Noise reduction mechanism of airfoils with leading-edge serrations and surface ridges inspired by owl wings

The remarkable characteristics of the silent flight of owls provide infinite inspiration for the low-noise operation of the rotating impeller machinery. However, how the characteristics of owl wings, including the leading-edge serrations, trailing-edge serrations, and plumed surfaces, affect the aerodynamic noise has not been studied comprehensively. According to previous research, the noise reduction level of an airfoil with an extensive sinusoidal profile is limited to a certain degree. In this paper, as a new coupling element, the surface ridge of owl wings is added to the airfoils with leading-edge serrations. Based on the NACA0012 (National Advisory Committee for Aeronautics) airfoil, the bionic airfoils with sinusoidal, serrated, and iron-shaped leading-edge serrations and surface ridges are reconstructed and studied to reveal the noise reduction mechanism of the coupled elements. The hybrid numerical method of large eddy simulation combined with the acoustic analog equations is adopted to predict the far-field acoustic characteristics. The vortex dynamic method is used to exposit the noise reduction mechanism of biomimetic flow control. The results demonstrate that the airfoil with iron-shaped leading-edge serrations has the best effect of noise reduction. Relative to the original airfoil, the sound pressure level is reduced by 14.3 dB. The change of streamwise vortices caused by the biomimetic structures leads the regular large-scale tubular vortices to separate into smaller horseshoe vortices. In addition, the correlation coefficient of spanwise is reduced, and the change of time-averaged vorticity in the space field promotes the attenuation effect of sound source caused by sound pressure fluctuation radiation.