Effect of sawtooth trailing edge serrations on the reductions of airfoil broadband noise

Abstract

The present paper examines the use of modified dual-wavelength sawtooth serrations introduced at the trailing edge (TE) of a National Advisory Committee for Aeronautics (NACA) 65 (12)-10 airfoil as a passive means for the control of airfoil broadband noise. The studies are conducted for different parameters such as serration wavelengths (λ), serration amplitudes (h) as well as (hꞌ) modified amplitude to determine the best serration parameter which provides higher noise reductions. The reduction of noise level brought by the modified dual-wavelength sawtooth serrations at the TE is about 4 dB, while it is about 3.3 dB for the simple ones. It reveals that the modified sawtooth can provide a substantial reduction of self-noise alongside the interaction noise over an extensive range of frequencies which indicates the existence of strong far-field destructive interference (i.e., feedback) from low to mid-frequency ranges (i.e., 1.5 - 5 kHz). The TE serrated airfoils show lower acoustic emissions as compared to baseline although they exhibit a common behavior for all emission angles. The large noise decrease provided by the dual-wavelength sawtooth may be because of the significant reductions in the scattering intensity of sound as well as strong far-field destructive interference owing to the presence of two roots between the two successive maximum amplitude peaks.