designs to the specific application at hand, using the already-acquired data to guide further exploration of the input space, instead of using a fixed sampling scheme defined a priori. Though mixed results have been reported, it has been argued that adaptive sampling techniques can be more efficient, yielding better surrogate models with less sampling points. In this paper, we address the problem of adaptive sampling for single and multi-response metamodels, with a focus on Multi-stage Multi-response Bayesian Surrogate Models (MMBSM). We compare distance-optimal latin hypercube sampling, an entropy-based criterion and the maximum cross-validation variance criterion, originally proposed for one-dimensional output spaces and implemented in this paper for multi-dimensional output spaces. Our results indicate that, both for single and multi-response surrogate models, the entropy-based adaptive sampling approach leads to models that are more robust to the initial experimental design and at least as accurate (or better) when compared with other sampling techniques using the same number of sampling points.
Narumanchi S.V.J., Murthy J.Y., Amon C.H., Boltzmann Transport Equation-based Thermal Modeling Approaches for Microelectronics, Heat and Mass Transfer/Waerme- und Stoffuebertragung 42(6): 478-491, 2006.
[Abstract] Fourier diffusion has been found to be inadequate for the prediction of heat conduction in modern microelectronics, where extreme miniaturization has led to feature sizes in the sub-micron range. Over the past decade, the phonon Boltzmann transport equation (BTE) in the relaxation time approximation has been employed to make thermal predictions in dielectrics and semiconductors at micro-scales and nano-scales. This paper presents a review of the BTE-based solution methods widely employed in the literature and recently developed by the authors. First, the solution approaches based on the gray formulation of the BTE are presented. The semi-gray approach, moments of the Boltzmann equation, the lattice Boltzmann approach, and the ballistic-diffusive approximation are also discussed. Models which incorporate greater details of phonon dispersion are also presented. Hotspot self-heating in sub-micron SOI transistors and transient electrostatic discharge in NMOS transistors are also examined. Results, which illustrate the differences between some of these models reveal the importance of developing models that incorporate substantial details of phonon physics. The impact of boundary conditions on thermal predictions is also investigated.
Smith B., Vanderbeek A., Amon C.H., Simultaneous Electro-Thermal Test Method for Pyroelectric Microsensors, Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference 2006: 1314-1323, 2006.
[Abstract] Pyroelectric film materials, including polyvinylidene fluoride (PVDF) and its copolymers (e.g., P(VDF/TrFe)), are attractive candidates for low-cost infrared detection and imaging applications due to their compatibility with CMOS processing and inexpensive packaging requirements compared to semiconductor-based detectors. The pyroelectric coefficient (p) describes the material's electric response to a change in sensor temperature and is the main contributor to the sensitivity and detectivity of the system. However, this value can vary greatly with film fabrication and poling processes, and its measurement is often highly coupled to the material's thermal diffusivity. This paper describes a new approach to film characterization that combines the popular "3-omega" technique for thermal characterization with a modified version of the laser intensity modulation method (LIMM) for determining the film's pyroelectric coefficient. The new method is capable of simultaneously measuring film conductivity, diffusivity, and pyroelectric coefficient. It could increase the accuracy of the pyroelectric measurements by providing in-situ thermal data to the electrical model instead of relying on published values or thermal measurements of a different sample. We also present a fabrication process that can be used to pole and measure a variety of pyroelectric materials and a mathematical framework to study the thermal phenomena of the setup. The thermal model is used to highlight the methodology's sensitivity to uncertainties in the geometric and material property values of the layers surrounding the pyroelectric film.
Murthy J.Y., Amon C.H., Nano Scale Thermal Transport Modeling, ITherm 2006, San Diego, CA, 2006.
Amon C.H., Goicochea J., Madrid M., Hierarchical Thermal Transport Modeling in Nano-structured Semiconductor Materials and Devices, EMCC-4, Proceedings of the 4th NSF Chemical Engineering Conference for Collaborative Research in Mediterranean Countries, pp. 209-212, Dead Sea, Israel, 2006.
Finol E.A., Shkolnik A.D., Scotti C.M., Amon C.H., Computational Modeling of Abdominal Aortic Aneurysms: An Assessment of Rupture Potential for Presurgical Planning, in Biomechanics Applied to Computer Assisted Surgery, ed. Y. Payan, Research Signpost Publisher, Kerala, India, pp. 243-260, 2005.
Finol E.A., Scotti C.M., Verdinelli I., Amon C.H., Wholey M.H., Performance Assessment of Embolic Protection Filters for Carotid Artery Stenting, WIT Transactions on Biomedicine and Health 8: 133-142, 2005.
[Abstract] Stroke is the third leading cause of death in the United States, accounting for 1.5 deaths reported per 1000 people. Carotid artery stenting (CAS) with cerebral protection is slowly becoming the gold standard for treatment of carotid artery occlusive disease in high risk patients. CAS is based on the selective cannulation of the common carotid artery by means of an introducer sheath or guiding catheter and the deployment of a wire mesh (stent) to treat the occluded artery segment. The goal for CAS is the prevention of stroke and its efficacy depends greatly on the periprocedural complications. The major concern with CAS is its potential to produce emboli that may translate into a severe neurological disorder. In this regard, several cerebral protection devices (CPDs) have been developed recently as an adjunct to CAS, with the primary function of capturing the plaque particles released from the site of vessel injury to prevent neurological events. A category of CPD that has received recent attention due to its ability to allow continued distal perfusion is the embolic protection filter. We tested in vitro one FDA approved (RX Accunet Embolic Protection System, Guidant Corporation, Indianapolis, IN) and two investigational (FilterWire EZ, Boston Scientific, Natick, MA and Angioguard XP, Cordis Corp., Coral Gables, FL) devices of this kind. The objective of this study was to assess the effectiveness of emboli capture of the devices, investigate potential intangible failure modes and complications, and set a baseline of desirable design parameters for future generations of embolic protection filters. None of the devices tested completely prevented embolization into the artery model. Overall, the RX Accunet device had the best filtration performance, failing to capture 0.16% of plaque particles when deployed in an artery model of 5.5 mm in diameter. Several complications related to device retrieval were detected in all devices on any given set of testing scenarios. Crossing profile, opening/closing mechanics and pore size were among the key design variables required for improved device designs.
Nain A.S., Amon C.H., Sitti M., Polymer Micro/Nanofiber Fabrication using Micro/Nanopipettes, 5th IEEE Conference on Nanotechnology 1: 547-550, 2005.
[Abstract] One of the most significant barriers for enabling the breakthroughs promised by nanotechnology is mass production of nanoscale structures, devices, and systems. One of the main challenges of nanomanufacturing systems is three dimensional customized manufacturing of micro/nanofibers. In this article we present a specialized tool developed for reproducible and controlled fabrication of micro/nano polymer fibers using micro/nanopipettes. Development of this tool will facilitate controlled deposition and shaping of polymer materials at the sub-micron scale in precisely determined locations. We present experimental results obtained using concentrated solutions of high molecular weight poly(methyl methacrylate) dissolved in chlorobenzene. Results indicate that it is feasible fabricating high aspect ratio (length to diameter ratio) polymer fibers having diameters approaching less than 200 nanometers using this approach.
Ghai S.S., Amon C.H., Kim W.T., Jhon M.S., Estimation of Anisotropic Thermal Conductivity in Nanoscale Confined Semiconductors via Lattice Boltzmann Method, American Society of Mechanical Engineers, Electronic and Photonic Packaging, EPP 5: 525-529, 2005.
[Abstract] A novel transient thermal transport model based on lattice Boltzmann method is developed to capture the sub-continuum effects including anisotropic thermal behavior of solids at nanoscale. Rigorous boundary condition treatment is incorporated via ghost boundary formulation. These sub-continuum effects deviate significantly from the bulk behavior and can not be accurately captured by the continuum based models such as Fourier equation. We observed that as the thickness of the semiconductor film is decreased to the scale of its carrier's mean free path, the thermal conductivity of the film reduces drastically from its bulk value and starts to show anisotropic behavior. In addition, a temperature jump, which does not exist at the bulk conditions, is observed at the interfaces. These sub-continuum effects are successfully captured by the lattice Boltzmann model and simple equations have been developed to accurately estimate these effects using the film geometry and properties.
Amon C.H., Yao S.C., Wu C.F., Hsieh C.C., Microelectromechanical System-Based Evaporative Thermal Management of High Heat Flux Electronics, Journal of Heat Transfer 127(1): 66-75, 2005.
[Abstract] This paper describes development of embedded droplet impingement for integrated cooling of electronics (EDIFICE), which seeks to develop an integrated droplet impingement cooling device for removing chip heat fluxes over 100 W/cm2, employing latent heat of vaporization of dielectric fluids. Micromanufacturing and microelectromechanical system are used as enabling technologies for developing innovative cooling schemes. Microspray nozzles are fabricated to produce 50-100µm droplets coupled with surface texturing on the backside of the chip to promote droplet spreading and effective evaporation. This paper examines jet impingement cooling of EDIFICE with a dielectric coolant and the influence of fluid properties, microspray characteristics, and surface evaporation. The development of micronozzles and microstructured surface texturing is discussed. Results of a prototype testing of swiss-roll swirl nozzles with dielectric fluid HFE-7200 on a notebook PC are presented. This paper also outlines the challenges to practical implementation and future research needs.
Campo A., Amon C.H., Attributes of a Derived Differential/Difference Energy Equation within the Platform of the Lévêque Problem, Heat and Mass Transfer/Waerme- und Stoffuebertragung 41(7): 577-582, 2005.
[Abstract] The main idea of the Lévêque approximation was to provide a simple asymptotic solution for the development of the thermal boundary layer in a laminar tube flow with fully established velocity and uniform inlet temperature. Inspired in the "flat plate passage" idealization, Lévêque assumed that the hydrodynamic boundary layer was confined to a thin annular region near the wall so that the velocity varied linearly with the distance y = R - r measured from the wall in an equivalent "parallel-plate passage". In contrast, this paper addresses the Lévêque problem adhering to the original energy conservation equation in cylindrical coordinates continually, but without making any geometric and/or hydrodynamic assumptions a priori. The semi-analytic solution procedure proposed here combines the Transversal Method Of Lines (TMOL) with the Fröbenius method, a particular case of the power series method.
Guzman A.M., Escobar R.A., Amon C.H., Flow Mixing Enhancement from Balloon Pulsations in an Intravenous Oxygenator, Journal of Biomechanical Engineering 127(3): 400-415, 2005.
[Abstract] Computational investigations of flow mixing and oxygen transfer characteristics in an intravenous membrane oxygenator (IMO) are performed by direct numerical simulations of the conservation of mass, momentum, and species equations. Three-dimensional computational models are developed to investigate flow-mixing and oxygen-transfer characteristics/or stationary and pulsating balloons, using the spectral element method. For a stationary balloon, the effect of the fiber placement within the fiber bundle and the number of fiber rings is investigated. In a pulsating balloon, the flow mixing characteristics are determined and the oxygen transfer rate is evaluated. For a stationary balloon, numerical simulations show two well-defined flow patterns that depend on the region of the IMO device. Successive increases of the Reynolds number raise the longitudinal velocity without creating secondary flow. This characteristic is not affected by staggered or non-staggered fiber placement within the fiber bundle. For a pulsating balloon, the flow mixing is enhanced by generating a three-dimensional time-dependent flow characterized by oscillatory radial, pulsatile longitudinal, and both oscillatory and random tangential velocities. This three-dimensional flow increases the flow mixing due to an active time-dependent secondary flow, particularly around the fibers. Analytical models show the fiber bundle placement effect on the pressure gradient and flow pattern. The oxygen transport from the fiber surface to the mean flow is due to a dominant radial diffusion mechanism, for the stationary balloon. The oxygen transfer rate reaches an asymptotic behavior at relatively low Reynolds numbers. For a pulsating balloon, the time-dependent oxygen-concentration field resembles the oscillatory and wavy nature of the time-dependent flow. Sherwood number evaluations demonstrate that balloon pulsations enhance the oxygen transfer rate, even for smaller flow rates.
Ghai S.S., Kim W.T., Escobar R.A., Amon C.H., Jhon M.S., A Novel Heat Transfer Model and its Application to Information Storage Systems, Journal of Applied Physics 97(10): 1-3, 2005.
[Abstract] Lattice Boltzmann method (LBM) based on Boltzmann transport equation is developed to simulate the nanoscale heat transport in solids. The LBM can simulate both the metals and semiconductors by properly incorporating the energy carriers. We found that boundary scattering of phonons results in an anisotropic thermal transport in nanoscale solids. The electron-phonon coupling is introduced to accurately describe the thermal behavior of nanoscale confined solids. Our numerical tool will be suitable for simulating complex multiscale systems involving multiple energy carriers with different length and time scales, and is useful in magnetic recording technology when the thermal response plays a crucial role such as for reliability of the head-disk interface and the heat assisted magnetic recording systems.
Weiss L.E., Amon C.H., Finger S., Miller E.D., Romero D., Verdinelli I., Walker L.M., Campbell P.G., Bayesian Computer-aided Experimental Design of Heterogeneous Scaffolds for Tissue Engineering, CAD Computer Aided Design 37(11): 1127-1139, 2005.
[Abstract] This paper presents a Bayesian methodology for computer-aided experimental design of heterogeneous scaffolds for tissue engineering applications. These heterogeneous scaffolds have spatial distributions of growth factors designed to induce and direct the growth of new tissue as the scaffolds degrade. While early scaffold designs have been essentially homogenous, new solid freeform fabrication (SFF) processes enable the fabrication of more complex, biologically inspired heterogeneous designs with controlled spatial distributions of growth factors and scaffold microstructures. SFF processes dramatically expand the number of design possibilities and significantly increase the experimental burden placed on tissue engineers in terms of time and cost. Therefore, we use a multi-stage Bayesian surrogate modeling methodology (MBSM) to build surrogate models that describe the relationship between the design parameters and the therapeutic response. This methodology is well suited for the early stages of the design process because we do not have accurate models of tissue growth, yet the success of our design depends on understanding the effect of the spatial distribution of growth factors on tissue growth. The MBSM process can guide experimental design more efficiently than traditional factorial methods. Using a simulated computer model of bone tissue regeneration, we demonstrate the advantages of Bayesian versus factorial methods for designing heterogeneous fibrin scaffolds with spatial distributions of growth factors enabled by a new SFF process.
Narumanchi S.V.J., Murthy J.Y., Amon C.H., Comparison of Different Phonon Transport Models in Predicting Heat Conduction in Sub-micron Silicon-on-Insulator Transistors, Journal of Heat Transfer 127(7): 713-723, 2005.
[Abstract] The problem of self-heating in microelectronic devices has begun to emerge as a bottleneck to device performance. Published models for phonon transport in microelectronics have used a gray Boltzmann transport equation (BTE) and do not account adequately for phonon dispersion or polarization. In this study; the problem of a hot spot in a submicron silicon-on-insulator transistor is addressed. A model based on the BTE incorporating full phonon dispersion effects is used. A structured finite volume approach is used to solve the BTE. The results from the full phonon dispersion model are compared to those obtained using a Fourier diffusion model. Comparisons are also made to previously published BTE models employing gray and semi-gray approximations. Significant differences are found in the maximum hot spot temperature predicted by the different models. Fourier diffusion underpredicts the hot spot temperature by as much as 350% with respect to predictions from the full phonon dispersion model. For the full phonon dispersion model, the longitudinal acoustic modes are found to carry a majority of the energy flux. The importance of accounting for phonon dispersion and polarization effects is clearly demonstrated.
Campo A., Amon C.H., Remarkable Improvement of the Lévêque Solution for Isoflux Heating with a Combination of the Transversal Method of Lines (TMOL) and a Computer-Extended Fröbenius Power Series, International Journal of Heat and Mass Transfer 48(10): 2110-2116, 2005.
[Abstract] This paper addresses the second Lévêque problem with uniform wall heat flux adhering to the original two-dimensional energy conservation equation with variable coefficients in cylindrical coordinates. The semi-analytic procedure to be proposed combines the transversal method of lines (TMOL) with the Fröbenius version of the power series method. The hybrid solution that emerges from this combination holds unique features that distinguish it from the traditional solution methods. In principle, due to the presence of a two-point backward formulation, the approximate analytic solution is considered first-order accurate. However, using as guidance the computed local convection coefficient at various transversal lines, it is demonstrated that the approximate analytic TMOL/Fröbenius solution is better than first-order accurate.
Amon C.H., Narumanchi S.V.J., Madrid M., Gomes C., Goicochea J., Hierarchical Modeling of Thermal Transport from Nano-to-Macroscales, in Microscale Heat Transfer - Fundamental and Applications, eds. S. Kakac, L.L. Vasiliev, Y. Bayazitoglu and Y. Yener, Springer, The Netherlands, pp. 379-400, 2005.
Guzman A.M., Escobar R.A., Amon C.H., Methodology for Predicting Oxygen Transport on an Intravenous Membrane Oxygenator Combining Computational and Analytical Models, Journal of Biomechanical Engineering 127(7): 1127-1140, 2005.
[Abstract] A computational methodology for accurately predicting flow and oxygen-transport characteristics and performance of an intravenous membrane oxygenator (IMO) device is developed, tested, and validated. This methodology uses extensive numerical simulations of three-dimensional computational models to determine flow-mixing characteristics and oxygen-transfer performance, and analytical models to indirectly validate numerical predictions with experimental data, using both blood and water as working fluids. Direct numerical simulations for IMO stationary and pulsating balloons predict flow field and oxygen transport performance in response to changes in the device length, number of fibers, and balloon pulsation frequency. Multifiber models are used to investigate interfiber interference and length effects for a stationary balloon whereas a single fiber model is used to analyze the effect of balloon pulsations on velocity and oxygen concentration fields and to evaluate oxygen transfer rates. An analytical lumped model is developed and validated by comparing its numerical predictions with experimental data. Numerical results demonstrate that oxygen transfer rates for a stationary balloon regime decrease with increasing number of fibers, independent of the fluid type. The oxygen transfer rate ratio obtained with blood and water is approximately two. Balloon pulsations show an effective and enhanced flow mixing, with time-dependent recirculating flows around the fibers regions which induce higher oxygen transfer rates. The mass transfer rates increase approximately 100% and 80%, with water and blood, respectively, compared with stationary balloon operation. Calculations with combinations of frequency, number of fibers, fiber length and diameter, and inlet volumetric flow rates, agree well with the reported experimental results, and provide a solid comparative base for analysis, predictions, and comparisons with numerical and experimental data.
Ghai S.S., Amon C.H., Kim W.T., Jhon M.S., Transient Thermal Responses of a Nanoscale Hot-spot in a Film with Alternating Materials, INTERMAG ASIA 2005: Digests of the IEEE International Magnetics Conference : 249-250, 2005.
[Abstract] An alternative model stemmed from the Boltzmann transport equation, the lattice Boltzmann method (LBM), is developed to successfully capture the transient thermal profile in a sub-continuum domain at a reduced computational cost. A film with alternating materials with different thermal characteristics is chosen to examine the transient thermal profile under the influence of a nanoscale hot-spot. For non-equilibrium conditions, the conventional definition of temperature breaks done so an equivalent temperature at which the total equilibrium energy of system is equal to the actual thermal energy. In order to efficiently formulate boundary effects which are useful in generalizing isolated domain to an alternating film, ghost particles are introduced. To simulate metallic solids, multi-grid simulation technique is used to simultaneously solve couple lattice Boltzmann equation for electrons and phonons. Simulation results show that reduction of the system size from the continuum to the sub-continuum domain, Fourier equation increasingly under-predict the peak temperature rise at the center of the hot-spot. Reducing the characteristic length, the sub-continuum effect of hot-spot confinement and high temperature rise is captured by LBM simulation while Fourier equation fails to capture these phenomena. For an alternating film case, the hot-spot in one domain interfere with the neighboring domains in a complex manner and domain interfaces strongly affect the thermal profile of the system.
Escobar R.A., Amon C.H., Lattice Boltzmann Modeling of the Thermal Response of Silicon-On-Insulator Transistors Under Joule Heating Including Phonon Dispersion, American Society of Mechanical Engineers, Electronic and Photonic Packaging, EPP 5: 489-499, 2005.
[Abstract] Lattice Boltzmann Method (LBM) simulations of phonon transport are performed in a computational model of an Silicon-on-Insulator (SOI) transistor to investigate the transient thermal response of the device under Joule heating conditions, which give origin to a non-equilibrium region of high temperature known as hotspot. The gray LBM based on the Debye assumption is compared to a dispersion LBM which incorporates nonlinear dispersion for all phonon .branches, including explicit treatment of optical phonons without simplifying assumptions. The simulations cover the effect of hotspot size, heat pulse duration, and source term modeling, as either a constant or frequency-dependent term. Results indicate that hotspot peak temperature levels found by both the dispersion and the gray LBM are higher than Fourier diffusion predictions. Additionally, proper modeling of the source term is found to be critical, in order to accurately predict peak hotspot temperatures.
Ghai S.S., Kim W.T., Amon C.H., Jhon M.S., Anisotropic Thermal Transport Estimation in Semiconductor Thin Films via Lattice Boltzmann Method, AIChE Annual Meeting, Conference Proceedings 01D03: 24995, 2005.
[Abstract] To achieve faster and smaller devices, regular advancements in microfabrication techniques have been achieved which has resulted in the device characteristic length to approach nanometer scale. At this nanoscale dimension, materials start to act characteristically different in comparison to the bulk system. This considerable change in the behavior results from the onset of sub-continuum regime where the long established continuum based models break down and more rigorous physics is required to capture the sub-continuum behavior of the system.
An important case where the sub-continuum effects are playing a critical role is the thermal transport in the nanoscale confined semiconductor films in the state-of-the-art electronic devices. In these devices, an increased emphasis has been laid on achieving smaller and faster systems, which has forced their characteristic length to nanometer scale. At these scales the sub-continuum effects of ballistic thermal transport, temperature slip at the boundaries, and anisotropic thermal conductivity become very prominent and energy management plays a crucial role in the operation and reliability of the system.
Continuum based Fourier equation have been proved inadequate to describe these phenomena and a rigorous physics based model e.g., Boltzmann transport equation (BTE), which can accurately capture these effects, is required. The BTE is based on phase-space formulation and thus is computationally quite intensive. This has lead to the development of an alternative model, stemmed from BTE, lattice Boltzmann method (LBM), to successfully capture the transient thermal profile at a reduced computational cost. The virtues of being inherently transient, easy to hybridize with other physical models and length scales, and inherently parallel in nature made LBM as our natural choice.
In sub-continuum domain the boundary conditions become very significant and they govern the effective mean free path of the carriers and thus control the transport properties of the solid. At the boundary, energy carriers are scattered both specularly and diffusively. Therefore, we incorporated a surface scattering factor, which is the fraction of carriers undergoing diffusive scattering at the boundary, and studied the thermal conductivity and temperature slip at the solid boundaries.
Using LBM, we studied silicon thin films of varying thicknesses ranging from few tens of nanometers to a few microns and observed that the thermal conductivity and temperature slip at the boundaries depends strongly on the surface scattering factor and the geometric dimensions in addition to the temperature of the film. Strong anisotropy in thermal conductivity and big temperature jumps at the boundaries has been observed for the thin films of silicon, which exhibits isotropic thermal behavior in the bulk. We constructed equations which can successfully model the anisotropic conductivity and temperature slip as a function of surface scattering factor and the thickness of the film. These expressions predict the numerical solution of LBM very closely and can act as correction terms for the existing numerical solvers based on Fourier equation.
Ghai S.S., Kim W.T., Amon C.H., Jhon M.S., Transient thermal modeling of a nanoscale hot spot in multilayered film, AIChE Annual Meeting, Conference Proceedings 01D04(25026), 2005.
[Abstract] In state-of-the-art data storage devices an increased emphasis has been laid on achieving higher areal density, which has forced the characteristic length of the devices to nanometer scale. At these scales the sub-continuum effects of ballistic thermal transport and temperature slip at the boundaries become very prominent and energy management plays a crucial role in the operation and reliability of the system. Continuum based Fourier equation is inadequate to describe these phenomena and a more rigorous physics based model e.g., Boltzmann transport equation (BTE), is required. Since the BTE, based on phase space formulation, is computationally intensive, an alternative model stemmed from BTE, lattice Boltzmann method (LBM), is developed to successfully capture the transient thermal behavior in a sub-continuum domain at a reduced computational cost. A film with alternating materials with different thermal characteristics is chosen to examine the transient thermal profile in the presence of a nanoscale hot-spot. This relatively simple system can provide essential physics of complex data storage systems e.g., heat assisted magnetic recording (HAMR), patterned media, and phase change media, where thermal effects play a subtle role.
Since LBM is inherently transient, easy to hybridize with other physical models and length scales, and parallel in nature, we have chosen LBM as our simulation tool. For non-equilibrium conditions, the conventional definition of temperature breaks down. Therefore, we assigned an equivalent temperature at which the total equilibrium energy of system is equal to the actual thermal energy. An emphasis has been laid on incorporating accurate boundary conditions, including diffusion and transmission of energy carriers at interfaces, which critically affects the thermal behavior. We extensively studied different length and time scales as well as different boundary conditions. Ghost particles are introduced, all around the real domain of the system, to efficiently simulate boundary effects and are useful in generalizing an isolated domain to an alternating film. Coupled lattice Boltzmann equation for electrons and phonons are solved simultaneously, using multi-grid simulation technique, to simulate metallic solids. LBM used in this work can not only predict the thermal behavior of the sub-continuum system but also provide an estimation of the effective conductivity.
Simulations have been performed on the domains containing a hot-spot and the LBM results are compared with the Fourier equation predictions for different length and time scales. It has been observed that Fourier equation increasingly under-predict the peak temperature rise at the center of the hot-spot as we reduce the system size from the continuum to the sub-continuum domain. When the characteristic length of the system is much greater than the mean free path, Fourier equation can successfully capture the temperature profile. But as the characteristic length is reduced the sub-continuum effect of hot-spot confinement and high temperature rise is captured only by our LBM simulation while Fourier equation fails to achieve so. For an alternating film case, the hot-spot in one domain interfere with the neighboring domains in a complex manner and domain interfaces strongly affect the thermal profile of the system. These phenomena are critical for the design and operation of the alternative data storage systems, such as HAMR and patterned media, by predicting the optimum heating pulse and intensity for the maximum acceptable interference and by accurately modeling the lubricant film's adsorption/desorption, replenishment, and diffusion and head contamination due to the energy transport from the hot-spot. The novel numerical tool developed in this work can successfully simulate complex multiscale systems involving multiple energy carriers with different length and time scales.
Gomes C.J., Goicochea J.V., Madrid M., Amon C.H., Silicon Thin Film Thermal Conductivity in Ballistic and Diffusive Regimes Predicted by Molecular Dynamics, Proceedings of the ASME Summer Heat Transfer Conference 4: 811-819, 2005.
[Abstract] The thermal conductivity of silicon thin films is predicted in the directions parallel and perpendicular to the film surfaces (in-plane and out-of-plane, respectively) using equilibrium molecular dynamics, the Green-Kubo relationship and the Stillinger-Weber interatomic potential. Film thicknesses range from 2 to 220 nm and temperatures from 300 to 1000 K. In this range of temperatures, the relation between the phonon mean free path (Λ) and the film thickness (ds) spans from the ballistic regime ( » ds) to the diffusive, bulk-like regime ( « ds). We show that equilibrium molecular dynamics and the Green-Kubo relationship can be applied to the study of the thermal conductivity of thin films in the ballistic, transitional and diffusive regimes. When the film is thin enough, the thermal conductivity becomes orthotropic and decreases with decreasing film thickness as a consequence of the scattering of phonons with the film boundaries. The in-plane thermal conductivity follows the trend observed experimentally at 300 K. In the ballistic limit, in accordance with the kinetic theory, the predicted out-of-plane thermal conductivity varies linearly with the film thickness and is temperature-independent for temperatures near or above Debye's temperature.
Smith B.R., Beutler P.D., Amon C.H., Thermal Transport Network Model for High-Porosity Materials: Application to Nanoporous Aerogels, Proceedings of the ASME Summer Heat Transfer Conference 4: 955-961, 2005.
[Abstract] Nanoporous thin films have received attention in the microelectronics field for their application as next-generation low-k inner-layer dielectric (ILD) materials due dielectric constants approaching 1.4. In addition, emerging applications as thermal insulation for microsystems aim to exploit the materials' unique thermal properties in sensor and component products. However, its thermal properties can vary greatly depending on fabrication processes and material morphology. In addition, a variety of transport phenomena are present and delineation among them is difficult. In this work, we examine heat transport in aerogel, one of the most common embodiments of nanoporous materials, to identify the main modes of energy transport. We employ a modified diffusion-limited cluster aggregation (DLCA) technique to simulate aerogel's highly porous, amorphous solid structure. Network models then simulate heat transport through the structure to extract effective thermal conductivity. The models are verified by comparing calculated bulk data to published aerogel literature. Preliminary models yield thermal conductivity on the order of 0.010 W/m*K, which is consistent with published data for aerogel films. These values vary inversely with porosity of the aerogel following an inverse power law often used to fit aerogel experimental data. This methodology is most useful for examining the sensitivity of thermal conductivity to salient structural features such as porosity, pore size distribution, solid thermal properties, average branch width, and sub-continuum phenomena. The results of this study can be used as a predictive tool in optimizing aerogel fabrication process to yield morphologies that best-suit the requirements of the application.
Smith B., Romero D., Agonafer D., Gu J., Amon C.H., Aerogel for Microsystems Thermal Insulation: System Design and Process Development, Proceedings of the ASME Summer Heat Transfer Conference 4: 753-762, 2005.
[Abstract] Extreme miniaturization in the microelectronics component market along with the emergence of system-on-chip applications has driven interest in correspondingly small-scale thermal management designs requiring novel material systems. This paper concentrates on aerogel, which is an amorphous, nanoporous dielectric oxide fabricated through a sol-gel process. Its extremely high porosity leads to very low thermal conductivity and dielectric constants. Significant research has been devoted to its electrical properties; however, there are several emerging applications that can leverage the thermal characteristics as well. Two promising applications are investigated in this paper: a monolithically integrated infrared sensor that requires thermal isolation between sensor and silicon substrate, and an ultra-miniature crystal oscillator device which demands thermal insulation of the crystal for low-power operation. This paper identifies the potential benefits of aerogel in these applications through system modeling, demonstrates aerogel's compatibility with standard low-cost microfabrication techniques, and presents results of thermal testing of aerogel films compared with other microelectronics insulators and available data in the literature. The goal is to explore system thermal design using aerogel while demonstrating its feasibility through experimentation. The combination of numerical simulations, Bayesian surrogate modeling, and process development helps to refine candidate aerogel applications and allow the designer to explore thermal designs which have not previously been possible in large-scale microelectronics system production.
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2000 - 2004
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Narumanchi S.V.J., Murthy J.Y., Amon C.H., Submicron Heat Transport Model in Silicon Accounting for Phonon Dispersion and Polarization, Journal of Heat Transfer 126(6): 946-955, 2004.
[Abstract] In recent years, the Boltzmann transport equation (BTE) has begun to be used for predicting thermal transport in dielectrics and semiconductors at the submicron scale. However, most published studies make a gray assumption and do not account for either dispersion or polarization. In this study, we propose a model based on the BTE, accounting for transverse acoustic and longitudinal acoustic phonons as well as optical phonons. This model incorporates realistic phonon dispersion curves for silicon. The interactions among the different phonon branches and different phonon frequencies are considered, and the proposed model satisfies energy conservation. Frequency-dependent relaxation times, obtained from perturbation theory, and accounting for phonon interaction rules, are used. In the present study, the BTE is numerically solved using a structured finite volume approach. For a problem involving a film with two boundaries at different temperatures, the numerical results match the analogous exact solutions from radiative transport literature for various acoustic thicknesses. For the same problem, the transient thermal response in the acoustically thick limit matches results from the solution to the parabolic Fourier diffusion equation. In the acoustically thick limit, the bulk experimental value of thermal conductivity of silicon at different temperatures is recovered from the model. Experimental in-plane thermal conductivity data for silicon thin films over a wide range of temperatures are also matched satisfactorily.
Amon C.H., Murthy J.Y., Narumanchi S.V.J., Modeling Nanoscale Thermal Transport via the Boltzmann Transport Equation, American Society of Mechanical Engineers, Electronic and Photonic Packaging, EPP 4: 93-103, 2004.
[Abstract] In modern microelectronics, where extreme miniaturization has led to feature sizes in the sub-micron and nanoscale range, Fourier diffusion has been found to be inadequate for the prediction of heat conduction. Over the past decade, the phonon Boltzmann transport equation (BTE) in the relaxation time approximation has been employed to make thermal predictions in dielectrics and semiconductors at micron and nanoscales. This paper presents a review of the BTE-based solution methods widely employed in the literature. Particular attention is given to the problem of self-heating (hotspot) in sub-micron transistors. First, the solution approaches based on the gray formulation of the BTE are presented. In this class of solution methods, phonons are characterized by one single group velocity and relaxation time. Phonon dispersion is not accounted for in any detail. This is the most widely employed approach in the literature. The semi-gray BTE approach, moments of the Boltzmann equation, the lattice Boltzmann approach, and the ballistic-diffusive approximation are presented. Models which incorporate greater details of phonon dispersion are also discussed. This includes a full phonon dispersion model developed recently by the authors. This full phonon dispersion model satisfies energy conservation, incorporates the different phonon modes, as well as the interactions between the different modes, and accounts for the frequency dependence for both the phonon group velocity and relaxation times. Results which illustrate the differences between some of these models reveal the importance of developing models that incorporate substantial details of phonon physics.
Amon C.H., Sub-Micron Heat Transport in Semiconductors Accounting for Phonon Dispersion and Polarization, NATO Advanced Study Institute, Microscale Heat Transfer-Fundamentals and Applications in Biological and Microelectromechanical Systems, pp. 58-61, Çesme, Turkey, 2004.
Boyalakuntla D.S., Murthy J.Y., Amon C.H., Computation of Natural Convection in Channels with Pin Fins, IEEE Transactions on Components and Packaging Technologies 27(1): 138-146, 2004.
[Abstract] In this paper, we numerically analyze the possibility of using buoyant flow in the display panel of a laptop for electronics cooling. Three-dimensional (3-D) channels with embedded pin fin arrays are analyzed using an unstructured finite volume method. Studies have been performed with a uniform heat flux boundary condition applied on the inner wall as well as for a constant inner wall temperature condition; the outer wall in all cases is exposed to the ambient. A single periodic module is selected in the lateral direction. In the axial mean flow direction, however, the entire height of the display channel is considered. Buoyancy has been modeled using Boussinesq approximation. A range of Rayleigh numbers, panel inclinations, and pin fin arrangements are considered. Local and global flow and heat transfer results are obtained including Nusselt numbers as well as local temperature and velocity fields. The results are useful in designing augmented cooling schemes in portable electronics.
Escobar R.A., Amon C.H., Lattice-Boltzmann Modeling of Sub-Continuum Energy Transport in Silicon-on-Insulator Microelectronics Including Phonon Dispersion Effects, Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference 2: 584-591, 2004.
[Abstract] Numerical simulations of time-dependent energy transport in semiconductor thin films are conducted by using the Lattice-Boltzmann method applied to phonon transport. The discrete Lattice-Boltzmann method is first derived from the continuous Boltzmann transport equation, where nonlinear phonon dispersion relations are used to find frequency-dependent phonon velocities for longitudinal acoustic phonons. A Silicon-on-Insulator (SOI) transistor is modeled as a thin film of silicon, with a hotspot simulated by imposing a heat generation term in a localized region of the computational domain. Results indicate that a transition from diffusive to ballistic energy transport is found as the characteristic length of the thin film becomes comparable to the phonon mean free path. This transition is present in heat conduction in thin films as well as in the transient thermal response of SOI transistors. Steady-state temperature distributions are then used to calculate size-dependent thermal conductivity values in silicon thin films.
Gomes C.J., Madrid M., Amon C.H., Thin Film In-plane Silicon Thermal Conductivity Dependence on Molecular Dynamics Surface Boundary Conditions, American Society of Mechanical Engineers, Electronic and Photonic Packaging, EPP 4: 345-352, 2004.
[Abstract] The in-plane thermal conductivity of thin silicon films is predicted using equilibrium molecular dynamics, the Stillinger-Weber potential and the Green-Kubo relationship. Film thicknesses range from 2 to 200 nm. Periodic boundary conditions are used in the directions parallel to the thin film surfaces. Two different strategies are evaluated to treat the atoms on the surfaces perpendicular to the thin film direction: adding four layers of atoms kept frozen at their crystallographic positions, or restraining the atoms near the surfaces with a repulsive potential. We show that when the thin-film thickness is smaller than the phonon mean free path, the predictions of the in-plane thermal conductivity at 1000K differ significantly depending on the potential applied to the atoms near the surfaces. In this limit, the experimentally observed trend of decreasing thermal conductivity with decreasing film thickness is predicted when the surface atoms are subject to a repulsive potential in addition to the Stillinger-Weber potential, but not when they are limited by frozen atoms.
Nain A.S., Amon C.H., Sitti M., Three-Dimensional Nanoscale Manipulation and Manufacturing using Proximal probes: Controlled Pulling of Polymer Micro-Nanofibers, Proceedings of the IEEE International Conference on Mechatronics 2004, ICM'04 : 224-230, 2004.
[Abstract] Besides imaging and characterization, proximal probes are proposed to be used as three-dimensional (3D) nanoscale manipulation and manufacturing tools in this paper. We propose 3D nanoscale pulling of liquid polymer micro/nano-fibers by precise positioning of Atomic Force Microscope (AFM) nanoprobes and control of polymer solidification. An AFM probe is used to pull or extrude thermoset and thermoplastic polymers precisely to fabricate 3D polymer nano-fiber structures. A liquid polymer fiber bridge between the probe tip and a substrate is maintained when pulling the probe from the surface with controlled speed and position. We present results of our pulling experiments in vertical, horizontal and arbitrary 3D pulling directions for Poly(methyl methacrylate): PMMA polymer fibers. Force-distance curves obtained using AFM for PMMA samples at different scan rates are presented. A preliminary study showing the effect of velocity profile on pulling liquid bridges using POLYFLOW® is presented.
Narumanchi S.V.J., Murthy J.Y., Amon C.H., Simulations of Heat Transport during Transient Electrostatic Discharge Events in a Sub-micron Transistor, Proceedings of the ASME Heat Transfer/Fluids Engineering Summer Conference 2004, HT/FED 2004 4: 347-359, 2004.
[Abstract] The thermal problem associated with the transient electrostatic discharge phenomena in sub-micron silicon transistors is fast becoming a major reliability concern in IC packages. Currently, Fourier diffusion and some simple models based on the solution to the phonon Boltzmann transport equation (BTE) are used to predict failure (melting of silicon) in these transistors. In this study, a more comprehensive model, based on the phonon BTE and incorporating considerable details of phonon physics, is proposed and used to study the ESD problem. Transient results from the model reveal very significant discrepancies when compared to results from the other models in the literature.
Romero D.A., Amon C.H., Finger S., Verdinelli I., Multi-stage Bayesian Surrogates for the Design of Time-Dependent Systems, Proceedings of the ASME Design Engineering Technical Conference 3: 405-414, 2004.
[Abstract] During the early stages of the design process, designers rarely have accurate models of system behavior, yet the success of their designs depends on understanding the effect of changes in the design parameters on the system response. When models are available, they are often expensive to evaluate and difficult to run, in large part due to the imprecise knowledge available at the beginning of the design process. To circumvent these problems, a common approach proposed in the literature is the use of surrogate models. In this paper, we propose a framework for building surrogate models in multiple stages for time-dependent systems. Because they are built in stages, the surrogate models can respond to changes in constraints and can be fine-tuned as design decisions are made. When designers have mathematical models available, they can perform repeated sensitivity analyses, explore trade-offs and perform optimization studies. In this framework, the observed responses are viewed as a set of time-correlated spatial processes. The framework uses optimal sampling techniques to improve the accuracy of the resulting surrogate model while keeping the number of samples to a minimum. A new non-stationary covariance structure is proposed and tested with an example design application. The resulting models are compared with the surrogates obtained with the stationary covariance structure proposed by Romero et al. The results show increased accuracy using the non- stationary covariance structure due to its superior interpolation capabilities.
Scotti C.M., Finol E.A., Amon C.H., Computational Fluid Dynamics and Wall Mechanics of Pre- and Post-Operative Abdominal Aortic Aneurysms, Computational Fluid Dynamics 2004 2006(Part VII): 329-334, 2004.
[Abstract] In the present work we numerically evaluated the biomechanical environment of pre- and post-operative aneurysms and examined the cumulative ffect of factors such as the presence of localized thrombus, non-uniform material properties, and patient-specific geometry. Flow disturbance was intensified through the pre-operative aneurysm sac with the onset of diastolic conditions and the obstruction of the localized thrombus. This led to increased flow-induced forces, particularly in the normal direction to the wall. Similarly, the thrombus localized the Von Mises stress where changes in geometry and material property occur, while simultaneously shielding the AAA wall from elevated mechanical forces.
Under post-operative conditions, the EVG restores normal blood flow through the arterial lumen. A region of increased wall shear stress gradient exists along the posterior neck of the graft attachment site due to angulation of the aneurysm neck. At peak systolic pressure, the flow-induced displacement force on the graft may be sufficient to cause graft migration. The implantation of the EVG reduces the stresses exerted on the aneurysm wall, decreasing the Von Mises stress within the aneurysm by 83% in comparison with the pre-operative conditions. The maximum Von Mises stress at the post-operative aneurysm wall is 96% lower than that experienced by the endovascular graft. Stress remained uniformly distributed on most of the graft, but localized at the proximal attachment site due to contact forces between the graft and the arterial wall.
Scotti C.M., Finol E.A., Viswanathan S., Shkolnik A., Dimartino E.S., Vorp D.A., Amon C.H., Computational fluid dynamics and solid mechanics analyses of a patient-specific AAA pre-and post-evar, Proceedings of the ASME 2004 International Mechanical Engineering Congress and Exposition : 63-64, 2004.
[Abstract] The establishment of a new pathway for blood flow immediately following endovascular aneurysm repair (EVAR) results in morphological changes and remodeling of the aneurismal sac. While EVAR is a minimally invasive surgical intervention, failure of the endovascular graft (EVG) may occur in which there is downstream migration and endoleak formation, creating a repressurization of the aneurismal sac and an increased risk of rupture. While the mechanism of aneurysm rupture and EVG failure is fundamental in nature, the factors that most significantly contribute to the end result are not yet fully understood. Mechanically, both are the consequence of an exerted force or disturbance exceeding the strength of a given material, whether it is the aneurismal arterial wall or the interaction that exists between the graft and wall. Embedded within this causal relationship are the contributions of arterial wall remodeling, intraluminal thrombus formation, and the dynamics that exists within the lumen. Several studies have been performed to examine these factors individually as they affect shear stress, the development of vortices, and the mechanical stress experienced along the arterial wall. However, a complete investigation is needed to study an anatomically realistic geometry operating under physiological conditions. The computational analyses conducted in this investigation address the confluence of these factors as they are modeled within an accurate patient-specific abdominal aortic aneurysm (AAA) reconstructed from CT scan data prior to and after EVAR. Our results verify the pressure-dominated characteristic of the flow and the negligible contribution of the dynamic and frictional force components; both are in good agreement with previously published results for analytical estimation of flow-induced forces in EVGs.
Scotti C.M., Gasbarro M.D., Finol E.A., DiMartino E.S., Shimada K., and Amon C.H., Flow-Induced Aneurysm Wall Stress and Thinning: A Fluid-Structure Interaction Study, Biomedical Engineering Society, 2004.
Trivic D.N., O'Brien T.J., Amon C.H., Modeling the Radiation of Anisotropically Scattering Media by Coupling Mie Theory with Finite Volume Method, International Journal of Heat and Mass Transfer 47(26): 5765-5780, 2004.
[Abstract] A new mathematical model and code for radiative heat transfer of participate media with anisotropic scattering for 2-D rectangular enclosure is developed. The model is based on the coupling of (i) finite volume method for the solution of radiative transfer equation with (ii) Mie equations for the evaluation of scattering phase function. It has not been done before to the authors' best knowledge. The predictions were compared against the only found results, published 15 years ago. For those results the S-N discrete ordinates method for the solution of radiative transfer equation and the Legendre polynomials expansions for the evaluation of scattering phase function were used. The agreement between the results is very good. The advantages of new model and code are in their straight forward application to any given particles parameters without the need for previously designed analytical expression for scattering phase function. In addition, that analytical expression, with generated expansion coefficients, is restricted and can be used only for that particular case of particle parameters. The new model was applied to the solid particles of several various coals and of an ash and the series of 2-D predictions are performed. The effects of particle size parameter and of scattering albedo on radiative heat flux and on incident radiation were analyzed. It was found that the model developed is reliable and very accurate and thus suitable for extension towards: (i) 3-D geometries, (ii) mixtures of non-gray gases with particles as well as for (iii) incorporation in computational fluid dynamics codes.
Wesner J.W., Garrett Jr. J.H., Subrahmanian E., Westerberg A.W., Amon C.H., Carnegie Mellon's Multidisciplinary Engineering Design Projects Course Serves a Variety of Students and Project Sponsors, ASEE Annual Conference Proceedings : 1627-1638, 2004.
[Abstract] The sponsering of various engineering design projects by Engineering Design Research Center (EDRC) to Mellon College of Engineering was discussed. The intent was to give the participating students a hands-on, integrative, multidisciplinary experience in the field of engineering design. The course was graded using five inputs which includes the quality of the final reports each team of students prepares, the faculty coach's assessment of student performance and the team member ratings of the team colleagues. It was observed that the response to the course had come from the non-engineering students who participated.
Fedder G., Tang X., Hsieh C., Alyousef Y., Vladimer M., Amon C. H., Thermo-Fluids Considerations in the Development of a Silicon-based Micro-scale Direct Methanol Fuel Cell, Proceedings of the 2nd ASME/ZSIS International Thermal Sciences Seminar (ITSS) : 171-180, 2004.
[Abstract] A silicon-based micro-scale Direct Methanol Fuel Cell (DMFC) system is under development at Carnegie Mellon University, as a substitute for lithium-ion batteries to power hand-held electronic devices. The DMFC is simple in design, operational in any orientation and environmentally benign. The air flow and the methanol circulation are both at a natural convection draft, while a passive gas bubble separator removes CO2 from the methanol chamber. The design and operation of the passive gas separator system, which has been successfully fabricated and tested, is described.
Micro-scale Direct Methanol Fuel Cells have great potential for early applications in portable electronics due to their higher tolerance of power cost. However, challenges in system integration have to be overcome. A major issue is the development of micro-fluidics, which includes the micro pump for anode liquid re-circulation and for recycling the excess water from the cathode back to the anode, as well as the passive CO2gas bubble separation from the anode side liquids. In addition, the seamless integration of various micro-fluidic components and the electronic control system has been an essential concern for system reliability and cost reduction.
The DMFC is designed using MEMS technology. To achieve high energy density, the excess water at the cathode is collected and pumped back to the anode. This micro fuel cell contains several unique features. A silicon wafer with an array of etched holes selectively coated with a non-wetting agent is used at the cathode to collect the water effectively. A silicon membrane micro pump is developed for pumping the collected water back to the anode. Finally, a passive micro-scale CO2 bubble separator is developed to remove the gas bubbles from the anode stream. All of these silicon-based components are fabricated with a set of common processes on the same silicon wafer, such that interconnections are eliminated and fabrication cost is minimized. The resulting micro-scale fuel cell has an energy density four times larger than that of current lithium-ion batteries.
Alawadhi E.M., Amon C.H., PCM Thermal Control Unit for Portable Electronic Devices: Experimental and Numerical Studies, IEEE Transactions on Components and Packaging Technologies 26(1): 116-125, 2003.
[Abstract] This paper investigates the effectiveness of a thermal control unit (TCU) for portable electronic devices by performing experimental and numerical analyses. The TCU objective is to improve thermal management of electronic devices when their operating time is limited to a few hours. It is composed of an organic phase change material (PCM) and a thermal conductivity enhancer (TCE). To overcome the relatively low thermal conductivity of the PCM, a TCE is incorporated into the PCM to boost its conductivity. The TCU structure is complex, and modeling an electronic device with it requires time and effort. Hence, this research develops approximate, yet effective, solutions for modeling the TCU, which employ effective thermo-physical properties. The TCU component properties are averaged and a single TCU material is considered. This approach is evaluated by comparing the numerical predictions with the experimental results. The numerical model is then used to study the effect of important parameters that are experimentally expensive to examine, such as the PCM latent heat, Stefan number, and heat source power. It is shown that the TCU can provide a reliable solution to portable electronic devices, which avoids overheating and thermally-induced fatigue, as well as a solution which satisfies the ergonomic requirement.
Amon C.H., Yao S.C., MEMS-based Spatial and Temporal Thermal Management of High Heat Flux Electronics, American Society of Mechanical Engineers, Electronic and Photonic Packaging, EPP 3: 729-739, 2003.
[Abstract] This presentation describes the development of EDIFICE: Embedded Droplet impingement For Integrated Cooling of Electronics. The EDIFICE project seeks to develop an integrated droplet impingement cooling device for removing chip heat fluxes over 100 W/cm2, employing latent heat of vaporization of dielectric fluids. Micro-manufacturing and MEMS (Micro Electro-Mechanical Systems) will be discussed as enabling technologies for innovative cooling schemes recently proposed. Micro-spray nozzles are fabricated to produce 50-100 micron droplets coupled with surface texturing on the backside of the chip to promote droplet spreading and effective evaporation. A novel feature to enable adaptive on-demand cooling is MEMS sensing (on-chip temperature, remote IR temperature and ultrasonic dielectric film thickness) and MEMS actuation. EDIFICE is integrated within the electronics package and fabricated using advanced micro-manufacturing technologies (e.g., Deep Reactive Ion Etching (DRIE) and CMOS CMU-MEMS). The development of EDIFICE involves modeling, CFD simulations, and physical experimentation on test beds. This lecture will then examine jet impingement cooling of EDIFICE with a dielectric coolant and the influence of fluid properties, micro spray characteristics, and surface evaporation. The development of micro nozzles, micro-structured surface texturing, and the system integration of the evaporator is discussed. Results of a prototype testing of swirl nozzles with dielectric fluid HFE-7200 on a notebook PC are presented. This paper also reviews liquid and evaporative cooling research applied to thermal management of electronics. It outlines the challenges to practical implementation and future research needs.
Amon C.H., Challenges on Computational Modeling of Sub-micron and Nano Heat Transfer, 2003 Colloquium on Micro/Nano Thermal Engineering, µTherm Micro Thermal System Research Center, Seoul National University, pp. 121-158, 2003 (Invited Paper and Lecture).
Amon C.H., MEMS-based Thermal Management of High Heat Flux Devices: EDIFICE: Embedded Droplet Impingement for Integrated Cooling of Electronics, Rohsenow Symposium on Future Trends in Heat Transfer , 2003.
[Abstract] Increases in microprocessor power dissipation coupled with reductions in feature sizes due to manufacturing process improvements have resulted in continuously increasing heat fluxes. The ever increasing chip-level heat flux has necessitated the development of thermal management devices based on spray and evaporative cooling. This lecture presents a comprehensive review of liquid and evaporative cooling research applied to thermal management of electronics. It also outlines the challenges to practical implementation and future research needs. This presentation also describes the development of EDIFICE: Embedded Droplet Impingement For Integrated Cooling of Electronics. The EDIFICE project seeks to develop an integrated droplet impingement cooling device for removing chip heat fluxes over 100 W/cm2, employing latent heat of vaporization of dielectric fluids. Micro-manufacturing and MEMS (Micro Electro-Mechanical Systems) will be discussed as enabling technologies for innovative cooling schemes recently proposed. Micro-spray nozzles are fabricated to produce 50-100 micron droplets coupled with surface texturing on the backside of the chip to promote droplet spreading and evaporation. A novel feature to enable adaptive on-demand cooling is MEMS sensing (on-chip temperature, remote IR temperature and ultrasonic dielectric film thickness) and MEMS actuation. EDIFICE is integrated within the electronics package and fabricated using advanced micro-manufacturing technologies (e.g., Deep Reactive Ion Etching (DRIE) and CMOS CMU-MEMS). The development of EDIFICE involves modeling, CFD simulations, and physical experimentation on test beds. This lecture will then examine jet impingement cooling of EDIFICE with a dielectric coolant and the influence of fluid properties, micro spray characteristics, and surface evaporation. The development of micro nozzles, micro-structured surface texturing, and system integration of the evaporator will also be discussed.
Escobar R.A., Amon C.H., Convective Oxygen Transport Enhancement in Intravenous Membrane Oxygenators, American Society of Mechanical Engineers, Bioengineering Division 55: 19-20, 2003.
[Abstract] Numerical simulations of blood and water flow and oxygen transport in a computational model of an intravenous membrane oxygenator including moving boundaries are presented. The simulations are compared to an analytical transport model which is validated by comparing its results to experimental data reported in the literature. Good agreement is found between numerical, analytical and experimental results.
Escobar R.A., Amon C.H., Ghai S.S., Jhon M.S., Time-Dependent Simulations of Sub-Continuum Heat Generation Effects in Electronic Devices Using the Latice Boltzmann Method, American Society of Mechanical Engineers, Micro-Electromechanical Systems Division Publication (MEMS) 5: 603-612, 2003.
[Abstract] The lattice Boltzmann method (LBM), which accounts for electron-phonon scattering, is used to investigate heat generation effects on silicon-on-insulator (SOI) transistors. The wave nature of the LBM is shown and its influence on sub-continuum dynamics is discussed. The implementation of boundary conditions for constant temperature and constant heat flux is described. SOI devices are modeled as thin films in one dimension. The LBM simulation results for diffusive, transitional, and ballistic regimes are compared with Fourier equation solutions and literature results. For transitional and ballistic regimes, Fourier equation results underpredict the temperature levels obtained by the LBM, which is consistent with the findings previously reported by different authors.
Finol E.A., Amon C.H., DiMartino E.S., Vorp D.A., Pressure and Wall Shear Stress Distribution in Abdominal Aortic Aneurysms: Patient-Specific Modeling, In Computer Methods in Biomechanics and Biomedical Engineering, 4th Edition, eds. Middleton, J., Jones, M., Shrive, N., and Pande, G., Gordon and Breach Science Publishers, Newark, NJ, pp. 719-724, 2003.
Finol E.A., Amon C.H., Flow Dynamics In Anatomical Models of Abdominal Aortic Aneurysms: Computational Analysis of Pulsatile Flow, Acta Cientifica Venezolana 54(1): 43-49, 2003.
[Abstract] Blood flow in human arteries is dominated by time-dependent transport phenomena. In particular, in the abdominal segment of the aorta under a patient's average resting conditions, blood exhibits laminar flow patterns that are influenced by secondary flows induced by adjacent branches and in irregular vessel geometries. The flow dynamics becomes more complex when there is a pathological condition that causes changes in the normal structural composition of the vessel wall, for example, in the presence of an aneurysm. An aneurysm is an irreversible dilation of a blood vessel accompanied by weakening of the vessel wall. This work examines the importance of hemodynamics in the characterization of pulsatile blood flow patterns in individual Abdominal Aortic Aneurysm (AAA) models. These patient-specific computational models have been developed for the numerical simulation of the momentum transport equations utilizing the Finite Element Method (FEM) for the spatial and temporal discretization. We characterize pulsatile flow dynamics in AAAs for average resting conditions by means of identifying regions of disturbed flow and quantifying the disturbance by evaluating wall pressure and wall shear stresses at the aneurysm wall.
Finol E.A., Amon C.H., Forces Induced by Peak Systolic Flows in Asymmetric Endovascular Grafts, American Society of Mechanical Engineers, Bioengineering Division 55: 73-74, 2003.
[Abstract] Endovascular repair (EVAR) has emerged as an alternative, less-invasive surgical technique for the treatment of patients diagnosed with abdominal aortic aneurysms (AAAs). The anatomical pathway of blood flow in the abdominal aorta is restored by the implantation of an endovascular graft (EVG), effectively depressurizing the aneurysm and initiating a remodeling process of the diseased aorta. The short-term results of endovascular grafting are promising, but its long-term success has been compromised by the occurrence of graft migration and detachment, which induce endoleaks or incomplete occlusion of the aneurysm from the blood circulation. The forces induced by the blood as it flows through the graft are believed to be a factor of probable cause in the partial detachment from its proximal and distal anchoring points and the migration of the graft downstream. The purpose of this study is to utilize analytical tools to provide an estimation of the forces required to secure the graft proximally when relying only on stresses induced by the flow.
Finol E.A., Amon C.H., Flujo Sanguineo Pulsatil en Aneurismas del Segmento Abdominal de la Arteria Aorta, in Bioingenieria en Iberoamerica: Avances y Desarrollos, eds. Cerrolaza, M. and Muller-Karger C.M., Sociedad Venezolana de Metodos Numericos en Ingenieria, Caracas, Venezuela, pp. 228-248, 2003.
Finol E.A., Di Martino E.S., Vorp D.A., Amon C.H., Fluid Structure Interaction and Structural Analyses of an Aneurysm Model, 2003 Summer ASME Bioengineering Conference, Key Biscayne, FL, ASME 2003 BED-Vol. 54, 2003.
Finol E.A., Marra K.G., Amon C.H., Analytical Estimation of Flow-induced Forces in Endovascular Grafts and Design Methodology for a Tissue-engineered Endovascular Attachment Mechanism, Advances in Computational Bioengineering 7: 231-241, 2003.
[Abstract] Endovascular repair has emerged as an alternative, less-invasive surgical technique for the treatment of patients diagnosed with abdominal aortic aneurysms (AAAs). The anatomical pathway of blood flow in the abdominal aorta is restored by the implantation of an endovascular graft (EVG), depressurizing the aneurysm and initiating a remodeling process of the diseased aortic tissue. The short-term results of endovascular grafting are promising, but its long-term success has been compromised by the occurrence of graft migration or detachment, which induce endoleaks or incomplete occlusion of the aneurysm from the blood circulation. The forces induced by the blood as it flows through the graft are believed to be a factor of probable cause in the migration of the graft downstream and the partial detachment of its proximal and distal anchoring points. The purpose of this study is to utilize analytical tools to provide an estimation of the forces required to secure the graft proximally when relying on flow-induced stresses alone, and to describe the design methodology of a tissue-engineered proximal attachment mechanism that is capable of withstanding these forces. Composites of synthetic and native biodegradable polymers are examined as biomaterials for the attachment mechanism of the graft.
Finol E.A., Keyhani K., Amon C.H., The Effect of Asymmetry in Abdominal Aortic Aneurysms under Physiologically Realistic Pulsatile Flow Conditions, Journal of Biomechanical Engineering 125(2): 207-217, 2003.
[Abstract] In the abdominal segment of the human aorta under a patient's average resting conditions, pulsatile blood flow exhibits complex laminar patterns with secondary flows induced by adjacent branches and irregular vessel geometries. The flow dynamics becomes more complex when there is a pathological condition that causes changes in the normal structural composition of the vessel wall, for example, in the presence of an aneurysm. This work examines the hemodynamics of pulsatile blood flow in hypothetical three-dimensional models of abdominal aortic aneurysms (AAAs). Numerical predictions of blood flow patterns and hemodynamic stresses in AAAs are performed in single-aneurysm, asymmetric, rigid wall models using the finite element method. We characterize pulsatile flow dynamics in AAAs for average resting conditions by means of identifying regions of disturbed flow and quantifying the disturbance by evaluating flow-induced stresses at the aneurysm wall, specifically wall pressure and wall shear stress. Physiologically realistic abdominal aortic blood flow is simulated under pulsatile conditions for the range of time-average Reynolds numbers 50«Rem«300, corresponding to a range of peak Reynolds numbers 262.5«Repeak«1575. The vortex dynamics induced by pulsatile flow in AAAs is depicted by a sequence of four different flow phases in one period of the cardiac pulse. Peak wall shear stress and peak wall pressure are reported as a function of the time-average Reynolds number and aneurysm asymmetry. The effect of asymmetry in hypothetically shaped AAAs is to increase the maximum wall shear stress at peak flow and to induce the appearance of secondary flows in late diastole.
Ghai S.S., Jhon M.S., Escobar R.A., Amon C.H., Sub-Continuum Heat Conduction in Electronics Using the Lattice-Boltzmann Method, Advances in Electronic Packaging 1: 257-264, 2003.
[Abstract] The lattice Boltzmann method (LBM) is used to examine multi-length scale, confined heat conduction problems in one dimension for which sub-continuum effects are important. This paper describes the implementation of the method and its application to electronic devices. A silicon-on-insulator device with internal heat generation is used as a case study to illustrate the advantages of the LBM. We compare our results with various hierarchical equations of heat transfer such as Fourier, Cattaneo, and Boltzmann transport equations, as well as with experimental and numerical data from the literature. Our results provide excellent agreement with other methodologies, at a far less computational effort.
Ghai S.S., Amon C.H., Jhon M.S., Hsia Y.T., Nanoscale Heat Transfer Modeling in Solids: Data Storage and Electronic Devices, American Society of Mechanical Engineers, Tribology Division (15): 93-98, 2003.
[Abstract] Lattice Boltzmann method (LBM), is used to examine multilength scale, confined heat conduction phenomena in solids for which sub-continuum regime is important This paper describes the implementation of the method and its application to cases pertinent to data storage and electronic devices. Thin solid films with internal heat generation and with temperature difference across the boundaries are used as case studies to illustrate the benefits of the LBM. We compare our results with various hierarchical equations of heat transfer such as Fourier, Cattaneo, and Boltzmann transport equations, as well as with experimental and numerical data from the literature. Our results exhibit a good agreement with other methodologies in one and two dimensions, at a much lower computational effort.
Ghai S.S., Escobar R.A., Amon C.H., Jhon M.S., Nanoscale Thermal Transport in Solids Using Lattice Boltzmann Method, 2003 AIChE Annual Meeting, San Francisco, CA, 2003.
Gomes C.J., Madrid M., Amon C.H., Parallel Molecular Dynamics Code Validation through Bulk Silicon Thermal Conductivity Calculations, American Society of Mechanical Engineers, Electronic and Photonic Packaging, EPP 3: 321-328, 2003.
[Abstract] We have implemented a parallel molecular dynamics algorithm, which incorporates the Stillinger-Weber interatomic potential. The code was parallelized using a ghost cell atomic division approach, ensuring scaling with the number of processors and a significant increase in speed with respect to the serial version. The methodology is validated by computing the thermal conductivity and phonon frequency spectra of bulk silicon single crystals for different domain sizes at 1000K. The predicted thermal conductivities are consistent with the experimental value at that temperature. In addition, the phonon frequency spectra capture the properties expected from the dispersion relations for silicon.
Guzman A.M., Amon C.H., Analisis y Diseño de Pulmones Artificiales: Investigaciones Computacionales de un Oxigenador de Membrana Intravenoso, Capítulo 10. en Bioingeniería en Iberoamérica: Avances y Desarrollos, C.M. Müller-Karger y M. Cerrolaza, Editores; Editorial: CIMNE, p. 594, 2003.
Narumanchi S.V.J., Murthy J.Y., Amon C.H., Simulation of Unsteady Small Heat Source Effects in Sub-micron Heat Conduction, Journal of Heat Transfer 125(5): 896-903, 2003.
[Abstract] In compact transistors, large electric fields near the drain side create hot spots whose dimensions are smaller than the phonon mean free path in the medium. In this paper, we present a study of unsteady hot spot behavior. The unsteady gray phonon Boltzmann transport equation (BTE) is solved in the relaxation time approximation using a finite volume method. Electron-phonon interaction is represented as a heat source term in the phonon BTE. The evolution of the temperature profile is governed by the interaction of four competing time scales: the phonon residence time in the hot spot and in the domain, the duration of the energy source, and the phonon relaxation time. The influence of these time scales on the temperature is investigated. Both boundary scattering and heat source localization effects are observed to have considerable impact on the thermal predictions. Comparison of BTE solutions with conventional Fourier diffusion analysis reveals significant discrepancies.
Narumanchi S.V., Murthy J.Y., Amon C.H., Computations of Heat Transport in Sub-micron Silicon Thin Films Accounting for Phonon Dispersion and Polarization, International Mechanical Engineering Congress and Exposition, IMECE 2003-42447, Washington D.C., 2003.
Narumanchi S.V.J., Murthy J.Y., Amon C.H., Computations of Sub-micron Heat Transport in Silicon Accounting For Phonon Dispersion, Proceedings of the ASME Summer Heat Transfer Conference : 569-578, 2003.
[Abstract] In recent years, the Boltzmann transport equation (BTE) has begun to be used for predicting thermal transport in dielectrics and semiconductors at the sub-micron scale. However, most published studies make a gray assumption and do not account for either dispersion or polarization. In this study, we propose a model based on the BTE, accounting for transverse acoustic (TA) and longitudinal acoustic (LA) phonons as well as optical phonons. This model incorporates realistic phonon dispersion curves for silicon. The interactions among the different phonon branches and different phonon frequencies are considered, and the proposed model satisfies energy conservation. Frequency-dependent relaxation times, obtained from perturbation theory, and accounting for phonon interaction rules, are used. In the present study, the BTE is numerically solved using a structured finite volume approach. For a problem involving a film with two boundaries at different temperatures, the numerical results match the analogous exact solutions from radiative transport literature for various acoustic thicknesses. For the same problem, the transient thermal response in the acoustically thick limit matches results from the solution to the parabolic Fourier diffusion equation. Also, in the acoustically thick limit, the bulk experimental value of thermal conductivity of silicon at different temperatures is recovered from the model even at coarse phonon frequency band discretization.
Narumanchi S.V.J., Amon C.H., Murthy J.Y., Influence of Pulsating Submerged Liquid Jets on Chip-Level Thermal Phenomena, Journal of Electronic Packaging 125(3): 354-361, 2003.
[Abstract] In this study, numerical investigations are performed to examine the influence of unsteady submerged dielectric liquid (HFE-8401HT) jets impinging on electronic chip surfaces. The problem considered here involves conjugate heat transfer. Two different jet inlet velocity waveforms are studied - step and sinusoidal - over the range of frequencies 0.03-4.0 Hz, with the Reynolds number, based on jet inlet width, of up to 100. Results for chip surface temperatures and average Nusselt numbers are presented for both velocity waveforms over the range of frequencies considered, and the trends are discussed. A lumped-capacitance analysis for the chip is presented in terms of nondimensional parameters. The chip temperatures obtained from the analysis are compared to results obtained numerically.
Pacheco J.E., Amon C.H., Finger S., Incorporating Information from Replications into Bayesian Surrogate Models, Proceedings of the ASME Design Engineering Technical Conference 3: 485-494, 2003.
[Abstract] In some design domains, particularly rapidly evolving domains such as tissue engineering, analytical representations of the system do not exist. In these domains, the design process can be facilitated by the development of surrogate models that provide an understanding of the interactions of parameters and their influence on system performance, even though the models do not explain the underlying phenomena. Often, physical experiments are the only method for obtaining information about such systems. In particular, in bioengineering design domains, experiments are expensive and must be replicated to account for biological variability. Surrogate models can reduce the number of experiments needed and increase the value of the information gained through experimentation. In this paper, we present a framework for incorporating information from replications (repeated experiments) into Bayesian surrogate models. Within this framework, we develop uncertainty measurements for the prediction of the surrogate model. We illustrate the framework with two test cases using analytical functions. We then present a biomedical example used in the design of scaffold materials for the regeneration of bone tissue to show the use of Bayesian surrogates in exploratory design.
Pacheco J.E., Amon C.H., Finger S., Bayesian Surrogates Applied to Conceptual Stages of the Engineering Design Process, Journal of Mechanical Design 125(4): 664-672, 2003.
[Abstract] During the conceptual design stages, designers often have incomplete knowledge about the interactions among design parameters. We are developing a methodology that will enable designers to create models with levels of detail and accuracy that correspond to the current state of the design process. Thus, designers can create a rough surrogate model when only a few data points are available and then refine the model as the design progresses and more information becomes available. These surrogates represent the system response when limited information is available and when few realizations of experiments or numerical simulations are possible. This paper presents a covariance-based approach for building multistage surrogates in the conceptual design stages when bounds for the response are not available a priori. We test the methodology using a one-dimensional analytical function and a heat transfer problem with an analytical solution, in order to obtain error measurements. We then illustrate the use of the methodology in a thermal design problem for wearable computers. The surrogate model enables the designer to understand the relationships among the design parameters.
Romero D.A., Amon C.H., Finger S., Modeling Time-Dependent Systems Using Multi-Stage Bayesian Surrogates, ASME International Mechanical Engineering Congress and Exposition, Proceedings : 47-57, 2003.
[Abstract] Multi-Stage Bayesian Surrogate Models (MBSM) are meta-models, constructed using data obtained from different sources, which have the ability to integrate information and responses with different levels of accuracy. In applications of surrogate models for time-dependent systems, the data obtained from physical or computational experiments is usually a sequence of response values over time, measured for different combinations of design parameters. For such applications, the traditional MBSM approach is impractical to incorporate all the observed data in a single model of the system, mainly due to the prohibitive computational effort involved. In this paper, we propose a framework for building surrogate models for time-dependent systems, based on the cokriging technique. The proposed framework regards the observations as a set of time-correlated spatial processes, with a stationary, separable cross-covariance structure of known functional form. Results show that for time-dependent systems, the proposed methodology outperforms joint space-time models built with the traditional MBSM approach both in terms of accuracy and computational effort.
Smith B.R., Amon C.H., Design of a Low-Cost Infrared Sensor Array Through Thermal System Modeling, Advances in Electronic Packaging 1: 513-519, 2003.
[Abstract] The responsivity, sensitivity (signal-to-noise ratio), and cross talk of pyroelectric infrared sensor arrays are directly related to the thermal performance of the interconnect between sensor elements and readout electronics. Conventional low-cost designs, employing a film of sensor material like polyvinylidenefluoride (PDVF) layered on top of a silicon substrate, function by reading the electronic signal generated in the sensor when infrared radiation causes the sensor to heat up proportional to the radiation intensity. However, the change in temperature of the sensor material, and therefore signal generated, is highly dependent on the thermal properties of the interconnect material between the sensor and silicon substrate. A numerical framework for evaluating the effect of thermal conductivity and specific heat on sensor responsivity, sensitivity, and cross talk is developed. This allows us to analyze the relationships between feature size, thermal properties, and system performance. Using this model, a selection of materials from epoxies and other conventional solutions to emerging material systems such as nanoporous silica (aerogel) can be analyzed. Aerogel is most interesting since its thermal properties are 1-3 orders-of-magnitude better than conventional interconnect materials. Recent developments have also shown its compatibility with low-cost microelectronics fabrication and packaging techniques. The numerical model illustrates the potential of highly miniaturized pyroelectric infrared sensor arrays that have comparable performance at dramatically lower fabrication cost compared to conventional infrared sensor array technology.
Smith B.R., Amon C.H., Effect of Sub-Continuum Energy Transport on Effective Thermal Conductivity in Nanoporous Silica (Aerogel), American Society of Mechanical Engineers, Electronic and Photonic Packaging 3: 311-319, 2003.
[Abstract] This paper analyzes the effect of Fourier vs. subcontinuum heat transport through thin layers of nanoporous silica (aerogel) in the framework of an infrared focal plane array (IRFPA) sensor system. Aerogel is introduced as a compatible material for emerging microsystems applications and the comparison between aerogel and conventional insulation systems is analyzed. Correlations between aerogel's macro-scale thermal properties and its nano-scale structure are discussed to address the effect of the material's amorphous structure and sub-continuum phonon transport phenomena on macro-scale thermal conductivity. Simulations using the Lattice Boltzmann Method (LBM) quantify the effect of phonon scattering on silica conductivity. Techniques for extending the analysis to a three-dimensional silica matrix are discussed in light of recent advances in the simulation of aerogel morphology.
Subrahmanian E., Westerberg A., Talukdar S., Garrett J., Jacobson A., Paredis C., Amon C.H., Herder P., Turk A., Integrating Social Aspects and Group Work Aspects in Engineering Design Education, International Journal of Engineering Education 19(1): 75-80, 2003.
[Abstract] This paper describes two design project courses we teach at CMU, both empnasiztng the importance of the social aspects of design. The first, taught collaboratively with Delft University, asks students to formulate, but never solve, a series of design problems. Students move from individually solving a series of simple problems to solving or interpreting larger problems as cross-Atlantic teams. A second product design projects course is available to all CMU students, with projects supported both technically and financially by industrial sponsors. Students learn the value of multidisciplinary teaming and of having real problems on which to work. We place emphasis on the teams discovering the 'right' problem on which to work. Projects span two terms, with the second term team members learning first hand about the difficulties of project hand-off. In both courses, students learn from each other through weekly project presentations. Also, in both courses, everyone-faculty, industrial partners and students-places, organizes and shares all information electronically within LIRE, our web-based document management system. Finally, for the first course, almost all CMU lectures and student presentations for the past two years are available as web-available streaming video movies.
Yao S.C., Fedder G., Amon C.H., Hsieh C.C., Tang X., Vladimer M., Micro-Fluidic Development for Portable Scale Direct Methanol Fuel Cells, Materials Research Society Meeting, Boston, MA, 2003.
Finol E.A., Marra K.G., and Amon C.H., Tissue Engineering and Computational Fluid Dynamics Modeling in Endovascular Grafting Applications, Proceedings of the Engineering Tissue Growth International Conference and Exposition - ETG 2003, 2003.
Alawadhi E.M., Amon C.H., Thermal Analyses of a PCM Thermal Control Unit for Portable Electronic Devices: Experimental and Numerical Studies, Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference : 466-475, 2002.
[Abstract] This paper investigates the effectiveness of a Thermal Control Unit (TCU) for portable electronic devices by performing experimental and numerical analyses. The TCU objective is to improve thermal management of electronic devices where their operating time is limited to few hours. It is composed of an organic Phase Change Material (PCM) and a Thermal Conductivity Enhancer (TCE). The TCU can provide a reliable solution to portable electronic devices, which avoids overheating and thermally-induced fatigue, as well as a solution which satisfies the ergonomic requirement. Since the thermal conductivity of the PCM is very low, a TCE is incorporated into the PCM to boost its conductivity. The TCU structure is complex, and modeling an electronic device with it requires time and effort. Hence, this research develops approximate, yet effective, solutions for the TCU. The TCU component properties are averaged and a single TCU material is considered. This approach is evaluated by comparing the numerical predictions with the experimental results. The numerical model is used to study the effect of some important parameters that are experimentally expensive to examine, such as the heat transfer coefficient, the PCM latent heat, the Stefan number, and the effect of the heat source power.
Amon C.H., Advances in Computational Modeling of Nano-Scale Heat Transfer, Invited Keynote Paper and Talk, Twelfth International Heat Transfer Conference, Grenoble, France, CD ISBN:2-84299-307-1, 02-KNL-02, 2002.
Amon C.H., Infrastructure for Collaborative Enterprises, Invited Plenary Talk, Proceedings of the 11th International Conference, IEEE Computer Society, pp. xiv-xviii, 2002.
Finol E.A., Hajiloo S., Keyhani K., Vorp D.A., Amon C.H., Flow-Induced Wall Pressure under Average Resting Hemodynamic Conditions for Patient-Specific Abdominal Aortic Aneurysms, American Society of Mechanical Engineers, Bioengineering Division (Publication) 54: 363-364, 2002.
[Abstract] A study was done on the flow-induced wall pressure under average resting hemodynamic conditions for patient-specific abdominal aortic aneurysms (AAA). The methodology was based on the geometric reconstruction of each patient's AAA, the generation of adequate finite element domains and the simulation of the momentum transport equations for physiologically realistic blood flow properties. It was found that peak hemodynamic pressure correlated positively with the size of the aneurysm.
Finol E.A., Amon C.H., Flow-Induced Wall Shear Stress in Abdominal Aortic Aneurysms: Part II - Pulsatile Flow Hemodynamics, Computer methods in biomechanics and biomedical engineering 5(4): 319-328, 2002.
[Abstract] In continuing the investigation of AAA hemodynamics, unsteady flow-induced stresses are presented for pulsatile blood flow through the double-aneurysm model described in Part I. Physiologically realistic aortic blood flow is simulated under pulsatile conditions for the range of time-average Reynolds numbers 50< or =Re(m) < or =300. Hemodynamic disturbance is evaluated for a modified set of indicator functions which include wall pressure (p(w)), wall shear stress (tau(w)), Wall Shear Stress Gradient (WSSG), time-average wall shear stress (tau(w)*), and time-average Wall Shear Stress Gradient WSSG*. At peak flow, the highest shear stress and WSSG levels are obtained at the distal end of both aneurysms, in a pattern similar to that of steady flow. The maximum values of wall shear stresses and wall shear stress gradients are evaluated as a function of the time-average Reynolds number resulting in a fourth order polynomial correlation. A comparison between numerical predictions for steady and pulsatile flow is presented, illustrating the importance of considering time-dependent flow for the evaluation of hemodynamic indicators.
Finol E.A., Amon C.H., Flow-Induced Wall Shear Stress in Abdominal Aortic Aneurysms: Part I - Steady Flow Hemodynamics, Computer methods in biomechanics and biomedical engineering 5(4): 309-318, 2002.
[Abstract] Numerical predictions of blood flow patterns and hemodynamic stresses in Abdominal Aortic Aneurysms (AAAs) are performed in a two-aneurysm, axisymmetric, rigid wall model using the spectral element method. Homogeneous, Newtonian blood flow is simulated under steady conditions for the range of Reynolds numbers 10 < or =Re < or =2265. Flow hemodynamics are quantified by calculating the distributions of wall pressure (p(w)), wall shear stress (tau(w)), Wall Shear Stress Gradient (WSSG). A correlation between maximum values of hemodynamic stresses and Reynolds number is established, and the spatial distribution of WSSG is considered as a hemodynamic force that may cause damage to the arterial wall at an intermediate stage of AAA growth. The temporal distribution of hemodynamic stresses in pulsatile flow and their physical implications in AAA rupture are discussed in Part II of this paper.
Finol E.A., Di Martino E.S., Vorp D.A., Amon C.H., Biomechanics of Patient Specific Abdominal Aortic Aneurysms: Computational Analysis of Fluid Flow, Bioengineering, Proceedings of the Northeast Conference : 191-192, 2002.
[Abstract] Computational fluid flow analysis to study biomechanics of patient specific abdominal aortic aneurysms was performed. The results of analysis were compared to those of static stress analysis performed on the aneurysm model. The vortex dynamics induced by blood flow was found to be patient-specific and was determined by the shape and size of the aneurysm.
Ghai S.S., Jeong J.H., Jhon M.S., Amon C.H., Asheghi M., Nanoscale Heat Transfer and its Application to Information Technology, 2002 AIChE Annual Meeting, Indianapolis, IN, 2002.
Klingbeil N.W., Beuth J.L., Chin R.K., Amon C.H., Residual Stress-Induced Warping in Direct Metal Solid Freeform Fabrication, International Journal of Mechanical Sciences 44(1): 57-77, 2002.
[Abstract] Tolerance loss due to residual stress-induced warping is a major concern in solid freeform fabrication (SFF) processes, particularly those which involve direct deposition of molten metals. An understanding of how residual stresses develop and how they lead to tolerance loss is a key issue in advancing these processes. In this paper, results are presented from warping experiments on plate-shaped specimens created by two direct metal deposition methods, which are utilized by a particular SFF process termed shape deposition manufacturing (SDM). Results from these experiments give insight into the differences between the two deposition methods, the role of preheating and insulating conditions during manufacture and the influence of deposition path on magnitudes and distributions of warping displacements. Results are then compared to numerical predictions from both one and two-dimensional residual stress models, which are applicable to SDM and similar direct metal deposition processes. Results from the experiments and numerical models suggest that a combination of initial substrate preheating and part insulation can be applied to SDM and similar SFF processes to limit warping deflections, which is substantially simpler than active control of part temperatures during manufacture. Results also suggest that 3-D mechanical constraints are important in achieving precise control of warping behavior in SFF processes.
Pacheco J.E., Amon C.H., Finger S., Using Bayesian Models in Preliminary Design, International Design Conference - Design, Dubrovnik, 2002.
Pacheco J.E., Amon C.H., Finger S., Flexible Multistage Bayesian Models for Use in Conceptual Design, Proceedings of the ASME Design Engineering Technical Conference 4: 241-250, 2002.
[Abstract] During conceptual design, designers need tools to help improve design decisions and reduce design times. We are working to develop techniques to create Bayesian surrogate models that respond to designers' needs during conceptual stages of the design process. Bayesian surrogate models give analytical form to the overall performance of a system and can evolve along with the design. Bayesian surrogate models provide a mathematically rigorous framework in which computational models can be updated based on previous outcomes. In this paper, we present techniques that allow the addition or suppression of parameters without discarding previously obtained information. We also present a case study that illustrates how a surrogate model is constructed in stages when parameters are added or suppressed during the design process. Visualization tools, such as plots of the main effects of parameters, can be derived from surrogate models. These tools can be used to provide knowledge about the parameters that influence the design. Finally, a design problem is used to illustrate how Bayesian surrogate models can inform the designer about tradeoffs that would not be apparent from simulation data alone.
Shwaish I.K., Amon C.H., Murthy J.Y., Thermal/Fluid Performance Evaluation of Serrated Plate Fin Heat Sinks, Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference : 267-275, 2002.
[Abstract] Cooling of the electronics in people movers and other rail transportation systems require the removal of high power dissipation from the electronic equipment to ensure their long term reliability and performance. Though serrated plate fin heat sinks are commonly used in such applications, it is unclear whether they offer a performance advantage. In this study, the thermal performance of in-service serrated plate fin heat sink is evaluated for a range of Reynolds number by means of fully three-dimensional numerical simulations of the air flow over the heat sink. The flow is considered to be turbulent and both hydrodynamically and thermally developing. Our computations yield local and global heat transfer and flow parameters such as temperature distribution in the fin, heat transfer coefficient, Nusselt number, pressure drop, and the maximum temperature of the heat sink. The results point to the directions for optimizing the heat sink performance. Hence, variations of design parameters around the existing configurations are investigated. These parameters are fin interruption and staggering, fin height, serration spacing, fin thickness and inter-fin spacing, fin base thickness, and clearance gap between the fin tips and the upper wall of the channel that encloses the heat sink. The computations yield the optimal designs of plate fin heat sinks for transportation applications.
Yao S.C., Fedder G.K., Amon C.H., Hsieh C.C., Tang X., Alyousef Y., Design of Direct Methanol Micro Fuel Cell Fluidic Systems, ASME International Mechanical Engineering Congress and Exposition, Proceedings 7: 431-436, 2002.
[Abstract] Direct methanol micro fuel cells offer higher power density than advanced batteries. In this system, micro scale fluid handling becomes necessary. Designs of advanced micro fluidic systems including micro valves, pumps and CO 2 Gas bubble-Liquid separators, and flow channels are provided. MEMS-based special fabrication processes are utilized. Since all the fluidic components are fabricated with a common procedure, they are all fabricated together on the same silicon wafer at the same time. This assures the full fluidic system integration, which shall reduce the cost and enhance the reliability of the direct methanol micro fuel cell system.
Amon C.H., Murthy J., Yao S.C., Narumanchi S., Wu C.F., Hsieh C.C., MEMs-Enabled Thermal Management of High-Heat-Flux Devices EDIFICE: Embedded Droplet Impingement for Integrated Cooling of Electronics, Experimental Thermal and Fluid Science 25(5): 231-242, 2001.
[Abstract] This paper reports the development of embedded droplet impingement for integrated cooling of electronics (EDIFICE). The EDIFICE project seeks to develop an integrated droplet impingement cooling device for removing chip heat fluxes in the range 70-100 W/cm2, employing latent heat of vaporization of dielectric fluids (50-100 µm droplets) to achieve these high heat removal rates. Micro-manufacturing and micro electro-mechanical systems (MEMS) will be discussed as enabling technologies for innovative cooling schemes recently proposed. A novel feature to enable adaptive on-demand cooling is MEMS sensing (on-chip temperature, remote IR temperature and ultrasonic dielectric film thickness) and MEMS actuation. EDIFICE will be integrated within the electronics package and fabricated using advanced micro-manufacturing technology (e.g., deep reactive ion etching (DRIE) and complementary metal-oxide-semiconductor (CMOS) CMU-MEMS). The development of EDIFICE involves modeling, CFD simulations, and physical experimentation on test beds. In this study, numerical simulations are performed to investigate EDIFICE jet impingement cooling with a dielectric coolant and the influence of several parameters such as jet diameter, jet velocity, and latent heat effects. This paper also presents flow visualization of micro-jet break-up, induced by MEMS micro-nozzles of irregular shapes and flow swirling to generate droplets with desirable dispersion. To enhance liquid spreading on the impingement surface and to create a thin film for effective evaporation, MEMS micro-structured surfaces are fabricated. All of these components are made from silicon and enabled by integrated-MEMS process technologies.
Amon C.H., Finol E.A., Spectral Element Methods for Unsteady Momentum Heat and Mass transfer in Complex Geometries, Proceedings of the 8th Latinamerican Congress on Heat and Mass Transfer - LATCYM 2001 eds. Prince Journal and Treviño C. pp. 504-511, 2001 (Keynote Paper).
Boyalakuntla D.S., Murthy J.Y., Amon C.H., Computation of Natural Convection in Channels with Staggered Pin Fins, American Society of Mechanical Engineers, Heat Transfer Division 369(7): 129-137, 2001.
[Abstract] In this paper we numerically analyze the possibility of using buoyant flow in the display panel of a laptop for electronics cooling. A three-dimensional channel with a staggered pin fin array is analyzed using an unstructured finite volume method. A uniform heat flux is applied on the inner wall; the outer wall is exposed to the ambient. A single periodic module is selected in the lateral direction. In the axial direction, however, the entire height of the display channel is considered. A range of Rayleigh numbers, panel inclinations and staggering arrangements are considered, and local and global flow and heat transfer results are obtained. The results are useful in designing augmented cooling schemes in portable electronics.
Chin R.K., Beuth J.L., Amon C.H., Successive Deposition of Metals in Solid Freeform Fabrication Processes Part II: Thermomechanical Models of Adjacent Droplets, Journal of Manufacturing Science and Engineering 123(4): 632-638, 2001.
[Abstract] Residual stress-induced tolerance losses are a principal barrier for using Solid Freeform Fabrication (SFF) processes to create functional parts out of engineering materials. In Part 1 of this paper, problems of successively deposited layers and droplets deposited in a column are considered for SFF processes. Models of these problems are used to detail thermal and mechanical interactions between existing and newly deposited material as well as their effects on final residual stress distributions on sub-layer (droplet) and multilayer scales. In the current study, sub-layer interactions are further considered using models of droplets deposited adjacent to one another. As in Part 1, models are applied to a particular SFF process; however, insights and conclusions are relevant to numerous similar SFF processes. Simulations of separated and connected droplets deposited onto a large substrate indicate very limited thermal interactions between adjacently deposited droplets. However, mechanical interactions between droplets can be significant, which is consistent with the directionality of warping observed in experiments. Results from deposition of droplets on a thin substrate demonstrate the importance of process-induced substrate preheating in reducing residual stresses.
Chin R.K., Beuth J.L., Amon C.H., Successive Deposition of Metals in Solid Freeform Fabrication Processes Part I: Thermomechanical Models of Layers and Droplet Columns, Journal of Manufacturing Science and Engineering 123(4): 623-631, 2001.
[Abstract] Solid Freeform Fabrication (SFF) processes allow the automated building of three-dimensional shapes by successively depositing material in layers. Residual stress-induced tolerance losses are principal concerns in using these processes to create functional parts. Thermomechanical models of temperatures and stresses are presented, which are relevant to controlling residual stress effects in SFF processes. Models are applied to a particular SFF process; however, insights and conclusions are applicable to a large number of related processes. The temporal evolution of temperatures and stresses is investigated at two levels of detail. The successive deposition of layers of material is investigated first using one-dimensional simulations, approximating the build-up of residual stress in a multi-layered part. The successive deposition of a column of molten metal droplets (a technique used to create thick layers) is then modeled using two-dimensional axisymmetric simulations. Insights are given into process changes that can minimize residual stress-related effects in manufactured parts, including part constraint and localized preheating near the point of deposition. Results for thermomechanical interactions between droplets deposited in a column provide the foundation for studying interactions between adjacently deposited droplets, which is addressed in Part 2.
Egan E., Amon C.H., Measuring Thermal Conductivity Enhancement of Polymer Composites: Application to Embedded Electronics Thermal Design, Journal of Enhanced Heat Transfer 8(2): 119-135, 2001.
[Abstract] The effect of volume fraction and type of conductive filler on the thermal conductivity enhancement of polymer composites is determined from a simplified experimental technique using both specimen measurements and numerical simulations. Two conductive fillers, boron nitride powder and fine-mesh aluminum fibers, are blended with two different polymers in volume percentages of up to 30 percent. The volume fraction and the particle distribution of the filler are found to be more critical than polymer selection for thermal conductivity enhancement. Inferences into the filler dispersion of the polymer composites is made by using analogies from thermal resistance networks. Using numerical simulations of an embedded electronic artifact, it is also shown that embedding heat-generating electronics within a thermally conductive polymer composite can significantly enhance its transient and steady-state thermal performance.
Finol E.A., Amon C.H., Secondary Flow and Wall Shear Stress in Three-Dimensional Steady Flow AAA Hemodynamics, American Society of Mechanical Engineers, Bioengineering Division 51: 27-28, 2001.
[Abstract] Secondary flow and wall shear stress in three-dimensional steady flow abdominal aortic aneurysms (AAA) hemodynamics were studied. Hemodynamic indicators evaluated at the wall were nondimensionalized using their corresponding magnitudes obtained for Poiseuille flow. Results showed that negative wall shear stress were present at the proximal end of the small aneurysm while a low, almost zero shear stress was obtained at its center.
Finol E.A., Amon C.H., Blood Flow in Abdominal Aortic Aneurysms: Pulsatile Flow Hemodynamics, Journal of Biomechanical Engineering 123(5): 474-484, 2001.
[Abstract] Numerical predictions of blood flow patterns and hemodynamic stresses in Abdominal Aortic Aneurysms (AAAs) are performed in a two-aneurysm, axisymmetric, rigid wall model using the spectral element method. Physiologically realistic aortic blood flow is simulated under pulsatile conditions for the range of time-averaged Reynolds numbers 50≤Rem≤300, corresponding to a range of peak Reynolds numbers 262.5≤Repeak≤1575. The vortex dynamics induced by pulsatile flow in AAAs is characterized by a sequence of five different flow phases in one period of the flow cycle. Hemodynamic disturbance is evaluated for a modified set of indicator functions, which include wall pressure (pw), wall shear stress (Tw), and Wall Shear Stress Gradient (WSSG). At peak flow, the highest shear stress and WSSG levels are obtained downstream of both aneurysms, in a pattern similar to that of steady flow. Maximum values of wall shear stresses and wall shear stress gradients obtained at peak flow are evaluated as a function of the time-average Reynolds number resulting in a fourth order polynomial correlation. A comparison between predictions for steady and pulsatile flow is presented, illustrating the importance of considering time-dependent flow for the evaluation of hemodynamic indicators.
Finol E.A., Amon C.H., Three-Dimensional Computational Modeling of Blood Flow in Abdominal Aortic Aneurysms, Proceedings of the V Latin American and Caribbean Congress on Fluid Mechanics - LACAFLUM 2001, pp. DHT9-1 to DHT9-9, 2001.
Finol E.A., Amon C.H., Blood Flow in Abdominal Aortic Aneurysms: Steady Flow Hemodynamics, Proceedings of the 8th Latinamerican Congress on Heat and Mass Transfer - LATCYM 2001, eds. Prince, J. and Treviño, C., pp. 496-503, 2001.
Guzman A.M., Escobar E.A., Amon C.H., Fluid Dynamics Characteristics in a 3-D Model of an Intravascular Oxygenator without Balloon Pulsations, Invited Presentation, ASME International Mechanical Engineering Congress and Exposition - IMECE 2001, New York, NY, 2001.
Jain A., Boyalakuntla D.S., Murthy J.Y., Amon C.H., Buoyancy-Driven Flows in Channels with In-Line Pin Fins, Proceedings of the National Heat Transfer Conference 1: 159-166, 2001.
[Abstract] In this paper we apply an unstructured finite volume method to solve for the buoyant flow in an inclined three-dimensional channel with pin fins on one wall. The inner wall of the channel has a uniform heat flux supplied; the outer wall is subjected to the ambient convection. A single periodic module is selected in the lateral direction. In the axial direction, however, the entire height of the channel is considered. Buoyancy is modeled using the Boussinesq approximation. A range of Raleigh numbers, inclinations and pin fin arrangements are considered. Local and global flow heat transfer results are obtained, including the local and global Nusselt numbers as well as local temperature and velocity fields. The results are useful in designing augmented cooling schemes in portable electronics.
Murthy J.Y., Amon C.H., Gabriel K., Kumta P., Yao S.C., Boyalakuntla D., Hsieh C.C., Jain A., Narumanchi S.V.J., Rebello K., Wu C.F., MEMs-Based Thermal Management of Electronics Using Spray Impingement, Advances in Electronic Packaging 2: 733-744, 2001.
[Abstract] The enormous increase in chip-level heat fluxes has necessitated the development of thermal management devices based on liquid cooling. This paper describes the development of EDIFICE (Embedded Droplet Impingement For Integrated Cooling of Electronics). The EDIFICE project seeks to develop an integrated droplet impingement cooling device for removing chip heat fluxes in the range 50-100 W/cm2, employing the latent heat of vaporization of dielectric fluids. Micro-spray nozzles are fabricated to produce 50-100 micron droplets coupled with surface texturing on the backside of the chip to promote spreading and boiling. EDIFICE will be integrated within the electronics package and fabricated using advanced micro-manufacturing technologies. Numerical modeling is used to study preliminary designs at both the device and system level. The paper describes progress made in the development of the device and reports preliminary results.
Narumanchi S.V.J., Murthy J.Y., Amon C.H., Small Heat Sources Effects in Sub-Micron Heat Conduction, American Society of Mechanical Engineers, Heat Transfer Division 369(7): 237-238, 2001.
[Abstract] Decreasing dimensions of integrated circuit devices is leading to increased importance of microscale heat transfer effects and the failure of Fourier's law in predicting sub-micron heat conduction. In compact transistors, large electric fields near the drain side create hot spots whose dimensions are smaller than the phonon mean free path in the medium. Under these conditions, the phonon Boltzmann equation (BTE) needs to be solved in order to resolve the non-local thermal conduction phenomena. In this paper, the problem of an unsteady heat source of size comparable to or smaller than the phonon mean free path is considered. The unsteady 2-D phonon Boltzmann transport equation in the relaxation time approximation is solved using a finite volume method. The interaction of the heat-up time constant with the phonon residence time in the hotspot and also its interaction with the time scales associated with scattering processes are studied. The results are useful in assessing the peak temperatures during unsteady operation in microelectronic devices.
Pacheco J.E., Amon C.H., Finger S., Developing Bayesian Surrogates for Use in Preliminary Design, Proceedings of the ASME Design Engineering Technical Conference 4: 187-196, 2001.
[Abstract] During the preliminary design stages, designers often have incomplete knowledge about the interactions among design parameters. We are developing a methodology that will enable designers to create models with levels of detail and accuracy that correspond to the current state of the design knowledge. The methodology uses Bayesian surrogate models that are updated sequentially in stages. Thus, designers can create a rough surrogate model when only a few data points are available and then refine the model as the design progresses and more information becomes available. These surrogates represent the system response when limited information is available and when few realizations of experiments or numerical simulations are possible. This paper presents a covariance-based approach for building surrogates in the preliminary design stages when bounds are not available a priori. We test the methodology using an analytical one-dimensional function and a heat transfer problem with an analytical solution, in order to obtain error measurements. We then illustrate the use of the methodology in a thermal design problem for wearable computers. In this problem, the underlying heat transfer phenomena make the system response non-intuitive. The surrogate model enables the designer to understand the relationships among the design parameters in order to specify a system with the desired behavior.
Shwaish I.K., Murthy J.Y., Amon C.H., Bains D., Performance evaluation of serrated plate fins for under-carriage electronics cooling in transportation application, American Society of Mechanical Engineers, Heat Transfer Division 369(7): 177-178, 2001.
[Abstract] The performance of serrated plate fins for under-carriage electronics cooling in transportation applications was studied. Fully three-dimensional hydrodynamically and thermally developing turbulent flow was considered. The heat transfer coefficients, Nusselt number, pressure drop, and maximum temperature of the sink for different fin heights were calculated. The maximum and average temperature of the heat sink were found to be relatively constant for different fin heights, and it was concluded that the fin height might be reduced by 30% without impunity.
Subrahmanian E., Westerberg A., Taludkar S., Garrett J., Amon C.H., Herder P., Turk A., Integrating Social Aspects and Group Work Aspects in Engineering Design Educationn, , MDW III Conference, 2001.
Yao S.C., Amon C.H., Gabriel K., Kumta P., Murthy J.Y., Wu C.F., Hsieh C.C., Boyalakuntla D., Narumanchi S.V.J., Rebello K., MEMs Enabled Micro Spray Cooling System for Thermal Control of Electronic Chips, American Society of Mechanical Engineers, Heat Transfer Division 369(7): 181-192, 2001.
[Abstract] Liquid cooling of electronic devices becomes necessary when the chip-level heat fluxes increase and traditional air cooling encounters ever-increasing difficulties. From all the liquid cooling processes, spray cooling appears more successful due to its high critical heat flux, relatively low liquid flow rates, highly controllable, and the non-existence of boiling incipient hysterisis. This paper describes the development of the EDIFICE project (Embedded Droplet Impingement For Integrated Cooling of Electronics), which seeks to develop an integrated droplet impingement cooling device for removing chip heat fluxes in the range 50-100 W/cm2, employing latent heat of vaporization of dielectric fluids. Micro-spray nozzles are fabricated on silicon using MEMS technology to produce 100 micron droplets with odd shaped nozzles and swirling nozzles. The effects of shape s.z., type of fluid, and swirling are tested and discussed. Spray heat transfer on silicon surfaces is studied with various surface texturing on the backside of the chip to promote spreading and evaporation of cold fluids as well as at heated conditions. The effects of configuration and fluids are revealed. Numerical modeling is used to study preliminary designs at both the device and system level. The paper describes progress made in the development of the EDIFICE device.
Alawadhi E.M., Amon C.H., PCM Thermal Storage Unit for Time-Dependent Thermal Management of Electronic Devices, Proceedings of the ASME 34th Heat Transfer Conference - NHTC 2000, eds. Yao, S. and Jones, NHTC2000-12195, 2000.
Alawadhi E.M., Amon C.H., Performance Analysis of an Enhanced PCM Thermal Control Unit, Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference 1: 283-289, 2000.
[Abstract] This paper reports the investigation of a Thermal Control Unit (TCU) implemented into an electronic device to improve energy management, absorb excessive heat generated by the heat source component in a quick manner and maintain surface temperature below critical limits. The TCU is made of an organic Phase Change Material (PCM) and a Thermal Conductivity Enhancer (TCE), composed of aluminum fins. The effect of the fin distribution on the performance of the TCU is investigated over a range of operation conductions. To quantify the improvements of the TCU with the TCE, it is compared with the baseline case of TCU without TCE. Results illustrate significant effects of the TCE for both constant and variable power operations. In a constant power operation, the TCE helps to keep the TCU temperature uniform and constant during PCM melting, in which overheating of the PCM is prevented. During variable power operations, the TCE helps to reduce fluctuating temperatures in the TCU, with a reduction in maximum temperature of 10°C.
Amon C.H., Thermal Management of High-Heat-Flux Devices EDIFICE: Embedded Droplet Impingement for Integrated Cooling of Electronics, ASME-ZSITS International Thermal Sciences, eds. Bergles and Golobic 1: 47-56, 2000 (Invited Plenary Talk Paper).
Amon C.H., Murthy J.Y., Simulation Needs for the NGTS Combustion Program, DOE AGTSR Meeting, Proceedings Combustion VII Workshop, Berkeley, CA, 2000 (Invited Presentation).
Choi T., Amon C.H., Shih I-P. T., Trigui N., CFD Shape Optimization Based on an Adjoint Variable Formulation of the Compressible Navier-Stokes Equations, 38th Aerospace Sciences, AIAA-2000-0457, 2000.
Egan E., Amon C.H., Thermal Management Strategies for Embedded Electronic Components of Wearable Computers, Journal of Electronic Packaging 122(2): 98-106, 2000.
[Abstract] Wearable computers are rugged, portable computers that can be comfortably worn on the body and easily operated for maintenance applications. The recently developed process of Shape Deposition Manufacturing has created the opportunity to embed the electronics of wearable computers in a polymer composite substrate. As both a protective outer case and a conductive heat dissipating medium, the substrate satisfies two basic constraints of wearable computer design: ruggedness and cooling efficiency. One such application of embedded electronics is the VuMan3R, a wearable computer designed and manufactured at Carnegie Mellon University for aircraft maintenance. This paper combines finite element numerical simulations, physical experimentation, and analytical models to understand the thermal phenomena of embedded electronic design and to explore the thermal design space. Numerical models ascertain the effect of heat spreaders and polymer composite substrates on the thermal performance, while physical experimentation of an embedded electronic artifact ensures the accuracy of the numerical simulations and the practicality of the thermal design. Analytical models using thermal resistance networks predict the heat flow paths within the embedded electronic artifact as well as the role of conductive fillers used in polymer composites.
Leoni N., Amon C.H., Bayesian Surrogates for Integrating Numerical Analytical and Experimental Data: Application to Inverse Heat Transfer in Wearable Computers, IEEE Transactions on Components and Packaging Technologies 23(1): 23-32, 2000.
[Abstract] Wearable computers are portable electronics worn on the body. The increasing thermal challenges facing these compact electronics systems have motivated new cooling strategies such as transient thermal management with thermal storage materials. The ability of building models to assess quickly the effect of different design parameters is critical for effectively incorporating innovative thermal strategies into new products. System models that enable design space exploration are built from different information sources such as numerical simulations, physical experiments, analytical solutions and heuristics. These models, called surrogates, are nonlinear, adaptive, and suitable for system responses where limited information is available and few realizations of experiments or numerical simulations are feasible. This paper applies a Bayesian surrogate framework to estimate values for unknown physical parameters of an embedded electronics system. Physical experiments and numerical simulations are performed on an embedded electronics prototype system of a wearable computer. Numerical models for the experimental prototype, which involve five and three unknown parameters, are implemented with and without thermal contact resistances. Through the use of orthogonal arrays and optimal sampling, an efficient exploration of the parameter space is performed to determine thermal conductivities, thermal contact resistances and heat transfer coefficients. Surrogate models are built that combine information obtained from numerical simulations, experimental model measurements and a thermal resistance network. The integration of several information sources reduces the number of large-scale numerical simulations needed to find reliable estimates of the system parameters. For the embedded electronics case, the use of prior information from the thermal resistance network model reduces significantly the computational effort required to investigate the solution space.
Finol E.A., Amon C.H., On the Calculation of Fluid Shear Stresses at the Wall of Dilated Large Arteries: Part II - Application to 3D Computational Models, American Society of Mechanical Engineers, Bioengineering Division 48: 13-14, 2000.
Finol E.A., Amon C.H., On the Calculation of Fluid Shear Stresses at the Wall of Dilated Large Arteries: Part I - Application to 2D Axisymmetric Computational Models, American Society of Mechanical Engineers, Bioengineering Division 48: 97-98, 2000.
[Abstract] A procedure for calculating fluid shear stresses at the wall of two-dimensional axisymmetric models of dilated large arteries is presented. The procedure is applied to a model of a double-aneurysm abdominal aorta under pulsatile flow conditions for a time-average Reynolds number Rem = 680. The method is extended to three-dimensional artery models in Part II of this paper.
Finol E.A., Amon C.H., Procedure for Calculating Fluid Shear Stresses at the Wall of Dilated Large Arteries: Application to 2D Axisymmetric and 3D Computational Models, Annals of Biomedical Engineering 28(1):S-74, 2000.
Finol E.A., Amon C.H., Momentum Transfer in Abdominal Aortic Aneurysms: The Effect of Aneurysm Size in Steady Flow Hemodynamics, Proceedings of the ASME 34th National Heat Transfer Conference - NHTC 2000, eds. Yao, S. and Jones, A., NHTC2000-12205, 2000.
Finol E.A., Amon C.H., Pulsatile Flow Hemodynamics in Abdominal Aortic Aneurysms, Proceedings of the V International Congress of Numerical Methods in Engineering and Applied Sciences - CIMENICS 2000, eds. Troyani, N. and Cerrolaza, M., pp. CI81-CI90, 2000.
Guzman A.M., Escobar, R. A., Amon C.H., Effect of Curved Boundaries of an Intravenous Membrane Oxygenator on the Fluid Dynamics and Mass Transfer Characteristics, Advances in Heat and Mass Transfer in Biotechnology, ASME HTD-Vol. 368/BED-Vol. 47, pp. 121-122, 2000.
Guzman A.M., Escobar R.A., Loyola H.J., Amon C.H., Pressure Drop and Mass Transfer in an Intravenous Membrane Oxygenator in a Pulsatile Flow Regime, Proceedings of the ASME 34th National Heat Transfer Conference, eds. Yao, S. and Jones, A., NHTC2000-12206, 2000.
Guzman A.M., Amon C.H., Flow and Oxygen Transfer Characteristics of an Intravenous Membrane Oxygenator: A Computational Study, Computer Methods in Biomechanics and Biomedical Engineering 3(2): 147-166, 2000.
[Abstract] Spectral element computational simulations of the conservation of mass, momentum and species equations are performed to investigate the flow and oxygen transfer characteristics of an Intravenous Membrane Oxygenator (IMO). The simulations consider a three-dimensional IMO computational model consisting of equally-spaced fibers, an elastic balloon with non-permeable walls positioned longitudinally within the vena cava, and a Newtonian and time-dependent incompressible flow. Flow characteristics and oxygen transfer parameters are determined for operating conditions of a stationary and a pulsating balloon. For the stationary balloon configuration the flow is two-dimensional, parallel, laminar and without secondary flows for the Reynolds number range of 5.7-455.2. Evaluations of the oxygen transfer characteristics for the stationary balloon indicate that the main transport mechanisms are diffusion and convection in the crosswise and streamwise directions, respectively. Additionally, evaluations of oxygen transfer rates and Sherwood numbers in this Reynolds number range indicate that the oxygen transfer rate reaches an asymptotic limit at relatively moderate Reynolds numbers. For the pulsating balloon, flow characteristic results demonstrate the existence of a strong secondary flow around the fiber, and between the balloon and the fiber. This secondary flow induces oscillatory crosswise and streamwise velocities and a seemingly random spanwise flow which enhances the flow mixing as well as the transport of oxygen from the fiber surface to the bulk flow.
Narumanchi S.V.J., Amon C.H., Murthy J., Dielectric Jet Impingement Cooling of Electronic Chips, Proceedings of the ASME 34th National Heat Transfer Conference, eds. Yao, S. and Jones, NHTC2000-12138, 2000.
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1995 - 1999
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Flores A.J., Amon C.H., Shih T. I-P., Davis D.O, Willis B.P., Boundary-Layer Bleed through Micro Holes, IAA99-0880, 1999.
Guzman A.M., Amon C.H., Mass Transfer Enhancement in an Intravenous Membrane Oxygenator Induced by a Pulsating Balloon, American Society of Mechanical Engineers, Bioengineering Division 44: 15-16, 1999.
[Abstract] An account is given on the investigation of the flow and oxygen transfer characteristics of the Intravenous Membrane Oxygenator (IMO) device. The numerically obtained flow patterns and the oxygen transfer characteristics for stationary and pulsating balloon regimes are described.
Schmaltz K.S., Zarzalejo L.J., Amon C.H., Molten Droplet Solidification and Substrate Remelting in Microcasting Part II: Parametric Study and Effect of Dissimilar Materials, Heat and Mass Transfer 35(1): 17-23, 1999.
[Abstract] This paper presents a parametric study of relevant processing parameters found in microcasting Shape Deposition Manufacturing (SDM). Microcasting SDM is a novel, layered manufacturing process capable of rapidly manufacturing near net shape, metal objects. The quality of artifacts built with this process depends on proper metallurgic bonding between impacting molten droplets and previously deposited substrate layers, as well as on the final microstructure of the artifact. Numerical simulations are performed to investigate the effect of operating conditions on the metallurgic bonding induced by substrate remelting and on the microstructure determined by the cooling rates during solidification. Particularly, the effect of droplet impinging temperatures, substrate initial temperatures and combinations of copper and stainless steel materials are investigated. Numerical predictions reveal that impinging droplet temperature variations, within the attainable range in microcasting SDM, have a minimal effect on the cooling rates during solidification. However, droplet temperature has a significant effect on the substrate remelting depth. Furthermore, this investigation quantifies the extent to which substrate preheating lowers the cooling rate during solidification and promotes substrate remelting. The study of the interaction between copper and stainless steel materials shows that the cooling rates during solidification of the deposition material and the occurrence of substrate remelting are both highly dependent on the combination of materials.
Zarzalejo L.J., Schmaltz K.S., Amon C.H., Molten Droplet Solidification and Substrate Remelting in Microcasting Part I: Numerical Modeling and Experimental Verification, Heat and Mass Transfer 34(6): 477-485, 1999.
[Abstract] A numerical model of a molten metal droplet impinging, solidifying and bonding to a solid substrate has been developed. The numerical model solves for the evolution of the temperature distribution in the droplet and substrate, predicts the position of the remelting and solidification fronts, and accounts for convective motion. The effect of the convection induced by the droplet spreading is modeled through a time-dependent effective thermal conductivity. High speed filming of the molten droplet impinging and spreading on the substrate is performed to obtain the required parameters to determine this time dependent effective conductivity. The accuracy of the model is investigated with experimental techniques.
Vesligaj M., Amon C.H., Transient Thermal Management of Temperature Fluctuations during Time Varying Workloads on Portable Electronics, IEEE Transactions on Components and Packaging Technologies 22(4): 541-550, 1999.
[Abstract] This paper describes the investigation of solid to liquid phase change materials (PCM's) for passive energy storage during the condition of time varying workloads on portable electronics. The model investigated includes a thermal control unit (TCU) embedded in an epoxy polymer. A TCU is an enclosure that contains phase change material (PCM) and a thermal conductivity enhancer, is located near the power source, and acts as an energy storage and heat-spreading module. Physical experiments were carried out to investigate the performance improvements of introducing a TCU into an embedded system and were used to validate the accuracy of the numerical model. Numerical simulations were performed to study the effect duty cycles and substrate thermal conductivities have on the thermal performance of the electronic wearable computer system with passive energy storage. Additionally, the TCU was numerically modeled to determine the influence of boundary conditions on TCU performance. To quantify the improvements of the system, metrics were developed from analyzing the thermal evolution of the TCU parameters, such as temperature fields, temperature bands, PCM characteristics, and power loads. Results indicate that using a TCU for passive energy storage significantly increases the portable electronics system's operational performance. Duty cycles with the same average power over the duration of the cycle do not influence the length of the PCM phase change time, but do impact the mean value of the temperature fluctuation bands
Vesligaj M.J., Amon C.H., Transient Thermal Management of Portable Electronics Using Phase Change Materials and Time Varying Power Dissipation, American Society of Mechanical Engineers, EEP 26: 2, 1999.
[Abstract] This paper describes the examination of passive energy storage systems during the condition of time varying workloads on portable electronics. The model investigated contains a Thermal Control Unit (TCU) embedded in an epoxy polymer. A TCU is an enclosure that contains Phase Change Material (PCM) and a thermal conductivity enhancer, is located at the power source, and acts as an energy storage and heat-spreading module. Physical experiments are performed to investigate the performance improvements of introducing a TCU into an embedded system and are used to validate the accuracy of the numerical model. Numerical simulations are used to illustrate the effect duty cycles and substrate thermal conductivities have on the thermal performance of the electronic wearable computer system with passive energy storage. Additionally, the TCU is numerically modeled to determine the influence of boundary conditions on TCU performance. To quantify the improvements of the system, metrics are developed from analyzing the thermal evolution of the TCU parameters, such as temperature fields, temperature bands, PCM characteristics, and power loads. Results show that using a TCU for passive energy storage significantly increases the portable system's operational performance. Duty cycles with the same average power over the duration of the cycle do not influence the length of the PCM phase change time, but do impact the mean value of the temperature fluctuation bands.
Amon C.H., Beuth J.L., Weiss L.E., Merz R., Prinz F.B., Shape Deposition Manufacturing with Microcasting; Processing Thermal and Mechanical Issues, ASME Journal of Manufacturing Science and Engineering 120: 656-667, 1998.
[Abstract] Shape deposition manufacturing (SDM) is a solid freeform fabrication (SFF) methodology for automatically building up material layers to form three-dimensional, complex-shaped, multi-material structures. Microcasting is a molten metal droplet deposition process which is able to create fully dense metal layers with controlled microstructures. SDM combines microcasting with other intermediate processing operations, such as CNC machining and shot peening, to create high quality metal parts. In this paper, a description is given of SDM and the microcasting process. An overview of thermal and mechanical issues associated with SDM and microcasting is presented, including the control of interlayer metallurgical bonding through substrate remelting, the control of cooling rates of both the substrate and the deposited material and the minimization of residual thermal stress effects. Thermal models are used to study the issue of localized remelting of previously deposited material by newly deposited molten droplets to achieve metallurgical bonding. Mechanical modeling provides insight into residual stress build-up during part manufacture and residual stress-driven debonding between deposited layers.
Campo A., Amon C.H., A Simple Way to Determine the Two Asymptotic Nusselt Number Expressions for In-Tube Laminar Forced Convective Flows Employing the Method of Lines, Computer Applications in Engineering Education 6(2): 79-87, 1998.
[Abstract] The principal goal of this educational article is to present an alternative simple way for the determination of the two asymptotic Nusselt number subdistributions for laminar forced convection tube flows subjected to a uniform surface temperature. Under the assumption of developed velocity and developing temperature, the generalized method of lines (MOL) has been adopted here because it is a powerful computational technique that facilitates the determination of the two asymptotic temperature solutions (one for x → 0 and the other for x → ∞). The semianalytic solution method, MOL, is quite different and possesses unique features that distinguish it from other standard solution methods used in the past. The two asymptotic Nusselt number subdistributions, Nux→0 and Nux→∞, blend themselves into an approximate Nusselt number distribution of high quality. The unprecedented ease of the computational procedure, along with its adequate results, has proved to be a valuable instructional technique in undergraduate courses on heat transfer.
Guzman A.M., Amon C.H., Convective Heat Transfer and Flow Mixing in Converging-Diverging Channel Flows, American Society of Mechanical Engineers, Heat Transfer Division 361-1: 61-68, 1998.
[Abstract] Mixing and heat transfer enhancement due to chaotic advection in a converging-diverging channel flow is characterized and quantified by spectral element direct numerical simulations of the time-dependent Navier-Stokes, continuity and energy equations. The computational simulations are performed in a two-dimensional converging-diverging channel model composed of sinusoidal walls. Results are presented for laminar and transitional flow regimes, in the Reynolds number range 110
Guzman A.M., Amon C.H., Mass Transfer Performance Evaluations of an Intravenous Membrane Oxygenator, American Society of Mechanical Engineers, Bioengineering Division 40: 149-154, 1998.
[Abstract] The mass transfer performance of an Intravenous Membrane Oxygenator is investigated by computational simulations of the conservation of mass, momentum and species equations. The Intravenous Membrane Oxygenator (IMO), is a device developed experimentally to provide consistent and reproducible oxygen and carbon dioxide exchange. The IMO is composed by an elastic and non-permeable pulsating balloon positioned within the vena cava, and micro-porous-membrane fibers that transport oxygen and carbon dioxide, located longitudinally between the balloon and vena cava. During the operation regime, the blood flow motion is originated by a blood pressure gradient and a pulsating balloon motion. A three-dimensional physical-computational model consisting of equally-spaced fibers and a Newtonian and time-dependent incompressible flow is used for the simulations. The numerical simulation results for the stationary balloon configuration, obtained using the spectral element method, demonstrate that the flow remains parallel, laminar and with absence of secondary flows in the whole domain. Evaluations of the mass transfer characteristics and parameters, such as the oxygen concentration profile around the fiber and the Sherwood number, for increasing Reynolds numbers, indicate that the parabolic flow regime increase the oxygen transfer rate until an asymptotic limit in the oxygen transfer capability is reached. A further increase in the Reynolds number beyond this asymptotic limit does not increase the oxygen transfer rate.
Guzman A.M., Amon C.H., Federspiel W.J., Hattler B.G., Spectral- Element Simulations of the Flow Kinematics in an Intravenous Membrane Oxygenator, Proceedings Fourth World Congress on Computational Mechanics, Vol. II, p. 1011, 1998. Also published in Computational Mechanics, New Trends and Applications, Idelsohn, Onate & Dvorkin (Eds.), CIMNE, pp. 1-14, 1998.
Klingbeil N.W., Beuth J.L., Chin R.K., Amon C.H., Measurement and Modeling of Residual Stress-Induced Warping in Direct Metal Deposition Processes, Proceedings Solid Freeform Fabrication, pp. 367-374, 1998.
Leoni N., Amon C.H., Bayesian Surrogates for Integrating Numerical Analytical and Experimental Data for Modeling and Design: Application to Embedded Electronics in Wearable Computers, Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference : 58-67, 1998.
[Abstract] The surrogate framework is applied to estimate values for unknown physical parameters of an embedded electronics systems. In particular, physical experiments and numerical simulations are performed on an embedded electronics prototype of the TIA (Technical Information Assistant) wearable computer. Numerical models involving five and three unknown parameters w.t., without thermal contact resistances are evaluated.
Oliveira J.C., Amon C.H., Pressure-Based Semi-Analytical Solution of Radial Stokes Flows, Journal of Mathematical Analysis and Applications 217(1): 95-114, 1998.
[Abstract] A semi-analytical methodology is proposed and implemented to approximate the solution of the partial differential equations that model the radial Stokes flow between two parallel disks. Two traditional approaches for solving partial differential equations of mathematical physics, namely, eigenfunction expansions and Green's function representations, are used to obtain directly the pressure distribution for a variety of inflow boundary conditions. We combine these standard techniques with a functional analytic framework to propose a proper representation for the pressure. This methodology has been implemented first in a symbolic software (Mathematica®) and then numerically. We provide both validation of the methodology when an exact solution exists and comparison with a spectral element solver for a test case where inflow boundary conditions render the existence of a recirculation zone close to the entrance of the domain. We briefly discuss how to extend the methodology to handle the Navier-Stokes equation in radial flow - a subject that will be presented in detail in a following paper.
Ambrose S.A., Amon C.H., Systematic Design of a First-Year Mechanical Engineering Course at Carnegie Mellon University, Journal of Engineering Education 86(2): 173-181, 1997.
[Abstract] Carnegie Mellon University offers a first-year course titled Fundamentals of Mechanical Engineering to introduce undergraduate students to the discipline of mechanical engineering. The goals of the course are to excite students about the field of mechanical engineering early in their careers, introduce basic mechanical engineering concepts in an integrated way, provide a link to the basic physics and mathematics courses, and present design and problem-solving skills as central engineering activities. These goals are met through a combination of real-world engineering examples, classroom demonstrations, and hands-on experience in assignments and laboratories. Over the eleven semesters that this course has been taught, teams of first-year students have designed and assembled energy conversion mechanisms using miniature steam engines and Meccano sets to drive a mobile vehicle or to generate electricity for lighting a bulb. This paper describes the systematic process used to design this course and emphasizes this process of carefully integrating lectures with classroom demonstrations, laboratory experiments and hands-on projects to encourage students' active learning.
Amon C.H., Finger S., Merz R., Prinz F., Schmaltz K., Weiss L., Shape Deposition Manufacturing with Microcasting, Invited Paper, J. of the Japan Welding Society, Vol. 66, No. 4, pp. 64-69, 1997.
Amon C.H., Egan E., Siewiorek D., Smailagic A., System-level Thermal Management and Concurrent Design of a Wearable Multicomputer System, IEEE Transactions Comps. Pack. Manuf. Technology, Vol. 20, No. 2, pp. 128-137, 1997.
Campbell M.I., Amon C.H., Cagan J., Optimal Three-Dimensional Placement of Heat Generating Electronic Components, Journal of Electronic Packaging 119(2): 106-113, 1997.
[Abstract] This work introduces an algorithm that uses simulated annealing to perform electronic component layout while incorporating constraints related to thermal performance. A hierarchical heat transfer analysis is developed which is used in conjunction with the simulated annealing algorithm to produce final layout configurations that are densely packed and operate within specified temperature ranges. Examples of three-dimensional component placement test cases are presented including an application to embedded wearable computers.
Egan E., Amon C.H., A Study in the Enhancement of the Thermal Conductivity of Several Polymer Composites for Embedded Electronics Applications, ASME-HTD, Vol. 344, No. 6, pp. 103-110, 1997.
Finger S., Amon C.H., Designing and Prototyping Interactive Fluid Dynamics Exhibits for the Carnegie Science Center: An Undergraduate Team Project Experience, Proceedings - Frontiers in Education Conference 1: 366-370, 1997.
[Abstract] This paper describes a team project experience at Carnegie Mellon University in which undergraduate students designed and prototyped interactive exhibits to teach principles of fluid dynamics. The client for the exhibits was the Carnegie Science Center (CSC) in Pittsburgh. The CSC promotes science awareness, focusing primarily on stimulating science appreciation in pre-teenagers through interactive exhibits that create an intuitive understanding of scientific principles. The students participating in this project first researched basic child development concepts, child motivation techniques, and the science museum's practices. Then students conducted brainstorming sessions to identify fluid dynamics principles and to create a list of design criteria for the exhibits. Twelve exhibit ideas were selected and presented to the CSC from which six were chosen to further analyze, design and prototype.
Guzman A.M., Moraga N.O., Amon C.H., Pulsatile Non-Newtonian Flow in a Double Aneurysm, American Society of Mechanical Engineers, Bioengineering Division 36: 87-88, 1997.
Guzman A., Moraga N., Munoz J., Amon C.H., Pulsatile Non-Newtonian Flow in a Converging-Diverging Tube, AIChE Symposium Series, ed. M.S. El-Genk, Vol. 93, pp. 288-294, 1997.
Leoni N., Amon C.H., Transient Thermal Design of Wearable Computers with Embedded Electronics using Phase Change Materials, American Society of Mechanical Engineers, Heat Transfer Division 343: 49-56, 1997.
[Abstract] The use of organic Phase Change Materials (PCMs) encapsulated within aluminum foam structures is investigated as Heat Storage Units for transient thermal management of embedded electronics for wearable computers manufactured by Shape Deposition Manufacturing (SDM) process. SDM is a free form manufacturing process currently under development at Carnegie Mellon University, which consists of selective deposition and machining of layers allowing multi-material structures to be formed. One of the applications of SDM is to rapidly build electronic housings and simultaneously embed circuitry and electronics to form a rugged, compact system. Embedded electronics are specially suited for wearable computer applications, where stringent requirements in performance, functionality and custom application demand a multidisciplinary approach within a short design cycle. A new generation of wearable computers, named the Technical Information Assistant (TIA), is being currently designed, which incorporates speech recognition and simultaneous translation -all in a small envelope. The TIA displays a high power dissipation to external area ratio. Due to its working environment, a fully sealed housing is required. Furthermore, only natural convection and radiation serve as the external means for dissipating about 7 Watts with a maximum surface area of 300 cm2. It is therefore unfeasible to accomplish an adequate thermal design that will satisfy the electronic components reliability and the user's ergonomic requirements at steady state operation. Since battery life and field applications also limit the operation time of these wearable systems, this paper proposes transient thermal management of the TIA wearable computer using Phase Change Materials (PCMs) and enhanced heat conduction internal paths. PCMs provide a viable alternative for transient energy storage, delaying steady state operating conditions. Experimental measurements and numerical simulations are performed to investigate the performance of these PCMs under different geometric layouts using aluminum foam structures embedded in a polymer matrix manufactured with SDM. The effect of using PCMs with different melting temperatures is also investigated. These simulations predict the performance of the embedded systems and determine the time to reach limiting temperatures. It is shown that the PCM melting temperature has an important effect in increasing the time to reach the maximum allowable temperature at the surface of the wearable computer.
Leoni N.J., Amon C.H., Thermal Design for Transient Operation of the TIA Wearable Computer, American Society of Mechanical Engineers, EEP 19(2): 2151-2161, 1997.
[Abstract] High-power portable electronics with external heat dissipation due only to natural convection and radiation pose significant challenges to passive steady state cooling strategies. The Technical Information Assistant (TIA) wearable computer is one of these systems for which steady state approaches are no longer sufficient to ensure reliable and safe operation. Through the use of simple thermal resistance models, we evaluate the performance of different cooling strategies for the TIA and identify design bottlenecks. Physical experiments are performed to determine appropriate boundary conditions and to validate Spectral Element numerical simulations. These studies show the need of transient thermal management to comply with the performance goals for the wearable system. Using numerical simulations, we investigate the application of transient thermal energy storage with Phase Change Materials (PCMs), to augment the available operation time of the wearable computer. Results show that the available operation time of the wearable computer is increased by 150% with the addition of a Heat Storage Unit that weighs about 80 grams.
Majumdar D., Amon C.H., Oscillatory momentum transport mechanisms in transitional complex geometry flows, Journal of Fluids Engineering 119(1): 29-35, 1997.
[Abstract] This work reports direct numerical simulations of transitional flows in communicating channels. Above a critical Reynolds number, the flow becomes fluctuating and self-sustained with vortical motions temporally synchronized with channel traveling waves. The energy transfer mechanism between the mean and the fluctuating flow is investigated along with the distributions of oscillatory shear stress and transitional viscosity. The kinetic energy equation for the fluctuating velocity is solved from DNS data to evaluate the contributions of the production term, viscous dissipation, work of dynamic pressure and work of viscous shear stresses.
Oms L.J., Torres E., Amon C.H., The Water Table: A Fluid Exhibit for the Carnegie Science Center, 27th Annual Conference Frontiers in Education Conference Proceedings 1: 515-520, 1997.
[Abstract] In the summer of 1995 the Carnegie Science Center, USA, a science museum for children, asked the Engineering Design Research Center (EDRC), a research and educational organization within Carnegie Mellon University, to assist them in the design of educational fluid exhibits. Among the ideas developed by the students working in the program was "The Water Table", a water channel display of variable speed water flow. This exhibit allows children to interactively play with a stimulating display while gaining insight into the flow behaviour. Specifically, children can compare and contrast laminar and turbulent flows, observe vortex shedding and flow separation as water travels around different shaped objects interrupting the flow. Several factors, both cognitive and technical, have been considered during the design process and have contributed to the learning experience. The educational goals of this project are most tangible when one looks at the multiple outcomes it has produced. One important outcome is the interactive fashion in which The Water Table encourages children in sharing, learning and understanding flow behaviour. Furthermore, this project has helped develop engineering and design skills of the undergraduate students involved. The objective of this paper is to describe the learning experience of the undergraduate students who designed, prototyped and built The Water Table exhibit.
Schmaltz K.S., Leoni N., Padmanabhan P., Amon C.H., Finger S., Weiss L.E., Investigation of Transport Phenomena in Microcasting Shape Deposition Manufacturing via Experiments Designed Using Optimal Sampling, Symposium on Manufacturing and Materials Processing, ASME HTD-Vol. 347, No. 9, pp. 241-250, 1997.
Amon C.H., Finger S., Siewiorek D.P., Smailagic A., Integrating Design Education Research and Practice at Carnegie Mellon: A Multi-disciplinary Course in Wearable Computers, ASEE J. Engineering Education, pp. 279-285, 1996.
Amon C.H., Guzman A.M., Morel B., Lagrangian Chaos Eulerian Chaos and Mixing Enhancement in Converging- Diverging Channel Flows, Physics of Fluids 8(5): 1192-1206, 1996.
[Abstract] A study of Lagrangian chaos, Eulerian chaos, and mixing enhancement in converging-diverging channel flows, using spectral element direct numerical simulations, is presented. The time-dependent, incompressible Navier-Stokes and continuity equations are solved for laminar, transitional, and chaotic flow regimes for 100≤Re≤850. Classical fluid dynamics representations and dynamical system techniques characterize Eulerian flows, whereas Lagrangian trajectories and finite-time Lagrangian Lyapunov exponents identify Lagrangian chaotic flow regimes and quantify mixing enhancement. Classical representations demonstrate that the flow evolution to an aperiodic chaotic regime occurs through a sequence of instabilities, leading to three successive supercritical Hopf bifurcations. Poincaré sections and Eulerian Lyapunov exponent evaluations verify the first Hopf bifurcation at 125
Amon C.H., Schmaltz K.S., Merz R., Prinz F.B., Numerical and Experimental Investigation of Interface Bonding Via Substrate Remelting of an Impinging Molten Metal Droplet, Journal of Heat Transfer 118(1): 164-172, 1996.
[Abstract] A molten metal droplet landing and bonding to a solid substrate is investigated with combined analytical, numerical, and experimental techniques. This research supports a novel, thermal spray shape deposition process, referred to as microcasting, capable of rapidly manufacturing near netshape, steel objects. Metallurgical bonding between the impacting droplet and the previous deposition layer improves the strength and material property continuity between the layers, producing high-quality metal objects. A thorough understanding of the interface heat transfer process is needed to optimize the microcast object properties by minimizing the impacting droplet temperature necessary for superficial substrate remelting, while controlling substrate and deposit material cooling rates, remelt depths, and residual thermal stresses. A mixed Lagrangian-Eulerian numerical model is developed to calculate substrate remelting and temperature histories for investigating the required deposition temperatures and the effect of operating conditions on remelting. Experimental and analytical approaches are used to determine initial conditions for the numerical simulations, to verify the numerical accuracy, and to identify the resultant microstructures. Numerical results indicate that droplet to substrate conduction is the dominant heat transfer mode during remelting and solidification. Furthermore, a highly time-dependent heat transfer coefficient at the droplet/substrate interface necessitates a combined numerical model of the droplet and substrate for accurate predictions of the substrate remelting. The remelting depth and cooling rate numerical results are also verified by optical metallography, and compare well with both the analytical solution for the initial deposition period and the temperature measurements during droplet solidification.
Amon C.H., Chae K., Egan E., Kasabach C., Siewiorek D.P., Smailagic A., Stivoric J., System-level Thermal Management and Concurrent System Design of a Wearable Multicomputer System, Thermal Phenomena in Electronic Systems -Proceedings of the Intersociety Conference : 46-55, 1996.
[Abstract] The concurrent system design and thermal management of the Navigator2 which is used as a computerized maintenance manual for aircraft inspection with speech recognition capabilities are described. The Navigator2 is a wearable computer that includes a novel dual architecture, spread spectrum radio, and VGA head-mounted display. The thermal design of the Navigator2 develops concurrently with the overall design in a series of stages. A simplified computational model is used to investigate the performance of thermal interface devices and the effect of the heat spreader design on the maximum electronic component temperatures.
Chin R.K., Beuth J.L., Amon C.H., Thermomechanical Modeling of Molten Metal Droplet Solidification Applied to Layered Manufacturing, Mechanics of Materials 24(4): 257-271, 1996.
[Abstract] Transient and steady-state distributions of temperature and stress along the centerline of a single, initially molten metal droplet deposited onto a comparatively large substrate are examined. After investigating droplet deposition onto a room temperature substrate, the effect of substrate preheating on residual thermal stresses is quantified. Also, deposition of a second droplet is modeled and the effect on residual stresses of localized preheating by the first deposited droplet is assessed. Temperature-dependent conductivity, specific heat and density are used in coupled thermal models of droplet and substrate domains. Mechanical models include temperature-dependent Young's modulus, linear expansion coefficient and creep. Two-dimensional (2D) axisymmetric thermal and mechanical results are compared to one-dimensional (1D) results which approximate conditions along the droplet/substrate centerline. It is found that the more computationally efficient 1D models aid in interpreting the 2D results and provide reliable estimates of maximum stress magnitudes. Methods and results from this investigation are relevant to processes in which molten superheated metal contacts solid metal, such as welding processes. The specific application of interest in this work is droplet-level thermal and mechanical modeling of the microcasting stage of shape deposition manufacturing, which is a layered manufacturing process for the automated manufacture of complex three-dimensional metal parts.
Chin R.K., Beuth J.L., Amon C.H., Droplet-Level Modeling of Thermal Stresses in Layered Manufacturing Methods, American Society of Mechanical Engineers (Paper) , 1996.
[Abstract] This work addresses droplet-level thermal and mechanical modeling of molten metal deposition in shape deposition manufacturing, which is a layered manufacturing process for the automated production of complex three-dimensional metal parts. Methods and results from this investigation are also relevant to other processes in which molten metal contacts solid metal, including other layered manufacturing methods and welding processes. Transient and steady-state distributions of temperature and stress along the centerline of a single, initially molten metal droplet deposited onto a comparatively large substrate are examined. Transient temperatures from thermal models are used as inputs to mechanical models to predict the evolution of residual stresses. Temperature-dependent conductivity, specific heat and density are used in the thermal models. Mechanical models include temperature-dependent Young's modulus, linear expansion coefficient and creep. After investigating droplet deposition onto a room temperature substrate, the effect of substrate preheating on residual stresses is quantified. Two-dimensional (2-D) axisymmetric thermal and mechanical results are compared to one-dimensional (1-D) results which approximate conditions along the droplet/substrate centerline. It is found that the more computationally efficient 1-D models aid in interpreting the 2-D results and provide reliable estimates of maximum stress magnitudes.
Chin R.K., Beuth J.L., Amon C.H., Thermomechanical Modeling of Successive Material Deposition in Layered Manufacturing, Solid Freeform Fabrication Proceedings, eds. Bourell et al., pp. 507-514, 1996.
Egan E., Amon C.H., Cooling Strategies for Embedded Electronic Components of Wearable Computers Fabricated by Shape Deposition Manufacturing, Thermal Phenomena in Electronic Systems -Proceedings of the Intersociety Conference : 13-20, 1996.
[Abstract] Rugged, portable computers that can be comfortably worn on the body and easily operated for maintenance applications are being designed and manufactured at Carnegie Mellon University. The recently developed process of Shape Deposition Manufacturing has created the opportunity to embed the electronics of wearable computers in a polymer composite substrate. As both a protective outer case and a conductive heat dissipating medium, the substrate satisfies two basic design constraints of wearable computers: design for ruggedness and cooling efficiency. One such application of embedded electronics is the VuMan3R, a wearable computer designed for aircraft maintenance. To understand thermal phenomena of embedded electronic design, a combination of spectral element numerical simulations, physical experimentation, and analytical models facilitate the exploration of the thermal design space. Numerical models with heat spreaders, air channels, and various substrate materials test the thermal performance, while physical experimentation of an embedded electronic artifact ensures the accuracy and practicality of the numerical simulations. Analytical models using thermal resistance networks are created to predict heat flow paths within the embedded electronic artifact as well as the role of conductive fillers used in polymer composites.
Finger S., Stivoric J., Amon C.H., Gursoz L., Prinz F., Siewiorek D., Smailagic A., Weiss L.E., Reflections on a Concurrent Design Methodology: A Case Study in Wearable Computer Design, Computer-Aided Design 28(5): 393-404, 1996.
[Abstract] At Carnegie Mellon, we have designed and manufactured three generations of wearable, mobile computers. Each new generation of wearable computer has been designed within approximately one semester by an interdisciplinary design class taught at the Engineering Design Research Center (EDRC). Over the semesters that the course has been taught, an interdisciplinary concurrent design methodology has evolved. In this paper, we briefly present the design process for the Navigator, the third generation of wearable computers. We use this process to illustrate the needs of a multidisciplinary design team, to anticipate the needs of a distributed design team using a novel manufacturing process, and to reflect on the interplay between the practice of design and the evolution of our design methods.
Guzman A.M., Amon C.H., Dynamical flow characterization of transitional and chaotic regimes in converging-diverging channels, Journal of Fluid Mechanics 321: 25-57, 1996.
[Abstract] Numerical investigation of laminar, transitional and chaotic flows in converging-diverging channels are performed by direct numerical simulations in the Reynolds number range 10 < Re < 850. The temporal flow evolution and the onset of turbulence are investigated by combining classical fluid dynamics representations with dynamical system flow characterizations. Modern dynamical system techniques such as time-delay reconstructions of pseudophase spaces, autocorrelation functions, fractal dimensions and Eulerian Lyapunov exponents are used for the dynamical flow characterization of laminar, transitional and chaotic flow regimes. As a consequence of these flow characterizations, it is verified that the transitional flow evolves through intermediate states of periodicity, two-frequency quasi-periodicity, frequency-locking periodicity, and multiple-frequency quasi-periodicity before reaching a non-periodic unpredictable behaviour corresponding to low-dimensional deterministic chaos. Qualitative and quantitative differences in Eulerian dynamical flow parameters are identified to determine the predictability of transitional flows and to characterize chaotic, weak turbulent flows in converging-diverging channels. Autocorrelation functions, pseudophase space representations and Poincaré maps are used for the qualitative identification of chaotic flows, assertion of their unpredictable nature, and recognition of the topological structure of the attractors for different flow regimes. The predictability of transitional flows is determined by analysing the autocorrelation functions and by representing their attractors in the reconstructed pseudophase spaces. The transitional flow behaviour is examined by the geometric visualization of the evolution of the attractors and Poincaré maps until the appearance of a strange attractor at the onset of chaos. Eulerian Lyapunov exponents and fractal dimensions are quantitative parameters to establish the onset of chaos, the persistence of chaotic flow behaviour, and the long-term persistent unpredictability of chaotic Eulerian flow regimes. Lastly, three-dimensional simulations for converging-diverging channel flow are performed to determine the effect of the spanwise direction on the route of transition to chaos.
Oliveira J.C., Amon C.H., Evolution of Nonlinear Instabilities in Radial Flows, Encit 96/Latcym 96, Florianopolis, Brasil, Vol 1., pp. 61-66 and Vol. 3, pp. 1803-07, 1996.
Osio I.G., Amon C.H., An Engineering Design Methodology with Bayesian Surrogates and Optimal Sampling, Research in Engineering Design - Theory, Applications, and Concurrent Engineering 8(4): 189-206, 1996.
[Abstract] This paper presents an adaptive, surrogate-based, engineering design methodology for the efficient use of numerical simulations of physical models. These surrogates are nonlinear regression models fitted with data obtained from deterministic numerical simulations using optimal sampling. A multistage Bayesian procedure is followed in the formulation of surrogates to support the evolutionary nature of engineering design. Information from computer simulations of different levels of accuracy and detail is integrated, updating surrogates sequentially to improve their accuracy. Data-adaptive optimal sampling is conducted by minimizing the sum of the eigenvalues of the prior covariance matrix. Metrics to quantify prediction errors are proposed and tested to evaluate surrogate accuracy given cost and time constraints. The proposed methodology is tested with a known analytical function to illustrate accuracy and cost tradeoffs. This methodology is then applied to the thermal design of embedded electronic packages with five design parameters. Temperature distributions of embedded electronic chip configurations are calculated using spectral element direct numerical simulations. Surrogates, built from 30 simulations in two stages, are used to predict responses of new design combinations and to minimize the maximum chip temperature.
Schmaltz K.S., Amon C.H., Thermal Issues in Microcasting Shape Deposition Manufacturing, TMS Annual Meeting : 145-157, 1995.
[Abstract] Shape Deposition Manufacturing (SDM) is a layered manufacturing process that combines the benefits of Solid Freeform Fabrication (SFF), thermal spray and other manufacturing processing operations for fabricating multi-material objects of arbitrary three-dimensional geometric complexity with controlled microstructures and for embedding electronic components and sensors in conformal shape structures. SDM involves molten metal droplet deposition, remelting and solidification as well as the use of sacrificial support materials. Therefore a thorough understanding of the thermal issues is required to select SDM process parameters and to enable the integration with SDM computer-based design systems. Important thermal issues in the production of high-quality SDM objects are the creation of inter-layer metallurgical bonding through substrate remelting, the control of cooling rates of both the substrate and the deposition material, and the minimization of residual thermal stress build-up which may induce warping and debonding between deposited layers. This paper presents brief descriptions of the thermal modeling approach, the numerical prediction of the cooling rates and substrate remelting depths of steel deposited on steel and on copper, and the analytical and experimental methods used to verify temperature histories. Through the integration of these different tools, accurate and useful information is available to assist in the selection of operating parameters during the manufacture of microcast artifacts.
Amon C.H., Finger S., Siewiorek D.P., Smailagic A., Integration of Design Education Research and Practice at Carnegie Mellon University: A Multi-disciplinary Course in Wearable Computer Design, Proceedings - Frontiers in Education Conference 2: 658-666, 1995.
[Abstract] The Engineering Design Research Center (EDRC) at Carnegie Mellon University created a two-semester design course that integrates research and education through industrially sponsored design projects. This two-semester Wearable Computer Design course has been taught for six semesters. Over this period, teams of undergraduate students have designed, fabricated, and delivered a new generation of Wearable Computers. This article describes the evolution of this design course, the integration of design education, design research and design practice in an interdepartmental course. It also describes the interplay between disciplines, between theory, practice and education, and between designers and users.
Amon C.H., Nigen J.S., Siewiorek D.P., Smailagic A., Stivoric J., Concurrent Design and Analysis of the Navigator Wearable Computer System: The Thermal Perspective, IEEE transactions on components, packaging, and manufacturing technology. Part A 18(3): 567-577, 1995.
[Abstract] This paper describes the concurrent design of a wearable computer, called the Navigator, developed and built at Carnegie Mellon University in a multidesigner, multidomain environment. The design effort for the Navigator involved nineteen designers, representing the disciplines of electrical engineering, industrial design, mechanical engineering, software engineering, and human-computer interaction. The concurrent design framework developed by the Navigator design team is outlined and the parallel activities within each design phase are described, including the synchronization and interactions among all design disciplines at the phase boundaries. The evolution of the interdisciplinary design of the Navigator wearable computer is presented, with particular emphasis placed upon the role of the thermal design group in the overall design process. Furthermore, the particular challenges associated with the concurrent thermal management of wearable computer systems are outlined.
Amon C.H., Spectral Element-Fourier Method for Unsteady Conjugate Heat Transfer in Complex Geometry Flows, Journal of thermophysics and heat transfer 9(2): 247-253, 1995.
[Abstract] A spectral-element Fourier method (SEFM) is presented for the direct numerical simulation of forced convective heat transfer and conjugate conduction/convection in transitional internal flows in complex geometries. The SEFM is employed for the spatial discretization of the unsteady, incompressible, three-dimensional Navier-Stokes and energy equations. The resulting discrete equations are solved by a semi-implicit method in time treating explicitly the convection operator and implicitly the remaining pressure and viscous contributions. This methodology is illustrated by performing direct numerical simulations to investigate forced convective heat transfer in supercritical self-sustained oscillatory flows and conjugate effects in multimaterial domains. Highly unsteady flows in complex geometries are considered, including modified channels with periodic inhomogeneities such as spanwise rectangular and triangular grooves encountered in electronic equipment and compact heat exchangers.
Campbell M.I., Amon C.H., Cagan J., Szykman S., Electronic Component Placement Using Simulated Annealing Under Thermal Constraints, American Society of Mechanical Engineers, Heat Transfer Division 319: 155-162, 1995.
[Abstract] This work introduces an algorithm that uses simulated annealing to perform electronic component layout while incorporating constraints related to heat transfer issues. A hierarchical heat transfer analysis approach is developed which is used in conjunction with the simulated annealing algorithm to produce final designs that are densely packed and operate within specified temperature ranges. Examples of two- and three-dimensional electronic component layouts are presented.
Chin R.K., Beuth J.L., Amon C.H., Control of Residual Thermal Stresses in Shape Deposition Manufacturing, Solid Freeform Fabrication Proceedings, eds. H.L. Marcus et al., pp. 221-228, 1995.
Nigen J.S., Amon C.H., Effect of Material Composition and Localized Heat Generation on Time-Dependent Conjugate Heat Transport, International Journal of Heat and Mass Transfer 38(9): 1565-1576, 1995.
[Abstract] Time-dependent conjugate conduction/convection direct numerical simulations of four rib configurations that differ in material composition and distribution of internal heat generation are conducted in laminar and transitional grooved-channel flows. The effect of material composition, concentration of heat generation and flow regime on the spatial distribution of temperature, heat flux and Nusselt number along the solid-fluid interface is investigated. Furthermore, the distribution of internal heat generation is found to affect strongly the convective resistance at the solid-fluid interface of the rib, leading to a nonmonotonic relationship between convective heat transport and Reynolds number for the range of parameters investigated in the configurations with local heat generation.
PPrinz F.B., Weiss L.E., Amon C.H., Beuth J.L., Processing Thermal and Mechanical Issues in Shape Deposition Manufacturing, Solid Freeform Fabrication Proceedings, eds. H.L. Marcus et al., pp. 118-129, 1995.
Schmaltz K.S., Amon C.H., Experimental Verification of an Impinging Molten Metal Droplet Numerical Simulation, American Society of Mechanical Engineers, Heat Transfer Division 317-1: 219-226, 1995.
[Abstract] Numerical modeling techniques and experimental verification have been used to investigate the remelting of a substrate by a molten metal droplet landing and solidifying on the substrate. Understanding the phenomenon of droplet/substrate bonding is relevant to a manufacturing process being developed at Carnegie Mellon University, called Shape Deposition Manufacturing microcasting, where successive layers of molten metal material are deposited to form three-dimensional objects. By remelting a thin layer of the solid substrate, an impinging droplet creates a stronger, metallurgical bond that greatly improves the deposited material properties. A combination of methods is required to optimize the process and ensure remelting, including a numerical approach due to the extreme thermal conditions of the process, analytical determination of the initial interface conditions, and experimental verification. Experiments validating the numerical results and determining the process operating conditions include calorimetry and thermocouple measurements, metallographic characterizations, and droplet pattern tests. Experiments have shown that despite the two-dimensional nature of the microcast droplets, a one-dimensional numerical model provides accurate information regarding the initial stage of the process relevant to remelting. Initial agreement between experimental droplet cooling and the numerical results also indicates that conduction heat transfer and latent heat release from the droplet to the substrate dominates over any convective effects during the solidification and cooling process. Substrate remelt depths measured from micrographic examinations compare well with the numerical simulations of droplet solidification, and droplet spreading patterns on various substrate materials concur with the analytical and numerical predictions.
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1988 - 1994
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Amon C.H., Numerical Prediction of Conjugate Conduction/Convection Heat Transfer in Multimaterial Electronic Components, American Society of Mechanical Engineers, Heat Transfer Division 292: 37-47, 1994.
[Abstract] A time-dependent conjugate conduction/convection numerical study of four electronic component configurations that differ in material composition and distribution of internal heat generation is conducted in parallel PCBs (Printed Circuit Boards). DNS (Direct Numerical Simulations) of the governing physical equations are performed using spectral element methodologies to predict thermal performance in the laminar and transitional Reynolds number flow regimes. Both material composition and concentration of heat generation are found to affect the spatial distribution of temperature, heat flux, and Nusselt number along the solid-fluid interface. Furthermore, it is found that the distribution of internal heat generation strongly affects the convective resistance at the solid-fluid interface of the component. This leads, for the configurations with local heat generation, to a non-monotonic relationship between convective heat transport and Reynolds number for the range of parameters investigated. It is also found that conjugate heat transfer can significantly affect the temperature distribution.
Amon C.H., Finger S., Prinz F.B., Weiss L.E., Modeling Novel Manufacturing Processes, American Society of Mechanical Engineers, Production Engineering Division (Publication) PED 68-2: 535-546, 1994.
[Abstract] This paper proposes a methodology for acquiring adaptable surrogate models of novel manufacturing processes using statistical methods, numerical simulation of the physical models, and experimentation. These surrogate models must contain different levels of abstraction, complexity, and accuracy to be useful from conceptual design through manufacturing process control. In addition, the proposed process modeling framework combines models of different subprocesses. This methodology is being developed in the context of the Shape Deposition fabrication process in which a part is built by successively depositing molten materials in thin layers. Creating each layer requires several manufacturing subprocesses such as microcasting, thermal spraying, shot peening, and machining. The governing physical equations of the microcasting thermal phenomena are simulated and numerical results are compared with experimental measurements.
Amon C.H., Merz R., Prinz F.B., Schmaltz K.S., Thermal Modeling and Experimental Testing of MD* Spray Shape Deposition Processes, Proceedings 10th International Heat Transfer Conference, Vol. 7, pp. 321-327, Brighton, UK, 1994.
Amon C.H., Computational Methodologies as a Research Tool for Concurrent Thermal Design Engineering, ISAC '94, International Symposium on Advanced Computing, Computational Mechanics for Electronic Devices/Components J.M., pp. 33-52, Invited Paper, Japan, 1994.
Guzman A.M., Amon C.H., Periodic Quasiperiodic and Chaotic Regimes in Converging-Diverging Open Channel Flows, American Society of Mechanical Engineers, Heat Transfer Division 298: 47-56, 1994.
[Abstract] Transition to chaos in converging-diverging open channel flows is investigated by direct numerical simulations of the time dependent incompressible Navier-Stokes equations. These are solved using a spectral element method for spatial discretization and a finite difference three-step, time splitting scheme for the temporal discretization, in the Reynolds number range between 125 and 850. Flow structures and evolutions of supercritical flow regimes are investigated. The strange attractor resulting from this transition process, associated with the chaotic flow regime is verified by both its fractal dimension and its representation in the pseudo phase space.
Guzman A.M., Amon C.H., Transition to Chaos in Converging-Diverging Channel Flows: Ruelle-Takens-Newhouse Scenario, Physics of Fluids 6(6): 1994-2002, 1994.
[Abstract] Direct numerical simulations of the transition process from laminar to chaotic flow in converging-diverging channels are presented. The chaotic flow regime is reached after a sequence of successive supercritical Hopf bifurcations to periodic, quasiperiodic, and chaotic self-sustained flow regimes. The numerical experiments reveal three distinct bifurcations as the Reynolds number is increased, each adding a new fundamental frequency to the velocity spectrum. In addition, frequency-locked periodic solutions with independent but synchronized periodic functions are obtained. A scenario similar to the Ruelle-Takens-Newhouse scenario of the onset of chaos is verified in this forced convective open system flow. The results are illustrated for different Reynolds numbers using time-velocity histories, Fourier power spectra, and phase space trajectories. The global structure of the self-sustained oscillatory flow for a periodic regime is also discussed.
Nigen J.S., Amon C.H., Time-Dependent Conjugate Heat Transfer Characteristics of Self-Sustained Oscillatory Flows in a Grooved Channel, Journal of Fluids Engineering 116(3): 499-506, 1994.
[Abstract] Convective heat transport in a grooved channel is numerically investigated using a time-dependent formulation. Conjugate conduction/convection and uniform heat-flux representations for the solid domain are considered. For the conjugate representation, the solid domain is composed of multiple materials and concentrated heat generation. The associated cooling flows include laminar and transitional regimes. Steady and time-dependent contours of the streamfunction and local skin-friction coefficients are presented. Additionally, local distributions of Nusselt number and surface temperature are displayed for both the conjugate and convection-only representations. These results are contrasted over the range of Reynolds numbers explored to demonstrate the significance of including time-dependency and conjugation in the study of convective heat transport. Such considerations are found to be important in the design and analysis of heat exchanger configurations with spatially varying material composition and concentrated heat generation.
Amon C., Beuth J., Kirchner H., Merz R., Prinz F., Schmaltz K., Weiss L., Material Issues in Layered Forming, Proceedings 1993 Solid Freeform Fabrication Symp., eds. H.L. Marcus et al., pp. 1-10, Austin, TX, 1993.
Amon C.H., Spectral Element-Fourier Method for Transitional Flows in Complex Geometries, AIAA journal 31(1): 42-48, 1993.
[Abstract] An efficient spectral element-Fourier method is presented for the direct numerical simulation of transitional internal flows in complex geometries. This method is applied for the spatial discretization of the unsteady, incompressible, three-dimensional Navier-Stokes equations in the velocity-pressure formulation. The resulting discrete equations are solved by a semi-implicit method in time, where the nonlinear convective term is treated explicitly. Direct numerical simulations are performed to investigate the spatial structure and temporal evolution of two- and three-dimensional transitional flows in grooved channels.
Amon C.H., Mikic B.B., Computational Methodologies as a Research Tool in Complex Forced Convection Heat Transfer Systems: Present Status and Future Expectations, 1991 US/JAPAN Joint Seminar on Computers in Heat Transfer Sciences. Reprinted in Computers and Computing in Heat Transfer Science and Engineering, pp. 61-86, CRC Press, Begell House, 1993.
Guzman A.M., Amon C.H., Flow Patterns and Forced Convective Heat Transfer in Converging-Diverging Channels American Society of Mechanical Engineers, Heat Transfer Division 237: 43-53, 1993.
[Abstract] Numerical investigations of the thermo-fluid phenomena and transition to turbulence in symmetric converging-diverging channels with sinusoidal walls are presented. The hydrodynamic-heat transfer results are obtained by direct numerical simulation of the unsteady Navier-Stokes and energy equations using a spectral element method for the spatial discretization and a finite difference time-stepping scheme for the temporal discretization. For Re≤20 there is no flow separation along the channel sinusoidal walls. As Re is increased, the symmetric vortices generated in each furrow move downstream as a consequence of the strength of the shear layer that increases with the velocity. For Re ≥Re ≈135, the vortices are located asymmetrically, and travelling waves are observed inducing self-sustained oscillatory flows which increase in amplitude and complexity as the Reynolds number is increased. These simulations resolve time-dependent flows, predict Rec for the onset of oscillatory flow, investigate the early transition process through a series of bifurcations from laminar to chaotic flow, and establish the flow conditions under which the periodic flow loses stability leading to a chaotic state prior to the fully-turbulent flow.
Majumdar D., Amon C.H., Physics of Heat and Momentum Transfer in Transitional Flows in Complex Geometries, American Society of Mechanical Engineers, Heat Transfer Division 246: 75-83, 1993.
[Abstract] This work reports a direct numerical simulation of transitional flows in complex geometries. Results are obtained by employing a spectral element method and are particularized for transitional incompressible flows in communicating channels. Above a critical Reynolds number, flows in these geometries bifurcate to a time-periodic, self-sustained oscillatory state. The physics of heat and momentum transfer in transitional self-sustained oscillatory flows is investigated and the different energy transfer mechanisms from the mean to the fluctuating flow and vice versa are analyzed. The kinetic energy equation for the fluctuating components of velocity and pressure is considered, and the correspondence among the various terms of that equation namely the production term, viscous dissipation term, work of the total dynamic pressure and work of the viscous shear stresses is established. To determine the contribution of transitional momentum transfer in these flows, attention is focussed on transitional viscosity similar to eddy viscosity in turbulent flows. The effect of the fluctuating components of velocity and temperature on heat transfer is analyzed through the consideration of the stream-wise and cross-stream oscillatory heat fluxes.
Nigen J.S., Amon C.H., Forced Convective Cooling Enhancement of Electronic Package Configurations through Self-Sustained Oscillatory Flows, Journal of Electronic Packaging 115(4): 356-365, 1993.
[Abstract] Two-dimensional arrangements of electronic packages surface mounted to a printed circuit board represent grooved-channel geometries. For a certain range of Reynolds numbers, these geometries excite and sustain instabilities that are normally damped in planar Poiseuille flows. This results in a bifurcation to a self-sustained oscillatory state, which improves mixing and thereby enhances convective heat transport. Numerical simulations of the flow field and heat transfer characteristics of oscillatory and nonoscillatory flows for five grooved channels are presented. Additionally, the numerically obtained flow field corresponding to a suspended electronic package is analyzed. The extent of heat transfer enhancement is gauged through direct comparison to results corresponding to the steady-flow regime. Local heat transfer coefficients are determined and used to calculate the temperature distribution within a surface-mounted package. Moreover, the importance of using locally-defined instead of spatially-averaged heat transfer coefficients for thermal design and analysis of electronic packages is discussed.
Amon C.H., Majumdar D., Mikic B.B., Herman C.V., Mayinger F., Sekulic D.P., Experimental and Numerical Investigation of Oscillatory Flow and Thermal Phenomena in Communicating Channels, American Society of Mechanical Engineers, Heat Transfer Division 170: 25-34, 1991.
[Abstract] A numerical and experimental study of the flow fields and convective heat transfer characteristics in communicating channels is performed to gain insight into the operation of compact heat exchange surfaces with interrupted plates. The geometric parameters are selected to excite and sustain the normally damped Tollmien-Schlichting modes. Travelling waves are observed at relatively low Reynolds numbers, inducing self-sustained oscillatory flows that significantly enhance mixing. The critical Reynolds number, at which periodic oscillations are first observed in the fully-developed region of the flow, is determined. The numerical results are obtained by direct numerical simulation of the time-dependent energy and Navier-Stokes equations using a spectral element-Fourier method. The oscillatory heat transfer phenomenon is visualized experimentally using real-time, holographic interferometry. For hydrodynamically fully-developed flow conditions, the temperature fields have been recorded using high-speed cinematography. The experimental visualizations of the thermal waves verify the numerical predictions of the thermal-hydraulic structure and evolution of communicating-channels flows.
Amon C.H., Heat Transfer Enhancement by Flow Destabilization in Electronic Chip Configurations, Journal of Electronic Packaging 114(1): 35-40, 1992.
[Abstract] Numerical simulations of the flow pattern and forced convective heat transfer in geometries such as those encountered in cooling systems for electronic devices are presented. For Reynolds numbers above the critical one, Rc, these flows exhibit a traveling wave structure with laminar self-sustained oscillations at the least stable Tollmien-Schlichting mode frequency. Supercritical oscillatory flow induces largescale convective patterns which lead to significant mixing and correspondingly heat transfer augmentation. Three techniques of heat transfer enhancement by flow destabilization are compared on an equal pumping basis: active flow modulation, passive flow modulation and supercritical flow destabilization. It is found that the best enhancement system regarding minimum power dissipation corresponds to passive flow modulation in the range of low Nusselt numbers. However, supercritical flow destabilization becomes competitive as the requirement for a higher Nusselt number begins to dominate the design choices.
Majumdar D., Amon C.H., Herman C.V., Mayinger F., Mikic B.B., Visualization of High-Frequency Thermo-Fluid Phenomena Using Real-Time Holographic Interferometry and Numerical Animation, in Computational and Experimental Flow Visualization, ASME HTD-Vol. 206, No. 1, pp. 25-31, 1992.
Majumdar D., Amon C.H., Heat and Momentum Transport in Oscillatory Viscous Flows, Journal of Heat Transfer 114(4): 866-873, 1992.
[Abstract] Heat and momentum transport in self-sustained oscillatory viscous flows is investigated by direct numerical simulation using the spectral element method. Above a critical Reynolds number, these flows bifurcate to a time-periodic, self-sustained oscillatory state. Traveling waves are observed, even at moderately low Reynolds numbers, inducing self-sustained oscillations that result in very well-mixed flows, which, in turn, lead to convective heat transfer augmentation. These oscillatory states are investigated and correlations between the time- and space-averaged Nusselt and Reynolds numbers are obtained. The transport phenomena of heat and momentum due to the oscillatory components of the flow are analyzed by looking at the phase portraits of velocity and temperature, investigating the behavior of the terms involving their fluctuations, as well as considering the correlation coefficients between the fluctuating components. Results are presented for laminar and transitional incompressible flows in communicating channels, composed of interrupted surfaces, leading to relatively thin thermal boundary layers that further contribute to heat transfer augmentation.
Nigen J., Amon C.H., Conjugate Forced Convective Effects of Complex Recirculating Self-Sustained Oscillatory Flow, in Fundamentals of Forced Convection Heat Transfer, ASME HTD-Vol. 210, pp. 91-98, 1992.
Nigen I.S., Amon C.H., Concurrent Thermal Designs of PCBs: Balancing Accuracy with Time Constraints, IEEE Transactions on Components, Hybrids, and Manufacturing Technology 15(5): 850-859, 1992.
[Abstract] A thermal design methodology suitable for concurrent design of cost-driven electronic systems is proposed and exemplified for a sample printed circuit board (PCB). The design methodology utilizes an evolutionary concept, in which the analysis tools are capable of adjusting their level of complexity as the design evolves, initiating with rough approximate analyses and culminating in a conjugate conduction/convection simulation for a portion of the sample board. The level of approximation included at each stage of the design is selected with consideration of both time and accuracy constraints. Furthermore, the importance of considering the conjugate problem in generating heat transfer correlations for electronic packages is discussed
Amon C.H., Mikic B.B., Spectral Element Simulations of Unsteady Forced Convective Heat Transfer: Application to Compact Heat Exchanger Geometries, Numerical Heat Transfer; Part A: Applications 19(1): 1-19, 1991.
[Abstract] Numerical investigations of the flow pattern and forced convective heat transfer in supercritical flows, such as those encountered in compact heat exchangers, are presented. These flows exhibit laminar self-sustained oscillations at the plane channel Tollmien-Schlichting frequency for Reynolds numbers above the critical one. These studies indicate that oscillatory separated flow results in large-scale convective patterns that are responsible for significant heat transfer enhancement and leads to a reduction in the pumping power required to achieve a given Nusselt number. The hydrodynamic-heat transfer numerical results are obtained by direct simulation of the unsteady energy and Navier-Stokes equations using a spectral element method for the spatial discretization. The spectral element method is a high-order weighted-residual technique that exploits both the common features and the competitive advantages of low-order finite element methods (versatility) and spectral techniques (accuracy and rapid convergence). It is shown that computational heat transfer and, in particular, direct numerical simulation can contribute significantly to exploration of the rich physics associated with heat transfer enhancement by flow destabilization.
Nigen J.S., Amon C.H., Forced Convective Cooling Enhancement of Surface-Mounted Electronic Package Configurations through Self-Sustained Oscillatory Flows, American Society of Mechanical Engineers, Heat Transfer Division 171: 39-46, 1991.
[Abstract] Grooved-channel geometries are formed when electronic components are directly mounted to a substrate. Some grooved-channel geometries have been found to excite and sustain the normally damped instabilities present in Poiseuille flows at lower Reynolds numbers than indicated by linear stability analysis. The resulting self-sustained oscillatory flows improve mixing and thereby enhance convective heat dissipation. Numerical simulations of the flow field and heat transfer characteristics of oscillatory and non-oscillatory flows for three grooved-channel geometries are presented. Each geometry differs only in groove width i.e., separation between packages. The extent of heat transfer enhancement is gauged through direct comparison to results corresponding to the steady-flow regime. Local heat transfer coefficients are determined and used to calculate the temperature distribution within a surface-mounted package. Furthermore, the importance of using locally-defined heat transfer coefficients instead of spatially-averaged coefficients for thermal design and analysis is discussed.
Amon C.H., Mikic B.B., Numerical and Experimental Visualization of Fast-Evolving Heat Transfer Phenomena in Self-Sustained Oscillatory Flows, Journal of Thermophysics and Heat Transfer 4(2): 239-246, 1990.
[Abstract] Numerical investigations of the flow pattern and heat transfer enhancement in supercritical grooved-channel and communicating-channels flows are presented. For Reynolds numbers above the critical one, Rc = 0(100), these flows exhibit laminar self-sustained oscillations at the plane channel Tollmien-Schlichting frequency. These ordered, very well-mixed flows require significantly less pumping power than the random fluctuating turbulent flows to achieve the same transport rates. Comparing different heat transfer augmentation schemes in grooved channels, it is shown that the best enhancement system regarding minimum power dissipation corresponds to passive flow modulation in the range of low Nusselt numbers. However, spontaneous supercritical flow destabilization becomes competitive as the Nusselt number is increased.
Amon C.H., Spectral Element-Fourier Approximation for the Navier-Stokes Equations: Some Aspects on Parallel Implementation, American Society of Mechanical Engineers, Heat Transfer Division 133: 11-18, 1990.
[Abstract] Domain decomposition methods constitute a powerful type of numerical techniques for the solution of partial differential equations that have become popular over the last years due to their potentiality for efficient implementation in parallel/vector computer architectures. Spectral element-Fourier method is a domain decomposition technique that combines the advantages of globally unstructured/locally structured spatial discretizations, where the global decomposition in macro-elements allows geometric flexibility and the local structure permits an efficient high-order approximation by spectral techniques with rapid convergence and high accuracy. In this paper we discuss the implementation of the spectral element-Fourier method (SEFM) for the solution of the incompressible, unsteady, three-dimensional Navier-Stokes equations in complex geometries. The time-stepping scheme used for the time discretization is reviewed and the fundamental ideas of SEFM for the spatial discretization are presented. Emphasis is placed on highlighting some aspects of the naturally effective parallel and vector implementation for the resulting linear equations and the limitations for the nonlinear equations. Direct numerical simulations of unsteady transitional flows in grooved channels are presented.
Amon C.H., Mikic B.B., Numerical Prediction of Convective Heat Transfer in Self-Sustained Oscillatory Flows, Journal of Thermophysics and Heat Transfer 4(2): 239-246, 1990.
[Abstract] Numerical investigations of the flow pattern and heat transfer enhancement in supercritical grooved-channel and communicating-channels flows are presented. For Reynolds numbers above the critical one, Rc = 0(100), these flows exhibit laminar self-sustained oscillations at the plane channel Tollmien-Schlichting frequency. These ordered, very well-mixed flows require significantly less pumping power than the random fluctuating turbulent flows to achieve the same transport rates. Comparing different heat transfer augmentation schemes in grooved channels, it is shown that the best enhancement system regarding minimum power dissipation corresponds to passive flow modulation in the range of low Nusselt numbers. However, spontaneous supercritical flow destabilization becomes competitive as the Nusselt number is increased.
Amon C.H., Patera A.T., Numerical Calculation of Stable Three-Dimensional Tertiary States in Grooved-Channel Flow, Physics of Fluids A 1(12): 2005-2009, 1989.
[Abstract] Numerical simulations of the early transition process in periodic grooved-channel flow are presented. For Reynolds numbers, R < Rc,1 = O(100), the two-dimensional steady flow is stable to all disturbances; at R = Rc,1 the flow undergoes a supercritical Hopf bifurcation to a nonlinear two-dimensional steady-periodic state; for R > Rc,2 > Rc,1 the wavy two-dimensional flow is unstable to a classical linear three-dimensional secondary instability; and for some range of Reynolds number above Rc,2 the secondary instability saturates in a steady-periodic, three-dimensional, low-order equilibrium. The three-dimensional equilibria owe their existence and stability to the narrow band nature of grooved-channel-flow secondary instability, which in turn reflects the low-Reynolds-number supercritical form of the grooved-channel-flow primary bifurcation. The contrast between the low-order, weak transition in "inflectional" complex-geometry channels and the abrupt, snap-through transition in (subcritical-primary broadband-secondary) planar channels illustrates the important role of primary criticality in the early transition process.
Amon C.H., Mikic B.B., Spectral Element Simulations of Forced Convective Heat Transfer: Application to Supercritical Slotted Channel Flows, American Society of Mechanical Engineers, Heat Transfer Division 110: 175-183, 1989.
[Abstract] Numerical investigations of the flow pattern and forced convective heat transfer in supercritical flows, such as those encountered in compact heat exchangers, are presented. These flows exhibit laminar self-sustained oscillations at the plane channel Tollmien-Schlichting frequency for Reynolds numbers above the critical one. These studies indicate that oscillatory separated flow results in large convective patterns which are responsible for significant heat transfer enhancement. The hydrodynamic-heat transfer numerical results are obtained by direct simulation of the unaveraged energy and Navier-Stokes equations using a spectral element method for the spatial discretization. The spectral element method is a high-order weighted-residual technique that exploits both the common features and the competitive advantages of low-order finite element methods (generality and geometry flexibility) and p-type spectral techniques (accuracy and rapid convergence).
Amon C.H., Mikic B.B., Flow Pattern and Heat Transfer Enhancement in Self-Sustained Oscillatory Flows, AIAA Paper 89-0428,Twenty-Seventh Aerospace Science Meeting R.n., Nevada, January 1989.
Amon C.H., Mikic B.B., Korin E., Effect of Oscillatory Flow on Heat Removal from Circular Fins, Cooling Technology for Electronic Equipment, ed. W. Aung, Hemisphere Publishing Corp., New York, pp. 125-138, 1988.
Karniadakis G.E., Amon C.H., Stability Calculations for Wall-Bounded Flows in Complex Geometries, Advances in Computer Methods for Partial Differential Equations VI, eds. R. Vichnevetsky and R.S. Stepleman, IMACS, New Jersey, pp. 525-534, 1988.
Ratts E., Amon C.H., Mikic B.B., Patera A.T., Cooling Enhancement of Forced Convection Air Cooled Chip Array through Flow Modulation Induced by Vortex-Shedding Cylinders in Cross-Flow, Cooling Technology for Electronic Equipment, ed. W. Aung, Hemisphere Publishing Corp., New York, pp. 183-194, 1988.
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