Experimental Study of Sound Generated by Backward-Facing Steps Under Wall Jet

Abstract

Sound radiation by a backward facing step under a plane wall jet is examined. The investigation is based on an experiment where the mean velocity, the step heights, and the cross-stream extent of the jet are varied. The comparison between the backward-facing-step ow and the corresponding wall jet shows an acoustic source near the step. Both near- and far- eld measurements indicate how the step changes the radiation pattern of this source. The sound level is signi cantly increased into the upstream directions and accompanied by strong low-frequency peaks in the spectra. Aerodynamic results show that the reattachment length is much shorter for a step placed under a wall jet than for a step placed in a channel and that the most turbulent ow regions, namely, the jet mixing layer and the step reattachment region, coincide with the acoustic source locations. Nomenclature C = far- eld array center c0 = speed of sound e = nozzle cross-streamwidth h = step height k = acoustic wave number l = jet half-width