Cette thèse, par une étude combinant modélisation et expériences, s'attache à comprendre l'interdépendance entre porosité, transport et adsorption dans les matériaux poreux hiérarchiques. Le transport et l'adsorption sont simulés dans les matériaux poreux à l'aide d'un modèle de Lattice Boltzmann étendu pour prendre en compte l'adsorption. Ce modèle mésoscopique permet de simuler le comportement d’un fluide présent au sein des pores du matériau ainsi que d'espèces en solution. La synthèse des matériaux poreux hiérarchiques par ice-templating et la caractérisation de la géométrie des échantillons par tomographie aux rayons X a permis de réaliser des simulations sur des géométries réelles. La comparaison des résultats de simulation et des résultats expérimentaux a prouvé la capacité du modèle à reproduire le comportement du fluide et des espèces dans le matériau. Grâce au modèle employé, l'interdépendance entre transport et adsorption a pu être mise en évidence, montrant notamment que le flux de fluide pouvait avoir une influence sur la densité adsorbée. Une étude sur l'influence de la géométrie et de la rugosité sur le couplage entre transport et adsorption a été menée sur des géométries bien caractérisées. Enfin, la modification du modèle de Lattice-Boltzmann pour rendre compte de l'effet d'un pic de traceurs a été développé. Cette modification permet notamment d'observer la cinétique hors régime stationnaire des traceurs dans le matériau et de rendre compte de l'effet du flux sur l'accessibilité des sites d'adsorption. / The aim of this work concerns the interplay between porosity, transport and adsorption in hierarchical porous materials. This work combine a simulation and an experimental approach. Transport and adsorption are simulated with a Lattice-Boltzmann model recently extended to take adsorption into account. This mesoscopic model allows to compute fluid and solute behavior in porous materials. The synthesis of hierarchical porous materials using ice-templating and the characterization of materials geometry with X-ray tomography allowed to compute simulations on real materials. The comparison between experimental and simualtion results have proved the efficency of the model to model fluid and species behavior in the materials. Using this model, the interplay between transport and adsorption have been studied highlighting an effect of the fluid flow on adsorbed density. The effect of roughness and material geometry have also been studied. Finally the modification of the modelallows now to simulate the kinematic of a pulse of species. This modification gives a tool to perform simulation in non-steady state flow and evaluate the adsorption site accessibility regarding to the flow field.

Esta dissertação apresenta o desenvolvimento e otimização de uma metodologia baseada em código comercial para previsão de ruído em um eslate, além do estudo da dependência do ruído ao variar a camada limite do aerofólio. Com a contínua redução do ruído produzido por motores em aeronaves e as sucessivas restrições nos níveis de certificação, o ruído produzido por dispositivos aerodinâmicos tem ganhado importância no projeto de uma aeronave. Durante o pouso, o ruído gerado pelos dispositivos hiper-sustentadores é classificado dentre os mais relevantes, sendo o eslate um de seus componentes. Este trabalho busca criar e otimizar uma metodologia baseada no código PowerFLOW, assim como estudar a influência das camadas limite do aerofólio na geração de ruído. Tal código é baseado na formulação de Lattice- Boltzmann. As fontes acústicas simuladas são propagadas utilizando uma analogia acústica de Ffowcs-Williams e Hawkings e, então, analisadas utilizando métodos estatísticos de análise de sinais. Estudos de validação e verificação do código baseados em soluções analíticas são apresentados, tais como uma camada de mistura periódica no espaço e a solução dos vórtices de Taylor-Green. A seguir, o aerofólio 30P30N é utilizado em todo o estudo relacionado a eslates, analisando primeiramente a independência da solução em relação ao nível de refinamento da malha e do tamanho do domínio empregados. Baseado nas recomendações de tal estudo, o resultado é comparado com simulações disponíveis na literatura. Com uma maior confiança na metodologia, o trabalho então apresenta estudos variando a camada limite em regiões do eslate, assim como removendo-a completamente em certas regiões do aerofólio ao empregar condições de contorno de livre-escorregamento. O trabalho mostra que a influência das camadas limite do aerofólio é desprezível em relação ao erro do método. Isso é causado pela aparente independência do ruído do eslate em relação ao escoamento perto de sua cúspide. Tal independência permite que a malha computacional seja otimizada, reduzindo o custo da simulação em até 60%. / This dissertation presents the development and optimization of a methodology based on a commercial software to predict slat noise, also studying noise dependency when varying airfoil boundary layers. Due to continuous reduction on aircraft engine noise and successive restrictions on noise certification levels, airframe noise has been gaining importance on aircraft design. During landing, high-lift noise is ranked as one of the most relevant ones, being slat noise one of its components. This work focuses on creating and optimizing a noise prediction methodology based on the software PowerFLOW, and also on studying the influence of airfoil boundary layers on noise generation. Such software is based on Lattice-Boltzmann formulation. The simulated sound sources are propagated using Ffowcs-Williams and Hawkings acoustic analogy and then analyzed by signal analysis methods. Code validation and verification studies based on analytical solutions are presented, such as the spacially-periodic mixing layer and the Taylor-Green vortices solutions. Following, the 30P30N airfoil is employed through the rest of this work, firstly studying the solution independency related to mesh refinement level and computational domain size. Based on recommendations from this study, the results are compared to simulations from the literature. With higher confidence levels on this methodology, the work then presents studies varying the slat boundary layer and also removing it completely by employing free-slip boundary conditions on certain airfoil regions. This work presents that the airfoil boundary layer influence is neglectable when compared to the method error. This is caused by the apparent slat noise independency related to the flowfield near the slat cusp. Such independency allows the computational mesh to be optimized, reducing the simulation cost by up to 60%.

Este trabalho aborda a utilização do método de Lattice Boltzmann como ferramenta de simulação para a área de dinâmica de fluidos. Além disso, apresenta o programa LBSim desenvolvido durante o período de pesquisa, construído para ser flexível e extensível (através do emprego de técnicas de orientação a objetos) e com uma interface gráfica mais amigável do que outros projetos semelhantes. Como resultado deste trabalho, o software LBSim implementa uma série de módulos diferentes que utilizam o método de Lattice Boltzmann como base, permitindo a simulação de casos monofásico, multifásico, multicomponente, com suporte à gravidade, meios porosos, difusão, transferência de calor e paredes deslizantes. / This work approaches the use of the Lattice Boltzmann method as a simulation tool for the fluid dynamics area. Beyond that, it presents the LBSim software, developed during the period of the research and built to be flexible and extensible (through the application of techniques of the object oriented paradigm) and with a graphical interface more friendly than other similar projects. As a result of this work, the LBSim software implements a series of different modules that uses the Lattice Boltzmann method as a base, allowing the simulations of the following cases: monophase, multiphase, multicomponent, with support of gravity, porous media, diffusion, heat transfer and moving walls.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-03-06T20:24:42Z No. of bitstreams: 1 joventinodeoliveiracampos.pdf: 3604904 bytes, checksum: aca8053f097ddcb9d96ba51186838610 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-03-06T20:27:39Z (GMT) No. of bitstreams: 1 joventinodeoliveiracampos.pdf: 3604904 bytes, checksum: aca8053f097ddcb9d96ba51186838610 (MD5) / Made available in DSpace on 2017-03-06T20:27:39Z (GMT). No. of bitstreams: 1 joventinodeoliveiracampos.pdf: 3604904 bytes, checksum: aca8053f097ddcb9d96ba51186838610 (MD5) Previous issue date: 2015-06-26 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Este trabalho apresenta o método de lattice Boltzmann (MLB) para simulações computacionais da atividade elétrica cardíaca usando o modelo monodomínio. Uma implementação otimizada do método de lattice Boltzmann é apresentada, a qual usa um modelo de colisão com múltiplos parâmetros de relaxação conhecido como multiple relaxation time (MRT), para considerar a anisotropia do tecido cardíaco. Com foco em simulações rápidas da dinâmica cardíaca, devido ao alto grau de paralelismo presente no MLB, uma implementação que executa em uma unidade de processamento gráfico (GPU) foi realizada e seu desempenho foi estudado através de domínios tridimensionais regulares e irregulares. Os resultados da implementação para simulações cardíacas mostraram fatores de aceleração tão altos quanto 500x para a simulação global e para o MLB um desempenho de 419 mega lattice update per second (MLUPS) foi alcançado. Com tempos de execução próximos ao tempo real em um único computador equipado com uma GPU moderna, estes resultados mostram que este trabalho é uma proposta promissora para aplicação em ambiente clínico. / This work presents the lattice Boltzmann method (LBM) for computational simulations of the cardiac electrical activity using monodomain model. An optimized implementation of the lattice Boltzmann method is presented which uses a collision model with multiple relaxation parameters known as multiple relaxation time (MRT) in order to consider the anisotropy of the cardiac tissue. With focus on fast simulations of cardiac dynamics, due to the high level of parallelism present in the LBM, a GPU parallelization was performed and its performance was studied under regular and irregular three-dimensional domains. The results of our optimized LBM GPU implementation for cardiac simulations shown acceleration factors as high as 500x for the overall simulation and for the LBM a performance of 419 mega lattice updates per second (MLUPS) was achieved. With near real time simulations in a single computer equipped with a modern GPU these results show that the proposed framework is a promising approach for application in a clinical workflow.

This thesis aims to explore possible implementations and optimizations of the lattice-Boltzmann method. This method allows modeling of fluid flow using a simulation of fictive particles. The thesis focuses on possible improvements of the existing tool HemeLB which  is designed and optimized for bloodflow modeling. Several vectorization and paralellization approaches that could be included in this tool are explored. An application focused on comparing chosen algorithms including optimizations for the lattice-Boltzmann method was implemented as a part of the thesis. A group of tests focused on comparing this algorithms according to performance, cache usage and overall memory usage was performed. The best performance achieved was 150 millions of lattice site updates per second.

The Lattice-Boltzmann-model is analyzed with regard to application to numerical solution of flow acoustic problems. In the first part of this study the description of sound wave propagation by common variants of the Lattice-Boltzmann-model is examined by calculation of phase velocity and effective viscosity for sound waves. Schemes with nine velocities in two dimensions and nineteen velocities in three dimensions are considered. For each of these a single relaxation time model (LBGK-model) and a multiple relaxation time model (MRT) is investigated. All schemes exhibit an almost isotropic error in phase speed of sound waves. With a spatial resolution of 10 or 30 grid spacings per wavelength the deviation of phase speed is less than 1 % or 0.1 %, respectively. The dissipation of sound waves is not simulated correctly by LBGK-models since there the bulk viscosity is fixed to the shear viscosity. Apart from that there is only very little numerical dissipation. The dissipation error therefor is negligible in the audible frequency range in air as long as the simulation volumes do not become very large, i.e. much more than some hundred wavelengths. The MRT-models allow to adjust the bulk viscosity by a suitable choice of relaxation parameters. However, if the bulk viscosity is set to a realistic value, stability of the scheme requires free relaxation parameter values which are close to the relaxation parameters that determine the viscosities. Then the gain in stability of MRT-models compared to LBGK-models is lost to some extent. All schemes considered here are able to reproduce the effect of sound wave convection in homogeneous background flows. Although additional numerical errors arise in transport coefficients, the overall errors are of the same order of magnitude as in the case with zero background flow and are not critical in practical applications. In the second part of the work numerical experiments are described which demonstrate the coupling of the flow- and sound field. Three test cases are considered: Sound generation by a single vortex interaction with the leading edge of a semi-infinite flat plate, sound generation by a grazing flow over a partially covered cavity and instationary flow around a half-cylinder with an attached wedge tail. The first test case is simulated in two dimensions with a self-written program. The sound calculated directly is compared to prediction based on an acoustic analogy. The observed amplitudes of the radiated sound agree quantitatively well for all flow and eddy velocities considered here. This implies, that the coupling of the sound and flow field is correct. In the case of the cavity the flow is computed in two dimensions with a self-written program as well as in three dimensions with the commercially available program PowerFLOW. The simulated pressure fluctuations in the cavity are compared to results of a wind tunnel experiment. Good agreement between simulation and wind tunnel experiment is found. The instationary flow around a half cylinder with an attached wedge tail is simulated in three dimensions using PowerFLOW. The radiated sound cannot be captured with PowerFLOW because of insufficient quantization of fluid density. However, pressure fluctuations on the surface of the body exhibit good agreement with the result of a wind tunnel test. Summarizing the results of this work it can concluded, that the Lattice-Boltzmann-model is well suited to numerical solutions of flow acoustic problems.

The excessive presence of liquid water in a gas diffusion layer (GDL) hinders the access of reactant gases to the active sites of the catalyst layer leading to decreased performance of a polymer electrolyte membrane fuel cell (PEMFC). Therefore, GDLs are usually treated with a hydrophobic agent to render their fibres more hydrophobic in order to facilitate gas transport and water removal. Numerous studies have been conducted to investigate water transport in PEMFCs in recent years; however, the behaviour of liquid water in a GDL at a pore-level is poorly understood. Macroscopic models fail to incorporate the influence of the structural morphology of GDLs on liquid water transport behaviour. Experimental methods are not conducive towards a good understanding at a microscopic level because of the diminutive size of the GDLs porous structure. Alternatively, the Lattice Boltzmann (LB) method has gathered interest as it is found to be particularly useful in fluid flow simulations in porous media due to its capability to incorporate the complex boundaries of actual GDL structures. To date, most studies on fluid transport in GDLs integrated artificial structures generated by stochastic simulation techniques to the LB models. The stochastic-based model, however, does not represent closely the microscopic features of the actual GDL as manufactured. In addition, comparison of liquid water transport behaviour in different GDL structures using the LB method is rare since only a single GDL material has been utilised in most of those studies. This thesis aims to develop our understanding of liquid water transport behaviour in GDLs with morphologically different structures under varying wettability conditions based on the LB method and the X-ray computed tomography (XCT) technique. GDLs with paper and felt structures were reconstructed into 3D digital volumetric models via the XCT process. The digital models were then incorporated into a LB solver to model water saturation distribution through the GDL domains. The GDL wettability was also altered so that the effect on liquid water behaviour in the GDL could be examined. This project is divided into three main sections. In the sensitivity analysis, the effect of image resolution on gas permeability through the X-ray reconstructed GDL was carried out using a single-phase LB model. It was found that the resolution variation could significantly affect the resulting gas permeability in both principal and off-principal directions, as well as computational time. An optimum resolution, however, exists at 2.72 μm/pixel, which consumed 400 times less computational time with less than 8% difference in the resulting permeability compared to the base resolution. This study also served as a guideline for selecting a resolution for generating the XCT images of the GDLs which were utilised in the following studies. In the structure analysis, the structures of the paper and felt GDLs were generated using the XCT and the key properties of each GDL, including thickness, porosity, permeability and tortuosity, were characterised. The thickness and the through-plane porosity distributions of each GDL were examined based on the tomography images. The resulting local through-plane porosity distributions were then used to calculate through-plane permeability and tortuosity distributions using an analytical model available in the literature. This study revealed the heterogeneity of the GDLs and how the heterogeneous nature of the GDL structures affects others properties of the GDLs. In this study, the absolute through-plane permeability and tortuosity of the X-ray-reconstructed GDL samples were also characterised using the single-phase LB model. The results from the two models were then compared and validated against data in the literature. In the water transport analysis, the two-phase LB model was employed to examine the effects of GDL structures on the behaviour of liquid water in the GDLs, including invasion patterns, saturation distribution and breakthrough behaviour under varying GDL wettability conditions. It was found that wettability was responsible for invasion patterns and water saturation levels whilst the GDL structure was mostly responsible for breakthrough occurrence and saturation distribution. It was observed that water travelled with stable displacement saturating all pores in hydrophilic GDLs, while it travelled with capillary fingering causing decreased saturation in hydrophobic GDLs, about 50% in the highly hydrophobic cases. The GDL structure was found to play a key role in breakthrough behaviour in the hydrophilic GDL as it was seen that the through-plane fibres in the felt structure and the through-plane binders in the paper structure encouraged water removal from the GDL in the thickness direction. Conversely, the GDL structure was found to have negligible influence on breakthrough in the hydrophobic GDL. Each GDL structure, however, contributed to a distinct difference in water distribution in the GDL with hydrophobic wettability. The work presented in this thesis contributes to the understanding of liquid water transport behaviour in the GDLs under the combined effects of the GDL structures and wettability conditions, which is essential for the development of effective PEMFC water management and the design of future GDL materials.

Understanding how the water produced in an operating polymer electrolyte membrane fuel cell (PEMFC) is transported in cathode catalyst layer (CCL) is crucial to improving performance and efficiency. In this thesis, a multiple-relaxation-time (MRT) lattice Boltzmann method (LBM) is employed to simulate the high density ratio, multiphase water transport in in the CCL. The three-dimensional structure of the catalyst layer is reconstructed based on experimental data acquired with a dual beam scanning electron microscope/focused ion beam system and a stochastic method using lower order statistical functions (e.g. porosity and two point correlation functions). Simulations of the water transport dynamics are performed to examine the effect of a range of physical parameters: wettability, viscosity ratio, pressure gradient, and surface tension. The water penetration patterns in the catalyst layers reveal a complex fingering process and transition of the water transport pattern from a capillary fingering regime to a stable displacement regime is observed when the wettability potential of the catalyst layer changes. The second part of the analysis focuses on quantifying the impact of liquid water distribution and accumulation in the catalyst layer on effective transport properties by coupling two numerical methods: the two-phase LBM is used to determine equilibrium liquid water distribution, and then a finite volume-based pore-scale model (FV-PSM) is used to compute transport of reactant and charged species in the CL accounting for the impact of liquid water saturation .The simulated results elucidate and quantify the significant impact of liquid water on the effective oxygen and water vapor diffusivity, and thermal conductivity in CLs. / Graduate

No description available.

The main sources of airframe noise in commercial aircrafts are the landing gear and the highlift devices. Among the high-lift devices, the slat deserves special attention since it represents a distributed source along the wing span. During approach and landing the noise generated by the slat can be comparable to the engine generated noise. For the design of quieter high-lift systems, it is important to understand the physics responsible for the slat noise generation. The objective of the work described in this thesis is to correlate the dynamics of large scale turbulent structures at different airfoil configurations with the characteristics of the noise generated by these structures. Four different configurations were investigated, ranging two airfoil angles of attack and three slat positions relative to the main element. The unsteady flow data was provided by a Lattice-Boltzmann based computational code. The Proper Orthogonal Decomposition technique was used for the objective identification of large scale structures in the slat region. Two different metrics were considered for the eduction of the coherent structures: one based on the Turbulent Kinetic Energy of the structures; and one based on their correlation to the noise emitted by the slat. The results of the transient simulations showed good agreement with wind tunnel measurements, providing confidence on the relevance of the analysis. The noise spectra of three of the cases simulated were dominated by a series of narrowband peaks at low frequency, while the spectrum of the remaining case was broadband in nature. Analysis of the averaged flow showed a large variation of the size and shape of the recirculating zone inside the slat cove and on the reattachment position of the mixing layer, between the simulated cases. The results indicated that, as the reattachment point approximates the region of the gap between the slat and the main element, the noise emission power increases. The large scale structures most correlated to the noise were typically two-dimensional and their shape suggests they resulted from the growth of disturbances in the mixing layer due to the inflectional instability. The dynamics of the noise correlated structures at the frequencies of the peaks was consistent with the existence of an acoustic feedback mechanism acting inside the slat cove. Based on the observation of the educed structures a model to predict the frequencies of peaks was proposed, showing good agreement with the frequencies computed from the unsteady flow data. / As principais fontes de ruído não propulsivo em aeronaves comerciais são os trens-de-pouso e os dispositivos híper-sustentadores. Entre os dispositivos híper-sustentadores, o eslate se destaca por constituir uma fonte distribuída ao longo da envergadura da asa. Durante a fase de aproximação e aterrissagem, o eslate pode gerar ruído com níveis comparáveis ao gerado pelos motores. Para viabilizar projetos de aerofólios com eslates menos ruidosos, é importante compreender os fenômenos fluidodinâmicos responsáveis pela geração desse ruído. O trabalho descrito neste texto tem por objetivo verificar se existe correlação entre o comportamento de grandes estruturas turbulentas em diferentes configurações do aerofólio com as características do ruído aeroacústico gerado por elas. O escoamento em quatro configurações diferentes foi estudado, abrangendo dois ângulos de ataque e três posições do eslate em relação ao elemento principal. Os dados do escoamento para análise foram gerados através de simulações numéricas não estacionárias utilizando um código comercial baseado no Método Lattice-Boltzmann. O método da Decomposição Ortogonal Apropriada foi utilizado para a identificação das estruturas de grande escala baseada em critérios objetivos. Duas métricas distintas foram utilizadas, uma baseada na energia cinética turbulenta e outra baseada na correlação com as ondas acústicas geradas a partir do eslate. Os resultados das simulações transientes apresentaram boa concordância com resultados experimentais. O espectro de ruído de três casos simulados são dominados por picos de baixa frequência, enquanto o espectro do quarto caso é tipicamente de banda larga. A análise do escoamento indica uma tendência de aumento do ruído à medida que o recolamento se aproxima do bordo de fuga do eslate. As estruturas mais correlacionadas com o ruído são tipicamente bi-dimensionais e seu formato indica que são resultado do crescimento de perturbações na camada de mistura devido à instabilidade inflexional. A dinâmica das estruturas correlecionadas com o ruído na frequência dos picos é consistentes com a existência de uma retro-alimentação das perturbações da camada de mistura por ondas acústica na cova do eslate. Um modelo para previsão das frequências dos picos foi proposto a partir da observação das estruturas identificadas pela Decomposição Ortogonal Apropriada, mostrando boa concordância com as frequências observadas nos espectros calculados com base nos dados transientes das simulações.