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Results from a series of experimental measurements are presented in order to investigate the influence of the ground-plane boundary layer on the overall characteristics of a scale model road vehicle. The wind tunnel model is a generic bluff body which has a streamlined forebody, simple wheel representation and interchangeable rear end sections. The aerodynamic forces and moments were measured via an external 3-component balance at a free stream velocity of 24 m/s. corresponding to Reynolds number of 5.5 × 105 based on model length, over a range of ride heights and yaw angles. The ground plane boundary layer thickness was varied artificially. The influence of wheels and underbody roughness were also investigated.

Ever since aerodynamics became an essential element of the automobile design process, the principal development tool for the vehicle aerodynamicist has been the full-scale wind tunnel. In the absence of a reliable alternative, it is expected that this will continue for many years. As a true simulation of the conditions on the road the conventional full-scale wind tunnel has limitations. The ground is fixed relative to the vehicle allowing an unrepresentative boundary layer to develop, the wheels of the test vehicle do not rotate and there is some uncertainty over the influences imposed by the tunnel walls. In addition, the aerodynamic data obtained from different wind tunnels shows a degree of scatter and even configuration changes do not necessarily produce consistent effects. With particular regard for aerodynamic drag, the aerodynamicist should ensure that gains obtained in the wind tunnel generate real benefits on the road.

It is not generally appreciated that the longitudinal position of a road vehicle model in a wind tunnel can have a significant influence on its measured aerodynamic drag. This paper explores the influence of the proximity of the end of the test section on measured aerodynamic drag, where the ‘end’ of the test section is defined by the start of the first diffuser or the end of a separate groundboard. Both flat plates and three-dimensional, automotive shapes were tested in three different model-scale and full-scale wind tunnels. It was found that the drag began to change from its upstream, undisturbed value when a vehicle model was closer than a distance of four times the square root of its base area from the end of the test section and that large changes occur when a vehicle model was closer than twice the square-root of its base area to the end of the test section. The effect is attributed to base pressure changes in the proximity of the diffuser or of the end of a groundboard.