Numerical investigation into generation mechanism of whistle sound due to narrow folding-gap flow of automobile side-mirror

The aerodynamic noise of automobiles becomes more and more important as the electric vehicles become more popular. The extruding shape of vehicle side-mirrors is one of the primary aerodynamic sources of automobiles. Therefore, a lot of studies have been carried out to investigate the wind noise due to the side-mirror. However, it was frequently reported that the high-frequency tonal sound, named 'whistle,' was generated by the narrow gap flow through the folding gap of the side-mirror, which is inevitable to the side-mirror because, when the vehicle is not in operation, the side-mirror should be folded. The purpose of this study is to investigate the mechanism of whistle sound generation by the narrow folding-gap flow of the vehicle side-mirror. First, the main structures inside the folding-gap of side-mirror responsible for the production of whistle sound were identified by using the experiments using the small wind tunnel. Then, the simplified model was manufactured to include only the defined main structures in the narrow gap of the side-mirror. The wind tunnel test was again performed to confirm the operating conditions, where the whistle sound of the simplified model was generated. Finally, the numerical simulation of the folding-gap flow of the simplified side-mirror model is conducted using the high-resolution LES techniques, which can directly solve the source flow region inside and the radiated acoustic field outside of the narrow folding-gap simultaneously. The numerical results were validated through the comparison of numerically predicted pressure spectrum with the measured ones. The whistle sound generation mechanism was presented by using vortex sound sources.