Clarification of acoustic source and propagation process of aerodynamic noise radiated from a forward step with an incoming turbulent boundary layer

Abstract

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 2.73×10 4 . 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.

Experimental study on aerodynamic sound generated from flow around a forward facing step

“…It is mentioned that two dimensional sound sources in span direction of the step due to large scale vortices correspond to low frequency sound of Strouhal number of 1 but three dimensional sound sources due to small scale vortices correspond to high frequency sound more than Strouhal number of 2 for H/δ = 0.9 and 3.4 by the previous research (Tadokoro et al, 2018). In this study, frequency corresponding to Strouhal number of 1 based on flow velocity and the step height is estimated of f = 2333, 1167, and 778 Hz for H/δ = 1.0, 2.0, and 3.0, respectively.…”

“…In order to simulate flow around the A-pillar on automobile on wind tunnel test, forward facing step model is installed in the test section as shown in Fig. 1 based on previous research (Tadokoro et al, 2018). The step is located 150 mm downstream from the nozzle exit.…”

“…Flow velocity measurement is performed under the condition of U0 = 35 m/s as well as the previous research (Tadokoro et al, 2018).…”

“…As a result, it is clarified that the acoustic component of high frequency sound contributes significantly to cabin noise. Tadokoro et al, (2018) perform a detailed survey on the pressure fluctuation of the acoustic component from numerical analysis and they investigate its behaviour and sound source. Then, they show that the sound source exists near the step in which separation occurs and near the reattachment point of the separated flow.…”

Sound radiated from low Mach number turbulent boundary layer flows（Turbulent boundary layer on a smooth plate and over a small forward facing step）

“…A selfsimilar solution of the compressible laminar boundary layer equations obtained by a shooting method is adopted for the initial flow velocity and temperature profiles of three dimensional DNS. In order to induce laminar-to-turbulent transition, a circular region of injection by a jet from the wall is introduced near the inflow, referring to the method used by Tadokoro et al (2018). The grid resolutions with the main parameters at the location in which reaches an equilibrium state of turbulent boundary layer are given in wall units in Table 1, where Δ + indicates the minimum spacing in the wall- Moriyama, Inoue and Maekawa, Journal of Fluid Science and Technology, Vol.16, No.2 (2021) normal direction.…”