On the Effect of Leading Edge Serrations on Aerofoil Noise Production

Abstract

This paper presents experimental results on the aeroacoustic performances of a NACA 65(12)-10 aerofoil subjected to serrated leading edges. The serration patterns of these leading edges are formed by cutting into the main body of the aerofoil, instead of extending the leading edges. Therefore these serrated leading edges, when attached to the main body of the aerofoil, will always result in the same overall chord length. The experiment was performed in an aeroacoustic wind tunnel facility. These serrated leading edges were investigated for their effectiveness in suppressing four different types of noise sources: laminar instability tonal noise, leading edge separation bubble noise, turbulence-leading edge interaction noise and trailing edge self-noise. Streamwise vortices produced by an optimised serrated leading edge can suppress the separation bubble at the trailing edge, thereby reducing the instability laminar tonal noise significantly. It is found that the most effective serration configuration is the one with the largest serration amplitude and smallest serration wavelength. Without even relying on the streamwise vortices, the sawtooth geometry of the serration itself can already be sufficient to suppress the leading edge separation bubble. Due to the special geometry of the NACA 65(12)-10, it is very effective in the production of laminar separation bubble noise at the leading edge. The use of serrated leading edge can therefore be an effective passive device to suppress this particular noise source. Similarly, the most effective serration geometry in the reduction of turbulenceleading edge interaction noise is the one with the largest serration amplitude and smallest serration wavelength. However, this configuration is also prone to generating superfluous noise at high frequency. Extensive boundary layer and very near wake measurements were performed to investigate the flow structures on the NACA 65(12)-10 aerofoil with a large serration amplitude leading edge. It can be concluded that the serrated leading edge is very disruptive to the hydrodynamic growth of the turbulent boundary layer at the trailing edge. Evidences on the reduction of boundary layer low-frequency turbulence at the trailing edge could support the hypothesis of a reduction in the low-frequency far field noise. This remains to be confirmed in the future studies.

Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil

“…As demonstrated in Figure 7a,b, the most effective serration configuration for the turbulence-leading edge broadband noise reduction is the combination of the smallest λ and largest A, although a slight noise increase at high frequency can also be detected. The A exerts a more dominant factor on the noise reduction compared to the λ, which has also been reported in [14,15]. Repeating the same set of serrated leading edges across a wider range of U essentially returns the same positive outcomes, as shown in Figure 8.…”

On the use of leading-edge serrations for noise control in a tandem airfoil configuration

“…The noise increasing at higher frequencies could be a result of the superfluous noise that the serrated leading edges are known to produce at high frequencies. 42 In general, the root plane is observed to cause more noise reduction than the tip plane. Moving further downstream closer to the trailing edge, at about xr/c = 0.35, the reduction in PSD levels due to use of serrations becomes clearly evident on both suction [ Fig. 14(e)] and pressure [ Fig.…”

Applicability of Aeroacoustic Scaling Laws of Leading Edge Serrations for Rotating Applications

“…The underlying effect is suspected to be an efficient reduction in small-scale separation due to the leading edge contour. This takes place via the vortex-generating character of the leading edge serrations [27,28], resulting in increased stability of the blades' boundary layer as well as in an efficient shift of coherent structures in the blade tip region towards lower flow coefficients, as described in previous studies of the authors [16]. The known decorrelation and interference effects of the leading edge serrations [3] are still present though play a minor role in terms of the overall noise reduction.…”