Minimize Vortex Drag of a Passenger Car

Licentiate thesis, 2010

The aerodynamic drag force is the single biggest resistance force for a passenger car to overcome at highway speeds. Since all driving resistance forces can be directly linked to fuel consumption this has become a significant priority in recent years. Increasing fuel prices and environmental issues are strong drivers for reducing these resistance forces. Vehicle manufacturers are today struggling to develop more energy efficient vehicles that will meet future emissions targets of CO2 (carbon dioxide). To do so it is essential to improve both efficiency of driveline and reduce resistance such as inertia and drag forces. Pressure forces from the exterior body of a passenger car are the dominating forces of the total drag force. This will classify a passenger car aerodynamically as a bluff body. The biggest pressure forces are associated to wake formations at the rear end of a bluff body. This is the reason for this study of simplified vehicle like bluff bodies focusing on the rear end. Detailed flow field investigations of wake flows behind the models and boundary layer flows close to the surfaces have been performed. The measurements were carried out with stationary ground simulation in the L2 scale model wind tunnel at Chalmers University of Technology in Gothenburg. The wake flow was measured with a small scale 12-hole omniprobe that is capable of capturing almost reversed flows. The purpose was to measure the wake flow of small vehicle-like bodies in ground proximity to determine preferred rear end bodywork geometries. The testing was carried out on four different rear end type of models labeled boat-tailed, fastback and square-back rear end. It is important to have a small and balanced wake to reduce drag. It is preferable to have a high pressure recovery to the rear part of the body and minimum vortices. The drag force is due to the pressure difference between the front and rear of the body. Computational Fluid Dynamics (CFD) simulations have been performed on the same bodies with the same boundary conditions as the wind tunnel tests. The numerical settings were selected to compare standard simulation methods generally used for external vehicle aerodynamics.