An aeroacoustic investigation into the effect of self-oscillating trailing edge flaplets

Abstract The aeroacoustics of a NACA 0012 aerofoil with an array of self-oscillating flexible flaplets attached to the trailing edge has been investigated at low to moderate chord based Reynolds number (50,000–350,000) and at geometric angles of attack from α g = 0 ∘ – 20°. Two distinct situations were tested: one in which the flaplets were attached to the pressure side, tangentially extending the surface down the trailing edge; the other is with the flaplets attached to the suction side and extending this surface, respectively. For the reference aerofoil, strong tonal peaks are observed. When the passive flaplets are attached to the pressure side, these tonal peaks are removed and the overall sound pressure level (OSPL) is reduced by up to 20 dB. If the flaplets are placed on the suction side, the noise reduction is still present but not as strong as compared to the other case. It is concluded that the case with flaplets on the pressure side is more beneficial in interrupting the feedback loop between the boundary layer instabilities and noise sources in the wake as it is seemingly modifying the laminar separation bubble situated on the pressure side of the aerofoil, the major tonal noise source. The somewhat lower overall noise reduction obtained by the case with flaplets attached on the suction side is suggested to result from the upstream stabilisation in the boundary layer on the suction side by the mechanism of lock-in as show in a recent paper by Talboys and Brucker (2018).