Laser Doppler Velocimetry Measurements of Acoustic–Turbulent Flow Interaction over Liner

The interaction between a grazing turbulent boundary layer at Mach 0.3 and tonal acoustic waves with an amplitude equal to 145 dB over a single-degree-of-freedom acoustic liner is investigated experimentally using laser Doppler velocimetry. Streamwise and wall-normal velocity profiles are measured along the liner, and the acoustic-induced velocity component is extracted from time-resolved measurements via a turbulence-rejection technique, allowing the separation of coherent acoustic and turbulent velocity fluctuations within the boundary layer. The influence of the acoustic source position, upstream or downstream of the liner, is examined. Results show that acoustic excitation near liner resonance frequency reduces the near-wall streamwise mean velocity while enhancing velocity fluctuations, producing a localized hump in second-order statistics. The spatial distribution and amplitude of the acoustic-induced velocity above the liner are strongly dependent on source position and exhibit significant distortions within the near-wall region, revealing a pronounced distortion of the plane wave acoustic field. These measurements provide direct experimental evidence that the turbulent boundary layer modifies the structure of the propagating acoustic wave. These observations highlight that source orientation significantly modulates the transfer of acoustic energy to the turbulent boundary layer, providing new insights into the mechanisms governing acoustic–turbulence interactions over lined surfaces.