Aeroacoustics of Turbulent Boundary-Layer Flow over Small Steps

Abstract

The aeroacoustics of low-Mach-number boundary-layer flow over small steps is investigated using large-eddy simulation and Lighthill’s theory. Step heights range from 0.83% to 53% of turbulent boundary layer thickness with Re� = 4, 100, and both backward and forward steps are considered. Sound predictions using Lighthill’s equation with an acoustically compact Green’s function show that forward step is noisier than backward step as observed in previous experiments, but the difference is significantly reduced as the step size decreases. The underlying reason is analyzed in terms of source and Green’s function distributions. The backward steps generate sound primarily through diffraction of the unsteady hydrodynamic pressure in the boundary layer, whereas the generation and modification of acoustic sources by forward steps play a major role in the forward-step sound generation. The effect of steps on the space-time characteristics is investigated. The forward steps induce significantly larger pressure fluctuations than backward steps, and the maximum occurs near the reattachment location when there is flow separation. Reattachment is shown to decorrelate wall pressure fluctuations. The frequency spectra of wall pressure fluctuations are used to study the flow recovery downstream of the step. It is found that the forward step flow recovers faster than the backward step flow.