Noise Reduction Mechanisms of Brush-Like Trailing-Edge Extensions on a Stalled Airfoil: Velocity-Dependent Effects

Aerodynamic noise from separated flows is a major concern for aviation and wind energy. This experimental study examines brush-like trailing-edge extensions as a passive treatment for a NACA0012 airfoil at α=20∘ (deep stall). Far-field sound spectra and directivity were measured for U∞=10–70 m/s, while surface-pressure and PIV diagnostics were acquired at U∞=10 m/s to elucidate mechanisms. The treatment exhibits a non-monotonic response: substantial noise reduction at 10–20 m/s and 40–70 m/s, but moderate amplification at 30 m/s. At 10 m/s, brushes reduce suction-side pressure fluctuations (with only minor increases on the pressure side), implying a net decrease in unsteady lift and weaker dipole radiation. PIV shows attenuation of near-wake velocity fluctuations, Reynolds shear stress and turbulent kinetic energy, accompanied by a weaker, less coherent distribution of a velocity-gradient-based acoustic-source estimator. A Mode-Decomposing Autoencoder applied to the source field indicates selective suppression of the dominant shedding-related mode and a collapse of the latent-space trajectory onto a lower-dimensional manifold, consistent with reduced coherent shedding and acoustic radiation. Flow diagnostics were not conducted at 15–70 m/s; therefore, mechanisms for the intermediate-speed amplification and high-speed reductions remain to be verified. Overall, compliant/permeable trailing-edge brushes can modulate separated-flow instabilities, but their performance must be assessed across operating conditions.