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This paper presents a series of observations of time-averaged wake asymmetry for a range of “notchback” vehicle geometries. The primary focus is on a reduced scale experiment using full-sized saloon geometry. Substantial flow asymmetry was observed in the vehicle “notch”. Similar asymmetries are reported for a full scale experiment on the same geometry along with others as diverse as production models of a luxury and mid-sized saloon; basic car shapes and a simple body. In one case a physical explanation is proposed, based on the degeneration of an unstable symmetric wake structure.

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.

An accurate simulation of a ground vehicle interacting with a crosswind gust can be achieved by using a moving model mounted on a track such that it can traverse the working section of a conventional atmospheric boundary layer wind tunnel. This paper will briefly describe the facility that is being developed at Cranfield University and detail comparisons between static and dynamic data from tests on three basic model configurations. Under the same nominal wind input, data from static tests compares well with that from dynamic tests at yaw angles below 15°. At higher yaw angles, after the onset of “large scale” separation, the dynamic values of the forces and moments become larger than the static values.

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.