On the reductions of airfoil–turbulence noise by curved wavy serrations

Abstract

The present study provides an extensive experimental investigation into the use of curved sinusoidal (or wavy) trailing-edge (TE) serrations as a passive means for augmenting the airfoil broadband noise reduction over a broad range of frequencies. The curved sinusoidal TE serrated airfoils with parameters i = 20°, h/Co = 0.167, λ/Co = 0.134, and R/Co = 0.334 provide significant noise reduction benefits of about 4 dB with respect to the wider uniform serrations (i.e., λ/Co = 0.2), particularly from mid- to high-frequency ranges, where i, Co, h, λ, and R are the inclination angle, mean chord, serration amplitude, serration wavelength, and radius of curvature, respectively. Also, they provide an additional noise reduction of about 2.5 dB with respect to the narrow uniform serrations (i.e., λ/Co = 0.033), particularly from mid- to high-frequency ranges. On the whole, the curved sinusoidal TE serrations having an inclination angle of i = 15° and i = 20° could offer an average additional noise reduction benefit of about 1 and 2 dB for the range of frequencies from 1 to 10 kHz at all jet velocities. The normalized sound reduction (ΔPWL′) provided by the curved sinusoidal TE serrated airfoils show linear dependence with the modified Strouhal number for all jet speeds. Furthermore, the sound power reduction performance provided by curved sinusoidal TE serrations follows a modified Strouhal number scaling law from mid- to high-frequency ranges, which indicates their universal behavior on the noise reduction performance. The flow near the TE of the curved sinusoidal TE serrated airfoil undergoes multiple breakups, thus leading to non-uniform acoustic radiation throughout the span. The flow structures along the curvature undergo large-scale disintegration due to the presence of the inclination section in the curved sinusoidal TE serration. Thus, the granular structure of the turbulent eddies is fragmented into the fine length scales due to the presence of multiple breakups near the TE of the curved wavy serrations. Furthermore, the inclined portion of the curved wavy TE serration increases the disruption of the vortex shedding and reduces the velocity fluctuations, which reduces the surface pressure fluctuations close to the TE and, hence, the far-field noise.