Aeroacoustics of turbulent flow over a forward–backward facing step

Abstract

This paper presents a combined experimental and numerical study on the aeroacoustics of low Mach number turbulent flow over a forward–backward facing step (FBS). The height of the step is fixed at 50% of the incoming boundary layer thickness. Multiple lengths of the step are considered with aspect ratios (step length to height) equal to 1,2,2.4,4, and 8. The Reynolds number based on the step height ranges from 1.2×104 to 2.8×104 in both experiments and simulations. Wall pressure measurement results show that the largest wall pressure fluctuations occur slightly downstream of the step leading edge, and the downstream flow field is influenced by the leading edge flow disturbance. Large-Eddy Simulation (LES) is performed to visualize the instantaneous and time-averaged flow field. Large-scale vortices formed at the leading edge are found to shed downstream directly when the step length is shorter than the shear layer reattachment length, which leads to lower level wall pressure fluctuations on the top surface. Spectral proper orthogonal decomposition (SPOD) is applied to identify the dynamic behaviours of coherent structures downstream of the step leading edge. Using simulation data, the fluctuating wall pressure spectra are also calculated via Poisson’s equation. The results indicate that both turbulence-mean-shear interaction and turbulence–turbulence interaction contribute to the wall pressure fluctuations in the on-step reattachment region. Aeroacoustic beamforming results show that the dominant noise source is located at the step leading edge, where the highest level of fluctuating wall pressure is recorded. Meanwhile, the integrated far-field acoustic spectra show that the levels of FBS noise are lower when there is no flow reattachment occurring on the top surface. A new scaling law for FBS sound spectra is proposed to address the influence of the step aspect ratio on noise production.