The Effect on Aerodynamic Drag of the Longitudinal Position of a Road Vehicle Model in a Wind Tunnel Test Section

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. It is suggested that this effect may have contributed to some of the lack of correlation observed between wind tunnels and that it may be partially responsible for the lower drag levels measured in short-test-section, open-jet wind tunnels, when compared with closed-wall wind tunnels.