Este trabalho apresenta um estudo experimental e numérico de escoamentos em canais compostos. Simulação de grandes escalas e método dos elementos finitos, em paralelo com medições utilizando anemômetros de fio quente em um canal aerodinâmico são realizadas. Canais compostos estão presentes em muitas aplicações de engenharia. Dispositivos eletrônicos, trocadores de calor, reatores nucleares, canais de irrigação e planícies de inundação são alguns dos desafios enfrentados pela engenharia. A combinação de simulação de grandes escalas e o método dos elementos finitos para a investigação de escoamentos turbulentos pode ser de grande importância para o estudo dos escoamentos na engenharia. No caso dos escoamentos através dos canais compostos, publicações neste tema são ainda raros. Os principais objetivos deste trabalho são: analisar o escoamento de um fluido viscoso, incompressível e isotérmicas em um canal composto, empregando um código de computação tridimensional apresentado por Petry em 2002, que realiza simulação de grandes escalas com o método dos elementos finitos, para comparar os resultados numéricos com os resultados experimentais do escoamento turbulento em um canal composto cuja geometria é exactamente reproduzida pela malha numérica, para verificar a validade do método numérico e o comportamento de modelos em escala subgrade para reproduzir o fluxo no canal composto investigado; e comparar a eficácia dos esquemas Taylor-Galerkin e dois passos para analisar os resultados. O canal investigado consiste em um canal principal com seção transversal retangular, conectado a uma fenda retangular estreita. No código numérico, o modelo clássico de Smargorinsky é comparado com o modelo dinâmico de viscosidade turbulenta, inicialmente proposto por Germano et al. 1991. A segunda filtragem do processo dinâmico é feita através dos elementos finitos independentes propostos por Petry, 2002. Para a implementação do algoritmo, o método dos elementos finitos é usado, Taylor-Galerkin e esquemas dois passos são usados para a discretização no tempo e no espaço e de ligação das equações governantes. O domínio computacional é discretizadas por intermédio de elementos lineares hexaédricos. Os resultados obtidos a partir simulações de grandes escalas, usando o modelo clássico de Smagorinsky e o modelo dinâmico de submalha; mostram o desenvolvimento de uma camada de cisalhamento na direção principal do escoamento com características dinâmicas regidas pelos perfis de velocidade média. Os resultados da simulação mostraram boa concordância com os dados experimentais dos perfis de velocidade média, intensidade de turbulência e tensão de cisalhamento turbulenta. Em geral, o modelo dinâmico com o esquema de duis passos foi mais eficiente para reproduzir estruturas turbulentas, em comparação com o modelo Smagorinsky e o esquema Taylor-Galerkin particularmente ao longo da região da fenda do canal. / This work presents an experimental and numerical study of turbulent flows in compound channels. Large eddy simulation and finite element method in parallel with hot wires measurements in an aerodynamic channel are employed. Compound channels are present in many engineering applications like in electronic devices, heat exchangers, nuclear reactors and irrigation channels and flooding plains are some of the challenges faced by mechanical engineering. The combination of large eddy simulation and the finite element method for the investigation of turbulent flows can be of great relevance to the study of engineering flows. In the case of flows through compound channels, publications in this subject are still rare. The main objectives in this work are: to analyze the flow of viscous, incompressible and isothermal fluids in a compound channel; employing a three-dimensional computation code presented by Petry, 2002, which performs large eddy simulation with the finite element method; to compare the numerical results with experimental results of the turbulent flow in a compound channel whose geometry is exactly reproduced by the numerical mesh; to check the validity of the numerical method and the behavior of subgrid scale models to reproduce the flow in the compound channel investigated and compare the efficacy of the Taylor-Galerkin and Two-Steps schemes in analyzing the results. The compound channel investigated consists of a rectangular channel connected to a rectangular shaped slot. In the numerical code, Smargorinsky´s classical model is compared to the dynamic model of turbulent viscosity, initially proposed by Germano et al. The second filtering of the dynamic process is made through the independent finite elements proposed by Petry, 2002. For the implementation of the algorithm, the finite element method is used, Taylor- Galerkin and Two-Steps schemes are used for discretization in time and space and to link governing equations. The computational domain is discretized by means of linear hexahedrical elements. The results obtained from large eddy simulations, using the classical model of Smagorinsky and the Dynamic subgrid scale model show the development of a shear layer in the main direction of flow with dynamic characteristics governed by the mean velocity profiles. The simulation results showed good agreement compared to experimental data, and analysis of the profiles of mean velocity, turbulence intensities and turbulent shear stress. In general, dynamic model with the two-steps scheme was more able to reproduce turbulent structures in comparison with the Smagorinsky model with Taylor-Galerkin scheme, particularly along the channel slot.

Cette thèse est une contribution à la simulation numérique des transferts de quantité de mouvement et de chaleur dans les écoulements turbulents de fluides non-Newtoniens dans une conduite cylindrique fixe. La viscosité du fluide utilisé est décrite par la loi d'Ostwald de Waele. Deux modèles sous-mailles dans l'approche des simulations des grandes échelles ont été considérés : le modèle dynamique de Germano et al. (1991) et le modèle de Smagorinsky non-Newtonien. Ils sont utilisés pour décrire les mécanismes physiques mis en jeu dans les écoulements isothermes de ces fluides à rhéologie complexe. Les transferts thermiques sont simulés avec le modèle de Smagorinsky non-Newtonien. Ces derniers sont traités en deux parties. La première concerne les échanges de chaleur sans influence sur la distribution des vitesses. Il s'agit des écoulements non-thermo dépendants ou écoulements isothermes. La deuxième partie concerne la résolution des écoulements thermo dépendants qui mettent l'accent sur les modifications induites par le couplage vitesse-température. Les champs turbulents sont analysés statistiquement en moyennant dans le temps et dans l'espace (suivant les directions périodiques) les champs instantanés de vitesse et de température pour établir les profils moyens de vitesse et de température, les rms, la tension de Reynolds, les flux de chaleur, les moments d'ordre plus élevé (coefficients de dissymétrie et d'aplatissement), les pdf (fonction de densité de probabilté), les jpdf (fonction de densité de probabilité jointe), le coefficient de frottement, le nombre de Nusselt... Ces différentes grandeurs sont analysées en fonction des divers paramètres qui gouvernent le problème: les nombres de Reynolds et de Prandtl, l’indice d'écoulement et le nombre de Pearson / This thesis is a numerical contribution of momentum and heat transfer of turbulent pipe flows of non-Newtonian fluids. The apparent viscosity of the fluid is modelled by a power-law (Ostwald de Waele model). Two models subgrid of LES were considered: the dynamic model of Germano et al. (1991) and model Smagorinsky non-Newtonian. They are used to describe the physical mechanisms involved in the isothermal flow of these complex rheology fluids. Heat transfer are simulated with the model of non-Newtonian Smagorinsky. These are processed in two parts. The first concerns the heat exchange without affecting the velocity distribution. This is non-thermo dependent flow or isothermal flow. The second part concerns the resolution of thermo dependent flows that focus on changes induced by the temperature-velocity coupling. The turbulent fields are analyzed statistically by averaging over time and space (according to the periodic directions) the instantaneous field of velocity and temperature to establish the average profiles of velocity and temperature, the root mean square of turbulent fluctuations (rms), Reynolds stress, the heat flow, the moments of higher order (skewness and flatness), the pdf (probability density function), the jpdf (attached probability density function), the coefficient of friction, the number of Nusselt ... These differents variables are analyzed for various parameters governing the problem: the Reynolds and Prandtl flow index and the number of Pearson

L'allumage d'un moteur fusée cryotechnique (carburants liquides) est une phase critique. La moindre anomalie dans la procédure d'allumage peut conduire à la destruction du lanceur. L'objectif de cette thèse est de développer une méthodologie s'appuyant sur la simulation aux grandes échelles (LES) pour étudier les phénomènes physiques impliqués dans un tel allumage. L'intérêt de la méthode LES est de pouvoir capturer les couplages instationnaires entre la turbulence, les processus diphasiques et la cinétique chimique. L'outil numérique est tout d'abord validé sur des cas académiques et expérimentaux, puis appliqué à un moteur fusée réel. Une approche graduelle est employée : les différents cas de validation présentent une complexité croissante, permettant d'isoler les processus physiques principaux. Ce travail de recherche montre que l'approche de la simulation aux grandes échelles, dans un contexte de calcul massivement parallèle, peut être utilisée pour étudier la séquence complète d'allumage dans un moteur fusée réel. / The ignition of a cryogenic rocket engine (liquid propellants) is a critical phase. The slightest anomaly in the ignition sequence can lead to the destruction of the entire launcher. The objective of this research work is to set a methodology based on the Large Eddy Simulation (LES) approach, to study the different physical phenomena involved in such ignition transient. The LES method can capture the unsteady processes such as turbulence, two-phase flow physics and chemical kinetics. The numerical tool is first validated in academic and experimental cases, and then applied to a real rocket engine. A gradual approach is employed : the complexity is increased between each validation case, so as to identify the main physical processes. This research work shows that the LES approach, in the context of massively parallel computing, can be used to study the whole ignition sequence of a real cryogenic rocket engine.

Ce travail de thèse se propose de déterminer la distribution de vitesse dans des canaux composés à travers une étude expérimentale et numérique. Afin de répondre à cette problématique, deux approches sont suivies répondant chacune à des enjeux scientifiques et techniques et sous la contrainte de verrous scientifiques. La première approche est une étude expérimentale dont le but est de combler le manque de connaissances sur la distribution de vitesse dans des canaux type réseaux. Une série d’expérimentations est effectuée dans le hall expérimental de l’équipe mécanique des fluides du laboratoire ICube. La deuxième approche est la modélisation 3D où il s’agit de modéliser les travaux de (Proust et al., 2013) pour en déduire la distribution de vitesse dans une section transversale. Cette deuxième approche a pour but de développer une méthodologie de modélisation 3D du champ de vitesse en canaux composés. En plus des modèles de turbulence classiques (Kepsilon, K-oméga, RSM …); la simulation des grandes échelles est utilisée. Les résultats obtenus numériquement seront ensuite comparés à des données expérimentales. Les deux approches suivies dans cette thèse s’autoalimentent : l’étude expérimentale construit une base de données servant à valider le modèle numérique alors que l’étude numérique permet d’étendre les résultats des expérimentations à d’autres géométries. / Compound channels are characterized by a main channel and a floodplain. Their hydraulic behavior has important applications in rivers and flood control but also in sewer system management. Two approaches are followed in this study. The first one is an experimental study that aims to fulfill the lack of data concerning velocity distribution in narrow and deep compound channels. The experiments were carried out in a 16m long and 0.6m wide compound channel located in the experimental hall of ICube laboratory. The second approach is a numerical one. The objective of this study is to develop a methodology for numerical modelling of velocity distribution in compound channels. To do the experimental data of (Proust et al., 2013) were modelled compared to the experimental data in order to determine the most suitable model to reproduce velocity distribution. In addition to this comparison, the large eddy simulation method were also utilized for two experiments of (Proust et al., 2013) and (Nezu et al., 1990). Two specific boundary conditions were implemented in order to compare the efficiency of each method.

A centrifugal compressor is a mechanical machine with purpose to convert kineticenergy from a rotating impeller wheel into the fluid medium by compressingit. One application involves supplying boost air pressure to downsized internalcombustion engines (ICE). This allows, for a given combustion chamber volume,more oxygen to the combustion process, which is key for an elevated energeticefficiency and reducing emissions. However, the centrifugal compressor is limitedat off-design operating conditions by the inception of flow instabilities causingrotating stall and/or surge. These instabilities appear at low flow rates andtypically leads to large vibrations and stress levels. Such instabilities affectthe operating life-time of the machine and are associated with significant noiselevels.The flow in centrifugal compressors is complex due to the presence of a widerange of temporal- and spatial-scales and flow instabilities. The success fromconverting basic technology into a working design depends on understandingthe flow instabilities at off-design operating conditions, which limit significantlythe performance of the compressor. Therefore, the thesis aims to elucidate theunderlying flow mechanisms leading to rotating stall and/or surge by means ofnumerical analysis. Such knowledge may allow improved centrifugal compressordesigns enabling them to operate more silent over a broader operating range.Centrifugal compressors may have complex shapes with a rotating partthat generate turbulent flow separation, shear-layers and wakes. These flowfeatures must be assessed if one wants to understand the interactions among theflow structures at different locations within the compressor. For high fidelityprediction of the complex flow field, the Large Eddy Simulation (LES) approachis employed, which enables capturing relevant flow-driven instabilities underoff-design conditions. The LES solution sensitivity to the grid resolution usedand to the time-step employed has been assessed. Available experimentaldata in terms of compressor performance parameters, time-averaged velocity,pressure data (time-averaged and spectra) were used for validation purposes.LES produces a substantial amount of temporal and spatial flow data. Thisnecessitates efficient post-processing and introduction of statistical averagingin order to extract useful information from the instantaneous chaotic data. Inthe thesis, flow mode decomposition techniques and statistical methods, suchas Fourier spectra analysis, Dynamic Mode Decomposition (DMD), ProperOrthogonal Decomposition (POD) and two-point correlations, respectively, areemployed. These methods allow quantifying large coherent flow structures atvfrequencies of interest. Among the main findings a dominant mode was foundassociated with surge, which is categorized as a filling and emptying processof the system as a whole. The computed LES data suggest that it is causedby substantial periodic oscillation of the impeller blade incidence flow angleleading to complete system flow reversal. The rotating stall flow mode occurringprior to surge and co-existing with it, was also captured. It shows rotating flowfeatures upstream of the impeller as well as in the diffuser. / <p>QC 20171117</p>

Ce mémoire présente une étude numérique du bruit aéroacoustique large bande d'une roue de ventilateur centrifuge. La recherche bibliographique démontre qu'il existe peu de méthod~s pour identifier les zones responsables de la propagation du bruit large bande des centrifuges due à la complexité de la géométrie du ventilateur. La connaissance de ces zones responsables de ce type de bruit orientera la conception d'un ventilateur silencieux. Afin d'accroître la gamme de méthode pour la localisation de ces zones, un code spécifique a été développé et présenté dans ce mémoire. L'approche utilisée vise à simuler d'une part, l'écoulement dans le ventilateur par la LES (Large Eddy Simulation) sous le logiciel Fluent. En effet, la LES permet d'avoir accès aux petites échelles responsables du bruit large bande. Ensuite, les fluctuations de pression pariétales émanant de cette simulation sont récupérées pour alimenter l'analogie acoustique. D'autre part, la puissance acoustique rayonnée est calculée par le biais du code spécifiquement développé. Ce code s'appuie sur la méthode de la décomposition modale (DOP). Finalement, on extrait les modes et les zones qui rayonnent le plus sur les pales de la roue (principale source de bruit large bande). Les résultats de validation numériques entre le code développé et le logiciel Fluent sont convaincants. En effet, les variations des champs acoustiques des deux codes sont comparables avec une différence en moyenne de 2.5dB. De plus, on obtient une atténuation du bruit par la distance qui correspond à la décroissance d'une onde plane en fonction de la distance. Les résultats d'identification des zones de la pale qui contribuent le plus au rayonnement acoustique sont également présentés dans ce mémoire. Cette technique permettra aux concepteurs d'aiguiller les modifications à faire pour rendre la roue de ventilation plus silencieuse.

Dans les chambres de combustion aéronautiques, le refroidissement par micro-percage est la technique privilégiée pour protéger les parois contre les gaz chauds. L’air frais provenant du contournement traverse des milliers de perforations inclinées et for- ment des micro-jets. Ces derniers coalescent en un film qui protège les parois du tube a flamme. Avec les moyens informatiques actuels, effectuer une simulation aux grandes échelles d’un moteur réel est impossible. En effet, le nombre de micro-trous est beaucoup trop important pour permettre une résolution détaillée de chacun. Des modèles numériques sont donc nécessaires. Le modèle homogène, développé en 2008, permet de simuler des plaques multiperforees avec des maillages dont la résolution est supérieure a celle du trou. Il ne permet cependant pas de représenter la pénétration ni le mélange des jets avec les gaz chauds. Pour remédier a cela, une approche hétérogène, appelée modèle a trou épaissi, a été développée au cours de cette thèse. La précision étant toujours relative au maillage, une méthode de maillage adaptatif augmentant automatiquement la résolution dans les zones clés a été propose afin d’obtenir de meilleurs résultats pour un faible surcoût. Predire la température des parois du tube a flamme est l’objectif final des ingénieurs. A cet effet, une méthodologie appelée Adiab2colo, permettant d’évaluer la température de paroi a partir d’un calcul adiabatique non résolu, a également été développée. Ces trois techniques sont maintenant couramment utilisées par Safran Helicopter Engine pour la conception des moteurs de demain. / Numerical simulation is progressively taking importance in the design of an aero- nautical engine. However, concerning the particular case of cooling devices, the high number of sub-millimetric cooling holes is an obstacle for computational sim- ulations. A classical approach goes through the modelling of the effusion cooling by homogenisation. It allows to simulate a full combustor but failsin representing the jet penetration and mixing. A new approach named thickened-hole model was developed during this thesis to overcome this issue. A work on improving the mesh resolution onkey areas thanks to an automatic adaptive method is also presented, leading to a clear breakthrough. In parallel, as the flame tube temperature is a cornerstone for the combustor durability,a low-cost approach is proposed to predict it. To meet the time-constraints of design, it is based on thermal modelling instead of a direct thermal resolution.

La prédiction des flux aux parois joue un rôle déterminant dans le cycle de vie des chambres de combustion. Le transfert de chaleur de la flamme aux parois est entraîné, outre la convection, également par le rayonnement des gaz chauds au sein de la chambre. Afin d’intégrer les contributions convectives et radiatives au flux pariétal il est nécessaire de résoudre simultanément l’équation de transfert radiatif et les équations régissant l’écoulement réactif. Quand les méthodes de Monte Carlo sont couplées aux simulations aux grandes échelles (LES), de telles simulations deviennent très coûteuses. L’objectif de cette thèse est donc d’investiguer une technique pour améliorer l’efficacité de la méthode MC, basée sur un mécanisme alternatif d’échantillonnage appelée intégration Quasi-Monte Carlo (QMC). Au cours de cette thèse, la méthode QMC a été couplée à une simulation LES dans une configuration où le rayonnement joue un rôle très important : la flamme méthane-air de la chambre Oxytec. La comparaison entre les simulations couplées et non couplées avec les données expérimentales montre que le rayonnement thermique a un impact sur la topologie de l’écoulement et de la flamme. Enfin, un bon accord est trouvé entre le flux de chaleur pariétal prédit par la simulation et les données expérimentales. / The prediction of wall fluxes is a significant aspect in the life cycle of combustors, since it allows to prevent eventual wall damages. Heat transfer from flame to the walls is driven, apart from convection, also by radiation of burnt gases inside the chamber. In order to correctly account for both convective and radiative contributions to wall fluxes, the simultaneous solution of the radiative transfer equation (RTE) and the governing equations for reactive flows is required. However, multi-physics simulations where MC methods are coupled to Large Eddy Simulation (LES), remain very costly. The purpose of this study is then to investigate improvements of MC methods, by using an alternative sampling mechanism for numerical integration usually referred to as Quasi-Monte Carlo (QMC) integration. In this study, QMC method is coupled to Large Eddy Simulation (LES) of a configuration where the radiation plays an important role: the methane-air flame investigated during the experimental campaign Oxytec. Coupled and non-coupled simulations are compared and their comparison with experimental data shows that thermal radiation has an impact on both flow and flame topology. Finally a good agreement is found between numerical wall fluxes and experimental conductive fluxes.

Direct and large-eddy simulations of wall-bounded turbulent flows in complex geometries are presented in the thesis. To avoid the challenging resolution requirements of the near-wall region, we develop a virtual wall model in generalized curvilinear coordinates and incorporate the non-equilibrium effects via proper treatment of the momentum equations. The wall-modeled large-eddy simulation (WMLES) framework is formulated based on the wall model, accomplished via the stretched-vortex subgrid scale (SGS) model for the LES region. Based on this, we develop high-resolution in-house CFD codes, including direct numerical simulation (DNS), wall-resolved simulation (WRLES) and WMLES for wall-bounded turbulence simulations in complex geometries. First, we present LES of flow past different airfoils with Rec, based on the free-stream velocity and airfoil chord length, ranging from 104 to 2.1106. The numerical results are verified with DNS at low Rec, and validated with experimental data at higher Rec, including typical aerodynamic properties such as pressure coefficient distributions, velocity components, and also more challenging measurements such as skin-friction coefficient and Reynolds stresses. The unsteady separation behavior is investigated with skin friction portraits, which reveal a monotonic shrinking of the near wall structure scale. Second, we present LES of turbulent flow in a channel constricted by streamwise periodically distributed hill-shaped protrusions. Two Reynolds number cases, i.e. Reh=10595 and 33000 (based on the hill height and bulk mean velocity through the hill crest), are utilized to verify and validate our WMLES results. All comparisons show reasonable agreement, which enables us to further probe simulation results at higher Reynolds number (Reh=105). The Reynolds number effects are investigated, with emphasis on the mean skin-friction coefficients, separation bubble size and pressure fluctuations. The flow field at the top wall is evaluated with the empirical friction law and log-law as in planar channel flows. Finally, we present DNS of flow past the NACA0012 airfoil (Rec=104, AoA=10) with wavy roughness elements located near the leading edge. The effects of 2D surface roughness on the aerodynamic performance are investigated. For k8, massive separation occurs and almost covers the suction side of the airfoil dominating the airfoil aerodynamic performance.

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