日本機械学会論文集
Online ISSN : 2187-9761
乱流境界層が流入する前方ステップから生じる空力騒音の音源および伝播プロセスの解明
田所 真樹横山 博史水野 雅隆飯田 明由若松 幹生Phan Vinh Long田中 博
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論文ID: 18-00199

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Aerodynamic noise generated around a step such as front pillars of automobiles, which is transmitted into the cabin through the window glass, can negatively affect the comfort of the passengers. For the reduction of this noise, it is necessary to clarify the acoustic source and process of the propagation of the acoustic waves from the step. To do this, direct aeroacoustic simulations were performed along with wind tunnel experiments for flows around a forward step in a turbulent boundary layer, where the freestream Mach number was 0.1 and the Reynolds number based on the incoming boundary layer thickness, δ, and the freestream velocity was 2.73×104. The computations were performed for the height of h/δ = 0.9 and 3.4, where the effects of the step height on the radiated sound were investigated. The radiated sound becomes more intense for the higher step of h/δ = 3.4 particularly in a low frequency range of St ≤ 1.5, where St is the non-dimensional frequency based on the step height and freestream velocity, due to the occurrence of the large-scale vortices in the separated flow. To elucidate the radiation of the acoustic waves, the pressure fluctuations were decomposed into non-radiating convective and radiating acoustic components by usage of two methods. One is utilizing wavenumber-frequency spectra and the other one is spatial Gaussian filtering for the fluctuations at each frequency, which is proposed in this paper. The results by the former method present that the level of convective components is larger than the acoustic components by 20 - 40 dB. Moreover, the radiating components by the proposed spatial filtering method show that the radiation occurs around the step edge and the reattachment point. It is indicated that the proposed method is effective for the investigation of the acoustic radiation for the pressure fluctuations composed of convective and acoustic components.

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