Magister Scientiae - MSc / The aim of my work is to model a segment of a unit cell of a fuel cell stack using numerical methods which is classified as computational fluid dynamics and implementing the work in a commercial computational fluid dynamics package, FLUENT. The focus of my work is to study the thermal distribution within this segment. The results of the work aid in a better understanding of the fuel cell operation in this temperature range. At the time of my investigation experimental results were unavailable for validation and therefore my results are compared to previously published results published. The outcome of the results corresponds to this, where the current flux density increases with the increasing of operating temperature and fixed operating voltage and the temperature variation across the fuel cell at varying operating voltages. It is in the anticipation of determining actual and or unique material input parameters that this work is done and at which point this studies results would contribute to the understanding high temperature PEM fuel cell thermal behaviour, significantly. / South Africa

This study seeks to understand the interplay of vorticity and elasto-dynamics that forms the basis for a fluttering flag and flapping wing propulsion, and factors that distinguish one from the other. The fluid dynamics is assumed two dimensional and incompressible, and comprises potential and viscous flow simulations. The elastic solid is one dimensional and governed by the Bernoulli-Euler flexure model. The fluid and elastic solid models are coupled using a predictor-corrector algorithm. Flutter of a flag or foil is associated with drag and we show that the pressure on the foil is predominantly circulatory in origin. The circulatory pressure generated on the foil depends primarily on the slope and curvature. The wake vorticity exhibits a wide range of behavior starting from a Kelvin-Helmholtz type instability to a von Kármán wake. Potential flow simulations do not capture the wake accurately both at high and low mass ratios. This is reflected in the flutter boundary and pressure over the foil when compared with viscous flow simulations. Thrust due to heaving of a flexible foil shows maxima at a set of discrete frequencies that coincide with the frequencies at which the flapping velocity of the foil tip is a maximum. The propulsive efficiency shows maxima at a set of discrete frequencies that are close but distinct from the thrust maxima set of frequencies. These discrete frequencies are close to the natural frequencies of vibration of a cantilevered foil vibrating in vacuum. At low frequencies thrust is a consequence of a strong leading edge vortex developed over the foil and it remains attached to the foil as it is convected due to the favorable pressure gradient presented by the time and spatially varying shape of the foil. At moderate and high frequencies of oscillation the pressure, and consequently the thrust, generated by the foil is non-circulatory in origin and they are high where the accelerations of the foil are high. At high frequencies the leading edge vortex is weak. Except in the low frequency range, potential flow simulations qualitatively compares well with viscous flow predictions. We show that thrust and drag on a flexible foil oscillating in a flow is caused by the phase difference between the slope of the foil and the fluid pressure on it. Propulsive efficiency though is governed by the phase difference between foil velocity and fluid pressure and inertia forces. Thus, the interplay of vorticity and elasto-dynamics determine the behavior of a flutter and propulsion of a flexible foil in a fluid flow.

Desde a década de 70, há um crescente interesse em simulações em computador de fenômenos físicos visto sua diversidade de aplicações. Dentre esses fenômenos, podem ser destacados a interação entre corpos rígidos, elásticos, plásticos, quebráveis e também fluidos. Neste trabalho realizamos a simulação de um desses fenômenos, o escoamento de fluidos, por um método conhecido como Smoothed Particles Hydrodynamics, uma abordagem lagrangeana baseada em partículas para resolução das equações que modelam o movimento do fluido. Várias são as vantagens de métodos lagrangeanos usando partículas sobre os que usam malhas, por exemplo, as propriedades do material transladam com as partículas como função do tempo, além da capacidade de lidar com grandes deformações. Dentre as desvantagem, destacamos uma deficiência relacionada ao ganho de energia total do sistema e estabilidade das partículas. Para lidar com isso, utilizamos uma abordagem baseada na lei da conservação da energia: em um sistema isolado a energia total se mantém constante e ela não pode ser criada ou destruida. Dessa forma, alterando o integrador temporal nós restringimos o aumento arbitrário de energia, tornando a simulação mais tolerante às condições iniciais / Since the late 70s, there is a growing interest in physically-based simulations due to its increasing range of application. Among these simulations, we may highlight interaction between rigid, elastic, plastic and breakable bodies and also fluids. In this work, one of these phenomena, fluid flow, is simulated using a technique known as Smoothed Particle Hydrodynamics, a meshless lagrangean method that solves the equations of the flow behavior of fluids. There are several advantages of meshless methods over mesh-based methods, for instance, the material properties are translated along with particles as a function of time and the ability to handle arbitrary deformations. Among the disadvantages, we may highlight a problem related to the gain of energy by the system and stability issues. In order to handle this, we used an approach based on the law of conservation of energy: in an isolated system the total energy remains constant and cannot be created or destroyed. Based on this, we used a technique that bounds the total energy and the simulation becomes less sensitive to initial conditions

Orientador: Caio Gorla Nogueira / Resumo: Este trabalho apresenta um conjunto de técnicas estatísticas básicas aplicadas à modelagem de problemas de fluxo em meios porosos fraturados e de estabilidade de taludes, com o objetivo de identificar as variáveis explicativas mais influentes sobre a variabilidade das variáveis resposta. Diferentes planejamentos de experimentos foram utilizados para possibilitar a construção de metamodelos polinomiais representativos dos fenômenos estudados. Uma modificação do planejamento do tipo Box-Behnken é apresentada e foi proposta pelo autor para analisar problemas que envolvem elevado número de variáveis explicativas (e.g. 30). Os metamodelos, obtidos via método dos mínimos quadrados, são também chamados de superfícies de resposta ou modelos de regressão e são indispensáveis à verificação da sensibilidade das variáveis explicativas. O conjunto de técnicas mostrou- se muito eficaz na identificação das variáveis explicativas que provocaram efeitos mais significativos sobre a variável resposta. Evidenciou-se também, por meio dos exemplos de estabilidade de taludes tratados, a possibilidade de se quantificar incertezas com o uso de metamodelos suficientemente adequados, uma opção que pode ser bastante útil no processo de quantificação de incertezas de problemas que não possuem soluções analíticas simples. / Abstract: This paper presents a set of basic statistical techniques applied to the modeling of flow problems in fractured porous and slope stability media, aiming to identify the most influential explanatory variables on the response variables variability. Different designs of experiments were used to enable the construction of polynomial metamodels representative of the studied phenomena. A Box-Behnken type design modification is presented and was proposed by the author to analyze problems involving high number of explanatory variables (e.g. 30). The metamodels, obtained by the least squares method, are also called response surfaces or regression models and are indispensable for verifying the sensitivity of the explanatory variables. The set of techniques was very effective in identifying the explanatory variables that had the most significant effects on the response variable. It was also evidenced, through the examples of stability of treated slopes, the possibility of quantifying uncertainties using sufficiently adequate metamodels, an option that can be very useful in the process of quantifying uncertainty of problems that do not have simple analytical solutions. / Mestre

In this research, an elastic cylinder that utilized vortex-induced vibration (VIV) was applied to improve convective heat transfer rates by disrupting the thermal boundary layer. Rigid and elastic cylinders were placed across a fluid channel. Vortex shedding around the cylinder led to the periodic vibration of the cylinder. As a result, the flow-structure interaction (FSI) increased the disruption of the thermal boundary layer, and therefore, improved the mixing process at the boundary. This study aims to improve convective heat transfer rate by increasing the perturbation in the fluid flow. A three-dimensional numerical model was constructed to simulate the effects of different flow channel geometries, including a channel with a stationary rigid cylinder, a channel with a elastic cylinder, a channel with two elastic cylinders of the same diameter, and a channel with two elastic cylinders of different diameters. Through the numerical simulations, the channel maximum wall temperature was found to be reduced by approximately 10% with a stationary cylinder and by around 17% when introducing an elastic cylinder in the channel compared with the channel without the cylinder. Channels with two-cylinder conditions were also studied in the current research. The additional cylinder with the same diameter in the fluid channel only reduced the surface wall temperature by 3% compared to the channel without any cylinders because the volume of the second cylinder could occupy some space, and therefore, reduce the effect of the convective heat transfer. By reducing the diameter of the second cylinder by 25% increased the effect of the convection heat transfer and reduced the maximum wall temperature by around 15%. Compared to the channel with no cylinder, the introduction of cylinders into the channel flow was found to increase the average Nusselt number by 55% with the insertion of a stationary rigid cylinder, by 85% with the insertion of an elastic cylinder, by 58% with the insertion of two cylinders of the same diameter, and by approximately 70% with the insertion of two cylinders of different diameters (the second cylinder having the smaller diameter). Furthermore, it was also found that the maximum local Nusselt number could be enhanced by around 200%-400% at the entrance of the fluid channel by using the elastic cylinders compared to the channel without cylinders.

No description available.

This master`s thesis analyzes the pressure distribution on the premises of scintillation detector secondary electron in the variant with three aperture. The aim of this study is analyzing the fluid flow depending on the application of the third aperture in the entrance of the detector, thus creating more self-pumped chamber, which is responsible for ensuring a better distribution of pressure in the premises of the detector. The result of the analysis would be determining how to change parameters within the detector. If it is found that application of the third aperture has a positive effect on water flow in the premises of the detector, this arrangement may be used for improvement of the detector. Master`s thesis is divided into several chapters. First describes the basic principles of electron microscopy, including sample preparation, conditions for the proper functioning of microscopes and sharing different types of electron microscopes. The next chapters describe briefly the physical descriptions of gases flow in low pressures and small apertures, the mathematical models and simulation software used in this analysis. The analysis is done in SolidWorks with the module called Cosmos FloSimulation. The conclusion summarizes the results of the analysis, including graphical representations of simulation.

The aim of this thesis is to study the issue of eniveromental scanning electron microscopy and pumping gas to create vacuum in the newly designed scintillation detector. Further, creating a model of recently proposed scintillation detector and simulating and analyzing pumping gas in differentially pumped chamber of detector. The theoretical part deals with electron microscopy, electron sources, electron optics and secondary electrons detectors. It is also presented which signals are generated by the electron beam on the surface of a solid. Further fluid flow issues and equations describing the flow in the solved chamber are dismantled. Furthermore, the impact of gaseous environment on the trajectory of primary electrons, because there are collisions of primary beam with atoms and molecules of gas. The following section discusses creating, quality and importance of the network in mathematical modelling. A method of a final volume used to calculate the differential equations describing the flow of gas at the premises of the detector is described . The practical part consists in creating a model of scintillation detector and analyzing the gas flow in drawing a vacuum in the newly designed scintillation detector. At the end the simulation results of gas flow are compared for different variants of apertures and various pressures on the neck of a scintillation detector designed for optimum performance of the detector. The outcome of this thesis is model of newly designed scintillation detector with optimized shapes of apertures according to functional requirements.

The aim of this thesis is to study the issue of eniveromental scanning electron microscopy and pumping gas to create a vacuum in the newly designed scintillation detector. Further, creating a model of recently proposed scintillation detector and simulating and analyzing pumping gas in differentially pumped chamber of detector and the results compare with the previous model. The theoretical part deals with electron microscopy, electron sources, electron optics and secondary electrons detectors. It is also presented which signals are generated by the electron beam on the surface of a solid. Further fluid flow issues and equations describing the flow in the solved chamber are dismantled. Furthermore, the impact of gaseous environment on the trajectory of primary electrons, because there are collisions of primary beam with atoms and molecules of gas. The following section discusses creating, quality and importance of the network in mathematical modelling. A method of a final volume used to calculate the differential equations describing the flow of gas at the premises of the microscope is described . The practical part consists in creating a model of scintillation detector and analyzing the gas flow in drawing a vacuum in the newly designed scintillation detector. Furthermore, the simulation results are compared with the results of simulations on the older type of scintillation detector. The output of this thesis is model of recently proposed scintillation detector with visualized simulation results.

Experimental data convincingly show that viscosity of a fluid may change significantly with pressure. This observation leads to various generalizations of well-known models, like Darcy's law, Stokes' law or the Navier-Stokes equations, among others. This thesis investigates three such models in a series of three published papers. Their unifying topic is development of existence theory and finding a weak solution to systems of partial differential equations stemming from the considered models.