Exploiting the misalignment of the serrated trailing edges for improved aerofoil broadband noise reduction

Abstract

This paper presents the experimental results of adding variable flap angles to the serrated trailing edges and their effects on the self-noise radiation of an aerofoil. The investigation included aeroacoustics and wake flow measurements on an NACA 65-(12)10 aerofoil. This paper explores further combinations of positive and negative serration flap angles,  U and  L respectively, across the spanwise extent of the trailing edge. A new parameter, the serration oscillation wavelength (λ), was also included. The study investigates five individual cases, Baseline (B), Straight-Serration (SS), sIngle-Flapped-Serration (IFS), Multi-FlappedSerration (MFS), Split-Flapped-Serration (SFS) and Spanwise-Wavy-Serration (SWS). The serration amplitude and wavelength remained consistent as 30 mm and 3.3 mm respectively. It is clear that all the “misaligned” trailing edges produce different noise characteristics compared to the conventional SS trailing edge. At low-to-mid frequency range, the conventional SS trailing edge still outperforms both the MFS and SFS trailing edges. However, from mid-to-high frequency, both the MFS and SFS trailing edges can achieve higher noise reduction than the SS trailing edge considerably. At very high frequency, where the SS trailing edge would otherwise experience noise increase, there is no noise increase for both the MFS and SFS trailing edges because the periodic misalignment reduces the crossflow component in the space between adjacent members of the serration. The SWS trailing edge produces the same level of noise reduction as the SS trailing edge across the low-to-mid frequency range. From mid-to-high frequency range, the SWS trailing edge outperforms the SS trailing edge, but the level of further noise reduction it can achieve is less than those of MFS and SFS trailing edges. At very high frequency, the SWS begins to experience slightly noise increase over the baseline, B, trailing edge. However, the increase in noise level by the SWS trailing edge at this high frequency range is still less than that by the SS trailing edge. The near-wake flow measurement results provide some explanations for the mechanisms underpinning the broadband noise reduction and high frequency noise suppression by these trailing edge devices.