Aerofoil self-noise radiations subjected to serration flap angles

P C Woodhead,T P Chong,P F Joseph,A Vathylakis

doi:10.1007/s00348-021-03239-y

Abstract

Besides the investigation of the aeroacoustics responses of an asymmetric aerofoil subjected to serrated trailing edge flap angles from negative (flap-down) to positive (flap-up), this paper also provides a new perspective on the physical mechanisms of broadband noise reduction by a serrated trailing edge. The blade-loading effect, which is a function of the length and flap angle for a straight/non-serrated trailing edge flat plate, plays a considerable role in the self-noise radiation that is hitherto less recognised. When the same trailing edge flat plate is cut into a sawtooth serration shape, the self-noise reduction will be underpinned simultaneously by both the serration effect (dominant) and the blade-loading effect. The results demonstrate that the far-field radiation of a serrated aerofoil can be manipulated significantly depending on the direction of the flap angle. In the flap-down configuration, the blade-loading will become a negative factor that causes a deterioration of the noise reduction performance across the entire frequency range. In the flap-up configuration, three spectral frequencies zones can be defined. At the low-frequency zone, the diminished cross-flow at the sawtooth gaps will impede the noise reduction capability. At the central-frequency zone, the re-distribution of the turbulence sources and reduction in the turbulence spanwise length scales will enhance the noise reduction performance. Improvement in the noise performance can also be achieved at the high-frequency zone owing to the lack of interaction between the cross-flow and sawtooth structure. A new concept is positively demonstrated by varying the serration flap angle as a periodic function across the spanwise direction (spanwise wavy serration). When compared to a non-flap serrated trailing edge, the spanwise wavy serration is found to further increase the noise reduction level between the central and high-frequency regions.Graphic abstract

Highlights

Self-noise emitted from the trailing edge of an aerofoil blade represents a major environmental and operational issue in the aviation, wind turbine and home appliance industries
The analysis will be based on the aeroacoustics performance of the flat plate trailing edges with and without flap angle
The narration will shift to the quantification of noise reduction/increase by the serrated trailing edge with amplitude H, which is measured against the baseline trailing edge of H/2

Summary

Introduction

Self-noise emitted from the trailing edge of an aerofoil blade represents a major environmental and operational issue in the aviation, wind turbine and home appliance industries. There have been continuous efforts in the research community to develop new flow control methods to improve the level of reduction for the trailing edge self-noise. One of the most commonly used methods to reduce the turbulent boundary layer trailing edge noise of aerofoil is inspired by the owl’s wing. The unique feature of trailing edge sawtooth serrations has been successfully demonstrated in wind tunnel experiments with broadband noise reduction typically up to 8 dB (Gruber et al 2011). Reduction in trailing edge noise has been witnessed through the addition of serrations on a real size wind turbine blade by Oerlemans et al (2009) and Hurault et al (2015). Noise reductions were found to be lower than that obtained under laboratory conditions, they still observe overall sound power level reductions on the wind turbine blade up to about 3 dB

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