Acoustic receptivity in the airfoil boundary layer: An experimental study in a closed wind tunnel

Airfoil trailing edge noise with a tonal frequency at a medium-Reynolds number (from 2 x 10 5 to 3 x 10 5 in this work) is related to periodic fluctuations in the airfoil boundary layer. Acoustic receptivity plays an important role, in that it constructs a feedback loop to induce ladder-structure phenomena and discrete peak frequencies. The present work is devoted to the experimental study of the acoustic receptivity in the airfoil boundary layer by employing a time-resolved particle image velocimetry method. The symmetric vortex shedding process is noticed, and a hysteresis phenomenon is discovered with the increasing and decreasing wind speed. The author applies the Hilbert transform to show a space-wavenumber spectrum of wall-normal velocity fluctuations to locate resonance points, where acoustic pressure resonates with fluctuations in the boundary layer. The results show that the acoustic reception can affect the local velocity to increase and decrease the wavenumber before and after reach points. The trailing edge noise impacts on the airfoil boundary layer to control the system states and follows the same acoustic feedback loop from Arbey and Bataille ["Noise generated by airfoil profiles placed in a uniform laminar flow," J. Fluid Mech. 134, 33-47 (1983)].