A wind tunnel based investigation into the effects of unsteady yaw angles on the aerodynamics of a simple automotive body has been carried out to increase the understanding of the effects of unsteady onset conditions similar to those experienced in normal driving conditions. Detailed flow field measurements have been made using surface pressure tappings and PIV around a simple automotive model in steady state conditions and these have been compared to measurements made whilst the model was oscillating in the yaw plane. The oscillating motion was created by a motored crank which was used to produce consistent and repeated motion which produced a reduced frequency that indicated that a quasi-static response should be expected. The PIV data are used to compare the wake flow structures and the surface pressures are used to infer aerodynamic loads and investigate the development of the flow structures across the surfaces of the model. This includes a comprehensive comparison of the surface pressures on the sides of the model during a transient and quasi-static yaw angel oscillation. These results show differences between the two test conditions with the oscillating model results containing hysteresis and the greatest differences in the flow field occurring on the leeside of the model. Two configurations of the same model with different rear pillar geometries were used to isolate model specific effects. Square rear pillars create strong and stable trailing vortices which are less affected by the model motion whereas radiused rear pillars created weaker and less steady vortices that mixed with the quasi-2D wake behind the model base and were affected to a greater extent by the model motion. The unsteadiness in the trailing vortex separation feeds upstream into the A-pillar vortex demonstrating that small geometry changes at the rear can affect the entire flow field around the model.

In this study, application of computational fluid dynamics to ground vehicle aerodynamics was investigated. Two types of vehicle models namely, Ahmed Body and MIRA Notchback Body and their scaled models were used. A commercial software &quot / Fluent&quot / was used and the effects of implementing different turbulence models with wall functions were observed. As a result, an appropriate turbulence model was selected to use in the study. The drag forces, surface pressure distributions and wake formations were investigated in simulation of various test cases available in the literature. The study was extended to simulate the aerodynamics of the vehicles in close-following situation. The results were then compared with available wind tunnel test data.

O presente trabalho apresenta um estudo numérico dos efeitos da aplicação de geometrias geradoras de vórtices, com intuito de controlar passivamente a camada limite, em um perfil aerodinâmico que integra a asa traseira de multi elementos de um veículo de Fórmula SAE. As equações de Navier-Stokes com médias de Reynolds foram resolvidas utilizando o modelo k-ω SST (Shear Stress Transport) para o problema de fechamento da turbulência. Uma metodologia numérica padrão foi definida e utilizada nos diferentes casos analisados. Domínio de cálculo, malha, condições de contorno e critério de convergência foram escolhidos com base em norma SAE para análise numérica de escoamento externo em veículos terrestres. As camadas de volumes prismáticos próximos as superfícies com não-deslizamento foram dimensionadas de forma a resultar em um tratamento de parede adequado ao modelo de turbulência aplicado. O método GCI (Grid Convergence Index) foi utilizado para avaliar a qualidade da malha. Com o intuito de reduzir o custo computacional nos testes com diferentes configurações de geradores de vórtices, apenas parte de interesse do domínio de cálculo foi resolvido, impondo perfis de velocidade, energia cinética da turbulência e dissipação específica em sua entrada. Estas condições foram importadas da simulação com domínio completo resolvida Para verificar a correta captação dos principais efeitos físicos envolvidos, comparações com resultados experimentais foram feitas para 2 casos com escoamentos representativos: o corpo de Ahmed e um perfil aerodinâmico com geradores de vórtices. Além disso, as diferenças entre resolver o domínio completo ou parcial foram estudadas em outro comparativo com resultados experimentais. Concluiu-se que a metodologia numérica foi capaz de obter os coeficientes aerodinâmicos, e suas tendências frente a mudanças de geometria, nos casos estudados. Resolver parcialmente o domínio, impondo perfis em sua entrada, acarretou em diferença nos coeficientes obtidos na ordem de 2% para o coeficiente de sustentação e 7% para o coeficiente de arrasto. O controle passivo via geradores de vórtices foi eficaz em atrasar a separação da camada limite no flap do veículo de Fórmula SAE, as melhoras nos coeficientes de arrasto e sustentação foram da ordem de 7% e 0,3%, respectivamente. / The present work is a numerical study of the effects of the application of vortex generating geometries, in order to passively control the boundary layer, in an aerodynamic profile that integrates a multi-element rear wing of a Formula SAE vehicle. The Reynolds Averaged Navier-Stokes equations were solved using the k-ω Shear Stress Transport model for the turbulence closure problem. A standard numerical methodology was defined and used in the different cases analyzed. Computational domain, mesh, boundary conditions and convergence criteria were chosen based on SAE standard for numerical analysis of external flow in land vehicles. The layers of prismatic volumes near the non-slip surfaces were dimensioned to result in a wall treatment suitable to the applied turbulence model. The Grid Convergence Index (GCI) method was applied to evaluate the mesh quality. In order to reduce the computational cost in tests with different vortex generators configurations, only the part of interest of the calculation domain was solved, imposing velocity, turbulent kinetic energy and specific dissipation profiles on its inlet These conditions were imported from the full domain simulation already solved. To verify the correct capture of the main physical effects involved, comparisons with experimental results were made for 2 cases with representative flows: the Ahmed body and an aerodynamic profile with vortex generators. In addition, the differences between solving the complete or partial domain were studied in another comparative with experimental results. It was concluded that the numerical methodology was able to obtain the aerodynamic coefficients, and their tendencies against changes of geometry, in the cases studied. Partially solving the domain, imposing profiles at its entrance, resulted in a difference in the coefficients obtained in the order of 2% for the lift coefficient and 7% for the drag coefficient. The passive control via vortex generators was effective in delaying the separation of the boundary layer on the flap of the Formula SAE vehicle, the improvements in drag and lift coefficients were of the order of 7% and 0,3%, respectively.

O presente trabalho apresenta um estudo numérico dos efeitos da aplicação de geometrias geradoras de vórtices, com intuito de controlar passivamente a camada limite, em um perfil aerodinâmico que integra a asa traseira de multi elementos de um veículo de Fórmula SAE. As equações de Navier-Stokes com médias de Reynolds foram resolvidas utilizando o modelo k-ω SST (Shear Stress Transport) para o problema de fechamento da turbulência. Uma metodologia numérica padrão foi definida e utilizada nos diferentes casos analisados. Domínio de cálculo, malha, condições de contorno e critério de convergência foram escolhidos com base em norma SAE para análise numérica de escoamento externo em veículos terrestres. As camadas de volumes prismáticos próximos as superfícies com não-deslizamento foram dimensionadas de forma a resultar em um tratamento de parede adequado ao modelo de turbulência aplicado. O método GCI (Grid Convergence Index) foi utilizado para avaliar a qualidade da malha. Com o intuito de reduzir o custo computacional nos testes com diferentes configurações de geradores de vórtices, apenas parte de interesse do domínio de cálculo foi resolvido, impondo perfis de velocidade, energia cinética da turbulência e dissipação específica em sua entrada. Estas condições foram importadas da simulação com domínio completo resolvida Para verificar a correta captação dos principais efeitos físicos envolvidos, comparações com resultados experimentais foram feitas para 2 casos com escoamentos representativos: o corpo de Ahmed e um perfil aerodinâmico com geradores de vórtices. Além disso, as diferenças entre resolver o domínio completo ou parcial foram estudadas em outro comparativo com resultados experimentais. Concluiu-se que a metodologia numérica foi capaz de obter os coeficientes aerodinâmicos, e suas tendências frente a mudanças de geometria, nos casos estudados. Resolver parcialmente o domínio, impondo perfis em sua entrada, acarretou em diferença nos coeficientes obtidos na ordem de 2% para o coeficiente de sustentação e 7% para o coeficiente de arrasto. O controle passivo via geradores de vórtices foi eficaz em atrasar a separação da camada limite no flap do veículo de Fórmula SAE, as melhoras nos coeficientes de arrasto e sustentação foram da ordem de 7% e 0,3%, respectivamente. / The present work is a numerical study of the effects of the application of vortex generating geometries, in order to passively control the boundary layer, in an aerodynamic profile that integrates a multi-element rear wing of a Formula SAE vehicle. The Reynolds Averaged Navier-Stokes equations were solved using the k-ω Shear Stress Transport model for the turbulence closure problem. A standard numerical methodology was defined and used in the different cases analyzed. Computational domain, mesh, boundary conditions and convergence criteria were chosen based on SAE standard for numerical analysis of external flow in land vehicles. The layers of prismatic volumes near the non-slip surfaces were dimensioned to result in a wall treatment suitable to the applied turbulence model. The Grid Convergence Index (GCI) method was applied to evaluate the mesh quality. In order to reduce the computational cost in tests with different vortex generators configurations, only the part of interest of the calculation domain was solved, imposing velocity, turbulent kinetic energy and specific dissipation profiles on its inlet These conditions were imported from the full domain simulation already solved. To verify the correct capture of the main physical effects involved, comparisons with experimental results were made for 2 cases with representative flows: the Ahmed body and an aerodynamic profile with vortex generators. In addition, the differences between solving the complete or partial domain were studied in another comparative with experimental results. It was concluded that the numerical methodology was able to obtain the aerodynamic coefficients, and their tendencies against changes of geometry, in the cases studied. Partially solving the domain, imposing profiles at its entrance, resulted in a difference in the coefficients obtained in the order of 2% for the lift coefficient and 7% for the drag coefficient. The passive control via vortex generators was effective in delaying the separation of the boundary layer on the flap of the Formula SAE vehicle, the improvements in drag and lift coefficients were of the order of 7% and 0,3%, respectively.

O presente trabalho apresenta um estudo numérico dos efeitos da aplicação de geometrias geradoras de vórtices, com intuito de controlar passivamente a camada limite, em um perfil aerodinâmico que integra a asa traseira de multi elementos de um veículo de Fórmula SAE. As equações de Navier-Stokes com médias de Reynolds foram resolvidas utilizando o modelo k-ω SST (Shear Stress Transport) para o problema de fechamento da turbulência. Uma metodologia numérica padrão foi definida e utilizada nos diferentes casos analisados. Domínio de cálculo, malha, condições de contorno e critério de convergência foram escolhidos com base em norma SAE para análise numérica de escoamento externo em veículos terrestres. As camadas de volumes prismáticos próximos as superfícies com não-deslizamento foram dimensionadas de forma a resultar em um tratamento de parede adequado ao modelo de turbulência aplicado. O método GCI (Grid Convergence Index) foi utilizado para avaliar a qualidade da malha. Com o intuito de reduzir o custo computacional nos testes com diferentes configurações de geradores de vórtices, apenas parte de interesse do domínio de cálculo foi resolvido, impondo perfis de velocidade, energia cinética da turbulência e dissipação específica em sua entrada. Estas condições foram importadas da simulação com domínio completo resolvida Para verificar a correta captação dos principais efeitos físicos envolvidos, comparações com resultados experimentais foram feitas para 2 casos com escoamentos representativos: o corpo de Ahmed e um perfil aerodinâmico com geradores de vórtices. Além disso, as diferenças entre resolver o domínio completo ou parcial foram estudadas em outro comparativo com resultados experimentais. Concluiu-se que a metodologia numérica foi capaz de obter os coeficientes aerodinâmicos, e suas tendências frente a mudanças de geometria, nos casos estudados. Resolver parcialmente o domínio, impondo perfis em sua entrada, acarretou em diferença nos coeficientes obtidos na ordem de 2% para o coeficiente de sustentação e 7% para o coeficiente de arrasto. O controle passivo via geradores de vórtices foi eficaz em atrasar a separação da camada limite no flap do veículo de Fórmula SAE, as melhoras nos coeficientes de arrasto e sustentação foram da ordem de 7% e 0,3%, respectivamente. / The present work is a numerical study of the effects of the application of vortex generating geometries, in order to passively control the boundary layer, in an aerodynamic profile that integrates a multi-element rear wing of a Formula SAE vehicle. The Reynolds Averaged Navier-Stokes equations were solved using the k-ω Shear Stress Transport model for the turbulence closure problem. A standard numerical methodology was defined and used in the different cases analyzed. Computational domain, mesh, boundary conditions and convergence criteria were chosen based on SAE standard for numerical analysis of external flow in land vehicles. The layers of prismatic volumes near the non-slip surfaces were dimensioned to result in a wall treatment suitable to the applied turbulence model. The Grid Convergence Index (GCI) method was applied to evaluate the mesh quality. In order to reduce the computational cost in tests with different vortex generators configurations, only the part of interest of the calculation domain was solved, imposing velocity, turbulent kinetic energy and specific dissipation profiles on its inlet These conditions were imported from the full domain simulation already solved. To verify the correct capture of the main physical effects involved, comparisons with experimental results were made for 2 cases with representative flows: the Ahmed body and an aerodynamic profile with vortex generators. In addition, the differences between solving the complete or partial domain were studied in another comparative with experimental results. It was concluded that the numerical methodology was able to obtain the aerodynamic coefficients, and their tendencies against changes of geometry, in the cases studied. Partially solving the domain, imposing profiles at its entrance, resulted in a difference in the coefficients obtained in the order of 2% for the lift coefficient and 7% for the drag coefficient. The passive control via vortex generators was effective in delaying the separation of the boundary layer on the flap of the Formula SAE vehicle, the improvements in drag and lift coefficients were of the order of 7% and 0,3%, respectively.

Le contrôle actif d’écoulement est actuellement étudié dans le but d’améliorer les performances aérodynamiques des véhicules aériens ou terrestres. La diminution de la traînée permettrait de réduire la consommation de carburants fossiles et donc l’émission des gaz à effet de serre des véhicules. Les actionneurs fluidiques sont utilisés comme dispositifs de contrôle depuis une quinzaine d’année. Le contrôle par jet synthétique semble être le plus adapté à une application sur un véhicule de série dans la mesure où l’actionneur ne doit pas être alimenté en fluide. Le travail présenté dans cette thèse combine l’expérimentation physique et la simulation numérique. Elle s’intéresse tout particulièrement au contrôle de l’écoulement autour d’un corps de Ahmed à culot droit à l’aide d’un actionneur de type jet synthétique. Les essais en soufflerie ont été essentiellement utilisés pour caractériser l’écoulement autour du corps de Ahmed et dans son sillage. L’écoulement autour de cette géométrie simplifiée de véhicule terrestre a été caractérisé par des pesées aérodynamiques, des mesures de pressions pariétales, des acquisitions des fluctuations de vitesse par anémomètrie à fil chaud et des mesures de champs de vitesse par Vélocimétrie par Images de Particules. Les grandeurs moyennes et instationnaires de l’écoulement ont ainsi pu être caractérisées. Les simulations numériques à l’aide du code de calcul elsA ont ensuite été réalisées sur une configuration similaire. Les résultats des simulations de l’écoulement non contrôlé ont été confrontés aux résultats expérimentaux. Dans le but d’agir sur la traînée, le contrôle à l’aide d’un actionneur de type jet synthétique a été réalisé sur la même géométrie. Les paramètres de contrôle tels que la quantité de mouvement, la fréquence d’actionnement et l’orientation des jets synthétiques ont été testés numériquement. Le contrôle à l’aide des paramètres testés, a entrainé une augmentation de la traînée qui est due à une réduction de la longueur de la zone de recirculation associée à une diminution de la pression pariétale au niveau du culot de la maquette. Il ressort de ce travail que le contrôle par jet synthétique à basse fréquence orienté selon le sens principal de l’écoulement semble être une voie à explorer. / Active flow control is currently studied in order to improve aerial or ground vehicle aerodynamics. Diminishing aerodynamic drag leads to a reduction of fuel consumption and so in greenhouse gas emissions of vehicles. Fluidic actuators have been used as control devices for about fifteen years. Considering the fact that the actuator does not need external fluid supply system, synthetic jet control seems to be the most suitable solution that can fit on production vehicles. This thesis combines experimental tests and numerical simulations. It tackles with the flow control around a square back Ahmed body with synthetic jet actuator. Wind tunnel tests have essentially been used to characterize the flow around and in the wake of the Ahmed body. Flow around this simplified geometry of ground vehicle has been characterized using hot wire anemometry, flush mounted pressure taps and two components Particular Image Velocimetry. The steady and unsteady features of the wake flow have thus been characterized. Simulations of this flow have been performed with the computation code elsA. Results of the simulations of the natural flow around the square back Ahmed body have been compared to experimental results. With a view to modifying the drag, flow control thanks to a synthetic jet actuator has been tested on a square back Ahmed body. Parameters of the flow control, such as momentum coefficient, actuation frequency and orientation of the synthetic jet have been numerically investigated. Results show a decrease of the circulation length leading to a diminution of the base pressure and hence to an increase of the drag. Flow control by using a low frequency with slots oriented along the mainstream seems to be a path to explore.