Manipulation of mid- and high-frequency wall-pressure sources by streamwise finlets

In the present research, the effect of streamwise finlets on the coherent structures of a turbulent boundary layer and their relation with pressure fluctuations and trailing-edge noise is investigated experimentally over a NACA0018 airfoil. A synthetic measurement is performed using time-resolved particle image velocimetry, wall-pressure transducers and a far-field microphone. The finlets induce strong momentum transport within the boundary layer, leading to the formation of a detached shear layer and backward flow separation. A strong velocity deficit is produced close to the wall. The instantaneous flow organisation reveals the formation of hairpin-like vortices on top of the finlets and spanwise rollers in the near-wall separation bubble. The newly generated vortices disrupt the turbulent coherent structures of the untreated case remarkably. An overall lift-up process of the unsteady turbulent structures is produced, bringing the most energetic turbulent structures away from the wall and reducing the near-wall shear stress. The spatial and temporal relation between instantaneous unsteady flow features and wall-pressure fluctuations is analysed quantitatively. A notable reduction of the correlation and coherence intensity in the mid- and high-frequency bands is achieved due to the modification of the turbulent structures. The former frequency ranges agree with that of the pressure fluctuations and far-field noise suppression, revealing the noise-reduction mechanisms.