Verification of acoustic and aerodynamic pressure fluctuations in near acoustic field by wavenumber-frequency spectrum

Abstract

To examine pressure fluctuations in the near acoustic field, the wavenumber-frequency spectrum method was applied to the flow field around a square cylinder placed near a flat plate. Since the pressure fluctuations in the near field contain both acoustic and fluid fluctuations, it is rather difficult to separate those components and estimate the fluctuation level of each component's fluctuation. However, they can be analyzed in the wavenumber domain by the statistical post processing method. That is, the velocity and wavenumber in the sound can be uniquely determined, while many wavenumbers can exist in the turbulent flow. In the present study, the flow structure and the sound field were investigated by applying Computational Fluid Dynamics (CFD) based on the Lattice Boltzmann Method (LBM). The wavenumber-frequency spectrum was calculated by using time series data on the pressure fluctuations from CFD results. The flow structure and the sound field in near field could be clarified from the wavenumber-frequency spectrum. The differences in the convective velocity, the directivity of sound waves and the levels of sound and turbulent flow pressure fluctuations were investigated by changing the distance between the square cylinder and the flat plate. Thus in this paper it is ensured that the wavenumber-frequency spectrum method is useful to analyze the time and spatial averaged flow and sound fields in the near acoustic field.