Shear Layer Correction Validation using a Non-Intrusive Acoustic Point Source

Microphone arrays can be used to localize and estimate the strengths of acoustic sources present in a region of interest.  In general, microphone array processing algorithms assume line-of-sight acoustic wave propagation from each source to each microphone.  However, when the microphone array is placed outside the jet in an open-jet test section, the presence of the shear layer refracts the acoustic waves and causes the wave propagation times (from each source to each microphone) to be different than in the case without the shear layer.  With a known source location in space, the change in propagation times can be determined by using Amiet’s method (J. Sound Vib., Vol. 58, 1978).  We investigate the effects of shear layer refraction on the conventional delay-and-sum beamforming algorithm by using a pulsed laser system to generate a plasma point source in space and time within the test section at several different test section flow speeds and configurations with varying levels of complexity.  The results indicate that Amiet’s method properly corrects for shear layer refraction effects when the source is in the inviscid core of the open-jet test section, as well as when the source is located in a simple thin wake.  When the source is in a complex wake region, the quantitative agreement degrades although overall trends are correct.