There is increasing concern about potential differences in aerodynamic behavior measured in steady flow wind tunnel conditions and that which occurs for vehicles on the road. As tools become available for better simulation of on road conditions there is a growing practical value in understanding what range of conditions are important to simulate.
Surface pressures measured on the sideglass of a European hatchback vehicle in the MIRA full scale wind tunnel are compared with those measured on-road. The on-road data corresponds to relatively calm, low yaw conditions and the time averaged pressure distributions on-road and in the wind tunnel at zero yaw were very similar.
Variations in instantaneous aerodynamic yaw angle produces fluctuations in surface pressures but the sensitivity of instantaneous pressures to yaw angle was lower for the on-road measurements compared with steady state wind tunnel tests. Also, surface pressure unsteadiness was larger than could be attributed to yaw angle fluctuations alone.
The response of instantaneous surface pressures to yaw angle fluctuations is reduced for higher frequency fluctuations as small scale turbulence gusts are less able to modify the flow over the entire vehicle. Examination of the spectral relationship between surface pressure and yaw angle indicates that the impact of yaw angle fluctuations at is reduced by a factor of 2 for reduced frequencies above unity.
