Leading-Edge Profiles for the Reduction of Airfoil Interaction Noise

Abstract

Airfoils operating in a turbulent flow are an efficient source of noise radiation by scattering vorticity into sound at the leading edge. Much work has been undertaken demonstrating the effectiveness by which serrations, or undulations, introduced onto the leading edge can substantially reduce broadband leading-edge interaction noise. However, most of this work is focused on sinusoidal leading-edge serration profiles. In this paper, a family of serration profiles is proposed that is capable of providing significantly greater noise reductions than single-wavelength serrations at optimal conditions. This new family of profiles will be shown to reduce interaction noise through a fundamentally different noise reduction mechanism than conventional single-wavelength profiles. Unlike single-wavelength profiles, which produce a single compact dominant source region per serration wavelength, these new profiles are designed to produce two or more dominant compact sources per serration wavelength of roughly the same source strength that are separated in the streamwise direction. Because these sources are arranged to be closer together than the turbulence length scale, they are highly coherent and, at certain frequencies, radiate exactly 180 deg out of phase, leading to very high levels of noise reduction in the far field. The experimental noise reduction spectra are first compared against an analytic model obtained as a solution of the acoustic wave equation using the Wiener–Hopf technique. This approach focuses solely on solving for an acoustic field, and not for any secondary vorticity induced by the edge. As such, this comparison, which shows disagreement, proves that secondary vorticity is a dominant mechanism for these new profiles. A simple supplementary analytical model has therefore been developed to explain this noise reduction mechanism for these new profiles, based on interference between sources distributed along the leading edge. Good qualitative agreement is obtained with the experimental results.