A parametric study of the effect of self-oscillating trailing-edge flaplets on aerofoil self-noise

This paper presents an acoustic study of a standard NACA 0012 aerofoil with additional self-oscillating passive flaplets deployed from the trailing edge for self-noise reduction, putting special emphasis on the potential reduction of tonal noise generated by the periodic shedding of vortices from the trailing edge. The flaplets protruding out of the trailing edge act as rectangular thin cantilever beams exited by the surrounding flow to oscillate in their dominant flexural bending mode. The noise attenuation performance is studied for different deployment length, width and inter-spacing to find the most dominant contributions at chord based Reynolds numbers Re-c from 100,000 to 900,000 and three geometric angles of attack alpha(g) = 0 degrees, 10 degrees and 15 degrees. It was observed that all flaplet configurations oscillate and thereby reduce the tonal noise. The range of the highest noise reduction in the low frequency range scales with the Strouhal number based on chord length, whereby this range can be set specifically by how far the flaplets protrude out of the trailing edge. This determines the free vibrating length and therefore the natural frequency of the oscillators. Laser-based measurements of the flaplet oscillations confirm the occurrence of a lock-in mechanism observed in previous studies, which effectively describes the oscillating flaplets as pacemaker to keep the fundamental instabilities of the flow in their linear state. It is concluded that this contributes mainly to the tonal noise reduction while the more broadband noise reduction stems from the geometry of the flaplets acting as 'slitted-serrations'. A larger width of the flaplets is most effective in the low-frequency range, while narrower flaplets are best addressing high-frequency noise components. The results further show that a smaller inter-spacing also benefits the noise reduction. The study paves the way for novel morphing techniques to target specific noise ranges during different flight manoeuvres. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the acoustic properties of a standard NACA 0012 airfoil with additional self-oscillating passive flaplets for self-noise reduction. Different flaplet configurations were observed to affect the oscillation and reduction of tonal noise.