Experimental and numerical study on noise reduction mechanisms of the airfoil with serrated trailing edge

Ji, Liang; Qiao, Weiyang; Tong, Fan; Xu, Kun; Chen, Weijie

doi:10.2514/6.2014-3297

Cited by 7 publications

(11 citation statements)

References 26 publications

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“…These particular flow features are a consequence of a funneling motion induced by a system of counter-rotating streamwise-oriented vortices generated at the root of the serrations (Figure 6). Similar findings were reported in presence of either blunt-edged serrations 28 or in thin-edged serrations.…”

Section: B Three-dimensional Flow Organizationsupporting

confidence: 78%

“…A pair of streamwise-oriented vortices with opposite orientation was found in the space in between the serrations. These structures cause a funneling motion, as previously discussed in the DNS computations of Ji et al 28 The primary effect of the funneling motion is a distortion of the mean flow that, according to Chong and Vathylakis, 7 causes a local variation of the effective angle seen by the turbulent flow approaching the serration edges. In particular, this variation may result in a higher local contribution to far-field broadband noise generation at the root with respect to the tip, when compared to the ideal case without flow distortion.…”

Section: Discussionmentioning

confidence: 68%

“…They observed that, in presence of serrated trailing edges, streamwise-oriented turbulent structures are more likely to develop into spanwise structures towards the tip of the serrations. Later studies of Arce León et al 8 and Ji et al 28 focused on the flow around thick and thin serrations at low Mach numbers. From their analysis a funneling motion, due to the generation of streamwise-oriented vortices at the root of the trailing-edge serrations, was responsible for the consistent alteration of the unperturbed path along the serration geometry.…”

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confidence: 99%

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Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

2016

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The three-dimensional flow field over the suction side of a NACA 0018 airfoil with trailing-edge serrations was studied by means of time-resolved tomographic particle image velocimetry. Mean flow results show that the boundary layer thickness decreases along the streamwise direction with a corresponding reduction of the size of the turbulent structures developing over the suction side of the serrations. At a positive angle of attack, streamwise-oriented and counter-rotating vortices aligned with the edge of the serrations are found to be the main features of the mean flow field. Their formation is attributed to the pressure imbalance between the two sides of the airfoil and the mixing layer at the edge. They locally modify the effective angle seen by the turbulent flow approaching the serrated edge. This effect may contribute to the serration underperformance in terms of noise reduction reported in literature. The spatial distribution of the spectra of the source term of the Poisson equation, which relates the velocity field to pressure fluctuations, suggests that the contribution of the serrations to far-field broadband noise is a function of the streamwise location. This observation is congruent with the spectra of the wall-normal and spanwise velocity fluctuations, which typically show low intensity close to the tips of the individual serrations. It follows that analytical models must take into account the local contribution to the far-field noise induced by the streamwise variation of the hydrodynamic pressure on the serration surface.

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Section: B Three-dimensional Flow Organizationsupporting

confidence: 78%

Section: Discussionmentioning

confidence: 68%

mentioning

confidence: 99%

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Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

2016

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“…For the aforementioned applications, the trailing edge noise is the most relevant noise source, especially at low Mach numbers since the turbulent fluctuations are scattered efficiently over a solid trailing edge [7]. For alleviating this dominant trailing edge noise, several passive noise-mitigation solutions such as trailing-edge brushes [8,9], sinusoidal and sawtooth serration [6,[10][11][12][13][14][15][16], slits [17,18], and porous treatments [19][20][21] have been proposed. Among these passive methods, sinusoidal and sawtooth trailing edge serrations have been of important interest for researchers [3,6].…”

Section: Introductionmentioning

confidence: 99%

Noise Reduction Mechanisms of an Airfoil with Trailing Edge Serrations at Low Mach Number

Tang

Lei

2019

Applied Sciences

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Trailing-edge serrations have proven to be valid applications of trailing edge noise mitigation for an airfoil, while the physical noise reduction mechanism has not been adequately studied. We performed simulations employing Large-eddy simulation and the Lighthill–Curle method to reveal the variation in the hydrodynamic field and sound source due to the trailing edge serrations. The grid resolution and computational results were validated against experimental data. The simulation results show that: the trailing edge serrations impede the growth of spanwise vortices and promote the development of streamwise vortices near the trailing edge and the wake; the velocity fluctuations in the vertical cross-section of the streamwise direction near the trailing edge are reduced for the serrated airfoil, thereby obviously reducing the strength of the pressure fluctuations near the trailing edge; and the trailing edge serrations decrease the distribution of the sound source near the trailing edge and reduce the local peak value of sound pressure level in a specific frequency range as well as the overall sound pressure level. Moreover, we observed that, in the flow around the NACA0012 airfoil, the location where the strong sound source distribution begins to appear is in good agreement with the location where the separated boundary layer reattaches. It is therefore effective to reduce trailing edge noise by applying serrations on the upstream of the reattachment point.

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“…However, due to the limitation of acoustic theory, the relationship between flow structure and the physical acoustic source has been paid less attention. Ji et al (2014) conducted both experiment and large-eddy simulation (LES) on the SD 2030 airfoil with serrations. The results show that the crushing process of shedding vortices was promoted by the spiral flows near the roots of the serrations, resulting in the break-up process of large eddies and the decrease of the correlation length in the spanwise direction.…”

Section: Introductionmentioning

confidence: 99%

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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Experimental and numerical study on noise reduction mechanisms of the airfoil with serrated trailing edge

Cited by 7 publications

References 26 publications

Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

Noise Reduction Mechanisms of an Airfoil with Trailing Edge Serrations at Low Mach Number

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

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