Minimize Vortex Drag of a Passenger Car

Abstract: The aerodynamic drag force is the single biggest resistance force for a passenger car toovercome at highway speeds. Since all driving resistance forces can be directly linked tofuel consumption this has become a significant priority in recent years. Increasing fuelprices and environmental issues are strong drivers for reducing these resistance forces.Vehicle manufacturers are today struggling to develop more energy efficient vehicles thatwill meet future emissions targets of CO2 (carbon dioxide). To do so it is essential toimprove 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 thetotal drag force. This will classify a passenger car aerodynamically as a bluff body. Thebiggest 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 therear end.Detailed flow field investigations of wake flows behind the models and boundary layerflows close to the surfaces have been performed. The measurements were carried outwith stationary ground simulation in the L2 scale model wind tunnel at ChalmersUniversity of Technology in Gothenburg. The wake flow was measured with a smallscale 12-hole omniprobe that is capable of capturing almost reversed flows. The purposewas to measure the wake flow of small vehicle-like bodies in ground proximity todetermine preferred rear end bodywork geometries. The testing was carried out on fourdifferent 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 ahigh pressure recovery to the rear part of the body and minimum vortices. The drag forceis due to the pressure difference between the front and rear of the body.Computational Fluid Dynamics (CFD) simulations have been performed on the samebodies with the same boundary conditions as the wind tunnel tests. The numericalsettings were selected to compare standard simulation methods generally used forexternal vehicle aerodynamics.

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