A Sensitivity Study of Some Numerical and Geometrical Parameters Affecting Lift Volvo Car Corporation

Volvo Car Corporation (VCC) uses Computational Fluid Dynamics (CFD) and wind tunnel during the aerodynamic development of new vehicles. In the past VCC main focus has been on the drag force correlation to the wind tunnel measurements but in recent years improved methods for lift force correlations has been highly wanted. Three objectives were considered in this study to improve the lift force correlation between the CFD simulations and wind tunnel measurements for geometrical configurations of the V60 and S60 models. Poor mesh resolution for the wall bounded flow existed for the VCC mesh method and therefore prisms layers were considered in this thesis to increase the mesh resolution inside the boundary layer. As slick tyres generally were used in the CFD simulations better geometrical correlation was wanted to be studied as it could improve the lift force correlation between CFD simulations and wind tunnel measurements. Therefore detailed tyres were considered in this study. As the coarsest surface mesh size was used for the underbody and the components inside the engine bay, where some of the highest flow velocities occurred, mesh refinements were investigated for engine bay and underbody in this study. The prisms layers improved the predicted behavior for the boundary layer as it captured the large velocity gradients more accurately. Due to this, the skin friction prediction was also improved. Different flow behavior around the front wheels and rear wake occurred due to earlier separation. The different flow field caused an improved correlation for the lift force but worsened correlation for the drag force due to increased pressure at the rear of the cars. However, the front lift force trend correlation for the considered configurations was improved with the prisms layer mesh method. The detailed tyres caused slight more disturbances for the underbody flow which caused more attached flow around the rear of the car hence lowered pressure. Earlier separation around the front wheels also occurred for the detailed tyre geometry as the disturbed flow around the wheels was increased. Slight improved correlation for the front and rear lift forces to the wind tunnel measurements could be seen with the detailed tyre compared to the slick tyre. The mesh refinements for the engine bay and underbody showed significant differences for the flow at the underbody which had significant impact on the flow at the rear wake for the V60 model. Minor differences could be seen for the aerodynamic …