A vehicle driving on the road experiences unsteady flow conditions which are not generally reproduced in the development environment. This paper investigates the potential importance of this difference to aeroacoustics and hence to occupant perception and proposes a methodology to enable better ranking of designs by taking account of wind noise modulation.
Two approaches of reproducing the effects of unsteady wind on aeroacoustics were investigated: an active wind tunnel Turbulence Generation System (TGS) and a quasi-steady approach based on measurements at a series of fixed yaw angles. A number of tools were used to investigate the onset flow and its impacts, including roof-mounted probe, acoustic heads and surface microphones. External noise measurements help to reveal the response of separate exterior noise sources to yaw.
The noise experienced by the driver or passenger ear facing the side-glass is dominated by increased sound pressure levels when the adjacent side-glass is the leeward side of the vehicle with some non-linear effects as leeward yaw produces first accelerated flow and then separation.
In part because of non-linearity in response to yaw, a challenging parameter for a wind tunnel simulation of dynamic yaw is achieving a wide enough variation in yaw angle and this work suggests that considering an appropriate range of yaw angles is more important than capturing the dynamics.
In terms of passenger perception, the most important effect of a time-varying onset flow was demonstrated to be the modulation of wind noise rather than the increase in time-averaged cabin noise. For the case considered, at 130 km/h, the impact of wind-noise modulation was found to be equivalent to an extra 1-2 dBA in terms of passenger perception, while the increment in time-averaged cabin noise would be only 0.2 dBA.