Investigation of Acoustic Effects of Leading-Edge Serrations on Airfoils

Hersh, A. S.; Soderman, P. T.; Hayden, Richard E.

doi:10.2514/3.59219

Cited by 129 publications

(75 citation statements)

References 3 publications

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“…The formation of streamwise vortices behind the roots of the sinusoidal LE profile were identified as being the reason for the reduction in tonal self-noise by breaking up the coherence of vortex generation at the trailing edge. Similar observations have also been made by Arndt & Nagel (1972), Hersh et al (1974) and Longhouse (1977) using sawtooth-type LE serrations.…”

Section: Introductionsupporting

confidence: 71%

On the reduction of aerofoil–turbulence interaction noise associated with wavy leading edges

2016

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An aerofoil leading-edge profile based on wavy (sinusoidal) protuberances/tubercles is investigated to understand the mechanisms by which they are able to reduce the noise produced through the interaction with turbulent mean flow. Numerical simulations are performed for non-lifting flat-plate aerofoils with straight and wavy leading edges (denoted by SLE and WLE, respectively) subjected to impinging turbulence that is synthetically generated in the upstream zone (freestream Mach number of 0.24). Full threedimensional Euler (inviscid) solutions are computed for this study thereby eliminating self-noise components. A high-order accurate finite-difference method and artefact-free boundary conditions are used in the current simulations. Various statistical analysis methods, including frequency spectra, are implemented to aid the understanding of the noise-reduction mechanisms. It is found with WLEs, unlike the SLE, that the surface pressure fluctuations along the leading edge exhibit a significant source cut-off effect due to geometric obliqueness which leads to reduced levels of radiated sound pressure. It is also found that there exists a phase interference effect particularly prevalent between the peak and the hill centre of the WLE geometry, which contributes to the noise reduction in the mid-to high-frequency range.

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Section: Introductionsupporting

confidence: 71%

On the reduction of aerofoil–turbulence interaction noise associated with wavy leading edges

2016

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“…Visualization of the flow showed that the serrated edges introduce vortices which energizes the boundary layer thereby delaying leading edge flow separation at higher angles of attack. Hersh et al (1974) have demonstrated the effectiveness of LE serrations in reducing the narrow band vortex shedding noise radiated from stationary and rotating aerofoils. Noise reductions of between 4 and 8 dB were observed at the peak shedding frequency, which they attributed to the formation of vortices breaking up the periodic structure of the wake.…”

Section: Introductionmentioning

confidence: 99%

Performance and mechanism of sinusoidal leading edge serrations for the reduction of turbulence–aerofoil interaction noise

et al. 2017

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This paper presents the results of a detailed experimental investigation into the effectiveness of sinusoidal leading edge serrations on aerofoils for the reduction of the noise generated by the interaction with turbulent flow. A detailed parametric study is performed to investigate the sensitivity of the noise reductions to the serration amplitude and wavelength. The study is primarily performed on flat plates in an idealized turbulent flow, which we demonstrate captures the same behaviour as when identical serrations are introduced onto 3D aerofoils. The influence on the noise reduction of the turbulence integral length-scale is also studied. An optimum serration wavelength is identified whereby maximum noise reductions are obtained, corresponding to when the transverse integral length-scale is roughly one-forth the serration wavelength. This paper proves that, at the optimum serration wavelength, adjacent valley sources are excited incoherently. One of the most important findings of this paper is that, at the optimum serration wavelength, the sound power radiation from the serrated aerofoil varies inversely proportional to the Strouhal number St h = f h U , where f , h and U are frequency, serration amplitude and flow speed, respectively. A simple model is proposed to explain this behaviour. Noise reductions are observed to generally increase with increasing frequency until the frequency at which aerofoil self-noise dominates the interaction noise. Leading edge serrations are also shown to reduce trailing edge self-noise. The mechanism for this phenomenon is explored through PIV measurements. Finally, the lift and drag of the serrated aerofoil are obtained through direct measurement and compared against the straight edge baseline aerofoil. It is shown that aerodynamic performance is not substantially degraded by the introduction of the leading edge serrations on the aerofoil.

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“…Hersh et al [4] have showed the effectiveness of LE serrations in reducing the narrow band vortex shedding noise emanating from stationary and rotating aerofoils. Noise reductions of between 4 to 8 dB were observed.…”

Section: Introductionmentioning

confidence: 99%

Airfoil noise reductions through leading edge serrations

et al. 2015

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This paper provides an experimental investigation into the use of leading edge serrations as a means of reducing the broadband noise generated due to the interaction between the aerofoil's leading edge (LE) and impinging turbulence. Experiments are performed on a flat plate in an open jet wind tunnel. Grids are used to generate isotropic homogeneous turbulence. The leading edge serrations are in the form of sinusoidal profiles of wavelengths, λ, and amplitudes, 2h. The frequency and amplitude characteristics are studied in detail in order to understand the effect of LE serrations on noise reduction characteristics and are compared with straight edge baseline flat plates. Noise reductions are found to be insignificant at low frequencies but significant in the mid frequency range (500 Hz to 8 kHz) for all the cases studied. The flat plate results are also compared to the noise reductions obtained on a serrated NACA-65 aerofoil with the same serration profile. Noise reductions are found to be significantly higher for the flat plates with a maximum noise reduction of around 9 dB compared with about 7 dB for the aerofoil. In general, it is observed that the sound power reduction level (∆PWL) is sensitive to the amplitude, 2h of the LE serrations but much less sensitive to the serration wavelength, λ. Thus, this paper sufficiently demonstrates that the LE amplitude act as a key parameter for enhancing the noise reduction levels in flat plates and aerofoils.

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Investigation of Acoustic Effects of Leading-Edge Serrations on Airfoils

Cited by 129 publications

References 3 publications

On the reduction of aerofoil–turbulence interaction noise associated with wavy leading edges

On the reduction of aerofoil–turbulence interaction noise associated with wavy leading edges

Performance and mechanism of sinusoidal leading edge serrations for the reduction of turbulence–aerofoil interaction noise

Airfoil noise reductions through leading edge serrations

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