The Effects of Car A-Pillar and Windshield Geometry on Local Flow and Noise

Abstract

A desirable requirement in the production of modern vehicles world-wide is the provision of a high level of driving comfort. An important aspect of this is the minimisation of aerodynamic noise. As structure-borne, engine, tyre, and power-train noise sources have been reduced in recent years, the aerodynamic noise is significant, especially at driving speeds exceeding 100 km/h. Prior experimental studies have revealed that the flow around a passenger car's A-pillar region is a primary source of aerodynamic noise, since the highest pressure fluctuation occurs here. Also, this region is closest to the driver's ears. Whilst a small part of the noise can come from aerodynamic noise generated by the mirror as the flow is first incident on the A-pillar, this study will only be addressing flow around the A-pillar. It is known that the area and strength of the A-pillar flow separation depend mainly on the local A-pillar and windshield geometry and yaw angle. However, the effects of scaling, local radii and yaw angle on the potential for noise generation are not well understood. Scaling is important so that model-scale results can be translated to the full-scale. Computational Fluid Dynamics methods (CFD) are not sufficiently developed either to predict the surface pressure fluctuations or the resulting acoustic waves with the required degree of accuracy. The objectives of this work were to investigate the scale effects, the influence of the local A-pillar and windshield radii on the flow characteristics, and the influence of yaw angle. In order to address these objectives, a series of experimental investigations was conducted using five 40% scale generic models with different A-pillar and windshield geometries and using three production vehicles. One model had a sharp-edged vertical windscreen. The other four models had a 60° inclined windscreen, which is a typical slant angle for contemporary production passenger cars, and various degrees of edge rounding including a model with a sharp edge. These models were used to measure the surface mean and fluctuating pressures in the A-pillar region at different speeds and steady yaw angles. Production vehicles were used to evaluate the surface mean and fluctuating pressures close to the A-pillar region, and the 'in-cabin noise' as a function of steady yaw angle and increased rounding of the A-pillar. The production vehicle tests were performed at different speeds and yaw angles in wind-tunnels and on-road. Flow visualisation was used to supplement the pressure data. The surface mean and fluctuating pressures were converted to non-dimensional pressure coefficients and the frequency content of the fluctuating pressure was investigated via the normalised power spectral density. Generally the surface mean and fluctuating pressure coefficients were found to be independent of Reynolds numbers. However, when yawed, a slight dependency was found to occur on the leeward side. This minor dependency was noted in the separated regions, but was not evident in the re-attached areas. The amplitudes and frequencies of the fluctuating pressures scaled well with velocity head and Strouhal number. Therefore, a scale model can be used for the prediction of the surface hydrodynamic pressures in the Apillar region of a future vehicle when suitable scaling laws are used. The magnitudes of fluctuating pressures and the area of flow separation close to the A-pillar region depended largely on the local radii. Most energy from the fluctuating pressures in the A-pillar region was between Strouhal numbers 5 to 12. The maximum hydrodynamic pressure fluctuation was found to be between the separated and reattached areas rather than at the re-attachment points as has been proposed by other researchers. Yaw could increase the area and magnitude of the flow separation on the leeward side by an order of magnitude compared to the windward side for the slanted sharp-edged model. However, the model shape with no slant angle (i.e., a vertical windshield) produced an intense but relatively small flow separation on the windward side when yawed. Negligible flow separation was found on the models with corner rounding and increase of yaw angle did not increase the separation substantially, even on the leeward side. However, future work is recommended on an additional model incorporating a smaller corner radius. For the production vehicles an increased rounding of the A-pillar significantly reduced the magnitude of the external fluctuating pressures, although the 'in-cabin noise' typically reduced by 2-3 dB. The amplitudes and frequencies of the fluctuating pressures scaled well with velocity head and Strouhal number. Atmospheric turbulence, correlation between the external pressure fluctuations and in-cabin noise, and boundary layer characteristics in the A-pillar region were not included in this work but are thought to be worthy of further investigation.