Some Aspects on On-Road Aerodynamics

The aerodynamics of modern day passenger cars are usually developed under constant and reproducible conditions in a wind tunnel. However, spontaneously occurring wind changes during on-road drive are of significant interest. Recently, BMW AG has been able to develop virtual methods enabling the investigation of such spontaneous on-road driving events. These methods have been used to assess driving in slipstream, cornering and the impact of naturally occurring wind gusts. During slipstream driving, a noticeable drag-reducing effect was found for a passenger vehicle behind a truck and behind different sized passenger cars. There was a significant effect on vehicle lift. Cornering conditions were examined by only changing the curvature of the streamlines in the flow field. Simulation results indicate a significant change on vehicle coefficients compared to straight line driving. Lastly, the flow in gusty wind conditions was investigated. Simulation results confirmed results from academic wind tunnel measurements. An increase in the aerodynamic yaw moment was found under transient oncoming flow conditions compared to the quasi-steady inflow. Overall, the results from the three investigated driving conditions have a significant relevance of the aerodynamic effects under real driving conditions.