Three different topics of turbulence research that cover modeling, theory and model computation categories are selected and studied in depth. In the first topic, "velocity gradient dynamics in turbulence" (modeling), the Lagrangian linear diffusion model that accounts for the viscous-effect is proposed to make the existing restricted-Euler velocity gradient dynamics model quantitatively useful. Results show good agreement with DNS data. In the second topic, "pressure-strain correlation in homogeneous anisotropic turbulence subject to rapid strain-dominated distortion" (theory), extensive rapid distortion calculation is performed for various anisotropic initial turbulence conditions in strain-dominated mean flows. The behavior of the rapid pressure-strain correlation is investigated and constraining criteria for the rapid pressure-strain correlation models are developed. In the last topic, "unsteady computation of turbulent flow past a square cylinder using partially-averaged Navier-Stokes method" (model computation), the basic philosophy of the PANS method is reviewed and a practical problem of flow past a square cylinder is computed for various levels of physical resolution. It is revealed that the PANS method can capture many important unsteady flow features at an affordable computational effort.

Three different topics of turbulence research that cover modeling, theory and model computation categories are selected and studied in depth. In the first topic, "velocity gradient dynamics in turbulence" (modeling), the Lagrangian linear diffusion model that accounts for the viscous-effect is proposed to make the existing restricted-Euler velocity gradient dynamics model quantitatively useful. Results show good agreement with DNS data. In the second topic, "pressure-strain correlation in homogeneous anisotropic turbulence subject to rapid strain-dominated distortion" (theory), extensive rapid distortion calculation is performed for various anisotropic initial turbulence conditions in strain-dominated mean flows. The behavior of the rapid pressure-strain correlation is investigated and constraining criteria for the rapid pressure-strain correlation models are developed. In the last topic, "unsteady computation of turbulent flow past a square cylinder using partially-averaged Navier-Stokes method" (model computation), the basic philosophy of the PANS method is reviewed and a practical problem of flow past a square cylinder is computed for various levels of physical resolution. It is revealed that the PANS method can capture many important unsteady flow features at an affordable computational effort.

Time integration methods are necessary for the solution of transient flow problems. In recent years, interest in transient flow problems has increased, leading to a need for better understanding of the costs and benefits of various time integration schemes. The present work investigates two common time integration schemes, namely the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) and the Fractional Step (FS) method. Three two-dimensional, transient, incompressible flow problems are solved using a cell centered, finite volume code. The three test cases are laminar flow in a lid-driven skewed cavity, laminar flow over a square cylinder, and turbulent flow over a square cylinder. Turbulence is modeled using wall functions and the k - ε turbulence model with the modifications suggested by Kato and Launder. Solution efficiency as measured by the effort carried out by the flow equation solver and CPU time is examined. Accuracy of the results, generated using the SIMPLE and FS time integration schemes, is analyzed through a comparison of the results with existing experimental and/or numerical solutions. Both the SIMPLE and FS algorithms are shown to be capable of solving benchmark flow problems with reasonable accuracy. The two schemes differ slightly in their prediction of flow evolution over time, especially when simulating very slowly changing flows. As the time step size decreases, the SIMPLE algorithm computational cost (CPU time) per time step remains approximately constant, while the FS method experiences a reduction in cost per time step. Also, the SIMPLE algorithm is numerically stable for time steps approaching infinity, while the FS scheme suffers from numerical instability if the time step size is too large. As a result, the SIMPLE algorithm is recommended to be used for transient simulations with large time steps or steady state problems while the FS scheme is better suited for small time step solutions, although both time-stepping schemes are found to be most efficient when their time steps are at their maximum stable value.

Time integration methods are necessary for the solution of transient flow problems. In recent years, interest in transient flow problems has increased, leading to a need for better understanding of the costs and benefits of various time integration schemes. The present work investigates two common time integration schemes, namely the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) and the Fractional Step (FS) method. Three two-dimensional, transient, incompressible flow problems are solved using a cell centered, finite volume code. The three test cases are laminar flow in a lid-driven skewed cavity, laminar flow over a square cylinder, and turbulent flow over a square cylinder. Turbulence is modeled using wall functions and the k - ε turbulence model with the modifications suggested by Kato and Launder. Solution efficiency as measured by the effort carried out by the flow equation solver and CPU time is examined. Accuracy of the results, generated using the SIMPLE and FS time integration schemes, is analyzed through a comparison of the results with existing experimental and/or numerical solutions. Both the SIMPLE and FS algorithms are shown to be capable of solving benchmark flow problems with reasonable accuracy. The two schemes differ slightly in their prediction of flow evolution over time, especially when simulating very slowly changing flows. As the time step size decreases, the SIMPLE algorithm computational cost (CPU time) per time step remains approximately constant, while the FS method experiences a reduction in cost per time step. Also, the SIMPLE algorithm is numerically stable for time steps approaching infinity, while the FS scheme suffers from numerical instability if the time step size is too large. As a result, the SIMPLE algorithm is recommended to be used for transient simulations with large time steps or steady state problems while the FS scheme is better suited for small time step solutions, although both time-stepping schemes are found to be most efficient when their time steps are at their maximum stable value.

Les décollements autour d’un corps en mouvement sont à l’origine de détériorations des performances aérodynamiques, de fatigues structurelles ou de nuisances sonores. La compréhension de ces phénomènes reste encore aujourd’hui l’un des enjeux majeurs de la recherche en aérodynamique. Le développement de systèmes permettant de contrôler l’écoulement et d’altérer ou de réduire les décollements apparaît comme une solution prometteuse en vue d’améliorer les performances aérodynamiques. On distingue les systèmes de contrôles passifs, simples d’utilisation mais incapables de s’adapter aux modifications de l’écoulement, des systèmes actifs qui disposent d’une grande adaptabilité mais nécessitent un apport extérieur d’énergie pour fonctionner. La stratégie du contrôle auto-adaptif s’apparente à un compromis entre ces deux méthodes. En s’inspirant de mécanismes présents dans la nature, elle permet d’associer amélioration des performances aérodynamiques, adaptabilité et autonomie. Ce travail de thèse porte sur l’étude expérimentale du contrôle du sillage turbulent d’un corps épais à l’aide d’actionneurs bio-inspirés avec un double objectif : i. déterminer les paramètres optimaux du dispositif de contrôle qui prend la forme d’un couple de volets flexibles, ii. Identifier les mécanismes physiques d’interactions entre l’actionnement et l’écoulement. Pour mener à bien cet objectif, de nombreux instruments de mesure complémentaires ont été mis en oeuvre. Une étude paramétrique a permis de démontrer l’efficacité du dispositif pour différentes configurations (fixes et auto-adaptatives) et d’identifier des configurations d’intérêt. La caractérisation de l’écoulement autour et dans le sillage du cylindre carré sans et avec contrôle a révélé un allongement de la longueur de recirculation à l’arrière du cylindre et la réduction de l’expansion du sillage. L’un des résultats majeurs de l’étude est que la réduction de traînée obtenue est principalement liée à une action du système sur l’anisotropie des fluctuations de l’écoulement et plus particulièrement sur l’entrainement du fluide dans le sillage de l’obstacle. / Flow separations around moving bodies lead to detrimental effects such as aerodynamic performances loss, structural fatigue and noises production. The understanding of these phenomena remains one of the most challenging issue of modern fluid dynamics. A promising solution to improve aerodynamic performances relies on the development of flow control devices able to prevent or mitigate the effects of separation. One can distinguish the passive flow control strategy, with easy to use devices but unable to adapt to the flow changes, from the active flow control strategy which benefits from a great adaptability but requires external power supply. Self-adaptive flow control appears to be a good compromise between those two strategies. Inspired from mechanisms at play in Nature, it combines good aerodynamic performances, self-adaptability and self-sustainability. This PhD thesis is dedicated to the experimental investigation of the turbulent flow over a bluff-body controlled by means of bio-inspired devices. The objective is two-folds : i. Design the control device which consists of a couple of compliant flaps, ii. Identify the physical mechanisms governing the interactions between the flow and the devices. A great number of complementary measurement techniques have been used in order to achieve these objectives. The efficiency of the devices for different configurations – locked and self-adaptive flaps - has been demonstrated through a parametric study. It has led to the identification of the main parameters involved in the control mechanism. The flow characterization around and in the wake of both uncontrolled and controlled cylinder revealed an increase in the length of the recirculation region and the reduction of the wake width. One of the major findings of this study is that the control essentially modifies the turbulent velocity field leading to a reduction of the lateral flow entrainment in the wake of the obstacle.

Les caractéristiques des transferts de masse et de chaleur par convection forcée lors de l'évaporation du film liquide dans le canal en présence d'un cylindre carré poreux parcouru par un écoulement transversal sont étudiées numériquement. L'objectif principal de la présente étude est d'évaluer l'effet de l'introduction d'un cylindre carré poreux sur le transfert de chaleur et de masse. Plus précisément, cette étude examine l'influence de paramètres tels que l'humidité relative de l'air ambiant, la température de l'air à l'entrée, le flux de chaleur imposé, la position du cylindre, le taux de blocage et le nombre de Reynolds. Une comparaison entre les deux configurations, avec et sans cylindre carré poreux a été réalisée pour mettre en évidence l'effet de son ajout. Pour modéliser ce phénomène nous avons résolu l'équation classique de convection forcée et le modèle de Darcy-Brinkman-Forchheimer dans les milieux poreux. Nous avons trouvé que l'insertion d'un cylindre carré poreux dans le canal pourrait perturber l'écoulement et améliorer de manière significative les taux de transfert de masse et de chaleur au niveau des parois du canal. Les transferts de chaleur et de masse deviennent plus importants avec la diminution du nombre de Darcy et lorsque l'obstacle poreux est placé au milieu du canal. Par contre, l'augmentation de l’humidité relative de l’air ambiant et de la température d'entrée réduit le transfert de masse. A Da = 10-6, l'écoulement ne pénètre plus à travers le cylindre poreux, la structure de l'écoulement est similaire à celui d'un cylindre carré plein. Enfin, nous proposons des lois de corrélations qui permettent de prédire les valeurs des nombres de Sherwood et de Nusselt en fonction des nombres de Reynolds, de Biot et du taux de blocage. / The characteristics of mass and heat transfers by forced convection during liquid film evaporation in the channel with a built in porous square cylinder in a cross flow are investigated numerically. The main objective of the present study is to evaluate the effect of introducing a porous square cylinder on the heat and mass transfer. Specifically, this study examines the influence of parameters such as the relative humidity of the ambient air, the air inlet temperature, the imposed heat flux, the variation of the cylinder position, blockage ratio and Reynolds number on the performance of the evaporation at the channel wall. A comparison between the two configurations, with and without, porous square cylinder has been performed to highlight the effect of its addition. To achieve this, we solved the classic equation of forced convection and the Darcy-Brinkman-Forchheimer model in the porous media. We find that the insertion of a porous square cylinder in the channel could make the flow more disturbed and significantly improve mass and heat transfer rates at the channel walls. The heat and mass transfer enhancements is greater with a decrease of the Darcy number and for γ=1 when the porous obstacle is placed in the middle of the channel. It is also greater with a decrease of the temperature and relative humidity of the air at the inlet. At Da = 10-6, the flow does not penetrate through the porous cylinder, the flow pattern is similar to that of a solid square cylinder. Finally, we propose correlations that allow us to define the Sherwood and Nusselt numbers based on the Reynolds, Biot numbers and the blockage ratio.