Abstract
The wind tunnel is likely to remain the primary experimental tool for evaluation of vehicle aerodynamic drag for the foreseeable future. Whilst full scale wind tunnel testing is now well established, concerns remain regarding the validity of the simulation, particularly with regard to the representation of atmospheric turbulence, ambient wind and the proximity of the ground-plane. In relation to the latter problem, moving ground systems have been shown to be an accurate simulation technique for vehicles at zero yaw.
Unfortunately the practical problems of mounting full scale vehicles above a full-width moving ground plane are likely to preclude the use of such a simulation for all but very specialist experimental investigations. The likelihood is that full-scale measurements will continue to be made above fixed ground planes (with upstream boundary layer control) for some time. Whilst recognising this it is also acknowledged that aerodynamic development of a vehicle underbody flow is an aspect of aerodynamic refinement in which significant advances can still be made for many vehicles. It is therefore important that the limitations of the wind tunnel simulation are well understood.
Full-scale track measurements are difficult and time consuming. This paper concerns the use of a relatively simple technique in which the pressure signature on the ground plane beneath a moving vehicle on a test track is compared with that obtained from full scale wind tunnel testing as a means of assessing the effectiveness of the correlation and its dependence on vehicle geometry.
The ground plane pressure signature is known to extend approximately one body length upstream of the vehicle nose and 1½ - 2½ body lengths downstream of the base (depending on vehicle geometry). The signature is shown to be remarkably sensitive to both underbody and rear upper-body geometry changes. This particular characteristic can be used to assess the correlation between wind tunnel and track measurements and to establish reasonable guidelines for the degree of refinement possible within a limited, fixed ground plane simulation.