Browsing by Subject "Numerical simulation"
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Item Consistent modeling of hypersonic nonequilibrium flows using direct simulation Monte Carlo(2013-08) Zhang, ChonglinHypersonic flows involve strong thermal and chemical nonequilibrium due to steep gradients in gas properties in the shock layer, wake, and next to vehicle surfaces. Accurate simulation of hypersonic nonequilibrium flows requires consideration of the molecular nature of the gas including internal energy excitation (translational, rotational, and vibrational energy modes) as well as chemical reaction processes such as dissociation. Both continuum and particle simulation methods are available to simulate such complex flow phenomena. Specifically, the direct simulation Monte Carlo (DSMC) method is widely used to model such complex nonequilibrium phenomena within a particle-based numerical method. This thesis describes in detail how the different types of DSMC thermochemical models should be implemented in a rigorous and consistent manner. In the process, new algorithms are developed including a new framework for phenomenological models able to incorporate results from computational chemistry. Using this framework, a new DSMC model for rotational energy exchange is constructed. General algorithms are developed for the various types of methods that inherently satisfy microscopic reversibility, detailed balance, and equipartition of energy in equilibrium. Furthermore, a new framework for developing rovibrational state-to-state DSMC collision models is proposed, and a vibrational state-to-state model is developed along the course. The overall result of this thesis is a rigorous and consistent approach to bridge molecular physics and computational chemistry through stochastic molecular simulation to continuum models for gases in strong thermochemical nonequilibrium.Item Corn stover steam gasification(2013-05) Zheng, HuixiaoBiomass corn stover is a promising renewable energy resource. One of the most widely used technologies of utilizing biomass is fluidized bed steam gasification due to its flexibility of handling fuels and the high-energy-content gas produced. In this study a complex unsteady-state two-phase kinetic model including fluid dynamics and reaction kinetics is developed. An un-reacted core shrinking model is employed to describe chemical reactions, and particle entrainment is considered. In addition, a pyrolysis model including the effect of particle size and temperature is developed and incorporated in the gasification model. This model is able to reflect the effect of particle size, temperature, pressure, the steam/biomass ratio, mass flow rate, and superficial gas velocity on gasifier performance. In addition, this model can provide detailed information on the evolution of gas, char, and particle size in the bed, and percentages of particles consumed during reactions, turned to fines by friction, or entrained out of the bed. There are many models with difference levels of complexity and modeling concepts for a fluidized bed but there are few studies available on comparing these models. Therefore, six other widely used models are also developed and compared to study the importance of modeling complexity on model selection. These models are the zero-dimensional non-stoichiometric equilibrium model, zero-dimensional stoichiometric equilibrium model, zero-dimensional kinetic model, one-dimensional one-phase kinetic model, one-dimensional two-phase kinetic model-all char in bed with particle size, and one-dimensional two-phase kinetic model-all char in bed without particle size. Gasification results show that the one-dimension two-phase kinetic model and the one-dimensional one-phase kinetic model are equivalent, and both predict the same gasification results in terms of the gas volumetric fraction, yields of char and dry tar-free gas, the history of the evolution of particles, and higher heating values (HHVs). Therefore, it can be concluded that the number of phases in the fluidized bed does not affect the simulation results. Since it takes less time to finish a run for the one-phase kinetic model than for the two-phase kinetic model, the one-dimensional one-phase kinetic model is better than the two-phase kinetic model. In addition, the one-dimensional two-phase kinetic models-all char in the bed predict the same gas volumetric fractions and the yields of char and dry tar-free gas at steady state, but predict different amounts of time for the bed to reach steady state and different amounts of char in the bed at steady state. The main reason is because different methods are used to calculate the reactive surface area for reaction rates. Through model comparison, it is found that models with similar modeling concepts tend to have similar results. Gasification models developed in this study are incorporated into a biomass integrated gasification combined cycle (BIGCC) system to provide heat and power for a corn ethanol plant. The effect of different gasification models on the overall BIGCC system performance is evaluated. Results show that BIGCC systems using different gasification models have similar but not identical overall system performances.Item Device Modeling and Characterization for CIGS Solar Cells(2013-06) Song, SangWe studied the way to achieve high efficiency and low cost of CuIn1-xGaxSe2 (CIGS) solar cells. The Fowler-Nordheim (F-N) tunneling currents at low bias decreased the shunt resistances and degraded the fill factor and efficiency. The activation energies of majority traps were directly related with F-N tunneling currents by the energy barriers. Air anneals decreased the efficiency from 7.74% to 5.18% after a 150�C, 1000 hour anneal. The decrease of shunt resistance due to F-N tunneling and the increase of series resistance degrade the efficiencies of solar cells. Air anneal reduces the free carrier densities by the newly generated Cu interstitial defects (Cui). Mobile Cui defects induce the metastability in CIGS solar cell. Since oxygen atoms are preferred to passivate the Se vacancies thus Cu interstitial defects explains well metastability of CIGS solar cells. Lattice mismatch and misfit stress between layers in CIGS solar cells can explain the particular effects of CIGS solar cells. The misfits of 35.08o rotated (220/204) CIGS to r-plane (102) MoSe2 layers are 1% ~ -4% lower than other orientation and the lattice constants of two layers in short direction are matched at Ga composition x=0.35. This explains well the preferred orientation and the maximum efficiency of Ga composition effects. Misfit between CIGS and CdS generated the dislocations in CdS layer as the interface traps. Thermionic emission currents due to interface traps limit the open circuit voltage at high Ga composition. The trap densities were calculated by critical thickness and dislocation spacing and the numerical device simulation results were well matched with the experimental results. A metal oxide broken-gap p-n heterojunction is suggested for tunnel junction for multi-junction polycrystalline solar cells and we examined the characteristics of broken-gap tunnel junction by numerical simulation. Ballistic transport mechanism explains well I-V characteristics of broken-gap junction. P-type Cu2O and n-type In2O3 broken-gap heterojunction is effective with the CIGS tandem solar cells. The junction has linear I-V characteristics with moderate carrier concentration (2�1017 cm-3) and the resistance is lower than GaAs tunnel junction. The efficiency of a CGS/CIS tandem solar cells was 24.1% with buffer layers. And no significant degradations are expected due to broken gap junction.Item Efficient and Robust ADMM Methods for Dynamics and Geometry Optimization(2022-01) Brown, GeorgeWe present novel ADMM-based methods for efficiently solving problems in a variety of applications in computer graphics. First, in the domain of physics-based animation we propose new techniques for simulating elastic bodies subject to dissipative forces. Second, in the field of geometry optimization we introduce a new algorithm for quasi-static deformation and surface parameterization. In each of these applications our proposed methods robustly converge to accurate solutions, and do so faster than existing algorithms. We achieve this by using key insights to address and overcome many limitations of standard solvers. Here we highlight the features of our two new algorithms for the aforementioned problems. Then we summarize our preliminary investigations into new techniques we designed in pursuit of faster and more reliable convergence in parameterization problems. Our first method is one for incorporating dissipative forces into optimization-based time integration schemes, which hitherto have been applied almost exclusively to systems with only conservative forces. We represent such forces using dissipation functions that may be nonlinear in both positions and velocities, enabling us to model a range of dissipative effects including Coulomb friction, Rayleigh damping, and power-law dissipation. To improve accuracy and minimize artificial damping, we provide an optimization-based version of the second-order accurate TR-BDF2 integrator. Finally, we present a method for modifying arbitrary dissipation functions to conserve linear and angular momentum, allowing us to eliminate the artificial angular momentum loss caused by Rayleigh damping. Our second method is designed to efficiently solve geometry optimization problems. We observe that in this domain existing local-global solvers such as ADMM struggle to resolve large rotations such as bending and twisting modes, and large distortions in the presence of barrier energies. We propose two improvements to address these challenges. First, we introduce a novel local-global splitting based on the polar decomposition that separates the geometric nonlinearity of rotations from the material nonlinearity of the deformation energy. The resulting ADMM-based algorithm is a combination of an L-BFGS solve in the global step and proximal updates of element stretches in the local step. We also introduce a novel method for dynamic reweighting that is used to adjust element weights at runtime for improved convergence. With both improved rotation handling and element weighting, our WRAPD algorithm is considerably faster than state-of-the-art approaches for quasi-static simulations. WRAPD is also much faster at making early progress in parameterization problems, making it valuable as an initializer to jump-start second-order algorithms. Finally, we investigate two possible extensions to WRAPD for accelerated convergence in parameterization problems. The first extension, P-WRAPD, leverages progressive reference shape updates similar to Liu et al. [2018] to bound distortion. We show that this yields minor improvements in a subset of examples. Our second extension, N-WRAPD, uses a non-scalar weighting scheme that independently assigns unique weights for every mode of deformation. This method shows promising preliminary results. Although slightly less stable, N-WRAPD generally converges significantly faster at a rate comparable to second-order algorithms.Item Interactions Between Fluid Flow, Heat Transfer, And Particle Transport In The Presence Of Jet-Axis Switching And Realistic Fluid Movers(2014-12) Gorman, JohnThe overarching goal of this thesis is to identify and quantify new processes and phenomena related to fluid flow, heat transfer, and particle transport interacting in unique modes. The research can be categorized into three modes of interaction: (a) heat transfer processes governed by the complex patterns of fluid flow provided by real-world fluid-moving devices, (b) heat transfer processes which are governed by a naturally occurring, extraordinary fluid-flow phenomenon, and (c) interacting fluid flow, particle transport, and heat transfer all of which are governed by the aforementioned extraordinary fluid-flow phenomenon. These categories are respectively treated in individual chapters of the thesis. The traditional approach to convective heat transfer is virtually devoid of realistic fluid flow models. As a consequence, traditional convective heat transfer analysis is oversimplified to the point of being out of step with reality. This assertion is proven here, and a new fundamentals-based model of high fidelity involving realistic fluid movers of is created. Next, the extraordinary fluid flow phenomenon designated as jet-axis switching is introduced and illustrated quantitatively. This phenomenon occurs whenever a non-circular jet passes into and through an unrestricted space. When the jet is involved in a process called jet-impingement heat transfer, the zone of jet incidence is highly altered due to the axis-switching process. The ignoring of the switching process, which has been standard in all previous work on non-circular-jet impingement heat transfer, has been shown here to be highly error prone. The major part of the thesis encompasses jet-axis-switching fluid mechanics, convective heat transfer, and particle transport. An all-encompassing simulation model was created which took account of fluid-particle, particle-particle, fluid--impingement plate, and particle--impingement plate interactions, all with heat transfer. It was found that jet-axis switching exerted a major effect on the trajectories of the particles, with a corresponding impact on the particle collection efficiency of the impactor plate. The transfer of heat between the fluid and the impingement plate was little affected by any alterations in the pattern of fluid flow caused by the presence of particles. On the hand, direct particle-to-plate heat transfer is substantial.Item Numerical Simulation and Real-time Prototyping Platform(2017-11) Raju, SiddharthOver the past decade, real-time control design has rapidly moved from the age-old process of developing code based firmware in embedded C, to platforms that auto generate the embedded C program from a higher level drag and drop, model based design. This has various advantages such as device independent system design, reduced possibility of user based code translation errors, limited effort needed in terms of code maintenance and most important of all, ability to design more complex systems without worrying about lower level implementation. Various softwares are commercially available such as Matlab, Plexim, PSIM, VisSim etc., for this purpose but they all have the same shortcoming, i.e., extremely unaffordable to individual users and small companies. In this thesis, a new numerical simulation platform has been developed from scratch. To this end, a new programming language with inbuilt matrix support was created. The implementation details of the various compiler components, such as lexer, parser, sematic analyzer etc., has been presented. In addition to this code based platform, a model based platform was developed, to enable drag and drop based system development and, the design details including solver development, tools supported, real-time controller peripheral support etc., are presented in detail. A common platform independent intermediate language was created, to which all model and code developed in this platform gets translated into. Multiple up compilers are made available with the platform to enable translation to other languages. One such supported language is C89 which is the most commonly supported language by real-time controller manufacture’s standalone compilers. The auto generated code adds all necessary peripheral and system configuration code and control code, compiles it to device specific hex code and programs it directly into the controller. This allows users to have no knowledge of the underlying device or any programming and makes the entire system more readable and easy to maintain. For optimized running of the generated code, a high-performance math library was created using minimax polynomials and implemented directly in supported device specific assembly. In addition to this method, other commonly used methods are analyzed in depth. In addition to this, various other forms of optimizations were implemented that are not available on other platforms. In the current version, only TI’s TMS320F28335 is supported with plans to extend it to more controllers. Two low cost rapid prototyping platforms were developed to enable real-time control using model developed in simulation. The solution was tested on multiple motor control and power electronics applications, the results of which are presented. This whole solution cuts down the cost of numerical simulation and rapid real-time prototyping to few thousand dollars which would have previously costed upwards of ten to hundred thousand dollar on a platform of similar capability.