In this thesis, two numerical modeling methods are used to investigate the thermal conductivity of the polymer electrolyte membrane (PEM) fuel cell gas diffusion layer (GDL). First, an analytical model is used to study the through-plane thermal conductivity from representative physical GDL models informed by microscale computed tomography imaging of four commercially available GDL materials. The effect of the heterogeneity of the through-plane porosity of the GDL and polytetrafluoroethylene (PTFE) treatment is studied and it is noted that the high porosity surface transition regions have a dominating effect over the addition of PTFE in impacting the overall thermal conductivity. Next, the lattice Boltzmann method (LBM) is employed to study both the in-plane and through-plane thermal conductivity of stochastic numerically generated GDL modeling domains. The effect of GDL compression, binder content, PTFE treatment, addition of a microporous layer (MPL), heterogeneous porosity distributions, and water saturation on the thermal conductivity are investigated.

A VOF based multiphase Lattice Boltzmann method that explicitly prescribes kinematic boundary conditions at the interface is developed. The advantage of the method is the direct control over the surface tension value. The details of the numerical method are presented. The Saffman instability, Taylor instability, and flow of deformable suspensions in a channel are used as example-problems to demonstrate the accuracy of the method. The method allows for relatively large viscosity and density ratios.

Magnetohydrodynamic (MHD) investigations of decaying isotropic turbulence and rectangular jets (RJ) are carried out. A novel MHD lattice Boltzmann scheme that combines multiple relaxation time (MRT) parameters for the velocity field with a single relaxation time (SRT) parameter for the Maxwell’s stress tensor is developed for this study. In the MHD homogeneous turbulence studies, the kinetic/magnetic energy and enstrophy decays, kinetic enstrophy evolution, and vorticity alignment with the strain-rate tensor are evaluated to assess the key physical MHD turbulence mechanisms. The magnetic and kinetic energies interact and exchange through the influence of the Lorentz force work. An initial random fluctuating magnetic field increases the vortex stretching and forward cascade mechanisms. A strong uniform mean magnetic field increases the anisotropy of the turbulent flow field and causes inverse cascading. In the RJ studies, an investigation into the MHD effects on velocity, instability, and the axis-switching phenomena is performed at various magnetic field strengths and Magnetic Reynolds Numbers. The magnetic field is found to decelerate the jet core, inhibit instability, and prevent axis-switching. The key physical mechanisms are: (i) the exchange of energy between kinetic and magnetic modes and (ii) the magnetic field effect on the vorticity evolution. From these studies, it is found that magnetic field influences momentum, vorticity, and energy evolution and the degree of modification depends on the field strength. This interaction changes vortex evolution, and alters turbulence processes and rectangular jet flow characteristics. Overall, this study provides more insight into the physics of MHD flows, which suggests possible applications of MHD Flow Control.

Thesis (Ph. D.)--Auburn University, 2008. / Abstract. Vita. Includes bibliographical references (p. 64-65).

Thesis (M.S.)--University of Delaware, 2009. / Principal faculty advisor: Xiaoming Li, Dept. of Electrical & Computer Engineering. Includes bibliographical references.

In this work, the lattice Boltzmann method (LBM) was validated for direct numerical simulation (DNS) of wall-bounded turbulent flows. The LBM is a discrete-particle-based method that numerically solves the Boltzmann equation as opposed to conventional DNS methods that are based on the Navier-Stokes (NS) equations. The advantages of the LBM are its simple implementation, its ability to handle complex geometries, and its scalability on modern high-performance computers. An LBM code was developed and used to simulate fully-developed turbulent channel flow. In order to validate the results, the turbulence statistics were compared to those calculated from a conventional NS-based finite difference (FD) simulation. In the present study, special care was taken to make sure the computational domains for LBM and FD simulations were the same. Similar validation studies in the literature have used LBM simulations with smaller computational domains in order to reduce the computational cost. However, reducing the size of the computational domain affects the turbulence statistics and confounds the results of the validation. The turbulence statistics calculated from the LBM and FD simulations were found to agree qualitatively; however, there were several significant deviations, particularly in the variance profiles. The largest discrepancy was in the variance of the pressure fluctuations, which differed by approximately 7%. Given that both the LBM and FD simulations resolved the full range of turbulent scales and no models were used, this error was deemed to be significant. The cause of the discrepancy in the pressure variance was found to be the compressibility of the LBM. The LBM allows the density to vary, while the FD method does not since it solves the incompressible form of the NS equations. The effect of the compressibility could be reduced by lowering the Mach number, but this would come at the cost of significantly increasing the computational cost. Therefore, the conclusion of this work is that, while the LBM is capable of producing accurate solutions for incompressible turbulent flows, it is significantly more expensive than conventional methods for simple wall-bounded turbulent flows. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2011-09-15 23:24:09.968

Thesis (M.S.)--California Polytechnic State University, 2009. / Title from PDF title page; viewed on September 22, 2009. Major professor: Zoé Wood, Ph.D. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Computer Science." "June 2009." Includes bibliographical references (p. 48-57). Also available on microfiche.

La fusion et la solidification couplées à des écoulements convectifs sont des processus fondamentaux dans le contexte géophysique, par exemple dans la formation des mares-mares arctiques. Ce système se caractérise par la présence d'écoulements instationnaires, chaotiques et souvent turbulents. Ce travail est motivé par des observations indiquant une réduction de la glace de mer arctique dans la mesure où le modèle global actuel ne pouvait pas prédire. Le but de ce travail est de fournir des informations sur les paramètres pertinents affectant la fusion / solidification dans les étangs de fonte des glaces de mer. La configuration idéalisée que nous considérons consiste en une couche de fluide chauffée par le bas et en contact avec une interface de fusion solide-liquide du côté supérieur. Nous étudions un tel système modèle au moyen d'outils numériques. Nous effectuons des simulations numériques directes par un algorithme Lattice Boltzmann basé sur l'enthalpie pour traiter la dynamique à long terme, ou de manière équivalente le régime à nombre élevé de Rayleigh, à la fois dans des configurations en deux et en trois dimensions. Nous montrons que le processus de convection et de fusion couplé n'améliore que faiblement le flux de chaleur et le mélange dans le système par rapport au réglage de Rayleigh-Bénard. Comme deux extensions au système de fusion, nous considérons l'effet de l'application de la vitesse sur la section liquide du système de fusion, l'effet de chauffage interne du système de fusion. / Melting and solidification coupled with convective flows are fundamental processes in the geophysical context, for instance in the Arctic melt-ponds formation. This system is characterized by the presence of unsteady, chaotic and often turbulent flows. This work is motivated by observations indicating reduction of Arctic sea-ice to the extent that present global model could not predict. The goal of this work is to provide information on the relevant parameters affecting the melting/solidification in sea ice melt ponds. The idealized setup we consider consists of a fluid layer heated from below and in contact with a solid-liquid melting interface on the top side. We investigate such a model system by means of numerical tools. We perform direct numerical simulations by an enthalpy based Lattice Boltzmann algorithm to address the long time dynamics, or equivalently the high Rayleigh number regime, both in two- and three-dimensional setups. We show that the coupled convection and melting process only weakly enhances heat flux and the mixing in the system as compared to the Rayleigh-Bénard setting. As two extensions to system of melting, we consider the effect of applying velocity on the liquid section of the melting system, which represents existence of wind-draft, and we consider the effect of internally heating the system of melting, which represents heating the system of melting through solar radiation.

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia Mecânica / Made available in DSpace on 2012-10-22T17:50:38Z (GMT). No. of bitstreams: 1 238516.pdf: 5298890 bytes, checksum: 0ef61b796a3cfc07504d13ed32e0748f (MD5) / O estudo de problemas que envolvem a molhabilidade e a capilaridade em meios porosos tem sido assunto de grande interesse científico e econômico. A importância desse estudo é revelada por muitos processos tecnológicos que incluem a aplicação direta de fluidos sobre diferentes tipos de superfícies. O principal objetivo desta tese de doutorado é o melhor entendimento destes fenômenos físicos. Com esse enfoque, um método Lattice-Boltzmann baseado em mediadores de campo é proposto para a simulação de fenômenos que envolvem a interação fluido-sólido, no qual os efeitos das forças de interação de longa distância são importantes e devem ser considerados para que a dinâmica macroscópica observada experimentalmente seja recuperada. A modelagem da interação fluido-fluido foi feita através de um modelo conhecido na literatura. Este modelo possibilita a simulação de equilíbrio de fases através de uma equação de estado que possui um comportamento semelhante à equação de van der Waals. Os resultados obtidos mostram que o ângulo de contato depende fortemente das interações de longa distância e o aumento das forças de adesão leva à diminuição do ângulo de contato, em concordância com simulações baseadas em dinâmica molecular. No caso de superfícies sólidas irregulares, observa-se a histerese de ângulo de contato. A universalidade da dinâmica de espalhamento sobre superfícies sólidas é observada numa escala característica, na qual os efeitos microscópicos parecem ser desprezíveis. A formação de um filme precursor deslocando-se mais rapidamente que o restante da gota é notada. Os resultados obtidos para a ascensão capilar entre placas paralelas em condições estáticas concordam com resultados teóricos e ilustram que o balanço de forças local em torno da linha de contato não é influenciado pela gravidade, definindo o ângulo de contato medido na presença do campo gravitacional como uma propriedade aparente. Em condições dinâmicas, a comparação entre resultados teóricos e simulados foi feita através da equação de Bosanquet. Verifica-se que o comportamento dinâmico previsto pela solução teórica é satisfatoriamente recuperado pelas simulações em condições de reservatório infinito, particularmente se a separação entre as placas é pequena. Quando é observada alguma discrepância, nota-se que teoria e simulação não coincidem nos primeiros estágios da ascensão devido ao modelo teórico não prever a formação inicial do menisco. É mostrado que a dependência entre o ângulo de contato dinâmico e o número capilar é representativa de superfícies lisas e homogêneas.

This thesis is dedicated to the derivation and the validation of a new collision model as a stabilization technique for high-order lattice Boltzmann methods (LBM). More specifically, it intends to stabilize simulations of: (1) isothermal and weakly compressible flows at high Reynolds numbers, and (2) fully compressible flows including discontinuities such as shock waves. The new collision model relies on an enhanced regularization step. The latter includes a recursive computation of nonequilibrium Hermite polynomial coefficients. These recursive formulas directly derive from the Chapman-Enskog expansion, and allow to properly filter out second- (and higher-) order nonhydrodynamic contributions in underresolved conditions. This approach is even more interesting since it is compatible with a very large number of velocity sets. This high-order LBM is first validated in the isothermal case, and for high-Reynolds number flows. The coupling with a shock-capturing technique allows to further extend its validity domain to the simulation of fully compressible flows including shockwaves. The present work ends with the linear stability analysis(LSA) of the new approach, in the isothermal case. This leads to a proper quantification of the impact induced by each discretization (velocity and numerical) on the spectral properties of the related set of equations. The LSA of the recursive regularized LBM finally confirms the drastic stability gain obtained with this new approach.