Predicting the effect of serrated trailing edges on aerofoil self-noise from the source to the far-field

Abstract

High-fidelity numerical simulations are carried out alongside analytical methods to investigate the effect of trailing-edge serrations on aerofoil self-noise. Serrated trailing-edges have long been studied as an effective means of noise reduction especially for wind turbine applications. This work aims to present an extensive understanding of the physical mechanisms behind the noise abatement. Two types of noise are investigated: i) the aerofoil tonal noise due to the acoustic feedback loop occurring between the trailing edge and the laminar-to-turbulent transition region and ii) the aerofoil broadband noise caused by the scattering of broadband wall pressure fluctuations at the trailing edge. Furthermore, the present study takes advantage of the accurate numerical data to develop a new approach to straight and serrated trailing-edge noise prediction. It is found that serrated trailing edges show a strong capability to reduce aerofoil tonal noise. The first tonal noise reduction mechanism is inferred to be the decreased intensity of wall pressure fluctuations in the transitional region in the presence of a serrated trailing edge which directly weakens the pressure scattering at the trailing edge. The second tonal noise reduction mechanism is presumably the early breakdown of the span wise coherence in the acoustic source pressure which is likely to create destructive interferences in the acoustic feedback process. Regarding the broadband trailing-edge noise, the dominant noise reduction mechanism is inferred to be the wall source distribution along the edge and particularly the decreased surface pressure fluctuations at the tip of the serration. Besides, Amiet’s trailing-edge noise radiation model is found to accurately capture the far-field sound from straight and serrated trailing edges in every direction via the addition of a directivity correction factor as long as a representative wall pressure source is used. This work reports that some optimised locations exist for the extraction of wall pressure fluctuations in the vicinity of the trailing edge used as the representative source in Amiet’s trailing-edge noise model. Finally, ideas towards a new wall pressure fluctuations model for serrated trailing edges are proposed. Its combination with Amiet’s theory provides a suitable and complete noise prediction method for straight and sawtooth-serrated trailing edges as far as the investigated cases are concerned.

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ORCID for Jae Kim: orcid.org/0000-0003-0476-2574

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