Dans cette dissertation, nous considérons l’écoulement des liquides conducteurs d’électricité dans un champ magnétique externe. De tels écoulements sont décrits par les équations de la magnétohydrodynamique (MHD) quasi-statique, et sont fréquemment rencontrés dans des applications pratiques. Il suit qu’il y a un intérêt fort pour des outils numérques qui peuvent simuler ces écoulements dans des géometries complexes.<p>La première partie de cette thèse (chapitres 2 et 3) est dédiée à la présentation de la machinerie numérique qui a été utilisée et implémentée afin de résoudre les équations de la MHD quasi-statistique (incompressible). Plus précisément, nous avons contribué au développement d’un solveur volumes finis non-structuré parallèle. La discussion sur ces méthodes est accompagnée d’une analyse numérique qui est aussi valable pour des mailles non-structurées. Dans le chapitre 3, nous vérifions notre implémentation par la simulation d’un certain nombre de cas tests avec un accent sur des écoulements dans un champ magnétique intense.<p>Dans la deuxième partie de cette thèse (chapitres 4-6), nous avons utilsé ce solveur pour étudier des écoulements MHD de proche paroi .La première géometrie considérée (chapitre 4) est celle d’une conduite circulaire infini d’axe à haut nombre de Hartmann. Nous avons investitgué la sensitivité des résultats numériques au schéma de discrétisation et à la topologie de la maille. Nos résultats permettent de caractériser in extenso l’écoulement MHD dans une conduite avec des bords bien conducteurs par moyen des lois d’échelle.<p>Le sujet du cinquième chapitre est l’écoulement dans une conduite toroïdale à section carée. Une étude du régime laminaire confirme une analyse asymptotique pour ce qui concerne les couches de cisaillement. Nous avons aussi effectué des simulations des écoulements turbulents afin d’évaluer l’effet d’un champ magnétique externe sur l’état des couches limites limites.<p>Finalement, dans le chapitre 6, nous investiguons l’écoulement MHD et dans un U-bend et dans un coude arrière. Nous expliquons comment générer une maille qui permet de toutes les couches de cisaillement à un coût computationelle acceptable. Nous comparons nos résultats aux solutions asymptotiques. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

<p>Intra-Arterial Chemotherapy (IAC) is a preferred treatment for the primary liver cancer, despite its adverse side-effects. During IAC, a mixture of chemotherapeutic drugs, e.g. Doxorubicin, is injected into an artery supplying the tumor. A fraction of Doxorubicin is absorbed by the tumor, but the remaining drug passes into systemic circulation, causing irreversible heart failure. The efficiency and safety of the IAC can be improved by chemical filtration of the excessive drugs with a catheter-based Chemofilter device, as proposed by a team of neuroradilogists. </p> <p>The objective of my work was to optimize the hemodynamic and drug binding performance of the Chemofilter device, using Computational Fluid Dynamics (CFD) modeling. For this, I investigated the performance of two distinct Chemofilter configurations: 1) a porous “Chemofilter basket” formed by a lattice of micro-cells and 2) a non-porous “honeycomb Chemofilter” consisting of parallel hexagonal channels. A multiscale modeling approach was developed to resolve the flow through a representative section of the porous membrane and subsequently characterize the overall performance of the device. A heat and mass transfer analogy was utilized to facilitate the comparison of alternative honeycomb configurations. </p> A multiphysics approach was developed for modeling the electrochemical binding of Doxorubicin to the anionic surface of the Chemofilter. An effective diffusion coefficient was derived based on dilute and concentrated solution theory, to account for the induced migration of ions. Computational predictions were supported by results of <i>in-vivo</i> studies performed by collaborators. CFD models showed that the honeycomb Chemofilter is the most advantageous configuration with 66.8% drug elimination and 2.9 mm-Hg pressure drop across the device. Another facet of the Chemofilter project was its surface design with shark-skin inspired texturing, which improves the binding performance by up to 3.5%. Computational modeling enables optimization of the chemofiltration device, thus allowing the increase of drug dose while reducing systemic toxicity of IAC.

Den pågående elektrifieringen av transport och samhälle kräver utveckling av nya metoder för återvinning av batterier. Hydrometallurgi som fokuserar på selektiv kristallisation av metaller är ett intressant alternativ för dessa ändamål. Dessa system kan studeras genom modellering och simulering. Många matematiska modeller finns tillgängliga för att beskriva de olika involverade processerna i kristallisationen av metaller. Dessa processer inkluderar övermättnad, nukleation, kristalltillväxt och aggregation. Denna rapport sammanställer ett antal av de tillgängliga matematiska modellerna och presenterar ett numeriskt tillvägagångssätt för modellering av den tidsberoende nummerdensiteten av partiklar genom en populationsbalansekvation. Populationsbalansen kan lösas med olika metoder såsom momentmetoden och metoden av viktade residualer. Här löses ekvationen genom diskretisering. Diskretisering av den inre koordinaten i ett flertal längdintervall möjliggör simulering av partikel-storleksfördelningen som en funktion av tid. Det numeriska tillvägagångssättet applicerades på bariumsulfatutfällning i en perfekt blandad satsreaktor och två- och tre-dimensionella T-mixer-system, såväl som en perfekt blandad satsreaktor för förträngningskristallisation av nickelsulfat med groddning. Den simulerade storleksfördelningens placering visade sig ha bra överenstämmelse med experimentell data vid låga Reynolds-tal. Här undersöktes även påverkan av en mängd parametrar såsom diskretisering, aggregation och magnituden av diffusion. Aggregation hade en märkbar inverkan på välblandade system. Inverkan av aggregation i diffusions-kontrollerade system med kort retentionstid var låg. Diffusionsmagnituden hade liten påverkan på den normaliserade distributionen men större på det totala antalet partiklar. / The currently ongoing electrification of society and transport necessitates the development of novel methods for battery recycling. Hydrometallurgy with a focus on selective metal crystallisation is an interesting prospect to these ends. The resource recovery systems of interest can be studied through simulation where many mathematical models are available to describe the varying processes involved. These processes include supersaturation generation, nucleation, growth and aggregation. This work compiles some of these mathematical models and presents a numerical approach for the modelling of the time-dependent particle number density with a population balance equation. The population balance equation can be solved using a variety of different methods such as method of moments and method of weighted residuals. Here, the balance equation was solved by discretisation. Discretising the inner coordinate (crystal length) into a number of length intervals allows for the particle size distribution to be modelled as a function of time for various crystallisation systems. The framework was successfully applied to barium sulphate precipitation in a perfectly mixed batch reactor and two- and three-dimensional T-mixer systems, as well as a seeded perfectly mixed nickel sulphate anti-solvent crystallisation system. The simulated size distribution showed promising similarity to experimental data at low Reynolds number. The influence of a variety of parameters such as aggregation and magnitude of diffusion was investigated. Aggregation had a significant impact on well-mixed systems increasing with retention time. The impact of aggregation on diffusion-controlled systems with low retention time was low. The magnitude of diffusion had little impact on the particle size distribution of the crystal population but a large impact on the total number of crystals.

<p dir="ltr">On many hypersonic vehicles, gaps are present on the outer surface of the vehicle and the interaction of the hypersonic freestream flow over these gaps can cause significant heat transfer to the vehicle. The project described in this thesis analyzed selected hypersonic gap problems and attempted to offer solutions to combat the heat transfer occurring in the gap. The first section of this thesis is a parametric study to understand the changes to the heat transfer and flow that modifications to the gap geometry can make. The second section is a comparison of the computational model to experimental data. The results of the studies show that adding a simple fillet or chamfer to the downstream step of the gap can reduce the maximum heat flux by over 90%. These results can be used to reduce the heat transfer caused by flow impingement in the gaps of hypersonic vehicles with a simple modification of the geometry and is consistent with the findings of other work in gap heating.</p>

The study of the flow of a fan blade was conducted to improve tonal fan noise reduction by optimizing an existing flow control configuration. The current configuration consisted of a trailing edge Slot with a flow control area of 0.045 in² per inch span with an exit angle of -3.3° with respect to the blade exit angle. Two other flow control configurations containing discrete jets were investigated. For the first configuration, the trailing edge jets (TEJ), the fan blade was modified with discrete jets spaced 0.3 inches apart with a flow control area of 0.01 in² per inch span positioned on the trailing edge aimed at -3.3° with respect to the blade exit angle. Similarly, discrete jets were also placed on the suction surface at 95.5% chord aimed at 15° with respect to the local blade surface. This configuration is referred to as the suction surface jet (SSJ). The discrete jets for both configurations were designed to be choked while injecting a mass flow rate of 1.00% of the fan through-flow. Computational Fluid Dynamics (CFD) was used to model new configurations and study subsequent changes in total pressure deficit using a blade design inlet Mach number of 0.73, Reynolds number based on chord length of 1.67 à 106, and design incidence angle of 0°. Experimental testing was later conducted in a 2D cascade tunnel. The TEJ and SSJ were tested at design blowing of 1.00% and at off-design conditions of 0.50%, 0.75%, and 1.25% fan through-flow. Results between the different flow control configurations were compared using a blowing coefficient. CFD showed the TEJ and SSJ offered aerodynamic improvement over the Slot configuration. Testing showed the SSJ outperformed the TEJ, as validated in CFD, producing wider and shallower wakes. SSJ area-averaged pressure losses were 25% less than TEJ at design. Noise predictions based on CFD findings showed that both TEJ and SSJ provided additional tonal sound power level attenuation over the Slot configuration at similar blowing coefficients, with the SSJ providing the most attenuation. Noise prediction based on experimental results concurred that the SSJ provided more total attenuation than the TEJ. Experimental results showed that the SSJ performed better aerodynamically and, based on analytical prediction, provided 2 dB more total attenuation than the TEJ. / Master of Science

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Zhongquan Zheng / A nano-material filter is an efficient device for improving indoor environmental quality (e.g. smoke reduction, air purification in buildings). Studying the effectiveness of nano-materials used in the device by computer simulation is challenging because very different size scales are involved. Therefore, numerical methods have to be developed to accommodate varying magnitudes of scales. In the current study, the simulation has been divided into three scales: macro-, micro- and nano-scale. The numerical schemes at each scale are targeted at a particular scale; however, the relationship of the general transport phenomena, physical mechanisms and properties among different scales are uniquely linked at the same time. The objective of the macro-scale simulation was to design and study a gas filter constructed with nano-material pellets. The filter was considered a packed-bed tube filled with manufactured nano-material pellets. Commercial computational fluid dynamics (CFD) packages were used along with the embedded programming macros. In the filtration process, we focused on the flow and species transport phenomena through the porous substrate. The mathematical/numerical models were built and tested based on the physical models used in the experimental setups for different materials that were tested. The results from the numerical models were validated and compared well to experimental data obtained from the pressure drop measurements and the adsorption (breakthrough) tests. In the micro-scale simulation, a modified immersed-boundary method (IBM) with the Zwikker-Kosten (ZK) porous model and the high-order schemes was validated and applied to simulate a representative porous unit that represented a periodic array of solid/porous cylinders. In the periodic unit, the solid cylinder case was used to validate the high-order schemes by comparing it to the results obtained from the commercial CFD software. The relationship between the pressure gradient and the porosity (Blake-Kozeny equation) was determined from this level and fed back to the macro-scale simulation, which provided a link between the two scales. In the porous cylinder case, both flow field and species transport were investigated with a porous model similar to the one used in the macro-scale. The species concentration change was calculated and found to be nonlinearly related to the adsorption coefficient. In the nano-scale simulation, a molecular dynamics (MD) simulation and a coupled molecular-continuum scheme were applied to solve the momentum and the mass transport problems at the molecular level at which the traditional continuum theory is no longer applicable. Both schemes were verified from the surface slip behavior study compared to the literature. The scale and shear effects in the Coutte flow were investigated, showing that in the micro-scale and macro-scale, the slip behavior could be neglected since it was only important in much smaller scales. The same hybrid scheme was then applied to a diffusion model with nano-pores constructed in the solid substrate. The adsorptions between various gases and the carbon substrate were simulated. The mass fluxes cross the fluid/solid interfaces were counted and both self-diffusivity and transport diffusivity were estimated and compared to their respective values found in the literature. The transport properties are closely related to the species transport (Fick’s law) in the macroscopic simulations. Linear concentration profiles in the channel were obtained based on those transport properties for various gases going through different sizes of nano-pores, which, as a connection to the continuum model, were to be used as boundary conditions in the continuum simulation.

Aujourd'hui divers systèmes de propulsion marins existent dont les systèmes de propulsion à jet. De nombreuses études ont été réalisées au cours des vingt dernières années et plus particulièrement par le biais de la simulation numérique ( Computational Fluid Dynamics , ( CFD )). Un modèle numérique simulant l'écoulement au travers un système de propulsion avec une pompe en rotation est développé dans ce projet. Il est également validé avec des résultats expérimentaux obtenus par d'autres chercheurs. Pour se [i.e. ce] faire, différentes étapes sont réalisées. En premier lieu, des paramètres tels que le maillage, le modèle de turbulence ou la modélisation de la rotation du rotor sont validés numériquement sur deux géométries. Le premier cas, bien documenté, correspond à un écoulement au travers une conduite en forme de"S" de section divergente et validé expérimentalement. Le second cas est une pompe où le rotor est en mouvement dans l'écoulement. Un modèle à multiple systèmes de référence ( Multiple Reference Frame,MFR ) est utilisé pour simuler la rotation de la pompe. Les paramètres numériques sont alors fixés pour la suite de l'étude. Ensuite un inodéle numérique d'un système de propulsion à jet d'une motomarine est développé dans un volume de contrôle réduit. Il prend en compte l'ensemble de la géométrie de la propulsion : la pompe en rotation, le venturi et une partie de la coque réelle du véhicule. Les conditions aux frontières sous la coque sont imposées grâce aux données d'une simulation complète du véhicule entier. Ce modèle est validé expérimentalement à deux vitesses (25 mph et 69 mph). Le comportement de l'écoulement est ensuite analysé. Finalement diverses variations géométriques sont effectuées telles que la supression [i.e. suppression] d'appendices dans la conduite ou le déplacement latéral de la lèvre de l'entrée d'eau à divers IVR, Inlet Velocity Ratio (rapport entre la vitesse du véhicule et celle de la pompe). Une augmentation des performances du système est observée à un certain IVR pour une supression [i.e. suppression] d'appendice donnée. La présence d'une plaque permettant le redressement de l'écoulement au niveau de la grille est néfaste à la poussée de même que la présence de l'arbre ou d'ailettes situées à l'entrée de la pompe. Le déplacement de la lèvre a pour objectif de déterminer la position optimale qui permet d'obtenir la meilleure augmentation de performance pour un IVR .

Depuis une vingtaine d’années, l’engouement pour les propulsions hydrojets n’a fait que croître et elles s’imposent aujourd’hui comme la propulsion marine incontournable pour les hautes vitesses. Dans un même temps, un outil permettant un gain considérable de temps et d’argent s’est lui aussi développé considérablement. En effet, la CFD (Computational Fluid Dynamics) est devenue une pratique courante lorsqu’il s’agit de prévoir le comportement d’un écoulement sans avoir à passer par un modèle réel. Elle sera utilisée tout au long du projet pour simuler le flux au travers de la propulsion. Le design d’une entrée d’eau est capital : une entrée d’eau mal conçue engendrera des zones de cavitation, de la recirculation sur la lèvre ou la rampe, des pertes importantes et un champ de vitesse non uniforme à la face de la pompe. Il en résultera une diminution du rendement de l’entrée, mais aussi une diminution du rendement de la pompe, puisqu’optmisée pour un flux uniforme. L’objectif de ce projet sera d’optimiser l’entrée d’eau pour augmenter le rendement global de la propulsion et ainsi réduire la consommation d’essence de 6 % sur un cycle donné. Actuellement, les conduites d’entrées sont conçues pour optimiser une vitesse de croisière moyenne. Dans ce projet, le but sera d’éviter d’avoir un compromis à faire entre les basses vitesses, la vitesse de croisière et la vitesse de pointe, et d’optimiser la géométrie de l’entrée pour une large plage de fonctionnement. Cela passe par une géométrie variable et donc un mécanisme asservi. Afin de concevoir un tel système, il sera nécessaire de trouver les géométries optimales pour les différents régimes de fonctionnement. Une étude CFD 2D paramétrable permettra de trouver les lignes directrices de ces géométries. Un modèle 3D devra ensuite être validé, puis utilisé pour pouvoir affiner les géométries optimales. Un système sera alors conçu puis testé sur le modèle CFD. Des tests expérimentaux viendront finaliser l’étude.

MScEng / Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / Research conducted on Marion Island aims at predicting the consequence of climate change in the Subantarctic region, as well as for other terrestrial ecosystems. The island has the ideal ecosystem, due to its size, isolation and relative simplicity, to be studied as a prediction model for the consequence of climate change and the interaction between various climate related parameters. The cushion plant, Azorella selago, is the focal point of this project due to the important functional roles it fulfils as well as its wide spread distribution over the island. A. selago grows in three different shapes; hemispherical, elliptical and crescent. The changes in airflow due to varying plant size are investigated as well as the exertion of force on the plant. The grass species Agrostis magellanica has the tendency to grow on top of A. selago. In addition it has been observed that the grass species grows more vibrantly on the leeward side of the cushion plant. In the light of this observation the particle deposition of grass seeds on A. selago is also investigated. Computational fluid dynamic analyses are conducted for various sizes of each shape of the plant. These studies are augmented by wind tunnel and in situ measurements and observation and experimental determining of particle drag coefficients. Time independent, incompressible, turbulent flow is modelled by means of a high Reynolds number turbulence model with a modified Law-of-the- Wall to accommodate for the significant surface roughness. Nine different dimensions over the various shapes were identified. Each shape is analysed and the patterns that emerged discussed. The windward pattern for all shapes display similar qualities. On the leeward side the shapes display distinctly different airflow patterns. The hemisphere shows two trailing lines typically associated with the horseshoe vortex phenomenon. The ellipse displays one distinctive trailing line. The most interesting flow patterns are found when analysing the crescent shape. Three trailing lines are observed, the side lines quickly dissipates with increasing height while the middle line remains distinctive. The complex recirculation patterns that emerge are further visualized by means of air particle tracks. The furthermost number of light particle deposition of A. magellanica seeds on A. selago are found at the windward location while physical evidence clearly indicates growth primarily on the leeward position. The leeward location on A. selago is protected from the harsh environmental conditions; wind speeds are minimal in this area. It will therefore be a more suitable site for the fostering of a vulnerable seedling that on the exposed windward side. A force analysis done on the plants reveals that the total force is two orders of magnitude higher that the shear force. Furthermore, the magnitude of the total force is directly proportional to the size of the plant. Three critical areas on the plant are common irrespective of shape or size: the stagnation point at the windward side, the apex region at the top and the leeward side of the plant.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: This study is aimed at the development of an efficient and reliable method of evaluating the performance of an air-cooled steam condenser (ACSC) under windy conditions, using computational fluid dynamics (CFD). A two-step modelling approach is employed as a result of computational limitations. The numerical ACSC model developed in this study makes use of the pressure jump fan model, amongst other approximations, in an attempt to minimize the computational expense of the performance evaluation. The accuracy of the numerical model is verified through a comparison of the numerical results to test data collected during full scale tests carried out on an operational ACSC. Good correlation is achieved between the numerical results and test data. Further verification is carried out through a comparison to previous numerical work. Satisfactory convergence is achieved for the most part and the few discrepancies in the results are explained. The effect of wind on ACSC performance at El Dorado Power Plant (Nevada, USA) is investigated and it is found that reduced fan performance due to distorted flow at the inlet of the upstream fans is the primary contributor to the reduction in performance associated with increased wind speed in this case. An attempt is subsequently made to identify effective wind effect mitigation measures. To this end the effects of wind screens, solid walkways and increasing the fan power are investigated. It is found that the installation of an appropriate wind screen configuration provides a useful means of reducing the negative effects of wind on ACSC performance and an improved wind screen configuration is suggested for El Dorado. Solid walkways are also shown to be beneficial to ACSC performance under windy conditions. It is further found that ACSC performance increases with walkway width but that the installation of excessively wide walkways is not justifiable. Finally, increasing the fan power during periods of unfavourable ambient conditions is shown to have limited benefit in this case. The model developed in this study has the potential to allow for the evaluation of large ACSC installations and provides a reliable platform from which further investigations into improving ACSC performance under windy conditions can be carried out. / AFRIKAANSE OPSOMMING: Hierdie studie is daarop gemik om die ontwikkeling van 'n geskikte en betroubare metode van evaluering van die verrigting van ’n lugverkoelde stoom-kondensator (air-cooled steam condenser, ACSC) onder winderige toestande, met behulp van numeriese vloei-dinamika. ’n Twee-stap modelleringsbenadering is aangewend as gevolg van rekenaar beperkings. Die numeriese ACSC-model wat in hierdie studie ontwikkel is, maak gebruik van die druksprong waaier model, asook ander benaderings, in ’n poging om die berekeningskoste van die verrigting-evaluering te verminder. Die akkuraatheid van die numeriese model is bevestig deur middel van ’n vergelyking van die numeriese resultate met toetsdata ingesamel tydens die volskaal toetse uitgevoer op ’n operasionele ACSC. Goeie korrelasie is bereik tussen die numeriese resultate en toetsdata. Verdere bevestiging is uitgevoer deur middel van ’n vergelyking met vorige numeriese werk. Bevredigende konvergensie is in die algemeen bereik en die paar verskille in die resultate word verduidelik. Die effek van wind op ACSC verrigting by El Dorado Power Plant (Nevada, VSA) is ondersoek, en daar is bevind dat verlaagde waaierverrigting, as gevolg van vervormde vloei by die inlaat van die stroomop waaiers, die primêre bydraer is tot die afname in ACSC werkverrigting geassosieer met verhoogde windsnelheid in hierdie geval. ’n Poging word dan aangewend om effektiewe wind-effek velagingsmaatreëls te identifiseer. Windskerms, soliede wandelvlakke en die verhoging van die waaierkrag word gevolglik ondersoek. Daar is bevind dat die installasie van ’n toepaslike windskerm-opset ’n nuttige middel tot ’n vermindering van die negatiewe effekte van wind op ACSC verrigting bied, en ’n verbeterde windskerm opset is voorgestel vir El Dorado. Soliede wandelvlakke word ook aanbeveel as voordelig vir ACSC verrigting onder winderige toestande. Dit is verder bevind dat die ACSC prestasie verhoog met wandelvlak breedte, maar dat die installasie van ’n te ruim wandelvlak nie regverdigbaar is nie. Ten slotte, word bewys dat die verhoging van die waaierkrag tydens periodes van ongunstige omgewingsomstandighede ’n beperkte voordeel in hierdie geval het. Die model wat ontwikkel is in hierdie studie het die potensiaal om voorsiening te maak vir die evaluering van groot ACSC- installasies en bied ’n betroubare platform vanwaar verdere ondersoeke tot die verbetering van ACSC verrigting onder winderige toestande uitgevoer kan word.