On the Reductions of Airfoil Broadband Noise through Sinusoidal Trailing-Edge Serrations

The present study investigates the efficacy of sinusoidal trailing-edge (TE) serrations as a passive means for the reductions of airfoil broadband noise, theoretically and experimentally. Comprehensive parametric studies were conducted to determine the effect of serration amplitudes and wavelengths on the noise reduction performance of a National Advisory Committee for Aeronautics (NACA) airfoil. Initially, the present paper shows the use of the trailing-edge noise (TNO) model for the accurate predictions of the surface pressure spectrum near the TE and hence the far-field noise using the Wiener-Hopf method. The predicted spectra and the noise reduction levels showed good agreement with the measurements for a wide range of frequencies. The present study reveals that the local maximums of the overall noise reductions occur when the transverse turbulence integral length scale is either 1.2 or 0.2 times the serration wavelength, which corresponds to $\lambda /{\mathrm{\Lambda }}_t=0.833$ or 5, where $\lambda$ and ${\mathrm{\Lambda }}_t$ are the serration wavelength and integral length scale. One of the key findings of the paper is that the serration wavelength at which the highest noise reductions occur when the acoustic emissions vary inversely with the modified Strouhal number ${\mathrm{S}}_{\mathrm{thm}}$ [i.e., ${\mathrm{w}}_{\mathrm{sste}}(\omega )/w_{bl}(\omega )\propto 1{/\mathrm{S}}_{\mathrm{thm}}$] for narrow (i.e., small wavelengths) and wider serrations (i.e., large wavelengths), where ${\mathrm{w}}_{\mathrm{sste}}$ and $w_{bl}$ are the acoustic emissions radiated from the serrated and baseline airfoils. Further, the TE serrations are also observed to reduce leading-edge (LE) noise along with the self-noise, which indicates the efficacy of TE serrations in reducing the total far-field noise.