O objetivo deste trabalho é estudar, dentro da Engenharia do Vento Computacional (EVC), a dispersão de poluentes em zonas urbanas, empregando-se um modelo numérico baseado em técnicas da Dinâmica dos Fluidos Computacional para escoamentos incompressíveis, não isotérmicos e com transporte de massa. Um esquema explícito de dois passos é usado para a discretização temporal das equações governantes, considerando expansões em séries de Taylor de segunda ordem para as derivadas no tempo. O processo de discretização espacial é realizado através da aplicação do Método dos Elementos Finitos (MEF), onde hexaedros de oito nós com um ponto de integração são utilizados. A turbulência é tratada numericamente através da Simulação de Grandes Escalas (LES) e os modelos clássico e dinâmico de Smagorinsky são empregados na modelagem das escalas inferiores à resolução da malha. Efeitos de temperatura sobre o escoamento são considerados na forma de forças de flutuação presentes na equação de balanço de momentum, as quais são calculadas a partir da aproximação de Boussinesq. Técnicas de paralelização em memória compartilhada (OpenMP) são também usadas a fim de melhorar a eficiência computacional do presente modelo para problemas com grande número de elementos. Exemplos clássicos de Dinâmica de Fluidos e Fenômenos de Transporte são inicialmente analisados para teste das ferramentas numéricas implementadas. Problemas de dispersão de poluentes com e sem a inclusão dos efeitos de temperatura são abordados para configurações geométricas bi e tridimensionais de street canyons, representando a unidade geométrica básica encontrada em centros urbanos de grandes cidades. / The main goal of the present work is to study the pollutant dispersion in urban areas using a numerical model based on techniques developed by Computational Fluid Dynamics, where applications of Computational Wind Engineering (CWE) are analyzed considering incompressible flows with heat and mass transport. A two-step explicit scheme is adopted for the time discretization of the governing equations considering second order Taylor series expansions of the time derivative terms. Spatial discretization is performed by applying the Finite Element Method (FEM), where eight-node hexahedral elements with one-point quadrature are utilized. Turbulence is numerically analyzed by using Large Eddy Simulation (LES) with the classical and dynamic Smagorinsky’s models for subgrid scale modeling. Thermal effects on the flow field are taken into account through buoyancy forces acting on the momentum balance equation, which are calculated considering the Boussinesq approximation. Shared memory parallelization techniques (OpenMP) are also employed in order to improve computational efficiency for problems with large number of elements. Classic examples of Fluid Dynamics and Transport Phenomena are first analyzed to verify the numerical tools implemented. Problems involving pollutant dispersion with and without the inclusion of thermal effects are investigated for two and three-dimensional geometric configurations of street canyons, which represent the basic geometric unit observed in urban centers of large cities.

O experimento Unsteady Aerodynamics Experiment Phase VI, realizado no ano de 2000 pelo Laboratório Nacional norte-americano para as Energias Renováveis (NREL) no túnel de vento Ames da NASA, foi reproduzido numericamente neste trabalho. O objetivo é o estudo das características da esteira aerodinâmica produzida pela turbina eólica de duas pás e 10 metros de diâmetro, operando à velocidade de rotação constante de 72 RPM, sujeita a uma velocidade de corrente livre do vento uniforme de 9 m/s, em um túnel de vento cuja seção de testes mede 36,6 m de largura por 24,4 m de altura e o comprimento mede 170 m. Para isso, foi utilizado o programa comercial ANSYS FLUENT versão 13.0, baseado no Método dos Volumes Finitos para a solução numérica das Equações de Navier-Stokes em regime transiente em conjunto com a Simulação de Grandes Escalas (SGE) para resolver a turbulência. As geometrias de todos os componentes da máquina foram criadas em software CAD. Um domínio móvel em forma de disco, contendo as pás do rotor e o hub da máquina, foi criado separadamente, e posteriormente inserido no domínio principal, estático, usando a ferramenta Moving Mesh disponível no software FLUENT. Ambos os domínios foram preenchidos por malhas compostas por tetraedros. Dados provenientes das simulações numéricas foram comparados aos dados experimentais de velocidade fornecidos por dois anemômetros sônicos instalados 5,8 m à jusante do rotor, ao que foi verificada boa concordância, com diferenças da ordem de 1% para o anemômetro 1 e 6% para o anemômetro 2. Resultados de velocidade na linha de centro do túnel e perfis de velocidade à jusante foram comparados com recente estudo numérico, e revelam diferenças importantes entre dados obtidos pela SGE, principalmente no que se refere à detecção de picos e flutuações relacionados às escalas turbulentas, e dados obtidos através da modelagem clássica da turbulência, RANS. As perturbações ultrapassaram a marca dos 10 diâmetros à jusante e atingiram o final do domínio localizado a 15 diâmetros. A esteira não apresentou simetria axial, e o ponto de maior redução na velocidade do escoamento foi detectado fora da linha de centro do rotor. / The Unsteady Aerodynamics Experiment Phase VI, which has been carried out in 2000 by the US National Renewable Energy Laboratory (NREL) at the NASA Ames wind tunnel, has been numerically reproduced. The purpose of this work is to study the characteristics of the wind wake produced by the 10 meter two bladed wind turbine, operating at a constant rotational speed of 72 RPM, subject to a free stream wind velocity of 9 m/s, inside a wind tunnel in which dimensions are 36.6 m in width, 24.4 m in height and length of 170 m. To achieve that, the ANSYS FLUENT version 13.0 commercial code, based in the Finite Volume Method to numerically solve the Navier-Stokes equations in transient state, has been used, together with the Large Eddy Simulation (LES) to characterize the turbulence. Geometries of all the machine components have been created in CAD software. A disc shaped moving domain, containing the blades and hub, has been created separately, and later inserted into the main, static domain, using the Moving Mesh tool available in the software. Both domains have been filled with meshes composed by tetrahedra. Data collected at the numerical simulations have been compared to experimental wind speed data provided by two sonic anemometers installed 5.8 m downstream from the rotor, for which a good agreement has been found, with differences of approximately 1% to the anemometer 1 and 6% to the anemometer 2. Results of wind velocity at the tunnel centerline and velocity profiles downstream have been compared with recent numerical study, and show important differences between data obtained by LES, especially with regard to the detection of peaks and fluctuations related to the turbulent scales, and data obtained by the classic turbulence modeling, RANS. Disturbances have passed the 10 diameter mark and reached the end at the domain located at 15 diameters. The wake did not show axial symmetry and the point of maximum reduction in the flow speed was detected outside the rotor centerline.

Made available in DSpace on 2014-08-20T14:25:46Z (GMT). No. of bitstreams: 1 dissertacao_rodrigo_lovato_paginas_pre_e_pos_textuais.pdf: 119173 bytes, checksum: e57cbc1af9ef2f4d32dabdf2e4714175 (MD5) Previous issue date: 2007-02-28 / Neste trabalho, nós estimamos a constante de Kolmogorov, C01 através da determinação de um conjunto de trajetórias Lagrangeanas em uma Camada Limite Estável simulada pelo modelo de simulação dos grandes turbilhões (Large-Eddy Simulation model). A partir do conjunto de trajetórias, a forma da função de estrutura da velocidade Lagrangeana no subintervalo inercial é avaliada, permitindo a determinação de C0. O valor de C0 encontrado é 3,70.

This thesis is concerned with studies on spatially developing high speed mixing layers with twin objectives: (a) to provide enhanced and detailed understanding of spatial development of two-dimensional mixing layer emanating from splitter plate through large eddy simulation (LES, from now on) technique and (b) to evolve a consistent strategy for Unsteady Reynolds Averaged Navier-Stokes (URANS) approach to mixing layer calculations. The inspiration for this work arose out of the explanations that were being developed for the reduction in the mixing layer thickness with compressibility (measured by a parameter called convective Mach number, Mc). The reasons centered around increased stability, increase in compressible dissipation that was later discounted in favor of reduction in production and pressure-strain terms (with Mc, of course). These were obtained with direct numerical simulations (DNS) or LES techniques with homogeneous shear flow or temporal mixing layer. As apart, there was also a wide held view that using RANS (steady) techniques did not capture the compressibility effects when used in a way described above and so classical industrial codes for computing mixing- layer-embedded flows are unsuitable for such applications. Other important aspects that come out of the examination of literature are: the mixing layer growth is controlled in the initial stages by the double- boundary layer profile over the splitter plate and results in the mixing layer growth that is somewhat irregular due to doubling and merging of vertical structures. The view point of a smooth growth of the mixing layer is a theo- retical approximation arising out of the use of a smooth tan-hyperbolic profile that results at larger distances from the splitter plate. For all practical applications, it is inferred that the initial development is what is important because the processes of ignition and stable combustion occur close to the splitter plate. For these reasons, it was thought that understanding the development of the mixing layer is best dealt with using accurate spatial simulation with the appropriate initial profile. The LES technique used here is drawn from an OpenFOAM approach for dissimilar gases and uses one-equation Eddy Model for SGS stresses. The temporal discretization is second order accurate backward Euler and spatial discretization is fourth order least squares; the algorithm used for solving the equations is PISO and the parallelized code uses domain decomposition approach to cover large spatial domain. The calculations are performed with boundary layer profiles over the splitter plate and an initial velocity field with white noise-like fluctuations to simulate the turbulence as in the experiments. Grid independence studies are performed and several experimental cases are considered for comparison with measured data on the velocity and temperature fields as well as turbulent statistics. These comparisons are excellent for the mean field behavior and moderately acceptable for turbulent kinetic energy and shear stress. To further benefit from the LES approach, the details of the mixing layer are calculated as a function of four independent parameters on which the growth depends: convective Mach number (Mc = (U1 -U2)/ (a1 +a2)), stream speed ratio (r = U2=U1), stream density ratio (s = p2/p1) and the average velocity of the two streams ((U1+U2)=2) and examine the various terms in the equations to enable answering the questions discussed earlier. It is uncovered that r has significant influence on the attainment of self similarity (which also implies on the rate of removal of velocity defect in the double-boundary layer profile) and other parameters have a very weak influence. The minimum velocity variation with distance from the splitter plate has the 1/paxial distance behavior like in wakes; however, after a distance, departure to linear rise occurs and the distance it takes for this to appear is delayed with Mc. Other features such as the coherent structures, their merger or break up, the area of the structures, convective velocity information extraction from the coherent structures, the behavior of the pressure field in the mixing layer through the field are elucidated in detail; the behavior of the correlations between parameters (like pressure, velocity etc) at different points is used to elucidate the coherence of their fluctuating field. The effects of the parameters on the energy spectra have expected trends. An examination of the kinetic energy budget terms reveals that • the production term is the main source of the xx turbulence stress, whereas it is not significant in the yy component. • A substantial portion of this is carried by the pressure-velocity coupling from the xx direction to the yy direction, which becomes the main source term in the yy component. • Both, the production term as well as the pressure-velocity term show a clear decrease with increase in Mc. The high point of the thesis is related to using the understanding derived from an analysis of various source terms in the kinetic energy balance to evolve an unsteady Reynolds Averaged Navier Stokes (URANS) model for calculating high speed mixing layers, a subject that has eluded international research till now. It recognizes that the key feature affected by ompressibility is related to the anisotropy of the stress tensor. The relationship between stress component (_Txy) and the velocity gradient (Sxy) as obtained from LES is set out in the form of a simple relationship accounting for the effects of other parameters obtained earlier in this thesis. A minor influence due to _Tyy is extracted by describing its dependence on Sxy again as gleaned from LES studies. The needed variation of Prandtl and Schmidt numbers through the field is extracted. While the detailed variations can in fact be taken into account in URANS simulations, a simple assumption of these values being around 0.3 is chosen for the present simulations of URANS. Introduction of these features into the momentum equation gives the much expected variation of the reduction in the growth rate of the mixing layer with convective Mach number as in experiments. The relationships that can be used in high speed mixing layers are Introduction of these features into the momentum equation gives the much expected variation of the reduction in the growth rate of the mixing layer with convective Mach number as in experiments. This is then a suggested new approach to solve high speed mixing layers. While it can be thought that the principal contributions of the thesis are complete here, an additional segment is presented related to entropy view of the mixing layer. This study that considers the mixing layer with two different species expresses various terms involved in the entropy conservation equation and obtains the contribution of various terms on the entropy change for various Mc. It is first verified that the entropy derived from the conservation equation matches with those calculated from fluid properties, entropy being a state variable. It is shown that irreversible diffusion comes down the most with convective Mach number. Left: This image shows pictorially the flow of source of turbulent stress from the axial to the cross wise turbulent stress. Production (Σ) of turbulence happens mainly in the xx direction, a part of it is carried by the pressure-velocity correlation to the yy direction, which itself has a low production. With increasing Mc, both the production as well as the pressure-velocity correlation decrease. Right: This image shows the growth rate obtained from simulations scaled with the incompressible growth rate, of LES and RANS in the background of experiments (others). As is clear, the growth rate obtained is well within the band of experimental results.

L’aspirateur d’une centrale hydroélectrique est l’organe hydraulique se situant en aval de la roue. Il a une forme divergente afin de récupérer l’énergie cinétique résiduelle en sortie de roue sous forme de pression statique et augmenter ainsi la chute nette de la centrale. Dans le cas des turbines de basse chute de type bulbe, les pertes de charge dans l’aspirateur influencent fortement le rendement global de la centrale. La prédiction correcte de ces pertes de charge au cours du dimensionnement de la turbine représente donc un enjeu majeur. La prédiction numérique des pertes de charge dans l’aspirateur est un réel challenge car l’écoulement dans l’aspirateur est dynamiquement complexe avec des nombres de Reynolds élevés, la présence de swirl et d’un gradient adverse de pression. Ces caractéristiques font que les approches de modélisation classiquement utilisées dans l’industrie sont mises en défaut. L’objectif de ce travail est double : (i) améliorer la prédiction de l’écoulement turbulent dans l’aspirateur en utilisant des approches instationnaires URANS et LES et en portant une attention particulière à la description des conditions d’entrée de l’aspirateur et (ii) réaliser une analyse fine des échanges énergétiques dans l’aspirateur pour mieux comprendre l’origine des pertes de charge. Une condition d’entrée instationnaire représentative de l’écoulement en sortie de roue est élaborée pour ces calculs. Les résultats de simulation sont comparés avec des mesures expérimentales afin d’évaluer la capacité prédictive de chaque approche de modélisation de la turbulence (URANS et LES). Cette étape de validation met en évidence l’importance d’une définition correcte des trois composantes de la vitesse en entrée d’aspirateur. L’influence des conditions aux limites du domaine de calcul, à savoir la rugosité de la paroi et la condition de sortie de l’aspirateur, sur les résultats de simulation est évaluée, notamment dans le cas d’une résolution LES. Grâce à une analyse détaillée du bilan d’énergie cinétique moyenne dans l’aspirateur, les phénomènes hydrodynamiques responsables des pertes de charge sont identifiés. Ceci permet d'analyser en détail les différences de prédiction de pertes de charge entre les calculs URANS et LES et d’identifier les pistes d’amélioration de la prédiction numérique de ces pertes. Enfin, cette analyse permet de comprendre l’évolution des pertes de charge observée entre plusieurs points de fonctionnement de la turbine. / The draft tube of a hydraulic turbine is the turbine element located downstream of the runner. It has a divergent shape in order to convert the residual kinetic energy leaving the runner into pressure and thus increase the effective head of the turbine. The performances of low head bulb turbines are highly influenced by the head losses in the draft tube. The prediction of these head losses in a design process is thereby a major issue. The numerical prediction of the head losses in the draft tube is a real challenge because the flow in the draft tube is dynamically complex with high Reynolds numbers, a swirl and an adverse pressure gradient. These characteristics render conventional industrial approaches not appropriate. The objective of this work is twofold: (i) to improve the numerical prediction of the turbulent flow in the draft tube by using URANS and LES unsteady approaches and paying special attention to the description of the inlet boundary conditions of the draft tube and (ii) to conduct a detailed analysis of the energy transfers in the draft tube in order to better understand the origin of the head losses. An unsteady inlet boundary condition for the simulations reproducing the flow field at the runner outlet is developed. Numerical results are compared to experimental measurements in order to evaluate the predictive capacity of each turbulence modelling approach (URANS and LES). This validation step highlights the importance of defining properly the three velocity components at the draft tube inlet. The influence on the numerical results of boundary conditions of the calculation domain, such as wall roughness and the outlet boundary condition, is evaluated, in particular in case of LES. Thanks to a detailed analysis of the mean kinetic energy balance in the draft tube, the hydrodynamic phenomena responsible for head losses are identified. The head losses prediction differences between URANS and LES are thus analyzed in detail and possible improvements for the head losses prediction are identified. Finally, this analysis enables to understand the head losses evolution observed between several operating points of the turbine.

Cette thèse vise à développer et évaluer la méthode de Galerkin discontinue (DG) pour la simulationdes grandes échelles (LES) des écoulements turbulents. L’approche DG présente un nombre d’avantages intéressants pour la LES : ordre élevé, stencil compact, prise en compte des maillages non structurés et expression de la solution numérique dans une base de polynômes permettant l’utilisation de modèles de turbulence multi-échelle. Parmi ce type de modèles, nous nous sommes intéressés ici à la méthode Variational Multiscale (VMS) qui consiste à séparer les échelles résolues dans la base de polynômes pour restreindre l’influence du modèle à une gamme réduite d’échelles. Les modèles considérés ont été paramétrés en prenant en compte les fonctions de transfert spécifiques aux discrétisations DG. La précision de la méthode pour la représentation de phénomènes turbulents variés a été évaluée à travers la réalisation de DNS de configurations académiques. Enfin, l’approche VMS/DGa été éprouvée sur des configurations simples à haut nombre de Reynolds. Il apparaît que cette méthodologie permet la représentation précise des phénomènes turbulents pour un coût réduit en terme de degrés de liberté. / This work focuses on the development of the Discontinuous Galerkin (DG) method for the large-eddy simulation (LES) of turbulents flows. The DG method shows some interesting properties for LES : high-order of accuracy, compact stencil, unstructured meshes and amodal polynomial basis which can be used to implement multiscale turbulence models. We consider in this work the Variational Multiscale approach (VMS), which consists in splitting the resolved scales into two components using the modal basis in order to restrict the action of the model to a given range of small scales. The models have been tuned using the transfer functions of the DG hp-discretizations. The accuracy of the DG method for the representation of turbulent phenomena has been assessed through DNS of free and wall-bounded canonical flows. Finally, the VMS/DG approach has been assessed for simple configurations at high Reynolds numbers. We have shown that this particular approach allows for an accurate representation of turbulent flows for coarse discretizations.

Dans les chambres de combustion des moteurs fusées cryotechniques, la pression excède la pression critique des réactifs. Les interactions moléculaires ne sont plus négligeables et le comportement du fluide n’est plus celui d’un gaz parfait. Le but de cette thèse est de développer un outil de Simulation des Grandes Echelles (SGE) pour étudier la combustion et la dynamique dans des géométries réalistes de moteur fusées. L’utilisation de l’équation d’état de Peng-Robinson, associée à une formulation thermodynamique généralisée, et des coefficients de transports appropriés permettent au code de SGE AVBP du CERFACS de simuler des systèmes réactifs à pression supercritique. Les changements thermodynamiques au sein d’AVBP nécessitent également l’adaptation des conditions limites et des schémas numériques. L’outil est validé sur une configuration mono-espèce à pression supercritique, puis sur un cas représentatif d’un injecteur coaxial de moteur-fusée. Les résultats obtenus sont en bon accord avec l’expérience et offrent des perspectives encourageantes pour des études futures, telles que des configurations multi-injecteurs ou l’analyse des instabilités de combustion haute fréquence. / In cryogenic engines combustion chambers, pressure exceeds the propellants critical pressure. Molecular interactions are generally no longer negligible and fluid behavior deviates from that of a perfect gas. The objective of this thesis is to develop a Large-Eddy Simulation (LES) tool to study combustion and dynamics in realistic geometries of rocket engines. The use of the Peng-Robinson equation of state, in conjunction with a generalized treatment of thermodynamics and appropriate transport coefficients, allows the CERFACS’ LES code AVBP to handle reactive systems at supercritical pressure. Change of the thermodynamics in AVBP necessarily leads to an adaptation of boundary conditions treatment and numerical schemes. The tool is validated on a mono-species configuration at supercritical pressure, and a reactive single coaxial injector, representative of a rocket injector. Results are in good agreement with experiments and provide encouraging perspectives for future studies, such as multi-injector configurations and high-frequency combustion instabilities.

Cette thèse se situe dans le cadre de la simulation numérique de la combustion turbulente à l’aide de méthodes de tabulation de la cinétique chimique. En approximant la structure fine des flammes turbulentes, ces méthodes prennent en compte des effets fins de cinétique chimique pour un faible coup dans les calculs numériques. Ceci permet de prédire les champs de température et d’espèces chimiques incluant les polluants. Le champ d’application de la chimie tabulée a d’abord été réservé à la simulation des écoulements moyens (RANS) dans une hypothèse de faible nombre de Mach pour une combustion dite "conventionnelle". Cependant, le développement actuel de nouvelles technologies de combustion ainsi que celui de modèles numériques plus avancés que les approches RANS nécessite d’étendre ce champ d’application. Les travaux de cette thèse ont mené au développement de nouveaux modèles de chimie tabulée afin de répondre à ces nouvelles exigences. L’émergence de nouvelles technologies comme la combustion sans flamme nécessite le développement de modèles dédiés. Ce mode de combustion présente en effet des structures de flamme mixtes. C’est pourquoi un modèle de tabulation de la cinétique chimique nommé UTaC (Unsteady flamelets Tabulated Chemistry) est proposé pour prédire la combustion diluée à haute température qui caractérise la combustion sans flamme. Le modèle est basé sur la tabulation de solutions instationnaires de flammelettes non-prémelangées qui s’auto-allument. Les pertes thermiques et la dilution variable des gaz brûlés sont négligés dans le cadre de cette thèse par soucis de simplification et de clarté de la validation du modèle. Le modèle est appliqué au cas d’un jet de combustible dilué dans un environnement de gaz vicié qui favorise l’auto-allumage comme moyen de stabilisation d’une flamme liftée. Plusieurs simulations RANS sont réalisées en faisant varier le combustible utilisé. Enfin, une simulation aux grandes échelles (LES) est aussi conduite pour le mélange méthane/air. Plusieurs codes numériques dédiés à la LES sont basés sur une formulation compressible des équations de Navier-Stokes. Cependant les méthodes de tabulation ne permettent pas directement de prendre en compte les effets acoustiques. Un modèle appelé TTC (Tabulated Thermo-chemistry for Compressible flows) a été créé afin d’introduire les méthodes de chimie tabulée dans les codes numériques compressibles. Pour cela, le calcul de la température est reformulé ainsi que le traitement des conditions aux limites à l’aide d’ondes caractéristiques. Enfin, l’application de modèle RANS de tabulation de la cinétique chimique à la LES est souvent faite sans tenir compte des spécificités de la simulation aux grandes échelles. Ainsi, les fonctions de densité de probabilités de type ß qui traduisent l’interaction de la combustion avec la turbulence en RANS sont utilisées telles quelles en LES. Nous montrerons que cette hypothèse est mauvaise car elle ne conserve pas l’intégrale du terme source dans une flamme prémélangée. Un nouveau modèle de chimie tabulée nommé F-TACLES (Filtered Tabulated Chemistry for Large Eddy Simulation) est alors développé spécifiquement pour la simulation aux grandes échelles de la combustion parfaitement prémélangée. Le modèle est basé sur le filtrage de flammes laminaires de prémélange mono-dimensionelles. / The thesis subject is located in the domain of numerical simulation of turbulent combustion through tabulated chemistry methods. These methods allow to include detailed chemistry effects at low cost in numerical simulation by approximating the fine scales structure of turbulent flames. Prediction of temperature and chemical species including pollutants becomes then possible. Tabulated chemistry models were first dedicated to low Mach-number RANS approaches for "conventional" combustion applications. However, the current uprising of new combustion configurations and of more precise numerical modeling than RANS approach requires to widen these range of applications. For that purpose, this thesis led to the development of new tabulated chemistry models. Flameless combustion is one of these new combustion technology that requires dedicated models. Indeed, complex flame structures are encountered in this combustion mode. That is why a tabulated chemistry model called UTaC (Unsteady flamelets Tabulated Chemistry) is derived to simulate high temperature diluted combustion which characterizes flameless combustion. The model lies on the tabulation of laminar unsteady non-premixed flamelets that auto-ignite. Heat losses and variation of dilution with burnt gases are neglected in the topic of this thesis for brevity and simplification of the model validation. The investigated configuration is a fuel jet diluted in a vitiated coflow. The hot coflow promotes auto-ignition in the lifted flame stabilization mechanism. Several RANS computations are performed by changing the fuel composition. Finally, a Large Eddy Simulation (LES) is also realized using a methane/air mixture as the impinging fuel stream. Several numerical codes for LES use a fully compressible formulation of Navier-Stokes equations. However, tabulated chemistry techniques do not take into account acoustic perturbations. A model called TTC (Tabulated Thermo-chemistry for Compressible flows) formalism is therefore developed in order to include tabulated chemistry in compressible CFD codes. TTC formalism consists in reformulating both temperature computation inside the numerical code and the characteristic boundary treatment. Finally, application of tabulated chemistry model to LES is usually done by a straightforward derivation from its RANS version without taking into account LES requirements. Indeed, ß-probability density functions which accounts for turbulence-chemistry interaction in RANS are used in LES although this technique does not conserve the source terms integral in premixed flames. A new model, F-TACLES (Filtered Tabulated Chemistry for Large Eddy Simulation), is then derived specifically for LES of perfectly premixed combustion. This model is based on filtering of 1D laminar premixed flamelets.

La réduction des émissions de dioxyde de carbone et de polluants réglementés (oxydes d'azote, hydrocarbures, particules...) dans les moteurs à allumage commandé est possible via l'apport de nouvelles technologies comme le downsizing, l'injection directe, la suralimentation, etc... Toutefois, les gains apportés par ces technologies, qui complexifient grandement le groupe motopropulseur, sont réduits du fait qu'elles induisent aussi une augmentation de l'intensité de certains phénomènes tels que les variations cycliques de la combustion (VCC) ou les combustions anormales. La compréhension de ces phénomènes est une clé dans l'amélioration des futurs moteurs à essence. L'objectif principal de cette thèse est de permettre la modélisation des phénomènes transitoires et acycliques dans des moteurs à structure de plus en plus complexe grâce au développement, à la validation puis à l'application d'une méthode de couplage entre la simulation aux grandes échelles (SGE ou LES pour Large-Eddy Simulation en anglais) et la simulation système. Ces travaux de thèse ont permis de démontrer qu'il est possible de simuler une configuration moteur et d'en étudier les comportements transitoires en 3D tout en prenant en compte la dynamique de tous les composants du moteur. Les simulations obtenues par cette approche pour le moteur du projet ANR SGEmac ont été comparées avec succès aux résultats expérimentaux et à des simulations tridimensionnelles : ces résultats constituent la dernière étape de validation du couplage et démontrent sa capacité à simuler des systèmes moteurs complets. Les codes couplés développés sont ensuite appliqués à l'étude des transitoires de charge et de régime pour le moteur du projet ANR ASTRIDE. La comparaison simulation/expérience montre que le solveur couplé permet bien de remplir les objectifs fixés de simulation des VCC et des transitoires. / The decrease of greenhouse gases and pollutant emissions (nitrous oxides, carbon oxides, particles...) for spark ignited engines goes through the development of new technologies such as direct injection, turbocharging, downsizing, etc. However, the benefits of these technologies, which complexify the engines, are limited by the phenomena they intensify such as Cyclic Combustion Variability (CCV) and abnormal combustions. A thorough understanding of these phenomena is a cornerstone for the improvement of future engines. The aim of this work is to predict acyclic and transient phenomena in increasingly complex engines through the development, the validation and the use of a coupling method between Large-Eddy Simulation (LES) and system simulation. This thesis has demonstrated that simulating a complete industrial engine in 3D with LES to study its transient behavior is possible. The methodology developed in the present work was used to study the engine of the national research agency project SGEmac and a good agreement was obtained between the experiments and the tridimensional simulations. These results are the last validation step of the coupling method and demonstrate the capacity of the coupled solver to simulate the whole engine. The coupling method is then applied to study engine load and regime transients for the national research agency project ASTRIDE. The comparison between simulations and experiments show that the coupled solver can simulate CCV and transients, thus fulfilling its initial goal.

Les vibrations induites par l'écoulement dans le coeur du REP peuvent provoquer une usure par frottement des crayons combustibles par friction au niveau des contacts entre la cellule de grille et les crayons des assemblages combustibles. Cela peut entraîner des dommages irréversibles de la gaine du crayon combustible et compromettre la première barrière de sûreté du réacteur. Assurer l'intégrité de la gaine est une préoccupation majeure dans la sûreté du réacteur. Cependant, les spectres d'excitation des forces fluides agissant sur les crayons ne sont pas bien connus. Le but de cette thèse est d'utiliser des éléments géométriques simples pour reproduire des cellules de grilles d'un REP. SGE ont été effectuées sur une conduite annulaire avec différents maillages en utilisant le code TrioCFD. Une étude de sensibilité de maillage a été réalisée afin de proposer un maillage reproduisant correctement les résultats dans la littérature. Ces informations de résolution de maillage ont été utilisées lors de la réalisation des simulations en utilisant divers obstacles géométriques intérieurs à la conduite, i.e., des ailettes de mélange, une grille circulaire et une combinaison de grille carrée et d'ailettes de mélange. Un maillage hybride a été utilisé dans le cas des ailettes de mélange et dans le cas de cellule de grille carrée. Les caractéristiques hydrauliques ainsi que la pression pariétale ont été analysées dans chaque cas. Il apparaît que la grille carrée est une combinaison approximative du cas des ailettes de mélange et du cas de la grille circulaire. Les simulations ont été comparés avec des mesures réalisées au CEA / Flow-induced vibrations in the pressurized water reactor core can cause fretting wear in the fuel rods. Due to friction, wear occurs at the contact locations between the spacer grid and the fuel rod. This may compromise the first safety barrier of the nuclear reactor by damaging the fuel rod cladding. In order to ensure the integrity of the cladding, it is necessary to know the random fluctuating forces acting on the rods. However, the spectra for these fluid forces are not well known. The goal of this thesis is to use simple geometrical elements to check the reproducibility of realistic PWR spacer grids. As a first step, LES were performed on annular pipe for different mesh refinements using the CFD code TrioCFD. A mesh sensitivity study was performed to propose a good mesh for reproducing standard literature results. This information on mesh resolution was used when carrying out simulations using various geometric obstacles inside the pipe-mixing vanes, circular spacer grid and a combination of square spacer grid with mixing vanes. Structured mesh was generated for the annular pipe case and circular grid case. An innovative hybrid mesh was used for the two remaining cases of the mixing vanes and the square grid; keeping unstructured mesh around the obstacles and structured mesh in the rest of the domain. Both hydraulic and wall pressure characteristics were analyzed for each case. The results for the square grid case were found to be an approximate combination of the mixing vane case and circular grid case. Simulation results were compared with experiments performed at CEA Cadarache. Some preliminary comparisons were also made with classical empirical models.