Drag Reduction of a Simple Bluff Body by Changing the Rear End and Use the Ground Effect

The flow field around bluff bodies constitutes a classic area within fluid dynamics and has been the topic for much research through the years. However, in the use for road vehicles with the effect of the ground, the behavior is changed very much from more classical aviation usage. In this paper we are investigating the drag force reduction on a vehicle like simplified model with rear open diffuser when stationary ground simulation is considered. The objective with this work was to study the rear end of a bluff body and optimize it for drag with ground vehicle like boundaries. Here the testing contains two common body variants, square back, boat tailed/fastback in generic forms. Scale model testing combined with simulations is used to explain behavior and flow field. The model testing is performed in the L2 scale model wind tunnel at Chalmers University of Technology in Gothenburg, Sweden. Simulations are done with the commercial CFD code Fluent. A diffuser on a car is normally used to create down force but here it is tested to see if the energy in the flow can be used to optimize reduction of drag. One part of the study is to show the potential in optimizing the rear end underbody for drag, by varying the diffuser angle. The results show a potential in drag reduction by using a diffuser and varying effect depending on other rear end geometries.