Comparison of a High Blockage Wind Tunnel, an Open Jet Wind Tunnel and On-road Testing with Respect to External Surface Pressures

Noise in passenger cars is dependent upon the fluctuating surface pressures on the exterior, particularly in the region of the A-pillar and the front side glass. The purpose of this work was to investigate whether the fluctuating surface pressure profile obtained in a typical full-size automotive wind tunnel can be duplicated within the limitations of high blockage tunnel. A further aim was to compare the data from both wind tunnels with road data. In order to investigate the spatial resolution of fluctuating pressures on the side window of a car, flush mounted microphones were used as fluctuating pressure transducers. Mean pressure coefficients were obtained from flush-mounted pressure taps in the same locations. Frequency based (spectral) analysis was carried out on the fluctuating pressure signal.

It was found that the regions of flow separation coincide with the regions of maximum fluctuating pressure. There is good correlation between the high blockage wind tunnel and the on-road tests, whilst there are discrepancies in those obtained in the open jet tunnel. Finally while the static and fluctuating pressures largely showed little velocity dependency, there was a noticeable deviation in some locations at high speed on road. The cause for this effect is not known at this stage, and is the subject of further investigation.