CHARACTERISTICS OF WALL PRESSURE FLUCTUATIONS FOR A BOUNDARY LAYER AFFECTED BY FLOW OVER AN IDEAL SIDE MIRROR

In low Mach number flows, wall pressure fluctuations induced by flows over a blunt body are the dominant source of aero- and vibroacoustic noise. Due to the complexity of flow patterns, their characteristics cannot be straightforwardly modeled with wall pressure models furnished by turbulent boundary layers (TBL). Therefore, studies on the flow over a blunt body and induced wall pressure play an essential role in predicting self- and radiated noises of underwater, automotive, and aerial vehicles. In this work, we employ large-eddy simulations (LES) with different sub-grid-scale (SGS) models to numerically investigate the flow over a generic side mirror of an automobile and space-time characteristics of the resulting wall pressure fluctuation. The statistical results from LES with both SGS models, including the pressure coefficient and frequency spectra, align well with published numerical and experimental results. In the spectral analysis, the downstream wall pressure fluctuation on the window is decomposed into three regimes in the wavenumber-frequency space via phase speed, including convection, recirculation, and acoustics. We observe that contours in the wavenumber spectrum of wall pressure in this work exhibit a bending effect compared to those in zero-pressure-gradient flat-plate TBLs. Based on the statistical analysis based on Taylor's hypothesis, we obtain the spatial distribution of the convection speed of wall pressure, which leads to three regions in physical space: recirculation, steady convection, and transition. The contours in the wavenumber spectra of wall pressure extracted from the steady convection region are free of bending effects. Moreover, the half-width of the spatial correlation function is proportional to the inverse of the corresponding frequency, which follows the attribute in the classical theory of TBL. This investigation indicates that the convection of wall pressure beneath a TBL deflects off the blunt body, while downstream wall pressure fluctuations in the steady-convection zone preserve the space-time characteristics of the TBL.