Sound radiated by three-dimensional discontinuities immersed in a turbulent boundary layer

Abstract

Measurements of the far-field sound from two types of three-dimensional discontinuities – swept forward-facing steps and a circular embossment – immersed in a low Mach number (M<0.18) turbulent boundary layer were made. The discontinuity height in each case was smaller than the height of the approaching, undisturbed boundary layer and the Reynolds number based on the discontinuity height ranged between 8300–92,000. The swept forward-facing steps were immersed in a wall-jet boundary layer, whereas the embossment was placed in a conventional boundary layer. For the embossment, measurements of surface pressure fluctuations were also performed beneath the separated flow on the vertical and the downstream surfaces. The wall-jet measurements show that the far-field sound from swept forward-facing step scales on the step-normal component of the boundary-layer edge velocity. The far-field sound from the circular embossment is qualitatively similar to that from a forward-facing step of the same height, a fact which is also reflected in the surface pressure fluctuation measurements. The step-normal velocity scaling obtained for the swept forward-facing steps was combined with an existing prediction formula for the sound radiated by a forward-facing step to yield a straight-forward prediction formula for the embossment noise. The prediction formula shows good agreement with the measured sound spectra if it is assumed that the dominant source for the embossment is the separation/reattachment region concentrated in the forward arc of the embossment. This assumption is supported by oil flow visualization which shows that the separated flow region is concentrated between approximately ±45°on either side of the spanwise center.