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.

Dans cette thèse nous explorons les possibilités offertes par l'implémentation de méthodes hybrides sur des machines de calcul hétérogènes dans le but de réaliser des simulations numériques de problèmes multiéchelles. La méthode hybride consiste à coupler des méthodes de diverses natures pour résoudre les différents aspects physiques et numériques des problèmes considérés. Elle repose sur une méthode particulaire avec remaillage qui combine les avantages des méthodes Lagrangiennes et Eulériennes. Les particules sont déplacées selon le champ de vitesse puis remaillées à chaque itération sur une grille en utilisant des formules de remaillage d'ordre élevés. Cette méthode semi-Lagrangienne bénéficie des avantages du maillage régulier mais n'est pas contrainte par une condition de CFL.Nous construisons une classe de méthodes d'ordre élevé pour lesquelles les preuves de convergence sont obtenues sous la seule contrainte de stabilité telle que les trajectoires des particules ne se croisent pas.Dans un contexte de calcul à haute performances, le développement du code de calcul a été axé sur la portabilité afin de supporter l'évolution rapide des architectures et leur nature hétérogène. Une étude des performances numériques de l'implémentation GPU de la méthode pour la résolution d'équations de transport est réalisée puis étendue au cas multi-GPU. La méthode hybride est appliquée à la simulation du transport d'un scalaire passif dans un écoulement turbulent 3D. Les deux sous-problèmes que sont l'écoulement turbulent et le transport du scalaire sont résolus simultanément sur des architectures multi-CPU et multi-GPU. / In this work, we investigate the implementation of hybrid methods on heterogeneous computers in order to achieve numerical simulations of multi-scale problems. The hybrid numerical method consists of coupling methods of different natures to solve the physical and numerical characteristics of the problem. It is based on a remeshed particle method that combines the advantages of Lagrangian and Eulerian methods. Particles are pushed by local velocities and remeshed at every time-step on a grid using high order interpolation formulas. This forward semi-lagrangian method takes advantage of the regular mesh on which particles are reinitialized but is not limited by CFL conditions.We derive a class of high order methods for which we are able to prove convergence results under the sole stability constraint that particle trajectories do not intersect.In the context of high performance computing, a strong portability constraint is applied to the code development in order to handle the rapid evolution of architectures and their heterogeneous nature. An analysis of the numerical efficiency of the GPU implementation of the method is performed and extended to multi-GPU platforms. The hybrid method is applied to the simulation of the transport of a passive scalar in a 3D turbulent flow. The two sub-problems of the flow and the scalar calculations are solved simultaneously on multi-CPU and multi-GPU architectures.

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.

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 porte sur la dynamique de particules dans des écoulements turbulents, en particulier sur l'apparition de structures. Deux situations physiques sont étudiées. D'une part, dans le cas du mouvement de traceurs, c'est-à-dire de particules fluides de même composition que le flot, transportés par un champ de vitesse turbulent bidimensionnel, un triplet de particules (un triangle) tend à se déformer en une structure très allongée sous l'action de l'écoulement. D'autre part, pour des particules inertielles de densité grande devant celle du fluide et soumises à une force de traînée, des distributions spatiales fortement inhomogènes peuvent apparaître, conduisant à la formation d' attracteurs étranges. L'approche suivie dans cette thèse consiste à modéliser l'action de l'écoulement turbulent en utilisant des outils de dynamique stochastique (équations de Langevin), qui permettent d'obtenir une description effective des comportements observés. Dans le cas des particules inertielles, les attracteurs sont caractérisés par une dimension fractale. L’ajout d’un bruit dans les équations du mouvement a permis d'étendre cette notion à des valeurs de dimension négatives, intrinsèques à la dynamique en l'absence de bruit. Cette thèse établit qu'il est possible de formuler les deux problèmes physiques étudiés en termes de processus stochastiques très généraux, dont le prototype est celui décrivant la sédimentation de particules en présence de bruit thermique. La détermination des caractéristiques de la solution requiert une nouvelle approche. La solution proposée ici est basée sur la théorie des grandes déviations. / This thesis deals with the dynamics of particles in turbulent flows and the formation of structures. Two physical situations are studied. First, we consider the dynamics of tracers, that is ideal fluid particles, transported by a turbulent velocity field. A triplet of such particles forms a triangle, which tends to be flattened under the action of the incompressible flow. Second, inertial particles of density higher than that of the fluid and subjected to a viscous drag force usually cluster on regions of high concentration, leading to the formation of strange attractors. The approach followed in this thesis consists in modeling the action of the turbulent flow using tools of stochastic dynamics (such as Langevin equations), which allow us to obtain a effective description of these phenomena. For inertial particles, the attractors are characterized by a non-integer fractal dimension. The addition of an external noise in the equations of motion lead to a generalization of this notion to negative values, intrinsic to the dynamics in the absence of noise. This thesis shows that it is possible to formulate the two problems in terms of very general stochastic processes, whose prototype is the one describing the sedimentation of particles in the presence of a thermal noise. The determination of the characteristics of the solution requires a new approach. The solution proposed here is based on the large deviation theory.

Bubbly turbulent flows occur in a variety of industrial, naval and geophysical problems. In these flows, the bubbles in the flow interact with turbulence and/or vortical structures present in the continuous phase, resulting in bubble motion and deformation, and at the same time modifying the turbulence and/or vortical structures. Despite the fact that this has been a subject of interest for some time, mechanisms of bubble break-up due to turbulence and turbulence modulation due to bubbles are not well understood. To help understand this two-way coupled problem, we study in this thesis, the interaction of single and multiple bubbles with vortical structures; the thesis being broadly divided in to three parts. In the first part, we study the interaction of a single bubble with a single vortical structure, namely a vortex ring, formed in the continuous phase (water). This may be thought of as a simplified case of the interaction of bubbles with vortical structures in any turbulent flow. We then increase the complexity and study the interaction of a single bubble with naturally occurring vortical structures present in a fully developed turbulent channel flow, and then finally to the case of a large number of bubbles injected in to a fully developed turbulent channel. The bubble motions and deformations in all three cases are directly imaged using high speed visualizations, while the flow field information is obtained using time-resolved Particle-Image Velocimetry (PIV) in the first two cases, and from pressure drop measurements within the channel in the latter case. The interaction of a single vortex ring with a bubble has been studied for a large range of vortex ring strengths, represented in terms of a Weber number (We). We find that in all cases, the bubble is first captured by the low pressure within the core of the ring, then stretched azimuthally within the core, and gradually broken up in to a number of smaller bubbles. Along with these bubble deformations, the vorticity within the core of the ring is also modified significantly due to bubble capture. In particular, at low We, we find that the core of the ring fragments as a result of the interaction resulting in a large reduction in the enstrophy of the ring and its convection speed. In the second part of the thesis, interaction of a single bubble with naturally occurring vortical structures present in a fully developed turbulent channel is studied. In this case, single bubbles of different sizes are injected either from bottom or top wall into a channel at Reynolds number of about 60,000. We study the trajectories of the single bubble, and also investigate the effect that such bubbles have on the naturally occurring vortical structures present in these flows. The injected bubble is found to have three broadly different types of bubble paths when injected from the bottom wall, which are sliding along the wall, bouncing motions and vertical escape from the vicinity of the wall. Even at the same bubble diameter Db and channel flow Re, we find that different realizations show considerable variations, with all three bubble paths being possible. PIV measurements of a bubble captured by a naturally occurring vortical structure in the flow, shows a more rapid decrease in enstrophy compared to naturally occurring structures in the absence of bubbles, as seen in the interaction of a bubble with a vortex ring. We also find that the bubble can interact with multiple vortical structures, depending on their strength and spatial distribution in the flow, resulting in a complex bouncing bubble motion. In the third part of the study, a large number of bubbles are injected in to the channel through porous plates fixed at the top and bottom channel walls. The main parameters here are the channel Re, bubble void fraction (α) and the orientation of injection. In this case, in addition to bubble visualizations, the pressure drop through the channel is measured at different vertical locations. These measurements show large vertical variations in the measured pressure drop due to the presence of bubbles. The overall drag reduction in these cases is obtained from an integral of the pressure drop variation along the vertical direction. The visualizations show a number of bubble dynamics regimes depending on the parameters, with possibilities of both increased and decreased drag compared to the reference no bubble case. From simultaneous measurements, we relate the variations in drag reduction to the different bubble dynamics regimes. We find that at the same void fraction (α), the drag reduction obtained can be very different due to changes in bubble dynamics regimes caused by changes in other parameters. Top wall injection is observed to give good drag reductions over a wide range of flow Re and α, but is seen to saturate beyond a threshold α. In contrast, the bottom wall injection case shows that drag reduction continuously increases with αat high Re. The present study shows a maximum of about 60% increase and a similar 60% reduction in wall drag over the entire range of conditions investigated.

The problem of calculating iso-surface statistics in turbulent flows is interesting for a number of reasons, some of them being combustion modeling, entrainment through turbulent/non-turbulent interfaces, calculating mass flux through iso-scalar surfaces and mapping of scalar fields. A fundamental effect of fuid turbulence is to wrinkle scalar iso-surfaces. A review of the literature shows that iso-surface calculations have primarily been done with geometric methods, which have challenges when used to calculate surfaces that have high complexity, such as in turbulent flows. In this thesis, we propose an alternative integral method and test it against analytical solutions. We present a parallelized algorithm and code to enable in-simulation calculation of isosurface area. We then use this code to calculate area statistics for data obtained from Direct Numerical Simulations and make predictions about the variation of the iso-scalar surface area with Taylor Peclet numbers between 9.8 and 4429 and Taylor Reynolds numbers between 98 and 633.

Sandstorms are frequently accompanied by intense electric fields and lightning. In a very narrow region close to the ground, sand particles undergo a charge exchange during which larger-sized sand grains become positively charged and smaller-sized sand grains become negatively charged and then all particles become suspended by the turbulent fluid motion. Although the association of intense electric fields with sandstorms has long been observed, the mechanism that causes these intense electric fields has not yet been described. Here, we hypothesize that differently sized sand particles are differentially transported by turbulence in the flow, resulting in a large-scale charge separation and a consequential large-scale electric field. To confirm our hypothesis, we combined a large-eddy simulation framework comprising a turbulent atmospheric boundary layer and movement of sand particles with an electrostatic Gauss law to investigate the physics of the electric fields in sandstorms. We varied the strength of the sandstorm from weak to strong as parametrized by the number density of the entrained sand particles. Our simulations reproduced observational measurements of both mean and root mean squared fluctuation values of the electric field. Our results allowed us to propose a law in which the electric field scales to two-thirds of the power of the concentration of the sand particles in weak-to-medium strength sandstorms. The underlying approach to simulate the solid particle-laden flow is Eulerian-Eulerian in which the particles are characterized by statistical descriptors. To explore the essential physics of the electric field generation in a sandstorm, we model the high-Reynolds-number atmospheric boundary-layer (ABL) using two different canonical turbulent flows: one model is that of a turbulent boundary-layer (TBL), and the second one is that of a turbulent half-channel flow. For the particle phase, the direct quadrature method of moments (DQMOM) is chosen in which the abscissas and weights of the quadrature method are tracked directly. The utilization of this framework is proposed to examine the transport of sand in sandstorms. Furthermore, the physical mechanisms necessary for production and sustenance of large-scale electric fields in sandstorms is investigated.

Depuis la découverte des modes asymétriques dans le sillage d'un corps simplifié d'automobile, réminiscents d'une bifurcation à bas nombre de Reynolds, se posent des questions propres au développement aérodynamique des véhicules terrestres telles que l'influence du vent latéral, de l'assiette et du rétreint d'arrière-corps couramment utilisé en phase d'optimisation. Notre travail s'attache à répondre expérimentalement à ces questions pour des géométries simplifiées mais aussi réelles. Les essais sont réalisés en soufflerie industrielle à l'échelle 2/5 pour le corps académique et en pleine échelle pour les monospaces. Nous montrons que le désalignement du véhicule par rapport à l'écoulement incident n'a pour effet que de modifier l'orientation du mode asymétrique sans en changer l'intensité. Nous construisons un modèle simple prédisant non seulement cette orientation mais aussi les conséquences sur les efforts aérodynamiques transverses. La contribution de l'instabilité sur les coefficients aérodynamiques de portance ou d'effort latéral est de l'ordre de 0,02 indépendamment du vent de travers et de l'assiette du véhicule. Les rétreints d'arrière-corps affectent également la dynamique du sillage et son orientation, mais l'instabilité n'est jamais supprimée. Ces résultats sont retrouvés pour des véhicules réels de type monospace dont le sillage est donc également soumis au même mode asymétrique, révélé sans ambigüité par des expériences de sensibilité en assiette. Nos résultats indiquent que, pour tous les véhicules considérés, le mode asymétrique de sillage est systématiquement présent dans l'enveloppe de conduite. Le contrôle ou la suppression de ce mode devrait offrir de nouvelles perspectives d'optimisation des véhicules à culot droit de type monospaces ou SUV. / Since the recent discovery of asymmetric modes in the wake of a simplified vehicle geometry, reminiscent from a bifurcation at low Reynolds numbers, some questions related to the aerodynamic development of ground vehicles such as the influence of lateral wind, pitch and afterbody boat-tail classically used during shape optimization remain unanswered. Our work is devoted to assess those questions experimentally for simplified but also real geometries. The tests are conducted in an industrial wind-tunnel, at the 2/5-scale for the academic body and at the full scale for the minivans. We show that the vehicle's misalignment only modifies the asymmetric mode's orientation without affecting its intensity. We build a model predicting not only this orientation but also the consequences on the cross-flow aerodynamic loading. The contribution of the instability to the lift or side force coefficients is of the order of 0,02 independently of lateral wind or of the vehicle's pitch. Afterbody boat-tails also impact the wake dynamics and its orientation but the instability is never suppressed. These results are retrieved for real vehicles such as minivans, whose wake is then subjected to the same asymmetric mode as well, revealed unambiguously with pitch sensitivity experiments. Our results indicate that, for all considered vehicles, the asymmetric wake mode is systematically present in the driving envelope. The control or the suppression of this mode should offer new optimization's perspectives for blunt based vehicles such as minivans or SUVs.

<div>High speed laser diagnostics are performed in two vastly differing swirl combustors at conditions relevant for industrial gas turbines. This high quality data can not only be used to elucidate key features of the flow field but also for validation of computational models simulating turbulence, chemistry, and their interactions. The first combustor is a piloted lean premixed prevaporized arrangement common in aviation applications. Fueling parameters are varied and sensitivity towards the pilot flame is observed. Conditioning to the stagnation line demonstrates increased fluctuations of shear and rotation in the inner shear layer when the pilot fueling is reduced.</div><div><br></div><div>The second flame has a simpler configuration with a single swirler and combusting natural gas. Thermoacoustic instability coupled to a helical precessing vortex core is found at certain conditions. Sparse Dynamic Mode Decomposition and phase averaging is applied to the velocity fields to create a three dimensional reconstruction of the helical vortex core in a non-precessing reference frame. Heat release is found to be correlated to the interaction strength of the central recirculation bubble and the helical vortex core. </div><div><br></div><div> </div><div>Finally, intermittent phenomena within a thermoacoustic instability are examined. The most prominent deviation is that the flame is observed to randomly lift and reattach. In addition, a convolutional neural network is employed to extract the morphology from otherwise qualitative OH species imaging. The average characteristics of the lifted and attached flame are compared and dramatic differences are found. All of the flow structures have been altered such as the precessing vortex core being greatly intensified during flame lift-off. Evaluating the average events before flame lift-off revealed the importance of conditions at the combustor inlet. However, evidence for a future reattachment event was only found with a less spatially confined perspective. In addition, transition to lift-off was very sudden while reattachment was far slower.</div>