Statistical-Empirical Modelling of Aerofoil Noise Subjected to Leading Edge Serrations and Aerodynamic Identification of Noise Reduction Mechanisms

Abstract

With the objective of reducing the broadband noise, emitted from the interaction of highly turbulent flow and aerofoil leading edge, sinusoidal leading edge serrations were analysed as an effective passive treatment. An extensive aeroacoustic study was performed in order to determine the main influences and interdependencies of factors, such as the Reynolds number (Re), turbulence intensity (Tu), serration amplitude (A/C) and wavelength (λ/C) as well as the angle of attack (AoA) on the noise reduction capability. A statistical-empirical model was developed to predict the overall sound pressure level and noise reduction of a NACA65(12)- 10 aerofoil with and without leading edge serrations in the analysed range of chord-based Reynolds numbers of 2.5·105 ≤ Re ≤ 6·105 and a geometrical angle of attack -10 deg ≤ α ≤ +10 deg. The observed main influencing factors match current research results to a high degree, and were quantified in a systematic order for the first time. Moreover, significant interdependencies of the turbulence intensity and the serration wavelength (Tu·λ/C), as well as the serration wavelength and the angle of attack (λ/C·AoA) were observed, validated and quantified. In order to study the noise reduction mechanisms, Particle Image Velocimetry (PIV) measurements were conducted upstream of the aerofoil leading edge and along the interstices of the leading edge serrations. Velocity, turbulence intensity and vorticity in the plane perpendicular to the main flow direction (y/z plane) were analysed and linked to the acoustic findings. It was observed that a noise reduction is accompanied by a reduction of the turbulence intensity within the serration interstices. The reduction in turbulence intensity is more pronounced with large serration amplitudes. However, the impact of the serration wavelength was found to be no function of the turbulence. It is more likely to be affected acoustically by spanwise de-correlation effects as a response to the incoming gusts.