An analytical and experimental study on the optimal serration geometry for leading-edge noise reduction*

Leading-edge noise refers to the sound generated due to the scattering of unsteady flow by the leading-edge of an airfoil. It is a common noise source in many multi-row rotor and stator systems, such as jet engines and contra-rotating open rotors. Using serrated leading edges has been shown to be capable of reducing leading-edge noise significantly, but the optimal serration geometry has not been known. In this study, by performing an asymptotic analysis, we show that in order to achieve greater noise reduction at high frequencies, the serration profile cannot have stationary points. Therefore, piecewise smooth profiles free of stationary points are more desirable. Moreover, we show that greater noise can be achieved in the high-frequency regime by using serrations that are sharper only around the non-smooth points. Based on these findings, a new type of serration profile is proposed. Both numerical evaluations and experimental tests confirm its improved acoustic performance. In particular, the trends predicted by the analytical study are well supported by the experimental results. It is expected that these findings can serve as an essential guide for designing serrations.