Gradient Effects on Drag Due to Boundary-Layer Suction in Automotive Wind Tunnels 2003-01-0655

A region with floor boundary-layer suction upstream of the vehicle to remove the oncoming boundary layer is often used in automotive wind tunnels. These suction systems inevitably change the empty-tunnel pressure gradient. In this paper, the empty-tunnel pressure gradient created by the use of boundary layer suction and its effect on measured drag are investigated. By using excess suction - more suction than necessary to remove the floor boundary layer – it was possible to show experimentally that the major part of the drag increase due to boundary layer suction is created by unintended gradient effects. Only a minor part of the drag increase is due to the increased flow velocities at the lower parts of the vehicle, or in other words, due to the improved ground simulation. A theoretical model, using the concept of horizontal buoyancy to predict the gradient effect, is proposed. The model is compared to the experimental results as well as to CFD calculations. The use of the analytical correction method to correct for the unintended gradient effects is expected to be a significant improvement for results obtained using boundary layer suction. Alternative methods to control the boundary layer thickness, which do not introduce strong empty-tunnel gradients, are recommended.