Sound Generated by Boundary-Layer Flow over Small Steps: Effect of Step Noncompactness

摘要

The effect of acoustic noncompactness of steps on sound generation in a turbulent boundary layer has been studied by comparing boundary-element solutions to the Lighthill equation with those obtained using a compact-step Green's function. When the ratio of acoustic wavelength to the step height is large good agreements between the two methods in predicted sound spectra and directivities are found thus validating the previous compact-Green's-function solutions. Discrepancies between the two results become significant with decreasing λ/h. For mildly noncompact step heights the sound directivity for forward steps exhibits asymmetry with the upstream side maintaining an approximate dipole shape with the rest significantly distorted. Dips in sound spectra and multiple lobes in directivity are observed along with significantly enhanced sound power when λ/h is sufficiently small indicating that edge scattering becomes the dominant source mechanism.