Leading edge noise reduction of thin aerofoil by the straight and curved serrations of the add-on type

Abstract

This paper presents the experimental results of the effect of add-on type leading edge serrations on the aeroacoustic performance of a symmetrical NACA0008 aerofoil. The focus is on the turbulence-leading edge interaction broadband noise. Tests have been conducted in an aeroacoustic open jet wind tunnel at Brunel University London. 25 serrated leading edges (straight) that can be adequately described by their serration wavelength  and serration amplitude h have been investigated at Reynolds numbers between 0.2 and 0.6 millions. It was found that those with large h are very effective in reducing the broadband noise levels up to about 9 dB. However, the serration designs with smaller h can produce noticeable levels of noise increase at high frequency. The effect of the  on the noise reduction depends on the corresponding h value. For example, a particular design with both the largest  and h has been shown to be very effective. Another one with a similar  , but the smallest h can actually degrade the performance significantly where noise increase over the baseline aerofoil becomes dominant. The curved-serration has been shown to be able to outperform the straight-serration counterpart by a further 5 dB broadband noise reduction when it is optimised properly. The mechanism is due to the increase of the “effective” serration amplitude h as a result of the curvature although its “normal” serration amplitude h actually remains the same as the straight-serration counterpart. Currently, there is no evidence to support the hypothesis that the peak of the curved-serration can prevent the interaction between the grid-generated turbulence structures and the serration troughs. Overall, the add-on type leading edge serration has been shown to be very effective in the reduction of the interaction broadband noise. The nature-inspired concept of a curvedserration at the leading edge provides an avenue for further research.