Effects of Wind-Tunnel Simulation of On-Road Conditions

Abstract

A road vehicle experiences different conditions on the road than the ones in the development environment such as wind-tunnel and CFD. In this work, on-road unsteadiness together with moving ground and rotating wheels were studied as the on-road environment. These conditions were then investigated using full-size and model-scaled vehicles, looking their impact on aerodynamic force coefficients, rear wake structures and in particular aeroacoustics.

Reproducing the effects of unsteady wind using a Turbulence Generation System(TGS) in the wind-tunnel showed that while the on-road yaw angle is varied, the time-averaged cabin noise will be increased but this increment is small. A more important effect of time-varying onset flow is the modulation of wind noise. The impact of the modulation for an assumed on-road environment was equivalent to an extra 1-2 dB(A) in cabin noise, from the point of view of passenger perception while the increment in time-averaged cabin noise would only be 0.2 dB(A).

The lack of ground effect simulation in the wind-tunnel could also result in different flow conditions from those on the road. This work showed that the underbody of the vehicle is very important especially for the passengers at the rear seat, with an influence up to 5 dB(A) particularly for lower frequencies below 1 kHz. Another important observation was that performing aeroacoustic measurements with moving ground would be possible across the spectrum, being able to identify any changes in cabin noise without any significant impact from mechanical noise masking effects.

Finally, the importance of unsteadiness to vehicle response was studied using different TGS modes representing the on-road environment intensities and length scales. The front bumper was found to be influenced by unsteadiness at frequencies 1, 2 and 4 Hz, while both waistline and base showed a more quasi-steady behaviour. For the side-glass and A-pillar, the harmonic modes showed unsteady behaviour more pronounced at the leeward side, closer to the mirror wake area where frequencies 1Hz < f < 4Hz, appeared to be key frequencies with respect to unsteadiness.