La perspective lagrangienne, décrivant un écoulement selon les trajectoires de traceurs fluides, est une approche naturelle pour étudier les phénomènes de dispersion dans les écoulements turbulents. En turbulence de paroi, le mouvement des traceurs est influencé par le cisaillement moyen et par une forte inhomogénéité et anisotropie en proche paroi. On étudie les propriétés lagrangiennes d’un écoulement de canal turbulent par simulation numérique directe à un nombre de Reynolds modéré. Les statistiques d’accélération lagrangienne sont comparées aux expériences de suivi de particules réalisées en parallèle à ce travail. Comme en turbulence homogène isotrope (THI), les composantes d’accélération le long des trajectoires lagrangiennes se décorrèlent sur des temps comparables aux plus petites échelles de l’écoulement, tandis que la norme de l’accélération reste corrélée plus longtemps. La persistance d’anisotropie à petite échelle loin de la paroi est constatée par l’existence d’une corrélation croisée non nulle entredeux composantes de l’accélération. On montre que, en conséquence des flux moyens d’énergie cinétique en turbulence de paroi, près des parois les traceurs se déplacent et s’étalent sur des plus grandes distances quand ils sont suivis en arrière dans le temps qu’en avant. La dispersion relative de paires de traceurs est aussi étudiée. Aux temps courts, la séparation des paires est balistique pour toutes les distances à la paroi. Comme en THI, les traceurs se séparent plus rapidement lorsqu’ils sont suivis en arrière dans le temps. Aux temps plus longs, le cisaillement moyen accélère la séparation dans la direction de l’écoulement moyen. Un modèle de cascade balistique initialement proposé pour la THI est adapté aux écoulements inhomogènes / The Lagrangian perspective, describing a flow from the trajectories of fluid tracers, isa natural framework for studying dispersion phenomena in turbulent flows. In wall-boundedturbulence, the motion of fluid tracers is affected by mean shear and by strong inhomogeneityand anisotropy near walls. We investigate the Lagrangian properties of a turbulent channel flowusing direct numerical simulations at a moderate Reynolds number. Lagrangian accelerationstatistics are compared to particle tracking experiments performed in parallel to this work. Asin homogeneous isotropic turbulence (HIT), the acceleration components along Lagrangianpaths decorrelate over time scales representative of the smallest scales of the flow, while theacceleration norm stays correlated for much longer. The persistence of small-scale anisotropy farfrom the wall is demonstrated in the form of a non-zero cross-correlation between accelerationcomponents. As a result of the average fluxes of kinetic energy in wall turbulence, tracers initiallylocated close to the wall travel and spread over longer distances when tracked backwardsin time than forwards. The relative dispersion of tracer pairs is finally investigated. At shorttimes, pair separation is ballistic for all wall distances. As in HIT, relative dispersion is timeasymmetric, with tracers separating faster when tracked backwards in time. At longer times,mean shear dominates leading to rapid separation in the mean flow direction. A ballisticcascade model previously proposed for HIT is adapted to inhomogeneous flows

Escoamentos sobre superfícies côncovas estão sujeitos à instabilidade centrífuga, dando origem a vórtices longitudinais, conhecidos como vórtices de Görtler. Esses vórtices são responsáveis por gerar distorções fortes nos perfis de velocidade. Como os vórtices são contra-rotativos, duas regiões surgem entre os mesmos: uma região de upwash e uma região de downwash. Na região de upwash o fluido próximo à parede é jogado para longe da mesma. Na região de downwash acontece o contrário, o fluido que se desloca a uma velocidade maior é jogado em direção à parede. Os vórtices se amplificam inicialmente de forma linear. À jusante na região não linear de desenvolvimento dos vórtices, a amplitude dos mesmos já é elevada, e há a formação de uma estrutura do tipo cogumelo com a distribuição da componente de velocidade na direção principal do escoamento . Essa nova distribuição de velocidade é tridimensional e difere em muito da camada limite obtida com a solução das equações de Blasius. Levando-se em consideração a camada limite térmica, já foi observado que, na média, há um aumento de transferência de calor na direção da parede. No presente trabalho, é verificado numericamente a transferência de calor na presença de vórtices de Görtler. Para tal, foi desenvolvido e implementado um código de simulação numérica direta espacial (DNS - do inglês Direct Numerical Simulation). Os resultados deste trabalho mostram a intensificação da transferência de calor através dos vórtices de Görtler, tanto no regime não-linear como na instabilidade secundária / Flows over concave surfaces are subject to centrifugal instability. It gives rise to stramwise vortices known as Görtler vortices. These vortices are responsible for generating strong distortions in the velocity profiles. As the vortices are counterrotating, two regions arise between them: a region of uowash and a region of downwash. In the upwash region, the fluid near the wall is convected away from it. In the downwash region the opposite happens, the fluid moving at a faster speed is moved towards the wall. The vortices initially amplify linearly in the downstream. When their amplitude is already high, in the non-linear development region, a mushroom-type structure, with the velocity distribution in the main flow direction, is formed. This new three-dimensional velocity distribution is different from the boundary layer obtained with the solution of Blasius equations. Taking into account a thermal boundary layer, on average, an increase in the heat transfer in the wall direction has been observed. In the present work, it is verified numerically the heat transfer in the presence of Görtler vortices. A simulation code was developed and implemanted usin Direct Numerical Simulation (DNS). The results of this work show the intensification of heat transfer through the Görtler vortices both in the non-linear regime and in the secondary instability

La première partie de cette thèse traite du développement du code de simulation numérique directe YWC, principalement du développement des conditions aux limites. En effet, une forte contribution scientifique a été apportée aux conditions aux limites appelées "Three dimensional Navier-Stokes characteristic boundary condtions" (3D-NSCBC). Premièrement, la formulation de ces conditions aux arêtes et coins a été complétée, ensuite une extension de la formulation a été proposée pour supprimer les déformations observées en sortie dans le cas d'écoulements non-perpendiculaires à la frontière. <p>De plus, ces conditions ont été étendues au cas des gaz réels et une nouvelle définition du facteur de relaxation pour la pression a été proposée. Ce nouveau facteur de relaxation permet de supprimer les déformations observées en sortie pour des écoulements transcritiques. <p>Les résultats obtenus avec le code YWC ont ensuite été utilisés dans la seconde partie de la thèse pour développer une nouvelle méthode de tabulation basée sur l'analyse en composantes principales. Par rapport aux méthodes existante telles que FPI ou SLFM, la technique proposée, permet une identification automatique des variables à transporter et n'est, de plus, pas lié à un régime de combustion spécifique. Cette technique a permis d'effectuer des calculs d'interaction flamme-vortex en ne transportant que 5 espèces à la place des 9 requises pour le calcul en chimie détaillée complète, sans pour autant perdre en précision. <p>Finalement, dans le but de réduire encore le nombre d'espèces transportées, les techniques T-BAKED et HT-BAKED PCA ont été introduites. En utilisant une pondération des points sous-représentés, ces deux techniques permettent d'augmenter la précision de l'analyse par composantes principales dans le cadre des phénomènes de combustion.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Cette étude s’inscrit dans les efforts faits par l’industrie aéronautique pour la protection environnementale.Pour cela, un des objectifs principaux est de réduire les émissions de polluants et les émissions sonores des aéronefs. Les émissions polluantes sont liées à la qualité de la combustion qui dépend elle même de la conception des chambres de combustion. Les émissions sonores sont réduites grâce à des traitements passifs comme les structures absorbantes placées en paroi des moteurs pour diminuer le bruit de soufflante. Pour ces deux applications différentes, on utilise des matériaux perforés. En effet, les matériaux absorbants sont constitués d’une structure en nid d’abeilles surmontée d’une plaque perforée. Dans les chambres de combustion, les perforations permettent l’injection d’un air froid protégeant les parois des hautes températures, mais leur présence peut modifier la caractérisation acoustique de la chambre. L’objectif de cette thèse est de modéliser la réponse acoustique d’un matériau perforé. La taille des perforations étant petite devant les longueurs d’ondes sonores, des techniques de modélisation asymptotique adaptées à la résolution de problèmes multi-échelle peuvent être mises en œuvre. En effet, ces méthodes permettent de faire le lien entre les effets présents à l’intérieur d’une perforation et la réponse acoustique homogénéisée de la plaque perforée. Dans ces travaux,ce sont les effets visqueux présents dans les perforations qui ont été essentiellement étudiés. Ensuite,des simulations numériques directes ont été réalisées pour vérifier la validité des hypothèses émises lors de la modélisation asymptotique. Ces travaux de thèse ont permis d’améliorer la compréhension de la modélisation de la réponse acoustique des matériaux perforés. De plus, la méthode analytique présentée peut être mise en œuvre pour des perforations de géométrie complexe. / This study is part of the effort made by the aeronautic industry to protect the environment. Thus,one of the main objectives is to reduce aircraft polluting emissions and sound emissions. The pollutinge missions are linked to the combustion quality which depends on the combustion chamber’s design.Noise pollution is reduced thanks to passive treatments such as absorbing structures placed on engine walls. For both applications, perforated plates are used. Indeed, absorbing materials are composed of honey comb structures topped by a perforated plate. In combustion chambers, cold air is injected through perforations to protect the walls from high temperatures, but the perforations can modify the chamber’s acoustic behavior. The objective of this thesis is to model the acoustic response of aperforated material. The perforation size is smaller than the sound wave length, therefore asymptotic modeling techniques adapted to resolving multiscale problems can be implemented. Indeed, thesemethods allow to link the effects inside the perforation with the homogenized acoustic response of theperforated plate. In this study, the viscous effects inside the perforation have been analysed. Moreover,direct numerical simulations have been performed to verify the asymptotic modeling hypotheses. The findings of this thesis allow to understand the acoustic modeling of perforated materials. Furthermore,the developed analytical method can be implemented for perforations with complex shape.

As aplicações aeroespaciais estão frequentemente associadas a escoamentos compressíveis com altíssimos números de Reynolds. No entanto, existem no contexto aeroespacial importantes aplicações que envolvem escoamentos compressíveis a Reynolds relativamente baixos. Entre eles se destacam o escoamento em pás de turbina a gás e ao redor de dispositivos de alta sustentação como eslates e flapes em grandes ângulos de ataque. Pode-se destacar também o processo de combustão supersônica que está intimamente ligado e é fortemente beneficiado pelo presente estudo. Nas aplicações aerodinâmicas em baixos números de Reynolds frequentemente uma parcela significativa do escoamento se apresenta no regime de transição para turbulência, ou nos estágios iniciais do escoamento turbulento. O objetivo do presente projeto é a simulação numérica direta de escoamentos compressíveis transicionais com desenvolvimento de um código para simulação em três dimensões de escoamentos alto subsônicos. O escoamento a ser estudado no projeto é a evolução linear e não linear de trens de onda e pacotes de onda em uma camada de mistura compressível. A solução das equações de Navier-Stokes é obtida através do método das diferenças finitas. As derivadas espaciais são resolvidas através de um método compacto de sexta ordem, enquanto que as derivadas temporais são resolvidas através do método de Runge-Kutta de quarta ordem. Os métodos de aproximação foram modificados para trabalhar com malhas não uniformes visando refinar a malha em pontos em que o fenômeno ocorre e, consequentemente, reduzir o custo computacional. A investigação numérica inicia-se com a análise da taxa de amplificação dos trens de ondas fortemente modulados em regime linear. Os resultados obtidos foram comparados favoravelmente com a teoria linear. Os testes foram estendidos para a análise não linear, e consequentemente, foi possível reproduzir os fenômenos clássicos de instabilidade hidrodinâmica através da evolução dos trens de ondas oblíquos. / Aerospace applications are frequently associated with compressible flows at relatively high Reynolds number. Nevertheless important applications involve compressible flows at relatively low Reynolds number in the aerospace context. Among them, the flow on gas turbine blades and high lift devices such as slats and flaps at high angle of attack are particulary important. Besides, progress in aeroespace research is dependent on developing more efficient propulsion systems. In aerodynamic applications at low Reynolds number, often a substancial portion of the flow is in the transition regime, or in the initial stages of a turbulent flow. The objective of the present study is the Direct Numerical Simulation of three-dimensional transition of compressible flows in a mixing layer. Inspired on the worked devoted to modulated waves, the current work investigates the linear and nonlinear temporal evolution of wavetrains in this phenomenon. The Navier-Stokes equations were solved with a sixth-order compact finite-difference schemes. The time integration was performed by a fourth-order Runge-Kutta scheme. Moreover, the methods to solve the spatial derivatives were modified to work with non-uniform grids. This technique was implemented with the objective to improve the resolution of the grid where the phenomenon occurs and to reduce the computational cost. The numerical investigation starts with an analysis of the growth rate of the wavetrains in linear regime to verify the numerical code. The results compared favourably with linear theory. Tests were also performed in the nonlinear regime to simulate the oblique wavetrains and it was possible to reproduce the classical hydrodynamic instability phenomena.

Faisant partie des recherches menées dans le cadre du développement du moteur cryogénique Vinci, prévu pour propulser le dernier étage d'Ariane 6, cette thèse a pour objectif la simulation numérique directe (DNS) de la croissance de bulles de vapeur en paroi.La réalisation de ce type de simulation nécessite que les effets physiques internes aux phases et les interactions entre phases soient correctement modélisés et résolus. Pour cela, des modèles et des schémas numériques adaptés à ce type d'écoulement sont mis au point et ce, couplés à des maillages suffisamment fins pour pouvoir résoudre la structure de l'écoulement et des zones de forts gradients, en particulier à l'interface. / Part of the researches concerning the Vinci cryogenic engine, created to propel the last part of Ariane 6, the main goal of this thesis is the direct numerical simulation of boiling flows.Simulation this kind of flow requires the modelling and the resolution of the multiple internal physical effects as well as phases interactions. To achieve this goal, we created models and numerical schemes suited to the boiling flows. The method is used on fine meshes in order to be able to resolve the flow structure, essentially at the interface.

Tsunamis are one of the most catastrophic natural events impacting coastal regions often generated by undersea earthquakes. Nevertheless, in enclosed basins, i.e., fjords, reservoirs and lakes, subaerial or submarine landslides can initiate devastating tsunamis with similar consequences. Although a subaerial or submarine landslide that impinges into a large water body can generate a tsunami, subaerial landslides are much more efficient tsunami generators than its counterpart. In this study we aim to integrate laboratory scale experiments of tsunami generation by subaerial landslide with numerical models. The work focuses on the numerical validation of two three-dimensional Navier-Stokes (3D-NS) models, FLOW-3D and our developed model TSUNAMI3D. The models are validated based on previous large scale laboratory experiments performed by a tsunami research team lead by Dr. Hermann Fritz, Georgia Institute of Technology. Three large scale landslide scenarios were selected from the set of laboratory experiments, namely, fjord like, headland and far field coastline. These scenarios showed that complex wave fields can be generated by subaerial landslides. The correct definition and evolution of the wave field are key to accurate modeling the ensuing tsunami and its effect in coastal regions. In this study, comparisons are performed between numerical results and laboratory experiments. Methodology and key parameters for soil rheology are defined for model validations. Results of the models are expected to be under the allowable errors indicated by the National Tsunami Hazard Mitigation Program (NTHMP), National Oceanic and Atmospheric Administration (NOAA) guidelines for validation of tsunami numerical models. The ultimate goal of this research is to obtain better tsunami calculation tools for real-world application of 3-D models for landslide tsunamis, which are necessary for the construction of inundation maps in the Gulf of Mexico and the Caribbean regions.

The Poiseuille Flow is the flow of a viscous incompressible fluid in a channel between two infinite parallel plates. The behaviour of flow is properly described by the well-known Navier-Stokes Equations. The fact that Navier-Stokes equations are partial differential equations makes their solution difficult. They can rarely be solved in closed form. On the other hand, numerical techniques can be applied successfully to the well-posed partial differential equations. In the present study pseudo-spectral method is implemented to analyze the Poiseuille Flow. The pseudo-spectral method is a high-accuracy numerical modelling technique. It is an optimum choice for the Poiseuille flow analysis due to the flows simple geometry. The method makes use of Fourier Transform and by handling operations in the Fourier space reduces the difficulty in the solution. Fewer terms are required in a pseudo-spectral orthogonal expansion to achieve the same accuracy as a lower order method. Karhunen-Lo&egrave / ve (KL) decomposition is widely used in computational fluid dynamics to achieve reduced storage requirements or construction of relatively low-dimensional models. In this study the KL basis is extracted from the flow field obtained from the direct numerical simulation of the Poiseuille flow.

La production pariétale de bulles de gaz et son impact sur la dynamique de la phase liquide en canal vertical est étudiée numériquement et expérimentalement. Dans un premier temps, un modèle de mélange 2D stationnaire est utilisé pour décrire l’évolution moyenne des panaches de gaz. Grâce à cette approche, un modèle de couche limite a pu être développé et a permis l’identification des nombres adimensionnels pertinents (analogues aux nombres de Rayleigh et de Prandtl pour la thermique) afin de caractériser les écoulements à bulles dispersées. Dans un second temps, un modèle Eulérien-Lagrangien 3D instationnaire, prenant en compte le couplage quadrilatéral (interactions bullesliquide et bullesbulles) est résolu par Simulation Numérique Directe (DNS) et permet ainsi une description plus fine de l’écoulement à l’échelle de la phase dispersée. Enfin, ces approches numériques sont complétées par des mesures de Spectroscopie d’Impédance Electrochimique (SIE) lors de la production de dihydrogène et de dioxygène par électrolyse alcaline. Les modèles d’écoulement proposés ici montrent globalement un très bon accord avec les résultats expérimentaux tirés de la littérature. Les approches homogènes et DNS présentent toutefois quelques disparités sur l’évaluation du taux de vide dans certaines conditions. Parallèlement, les mesures et simulations de SIE ont montré être clairement affectées par les évolutions du panache de bulles, les spectres d’impédance ont notamment mis en évidence une contribution basse fréquence fortement dépendante de la nature de la phase dispersée (taille de bulle et lois de dispersion). Les trois approches (modèle homogène, DNS et SIE) menées conjointement sont donc fortement complémentaires. Elles permettent non seulement une meilleure compréhension de la physique de l’écoulement diphasique, mais offrent aussi une capacité d’analyse de la pertinence des modèles existants tout en ouvrant la voie à leurs futures améliorations / The wall production of gas bubbles and its impact on the liquid dynamics in a vertical channel is studied by means of numerical simulations and experimentation. First, a 2D stationary mixture model is used to describe the averaged plumes evolutions. Through this approach, a boundary layer model has been developed and identified dimensionless numbers (Raleigh-like and Prandtl-like) characteristic of bubbly flows. Secondly, a 3D non-stationary four-way coupled (with bubblesliquid and bubblesbubbles interactions) Eulerian-Lagrangian model is solved by Direct Numerical Simulation (DNS) and allows a finer description of the two-phase flows at bubble-scale. Finally, the numerical methods are completed by Electrochemical Impedance Spectroscopy (EIS) measurements during hydrogen and oxygen production by alkaline electrolysis.The two-phase flow models are in good agreement with experimental results from literature. There are still some disparities between the homogeneous model and the DNS about the void fraction calculation under certain conditions, though. In the meantime, both EIS measurements and simulations were clearly affected by bubbles plume evolutions, the impedance spectra highlighted a low frequency contribution highly sensible to the nature of the dispersed phase (bubble size a dispersion laws). The 3 approaches (homogeneous model, DNS and EIS) used collectively are strongly complementary. They allow not only a better comprehension of the physics of the two-phase flow, but also serve the analysis of existing models while leading the way for further improvements

Submitted by Laison Junio da Silva Furlan (laisonfurlan@gmail.com) on 2018-09-11T21:45:47Z No. of bitstreams: 1 dissertacao_final.pdf: 2611301 bytes, checksum: b4c51f4e16b1f3e612c4d3a044c777c5 (MD5) / Approved for entry into archive by Claudia Adriana Spindola null (claudia@fct.unesp.br) on 2018-09-12T11:25:26Z (GMT) No. of bitstreams: 1 furlan_ljs_me_prud.pdf: 2445202 bytes, checksum: 1f1f2699158710f217b33ed602d0f51e (MD5) / Approved for entry into archive by Claudia Adriana Spindola null (claudia@fct.unesp.br) on 2018-09-12T11:36:28Z (GMT) No. of bitstreams: 1 furlan_ljs_me_prud.pdf: 2445202 bytes, checksum: 1f1f2699158710f217b33ed602d0f51e (MD5) / Made available in DSpace on 2018-09-12T11:36:28Z (GMT). No. of bitstreams: 1 furlan_ljs_me_prud.pdf: 2445202 bytes, checksum: 1f1f2699158710f217b33ed602d0f51e (MD5) Previous issue date: 2018-08-02 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O presente trabalho investiga a transição laminar-turbulenta devido a ondas de Tollmien-Schlichting para o escoamento de Poiseuille incompressível, bidimensional, de um fluido viscoelástico, utilizando a equação constitutiva Giesekus. A Teoria de Estabilidade Linear e a Simulação Numérica Direta são utilizadas para verificar a estabilidade de escoamentos de fluidos viscoelásticos a perturbações não estacionárias. Na análise LST a equação de Orr-Sommerfeld é modificada para um fluido viscoelástico e resolvida pelo método da estimativa (Shooting). Enquanto que, na formulação DNS, as equações de Navier-Stokes, juntamente com a equação constitutiva Giesekus, são resolvidas utilizando métodos de diferenças finitas compactas de alta ordem. Com o objetivo de avaliar as curvas neutras de estabilidade e as taxas de amplificação, diferentes simulações numéricas são realizadas variando-se os parâmetros adimensionais no modelo Giesekus e comparando com o fluido Newtoniano. As técnicas LST e DNS mostraram-se ferramentas eficientes na análise espacial da estabilidade de escoamentos viscoelásticos do tipo Giesekus, permitindo uma melhor compreensão da influência dos parâmetros adimensionais desses escoamentos e contribuindo com resultados originais na verificação da estabilidade de escoamentos viscoelásticos utilizando o fluido Giesekus. / The present work investigates the laminar-turbulent transition due to Tollmien-Schlichting waves for the incompressible two-dimensional Poiseuille flow of a viscoelastic fluid, using the Giesekus constitutive equation. Linear Stability Theory and Direct Numerical Simulation are used to verify the stability of viscoelastic fluid flows to unsteady disturbances. In the LST analysis, the Orr-Sommerfeld equation is modified to a viscoelastic fluid and solved by Shooting method. Whereas, in the DNS formulation, the Navier-Stokes equations with the Giesekus constitutive equation are solved using high-order compact finite difference methods. In order to evaluate the neutral stability curves and the amplification rates, different numerical simulations are performed by varying the dimensionless parameters in the Giesekus model and their results are compared with the Newtonian fluid. The LST and DNS techniques proved to be efficient tools to the spatial stability analysis of viscoelastic fluid flows of the Giesekus type, allowing a better comprehension of the dimensionless parameters influence of those flows, contributing with originals results to verification of the viscoelastics fluid flows stability using Giesekus fluid. / FAPESP: 2017/11068-6