Experimental Evaluation of Noise due to Flow Over Surface Steps

Abstract This paper presents results obtained from an experimental investigation of noise generated by the passage of a turbulent boundary layer over two generic surface irregularities: a forward-facing step and a backward-facing step. Acoustic measurements were made in a low-noise wind tunnel using a directional microphone system. The experimental arrangement used is similar to that used for earlier studies of the surface pressures upstream and downstream of steps. Two step heights were evaluated and data were acquired for two free-stream velocities. One important finding is that although noise from the backward-facing step could not be measured above background levels, noise from the forward-facing step was readily detected and broadband in nature. Noise from the forward-facing step is found to originate from the immediate vicinity of the step and scales on the seventh power of velocity. The results from this study also demonstrate the utility of a directional microphone for radiated noise measurements in a wind tunnel.