Scattering of Acoustic Waves by a Turbulent Shear Layer: New Insights into Haystacking and Implications for Acoustic Imaging Corrections

The scattering of acoustic waves by the turbulent shear layer that develops from the side walls of an open jet wind tunnel is investigated experimentally. A microphone array measures the acoustic field, while particle image velocimetry (PIV) characterizes the shear layer. Microphone measurements reveal a broadening of the power spectral density and two sidelobes around the source tone (haystacking). The bicoherence spectrum computed from the microphone signals confirms a nonlinear interaction between the source tone and low-frequency shear-layer dynamics. These low-frequency fluctuations correspond to the passage of large coherent structures in the turbulent shear layer, with a Strouhal number of approximately 0.2 based on vorticity thickness. Synchronized microphone array and PIV measurements demonstrate a direct correlation between the transverse velocity component of the shear layer and coherence loss between microphones. This suggests that transverse velocity fluctuations primarily drive the interaction with acoustic waves, modifying acoustic transmission. The findings have implications for improving acoustic imaging techniques in industrial settings. A compensation technique is proposed, leveraging velocity field data to mitigate coherence loss. This method can be applied even with a single microphone array if positioned close enough to the shear layer to capture hydrodynamic pressure fluctuations.