Effect of serrated trailing edges on aerofoil tonal noise

Gelot, Matthieu; Kim, Jae Wook

doi:10.1017/jfm.2020.724

Cited by 28 publications

(13 citation statements)

References 32 publications

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“…This also confirms that two-dimensional simulations cannot fully assess the main features of aerofoil tones, as suggested by Tam & Ju (2012) and de Pando et al (2014). Trailing-edge serrations or liner-type porous appendages both create modulations in span, and have been successfully used to mitigate aerofoil tones in the past (Moreau et al 2016;Gelot & Kim 2020).…”

Section: Discussionmentioning

confidence: 99%

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Experimental investigation of aerofoil tonal noise at low Mach number

et al. 2021

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This paper presents an experimental investigation of aerofoil tones emitted by a controlled-diffusion aerofoil at low Mach number ( $0.05$ ), moderate Reynolds number based on the chord length ( $1.4 \times 10^{5}$ ) and moderate incidence ( $5^{\circ }$ angle of attack). Wall-pressure measurements have been performed along the suction side of the aerofoil to reveal the acoustic source mechanisms. In particular, a feedback loop is found to extend from the aerofoil trailing edge to the regions near the leading edge where the flow encounters a mean favourable pressure gradient, and consists of acoustic disturbances travelling upstream. Simultaneous wall-pressure, velocity and far-field acoustic measurements have been performed to identify the boundary-layer instability responsible for tonal noise generation. Causality correlation between far-field acoustic pressure and wall-normal velocity fluctuations has been performed, which reveals the presence of a Kelvin–Helmholtz-type modal shape within the velocity disturbance field. Tomographic particle image velocimetry measurements have been performed to understand the three-dimensional aspects of this flow instability. These measurements confirm the presence of large two-dimensional rollers that undergo three-dimensional breakdown just upstream of the trailing edge. Finally, modal decomposition of the flow has been carried out using proper orthogonal decomposition, which demonstrates that the normal modes are responsible for aerofoil tonal noise. The higher normal modes are found to undergo regular modulations in the spanwise direction. Based on the observed modal shape, an explanation of aerofoil tonal noise amplitude reduction is given, which has been previously reported in modular or serrated trailing-edge aerofoils.

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Section: Discussionmentioning

confidence: 99%

“…Nash et al (1999) laid a hypothesis that large scale hydrodynamic oscillations are responsible for that frequency selection, which results in acoustic tones. Gelot & Kim (2020) gave indirect evidence of acoustic feedback using Arbey & Bataille's (? )emi-empirical formula.…”

Section: Introductionmentioning

confidence: 99%

Experimental investigation of aerofoil tonal noise at low Mach number

et al. 2021

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“…2016 b , c ; Avallone et al. 2018; Gelot & Kim 2020). If the thickness of the serrations is controlled to be thin, the perturbation to the incoming flow in the boundary layer will be weak.…”

Section: Numerical Proceduresmentioning

confidence: 99%

“…Even though the cut-in serrations can reduce noise in the far field, they may also cause external tonal noise and aerodynamic deficit (Ji et al 2014;Qiao et al 2014). The other one is the insert-type serration, which is also effective in noise reduction (Gruber, Azarpeyvand & Joseph 2010a;Gruber, Joseph & Chong 2010bFinez et al 2011;Jones & Sandberg 2012;León et al 2016b,c;Avallone et al 2018;Gelot & Kim 2020). If the thickness of the serrations is controlled to be thin, the perturbation to the incoming flow in the boundary layer will be weak.…”

Section: The Computational Cases and Validationmentioning

confidence: 99%

“…Moreover, Ayton's model predicted both the noise reduction and the frequency correctly. Compared with the many numerical studies on turbulent broadband noise, Gelot & Kim (2020) investigated the significant reduction of tonal noise on the Joukowski airfoil when serrations were applied. The serrations, in fact, suppressed the strength of the acoustic pressure in the transitional area, and a more destructive phase appeared near the trailing edges, which can finally affect the feedback loop and lead to low noise radiation.…”

Section: Introductionmentioning

confidence: 99%

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Noise reduction mechanisms for insert-type serrations of the NACA-0012 airfoil

Wan

et al. 2022

J. Fluid Mech.

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Trailing-edge serrations inspired by owls are capable of reducing broadband noise. In this study, the wall-resolved large-eddy simulations (LES) are carried out on the flow over NACA-0012 airfoil with additional serrated trailing edges. The computations are conducted with the high-order flux reconstruction method on unstructured meshes. Three kinds of serrations with different lengths are studied and compared with the straight trailing-edge case, and all three types of serration achieved a certain degree of noise reduction. Presently, the medium-length serration achieves the best noise reduction effect. The maximum decrease of overall sound pressure level is approximately 2.4 dB, implying that the length of serration has a substantial impact on the noise reduction effect. The serration has no significant effect on the upstream turbulence statistics, but it changes the flow structure near the serration, such as inducing side vortex pairs attached to the serration edges. Moreover, dynamic mode decomposition shows that the pressure structures vary with the serration length. For the most unstable hydrodynamic wave, the spanwise coherence of the mode structure of pressure in the upstream boundary layer is weakened. In addition, serrations can redistribute the dipole sources on the surfaces of airfoil and serrations. The destructive interference is enhanced to some extent, which is favourable for noise reduction. In contrast with LES simulations, the pure dipole analysis shows that the longest serration case seems to be the best. Furthermore, a recently developed noise theory is used to evaluate the influence of serrations on the flow noise sources qualitatively and quantitatively. It is found that the serrations can mitigate noise source intensity near the serration edges but increase the source intensity in the near wake. The combined effect of serration on the dipole source and flow noise source determines the overall noise reduction effect. To conclude, destructive interference plays a primary role in suppressing noise radiation by serrated trailing edges, and the dual effect of flow noise sources should be considered in future serration designs. As the influence of turbulence structure will make it more difficult to find the optimal serration parameters, the position of high-fidelity simulation will become increasingly important.

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