Ce travail de recherche s’intéresse au problème de la stabilisation de flammes, et du transfert de chaleur résultant, dans les moteurs fusées cryogéniques. La stabilisation de flamme dans un moteur fusée est un phénomène critique, et toute instabilité peut conduire à des dégâts importants, voire à la destruction du lanceur et des satellites embarqués. Les moteurs (Vulcain 2 et Vinci) qui équipent Ariane 5, et la future Ariane 6, utilisent le couple hydrogène / oxygène, dont la grande impulsion spécifique permet, en minimisant la masse des ergols par rapport à la masse de la charge, une meilleure performance du lanceur. Pour réduire le volume de stockage, les ergols sont refroidis à des températures très basses de l’ordre de quelques dizaines de Kelvin. Ils alimentent une flamme dont la température maximale peut atteindre 3500K, générant de très forts gradients de température dans le fluide, et des flux de chaleur extrêmes dans les parties solides de l’injecteur. Pour prédire les flux de chaleur entre la flamme, l’injecteur et les ergols froids, l’approche de Simulation aux Grandes Echelles (SGE), pour reproduire l’écoulement réactif turbulent instationnaire, est couplée au calcul de thermique du solide dans l’injecteur. Cette approche est d’abord validée par comparaison à une expérience en conditions ambiantes, menée au Laboratoire EM2C (Paris). L’interaction flamme-paroi en présence de transfert de chaleur, qui est un mécanisme de base de la stabilisation de flamme, est ensuite étudiée pour différents niveaux de pression. Finalement une configuration représentative d’un injecteur coaxial de moteur fusée est simulée pour étudier la structure et les mécanismes de stabilisation de la flamme, ainsi que les flux de chaleur reçus par l’injecteur, en vue d’évaluer la fatigue thermique du système. / This research work deals with the problem of the flame stabilization in the context of high pressure liquid rocket engines. Flame stabilization in a rocket engine is a critical feature. An instability can lead to important damages of the engine or the destruction of the launcher and the satellite. The engines (Vulcain 2 and Vinci) of the Ariane 5, and the future Ariane 6, use the hydrogen/oxygen propellants. One characteristic of this couple is its high specific impulse. The launcher performance is linked to the ratio of the payload to the total mass of propellants. For volume reasons the propellants are stored at low temperature of the order of a few tens of Kelvin. When injected in the combustion chamber, their combustion releases a huge amount of heat leading to temperature of 3500K. In order to predict the heat transfer between the flame, the solid injector and the cold propellants the Large Eddy Simulation, which allows to capture the unsteady features of the flow, is used in association with a thermal solver for the injector. This approach is validated with a low pressure experiment conducted at Centrale Paris, then a basic 1D configuration allows to understand the phenomena of high pressure flame-wall interaction. Finally a configuration representative of a coaxial rocket engine injector allows to evaluate the structure and the stabilization mechanisms of a cryogenic flame, the heat flux and the temperature of the injector.

Afin de répondre aux enjeux environnementaux, les fabricants de turbine à gaz ont mis au point de nouveaux concepts de chambre de combustion plus propres et moins consommateurs. Ces technologies sont cependant plus sensibles aux instabilités de combustion, un couplage entre acoustique et flamme pouvant conduire à des niveaux dangereux de fluctuations de pression et de dégagement de chaleur. Les solveurs de Helmholtz sont une méthode numérique efficace pour prédire ces instabilités de combustion. Ils reposent sur la description d'un fluide non visqueux au repos, dont le comportement acoustique est régi par une équation d'Helmholtz thermoacoustique, résolue dans le domaine fréquentiel comme un problème aux valeurs propres. Le couplage flamme/acoustique est modélisé par une fonction de transfert du premier ordre entre les perturbations de dégagement de chaleur et la vitesse acoustique en un point de référence. Bien que performants, les solveurs de Helmholtz négligent l'interaction entre acoustique et vorticité aux coins, car celle-ci dépend d'effets visqueux. Cette interaction pourrait fortement amortir l'acoustique d'une chambre de combustion et la négliger revient à faire des prédictions trop pessimistes voire erronées. Par conséquent, une méthodologie a été mise au point afin d'inclure dans un solveur de Helmholtz l'effet d'interactions complexes entre acoustique et écoulement. Ces interactions étant compactes, elles sont modélisées par des matrices 2x2 et ajoutées au solveur comme des paires de conditions limites : les conditions limites de matrice. Grâce à cette méthodologie, les fréquences et modes d'une configuration académique non-réactive sont correctement calculées en présence de deux éléments où une telle interaction est forte: un orifice et un tourbilloneur. Afin d'être applicable aux chambres industrielles, deux extensions sont nécessaires. Premièrement, les surfaces de matrices doivent pouvoir être non-planes, afin de s'adapter aux géométries industrielles complexes. Pour cela, une procédure d'ajustement a été mise en place. La matrice est mesurée sur des surfaces planes et des transformations nondissipatives lui sont appliquées afin de la déplacer sur les surfaces non planes. Ces transformations peuvent être déterminées analytiquement ou calculées avec un solveur de propagation acoustique. Le deuxième point concerne le point de référence du modèle de flamme. En effet, celui-ci est souvent choisi à l'intérieur de l'injecteur ce qui pose problème si celui-ci est retiré du domaine de calcul et remplacé par sa matrice. Dans cette thèse, le point de référence est remplacé par une surface de référence. La méthodologie étendue est validée sur des configurations académiques puis appliquée à une chambre annulaire de Safran. Cette nouvelle méthodologie permet de constater que l'interaction écoulement/acoustique au niveau des trous de dilution et des injecteurs joue un effet important sur la stabilité de la chambre mais aussi sur la structure des modes. Les premiers résultats avec une surface de référence pour la flamme sont encourageants. / Environmental concerns have motivated turbine engine manufacturers to create new combustor designs with reduced fuel consumption and pollutant emissions. These designs are however more sensitive to a mechanism known as combustion instabilities, a coupling between flame and acoustics that can generate dangerous levels of heat release and pressure fluctuations. Combustion instabilities can be predicted at an attractive cost by Helmholtz solvers. These solvers describe the acoustic behavior of an inviscid fluid at rest with a thermoacoustic Helmholtz equation, that can be solved in the frequency domain as an eigenvalue problem. The flame/acoustics coupling is modeled, often with a first order transfer function relating heat release fluctuations to the acoustic velocity at a reference point. One limitation of Helmholtz solvers is that they cannot account for the interaction between acoustics and vorticity at sharp edges. Indeed, this interaction relies on viscous processes at the tip of the edge and is suspected to play a strong damping role in a combustor. Neglecting it results in overly pessimistic stability predictions but can also affect the spatial structure of the unstable modes. In this thesis, a methodology was developed to include the effect of complex flow-acoustic interactions into a Helmholtz solver. It takes advantage of the compactness of these interactions and models them as 2-port matrices, introduced in the Helmholtz solver as a pair of coupled boundary conditions: the Matrix Boundary Conditions. This methodology correctly predicts the frequencies and mode shapes of a nonreactive academic configuration with either an orifice or a swirler, two elements where flowacoustic interactions are important. For industrial combustors, the matrix methodology must be extended for two reasons. First, industrial geometries are complex, and the Matrix Boundary Conditions must be applied to non-plane surfaces. This limitation is overcome thanks to an adjustment procedure. The matrix data on non-plane surfaces is obtained from the well-defined data on plane surfaces, by applying non-dissipative transformations determined either analytically or from an acoustics propagation solver. Second, the reference point of the flame/acoustics model is often chosen inside the injector and a new reference location must be defined if the injector is removed and replaced by its equivalent matrix. In this work, the reference point is replaced by a reference surface, chosen as the upstream matrix surface of the injector. The extended matrix methodology is successfully validated on academic configurations. It is then applied to study the stability of an annular combustor from Safran. Compared to standard Helmholtz computations, it is found that complex flow-acoustic features at dilution holes and injectors play an important role on the combustor stability and mode shapes. First encouraging results are obtained with surfacebased flame models.

La conception de nouveaux moteurs impose de respecter des normes de sécurité concernant les performances d'allumage et de ré-allumage en conditions critiques. Des campagnes d'essais étant onéreuses, les industriels cherchent donc à disposer d'outils numériques fiables. Afin d'améliorer la simulation des écoulements, le caractère multicomposant du carburant doit être pris en compte. L'objectif de cette thèse est d'étudier l'influence de l'évaporation d'un brouillard de gouttes sur un écoulement réactif. Pour cela, une étude de la propagation d'une flamme laminaire 1D est réalisée à l'aide d'un code de calcul multiphysique (CEDRE). Un train continu de gouttes monodisperse est injecté, les gouttes étant mono ou bicomposant. L'influence de la dynamique d'évaporation sur la combustion est étudiée. Deux cinétiques chimiques réduites multicomposant sont comparées. La composition, le diamètre et la richesse initiale des gouttes ont un impact sur la structure de flamme, la vitesse de flamme et la composition des gaz brûlés. Ensuite, l'effet de l'évaporation est étudié en phase d'allumage pour un brouillard de gouttes polydisperses monocomposant avec un modèle de noyau d'allumage local. L’écoulement instationnaire non-réactif dans un secteur de chambre industriel (MERCATO) est calculé avec une approche LES. Le caractère instationnaire, voire périodique, de la phase dispersée est mis en évidence en certains points de l'écoulement. Les résultats, associés au modèle d'allumage et à des critères, sont utilisées pour réaliser une carte de probabilité d'allumage. Des essais de calcul d'allumage complet de la chambre sont réalisés. Les résultats indiquent une surestimation des termes sources liés à l'évaporation de la phase dispersée et à la combustion. / The design of new aircraft engines needs in particular to comply with safety standards for the performance of stabilized combustion and ignition or re-ignition under critical conditions. Experimental campaigns are expensive, so numerical tools are needed. To improve the accuracy of the models used to simulate flow, the multicomponent nature of the fuel must be taken into account, whether it is kerosene or alternative fuel. The objective of this thesis is to study the influence of a droplet mist vaporization on a reactive flow. For this, an academic study of the propagation of a 1D laminar flame is performed using a CFD code {CEDRE). A continuous stream of monodisperse droplets is injected, the droplets being mono or bicomponent. The influence of the dynamics of evaporation on combustion is particularly studied. Two reduced multicomponent chemical kinetics are compared. The composition, the diameter and the initial equivalent ratio of droplets have an impact on the structure of the flame, the flame speed and composition of the burnt gases. A local ignition kernel model is applied to study the influence ofvaporization on ignition in the case of monocomponent, polydisperse droplets. Experimental data are available for a monosector combustion chamber (MERCATO) so the non-reactive unsteady flow is simulated with a LES approach. The unsteady, sometimes periodic, nature of the dispersed phase is highlighted in some points of the flow. A ignition model is applied to instantaneous flow fields and criteria are analysed to realise an ignition probability map which validates the approach. Finally, ignition of a combustion chamber is tested. The results point out an overestimation of source terms related to the evaporation of the dispersed phase and combustion.

L'impact environnemental du trafic aérien fait maintenant l'objet d'une réglementation qui tend à se sévériser. Dans ce contexte, les industriels misent sur l'amélioration des technologies afin de réduire la consommation de carburant et l'émission de polluants. Ces phénomènes dépendent en grande partie des chemins réactionnels sous-jacents, qui peuvent s'avérer très complexes. La Simulation aux Grandes Échelles (SGE) est un outil intéressant afin d'étudier ces phénomènes pour un coût de calcul qui reste raisonnable. Cependant, les processus chimiques, s'ils sont considérés sans simplification, font intervenir des centaines d'espèces aux temps caractéristiques très différents au sein de processus non-linéaires qui induisent une forte raideur dans le système d'équations, et un coût de calcul prohibitif. Permettant de s'absoudre de ces problèmes tout en conservant une bonne capacité de prédiction des polluants, les Chimies Analytiquement Réduites (CAR) font l'objet d'une attention grandissante au sein de la communauté. Les CAR permettent de conserver la physique du problème considéré, en conservant les espèces et voies réactionnelles les plus importantes. Grâce à l'évolution toujours croissante des moyens de calculs, les CAR sont appliqués dans des configurations de plus en plus complexes. Les travaux de thèse ont principalement portés sur deux sujets. Premièrement, une étude poussée des techniques et outils permettant une réduction efficace et systématique de chimies détaillées. L'outil de réduction multiétapes YARC est retenu et exhaustivement employé dans la dérivation et la validation d'une série de CAR préservant la description de la structure de flamme. Ensuite, une investigation de la faisabilité et des bénéfices qu'apportent l'utilisation de CAR en LES, comparé a des approches plus classiques, sur des cas tests de complexité croissante. La première configuration étudiée est une chambre de combustion partiellement pré-mélangée brûlant de l'éthylène, étudiée expérimentalement au DLR. Différentes modélisations de la chimie sont considérées, dont un CAR développé spécifiquement pour ce cas test, et les résultats démontrent qu'une prise en compte des interactions flamme-écoulement est cruciale pour une prédiction juste de la structure de la flamme et des niveaux de suies. La seconde configuration est un brûleur diphasique, avec une injection directe pauvre, brûlant du Jet-A2. Dans cette étude, une approche novatrice pour la prise en compte de la complexité du fuel réel (HyChem) est considérée, permettant la dérivation d’un CAR. Les résultats sont excellents et valident la méthodologie tout en fournissant une analyse précieuse des interactions flamme-spray et de la formation de polluants (NOx) dans des flammes à la structure complexe. / Recent implementation of emission control regulations has resulted in a considerable demand from industry to improve the efficiency while minimizing the consumption and pollutant emissions of the next generation of aero-engine combustors. Those phenomena are shown to strongly depend upon the underlying complex chemical pathways and their interaction with turbulence. Large Eddy Simulation (LES) is an attractive tool to address those issues with high accuracy at a reasonable computing cost. However, the computation of accurate combustion chemistry remains a challenge. Indeed, combustion proceeds through complex and highly non-linear processes that involve up to hundreds of different chemical compounds, which significantly increases the computational time and often induces stiffness in the resolved equations. As a mean to circumvent these drawbacks while retaining the necessary kinetics for the prediction of pollutants, Analytically Reduced Chemistry (ARC) has recently received high interest in the Computational Fluid Dynamics (CFD) community. ARC is a strategy for the description of combustion chemistry where only the most important species and reactions are retained, in a "physically-oriented way". ARC is on the verge of becoming affordable at a design stage, thanks to the continuously increasing available computational resources. The goal of the present work is twofold. A first objective is to test and validate efficient techniques and tools by which detailed chemistries are reduced to an LES-compliant format. To do so, the multi-step reduction tool YARC is selected and employed to derive and validate a series of ARC specifically designed to retrieve correct flame structures. A second objective is to investigate the overall feasibility and benefits of using ARC, combined to the Thickened Flame model (DTFLES), in performing LES of configurations of increasing complexity. The first configuration is a sooting swirl-stabilized non-premixed aero-engine combustor experimentally studied at DLR, burning ethylene. LES of this configuration is performed with the AVBP solver, in which ARC has been implemented. By comparison with global chemistry and tabulated chemistry, results highlight the importance of accurately capturing the flow-flame interactions for a good prediction of pollutants and soot. The second configuration is a swirled twophase flow burner featuring a lean direct injection system and burning Jet-A2. A novel methodology to real fuel modeling (HyChem approach) is employed, which allows subsequent ARC derivation. The excellent results in comparison with measurements constitute an additional validation of the methodology, and provide valuable qualitative and quantitative insights on the flame-spray interactions and on the pollutant formation (NOx) mechanisms in complex flame configurations.

A interação turbulência-radiação (TRI, do inglês Turbulence-Radiation Interaction) resulta do acoplamento altamente não linear entre flutuações da intensidade de radiação e flutuações da temperatura e da composição química do meio, e tem-se demonstrado experimentalmente, teoricamente e numericamente que este é um fenômeno relevante em diversas aplicações envolvendo altas temperaturas, especialmente em problemas reativos. Neste trabalho, o TRI é analisado em um escoamento interno não reativo de um gás participante que se desenvolve em um duto de seção transversal quadrada, para diferentes intensidades de turbulência do escoamento e considerando duas espécies distintas para a composição do fluido de trabalho (dióxido de carbono e vapor de água). O objetivo central é avaliar como a inclusão ou não da variação espectral das propriedades radiativas do meio no cálculo influencia a magnitude do TRI. Isso é feito através de simulações numéricas no código de dinâmica dos fluidos computacional Fire Dynamics Simulator (FDS), que resolve, através do método dos volumes finitos, as equações fundamentais que regem o problema – isto é, os balanços de massa, de quantidade de movimento e de energia e a equação de estado – em uma formulação adequada para baixos números de Mach, utilizando um algoritmo de solução explícito e de segunda ordem no tempo e no espaço. A turbulência é modelada através da simulação de grandes escalas (LES, do inglês Large Eddy Simulation), empregando-se o modelo de Smagorinsky dinâmico para o fechamento dos termos submalha; para a radiação térmica, o método dos volumes finitos é utilizado na discretização da equação da transferência radiativa e os modelos do gás cinza e da soma-ponderada-de-gases-cinza (WSGG, do inglês Weighted-Sum-of-Gray-Gases) são implementados como forma de desconsiderar e de incluir a dependência espectral das propriedades radiativas, respectivamente. A magnitude do TRI sobre o problema é avaliada através de diferenças entre as médias temporais dos fluxos de calor superficiais e do termo fonte radiativo obtidas em cálculos que consideram os efeitos do fenômeno e cálculos que os negligenciam. Em geral, a interação turbulência-radiação mostrou ser pouco importante em todos os casos considerados, o que concorda com resultados de outros estudos sobre o tema em escoamento não reativos. Com o modelo WSGG, as contribuições do fenômeno foram maiores do que com a hipótese do gás cinza, evidenciando que a inclusão da variação espectral na solução do problema radiativo tem um impacto sobre a magnitude dos efeitos do TRI. Além disso, é feita uma discussão, em parte inédita no contexto do TRI, sobre diferentes metodologias para a análise do fenômeno. Finalmente, é proposto um fator de correção para o termo fonte radiativo médio no modelo WSGG, que é validado através de sua implementação nos casos simulados. Em estudos futuros, uma análise de sensibilidade sobre os termos constituintes desse fator de correção pode levar a um melhor entendimento de como as flutuações de temperatura se correlacionam com o fenômeno da interação turbulência-radiação. / Turbulence-radiation interaction (TRI) results from the highly non-linear coupling between fluctuations of radiation intensity and fluctuations of temperature and chemical composition of the medium, and its relevance in a number of high-temperature problems, especially when chemical reactions are included, has been demonstrated experimentally, theoretically, and numerically. In the present study, the TRI is analyzed in a channel flow of a non-reactive participating gas for different turbulence intensities of the flow at the inlet and considering two distinct species for the medium composition (carbon dioxide and water vapor). The central objective is to evaluate how the inclusion or not of the spectral variation of the radiative properties of a participating gas in the radiative transfer calculations affects the turbulence-radiation interaction. With this purpose, numerical simulations are performed using the computational fluid dynamics Fortranbased code Fire Dynamics Simulator, that employs the finite volume method to solve a form of the fundamental equations – i.e., the mass, momentum and energy balances and the state equation – appropriate for low Mach number flows, through an explicit second-order (both in time and in space) core algorithm. Turbulence is modeled by the large eddy simulation approach (LES), using the dynamic Smagorinsky model to close the subgrid-scale terms; for the thermal radiation part of the problem, the finite volume method is used for the discretization of the radiative transfer equation and the gray gas and weighted-sum-of-gray-gases (WSGG) models are implemented as a way to omit and consider the spectral dependence of the radiative properties, respectively. The TRI magnitude in the problem is evaluated by differences between values for the time-averaged heat fluxes at the wall (convective and radiative) and for the time-averaged radiative heat source calculated accounting for and neglecting the turbulence-radiation interaction effects. In general, TRI had little importance over all the considered cases, a conclusion that agrees with results of previous studies. When using the WSGG model, the contributions of the phenomenon were greater that with the gray gas hypothesis, demonstrating that the inclusion of the spectral variance in the solution of the radiative problem has an impact in the TRI effects. Furthermore, this paper presents a discussion, partly unprecedented in the context of the turbulence-radiation interaction, about the different methodologies that can be used for the TRI analysis. Finally, a correction factor is proposed for the time-averaged radiative heat source in the WSGG model, which is then validated by its implementation in the simulated cases. In future studies, a sensibility analysis on the terms that compose this factor can lead to a better understanding of how fluctuations of temperature correlate with the turbulence-radiation interaction phenomenon.

O presente trabalho tem como objetivo estudar escoamentos de tornados e sua ação sobre corpos imersos empregando ferramentas numéricas da Engenharia do Vento Computacional (EVC). Os tornados constituem-se atualmente em uma das causas de desastres naturais no Brasil, especialmente nas regiões sul e sudeste do país, como também em alguns países vizinhos. Os efeitos gerados são geralmente localizados e de curta duração, podendo ser devastadores dependendo da escala do tornado. Tais características dificultam a realização de estudos detalhados a partir de eventos reais, o que levou ao desenvolvimento de modelos experimentais e numéricos. A abordagem numérica é utilizada neste trabalho para a simulação de tornados, a qual se baseia nas equações de Navier-Stokes e na equação de conservação de massa, considerando a hipótese de pseudo-compressibilidade e condições isotérmicas. Para escoamentos com turbulência utiliza-se a Simulação Direta de Grandes Escalas com o modelo clássico de Smagorinsky para as escalas inferiores à resolução da malha (Large Eddy Simulation ou LES em inglês). A discretização das equações fundamentais do escoamento se realiza com um esquema explícito de dois passos de Taylor-Galerkin, onde o Método dos Elementos Finitos é empregado na discretização espacial utilizando-se o elemento hexaédrico trilinear isoparamétrico com um ponto de integração e controle de modos espúrios Na presença de corpos imersos que se movem para simular os deslocamentos dos tornados, o escoamento é descrito cinematicamente através de uma formulação Arbitrária Lagrangeana-Euleriana (ALE) que inclui um esquema de movimento de malha. Tornados são reproduzidos através da simulação numérica de dispositivos experimentais e do Modelo de Vórtice Combinado de Rankine (RCVM). Exemplos clássicos da Dinâmica dos Fluidos Computacional são apresentados inicialmente para a verificação das ferramentas numéricas implementadas. Finalmente, problemas envolvendo tornados móveis e estacionários são analisados, incluindo sua ação sobre corpos imersos. Nos modelos baseados em experimentos, a variação da relação de redemoinho determinou os diferentes padrões de escoamento observados no laboratório. Nos exemplos de modelo de vórtice, quando o tornado impactou o corpo imerso gerou picos de forças em todas as direções e, após a passar pelo mesmo, produziu uma alteração significativa na estrutura do vórtice. / Analyses of tornado flows and its action on immersed bodies using numerical tools of Computational Wind Engineering (CWE) are the main aims of the present work. Tornadoes are currently one of the causes of natural disasters in Brazil, occurring more frequently in the southern and southeastern regions of the country, as well as in some neighboring countries. Effects are usually localized, presenting a short time interval, which can be devastating depending on the scale of the tornado. These characteristics difficult to carry out detailed studies based on real events, leading to the development of experimental and numerical models. The numerical approach is used in this work for the simulation of tornadoes, which is based on the Navier-Stokes equations and the mass conservation equation, considering the hypothesis of pseudo-compressibility and isothermal conditions. For turbulent flows, Large Eddy Simulation (LES) is used with the classical Smagorinsky model for sub-grid scales Discretization is performed the explicit two-step Taylor-Galerkin scheme, where the Finite Element Method is used in spatial discretization using isoparametric trilinear hexahedral elements with one-point quadrature and hourglass control. In the presence of immersed bodies that are moving in order to simulate translating tornadoes, the flow is kinematically described through a Lagrangian-Eulerian Arbitrary (ALE) formulation, which includes a mesh motion scheme. Tornadoes are reproduced using numerical simulation of experimental devices and the Rankine Combined Vortex Model (RCVM). Classical examples of Computational Fluid Dynamics are presented initially for the verification of the numerical tools implemented here. Finally, problems involving moving and stationary tornadoes are analyzed, including their actions on immersed bodies. For models based on experiments, the variation of the swirl ratio determined the different flow patterns observed in the laboratory. In the vortex model examples, when the tornado impacted on the immersed body, peaks of forces were generated in all directions and, after passing over it, a significant change in the structure of the vortex was produced.

abstract: This thesis focuses on an improved understanding of the dynamics at different length scales of wind farms in an atmospheric boundary layer (ABL) using a series of visualization studies and Fourier, wavelet based spectral analysis using high fidelity large eddy simulation (LES). For this purpose, a robust LES based neutral ABL model at very high Reynolds number has been developed using a high order spectral element method which has been validated against the previous literature. This ABL methodology has been used as a building block to drive large wind turbine arrays or wind farms residing inside the boundary layer as documented in the subsequent work. Studies conducted in the thesis involving massive periodic wind farms with neutral ABL have indicated towards the presence of large scale coherent structures that contribute to the power generated by the wind turbines via downdraft mechanisms which are also responsible for the modulation of near wall dynamics. This key idea about the modulation of large scales have seen a lot of promise in the application of flow past vertically staggered wind farms with turbines at different scales. Eventually, studies involving wind farms have been progressively evolved in a framework of inflow-outflow where the turbulent inflow is being fed from the precursor ABL using a spectral interpolation technique. This methodology has been used to enhance the understanding related to the multiscale physics of wind farm ABL interaction, where phenomenon like the growth of the inner layer, and wake impingement effects in the subsequent rows of wind turbines are important owing to the streamwise heterogeneity of the flow. Finally, the presence of realistic geophysical effects in the turbulent inflow have been investigated that influence the flow past the wind turbine arrays. Some of the geophysical effects that have been considered include the presence of the Coriolis forces as well as the temporal variation of mean wind magnitude and direction that might occur due to mesoscale dynamics. This study has been compared against field experimental results which provides an important step towards understanding the capability of the mean data driven LES methodology in predicting realistic flow structures. / Dissertation/Thesis / Turbulent flow past a large periodic wind farm / Doctoral Dissertation Mechanical Engineering 2018

Orientador: João Batista Campos Silva / Banca: Ricardo Alan Verdú Ramos / Banca: Edson Luiz Zaparoli / Resumo: O objetivo principal deste trabalho é a simulação numérica de escoamentos de fluidos incompressíveis pelo método de elementos finitos baseado em volumes de controle (CVFEM) utilizando a metodologia de simulação das grandes escalas. As equações governantes são filtradas para a simulação das variáveis de grandes escalas e as escalas sub-malhas, que aparecem devido ao processo de filtragem, são modeladas por meio do modelo de viscosidade turbulenta de Smagorinsky. O domínio é discretizado em malha não estruturada formada por elementos finitos triangulares de seis nós e as equações são integradas em volumes de controle formados em torno dos nós dos elementos. O presente código numérico foi validado aplicando-o a alguns problemas-testes e os resultados, comparados com os disponíveis na literatura. Os casos testes foram o escoamento em uma cavidade quadrada induzido pelo movimento da parede superior, e escoamento por convecção natural em uma cavidade quadrada. Os resultados obtidos, no presente trabalho, concordaram com os resultados da literatura. / Abstract: The main purpose of this work is the numerical simulation of incompressible fluid flows by a control volume finite element method (CVFEM) using the methodology of large-eddy simulation. The domain is discretized using unstructured mesh of six-noded triangular finite elements and the equations are integrated into control volumes around the nodes of the finite elements. The government equations are filtered for simulation of the large scales variables and the sub-grid scales appearing due to the filtering process are modeled through the eddy viscosity model of Smagorinsky. Two-dimensional benchmark problems are solved to validate the numerical code and the results are presented and compared with available results from the literature. The test cases were the lid-driven cavity flow and natural convection flow inside a square cavity. The obtained results, in the present work, agree with results from the literature. / Mestre

Made available in DSpace on 2014-06-11T19:23:39Z (GMT). No. of bitstreams: 0 Previous issue date: 2005-02-22Bitstream added on 2014-06-13T20:50:45Z : No. of bitstreams: 1 lima_rc_me_ilha.pdf: 3645930 bytes, checksum: d7ee96dea4a2ba8427f6366d87a818cc (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O objetivo principal deste trabalho é a simulação numérica de escoamentos viscosos, incompressíveis e transientes, com transferência de calor e massa; através do método de elementos finitos de subdomínio; usando a metodologia de simulação de grandes escalas para a modelagem da turbulência. Algumas aplicações de interesse são as simulações de escoamentos com transporte de um escalar, como nos casos de dispersão de poluentes induzida pelo movimento do ar atmosférico. O domínio é discretizado usando elementos finitos quadrilaterais de nove nós e as equações são integradas em volumes de controle ao redor dos nós dos elementos finitos. As equações governantes passam por um processo de filtragem, devido à metodologia aplicada, Simulação de Grandes Escalas (LES - Large-Eddy Simulation), e desta forma as maiores escalas são resolvidas diretamente através da solução das equações de Navier-Stokes filtradas, enquanto que as menores escalas ou escalas submalhas são modeladas, pelo modelo de viscosidade turbulenta de Smagorisnky. Alguns casos testes bidimensionais clássicos são resolvidos para validação do código e os resultados são apresentados e comparados com resultados disponíveis na literatura. Alguns poucos casos de dispersão de poluentes em geometrias que simulam cânions de ruas (urban street canyons) foram também simulados. / The main purpose of this work is the numerical simulation of viscous, incompressible and unsteady fluid flows by a sub-domain finite element method, using the methodology of large-eddy simulation (LES) for turbulence modeling. Some applications of interest are isothermal and thermal flows with transport of scalar variable such as the pollutant dispersion in the atmosphere by airflow. The domain is discretized using nine-nodes quadrilateral finite elements and the equations are integrated into control volumes around the nodes of the finite elements. The government equations are submitted to a filtering process for application of LES methodology, in which the large scales are directly solved using the filtered Navier-Stokes equations, while the small or sub-grid scales are modeled by the eddy viscosity model of Smagorinsky. Two-dimensional benchmark problems are solved to validate the numerical code and the results are presented and compared with available results from the literature. Some cases of pollutant dispersion in geometries that simulate urban street canyons have been also simulated.

Made available in DSpace on 2014-06-11T19:23:39Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-12-18Bitstream added on 2014-06-13T18:48:30Z : No. of bitstreams: 1 zachi_jm_me_ilha.pdf: 1606668 bytes, checksum: df36f4d5d30c079b2b8faf62fd202730 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O objetivo principal deste trabalho é a simulação numérica de escoamentos de fluidos incompressíveis pelo método de elementos finitos baseado em volumes de controle (CVFEM) utilizando a metodologia de simulação das grandes escalas. As equações governantes são filtradas para a simulação das variáveis de grandes escalas e as escalas sub-malhas, que aparecem devido ao processo de filtragem, são modeladas por meio do modelo de viscosidade turbulenta de Smagorinsky. O domínio é discretizado em malha não estruturada formada por elementos finitos triangulares de seis nós e as equações são integradas em volumes de controle formados em torno dos nós dos elementos. O presente código numérico foi validado aplicando-o a alguns problemas-testes e os resultados, comparados com os disponíveis na literatura. Os casos testes foram o escoamento em uma cavidade quadrada induzido pelo movimento da parede superior, e escoamento por convecção natural em uma cavidade quadrada. Os resultados obtidos, no presente trabalho, concordaram com os resultados da literatura. / The main purpose of this work is the numerical simulation of incompressible fluid flows by a control volume finite element method (CVFEM) using the methodology of large-eddy simulation. The domain is discretized using unstructured mesh of six-noded triangular finite elements and the equations are integrated into control volumes around the nodes of the finite elements. The government equations are filtered for simulation of the large scales variables and the sub-grid scales appearing due to the filtering process are modeled through the eddy viscosity model of Smagorinsky. Two-dimensional benchmark problems are solved to validate the numerical code and the results are presented and compared with available results from the literature. The test cases were the lid-driven cavity flow and natural convection flow inside a square cavity. The obtained results, in the present work, agree with results from the literature.