Control of airfoil broadband noise through non-uniform sinusoidal trailing-edge serrations

Abstract

This study provides experimental and analytical investigations on the use of non-uniform sinusoidal trailing edge (TE) serrations as a passive means for the control of airfoil broadband noise over a wide range of frequencies. Combinations of sharper/wider non-uniform TE serrations provide higher noise reductions up to about 5 dB over the uniform ones. The normalized sound power reductions (ΔPWL/) of non-uniform sinusoidal TE serrated airfoils show linear dependence with the corrected Strouhal number, i.e., ΔPWL/ = a Stm + b, where a and b are the arbitrary constants and Stm is the modified Strouhal number. It reveals that the presence of non-uniform wavy TE serrations shows superior noise reduction performance over uniform ones from mid to high frequencies when λ2 (wide) > λ1 (narrow), which is indicated by the good coalesce of ΔPWL/ with Stm. Furthermore, the modified Strouhal number scaling law for non-uniform sinusoidal TE serrated airfoils indicates the universal behavior of the noise reduction performance. The highest overall noise reductions provided by the non-uniform wavy TE serrations occur when the transverse turbulence integral length scale (Λt) is 0.5 times the geometric mean of the wavelengths of two individual serrations. The flow visualization clearly shows the breakup of eddies by the tip of serrations, and the pairing of the vortices evolved from the root/tip of the serrations. The presence of higher span-wise de-coherence/phase interference provided by the non-uniform TE serrated airfoils leads to higher noise reductions over uniform ones.