Effect of the asymmetric bio-inspired trailing-edge serrations on sound suppression in a coupled owl-based airfoil

Abstract

Trailing-edge noise plays a dominant part in blades coping with low inflow turbulence at the low Mach number. An important reason for the dramatic noise reduction of owls is on account of the distinct wing morphology of the trailing edge. In particular, its excellent hunting advantages are closely related to the angle of attack. Based on that, the bio-inspired trailing-edge design with oblique-arc profile were retrofitted to the coupled airfoil with 40% cross section of the owl wing in this study, which is contributed to break the limitation of the sound suppression of the conventional serration. High-resolution large eddy simulations combined with the acoustic analogy method are employed to perform the simulation under the specific attack angles with 0° and 15° . The results show that the potential of high-efficiency and low-noise bionic airfoil is exceedingly realized at the attack angle with 0° . The overall sound power level of the coupled airfoil is reduced by 5.47 dB compared with the smooth airfoil. An additional 3.68 dB noise reduction advantage over conventional serrations. Affected by the trailing-edge serrations, the coherent vortex structures with tube-shaped vortices in spanwise direction are dissipated into the horseshoe-type vortices, which implies that more acoustic energy of the coupled bionic airfoil is dissipated. In contrast, the wake gap flow with attack angle of 15° is conducive to intensify the turbulent wake flow that is prone to generate the V-shaped pressure fluctuations. Extra stronger broadband noise is emitted by the coupled airfoil in the longitudinal direction when the regular large-scale wake vortices of the smooth airfoil are broken into the more widespread out-of-order vortices. Consequently, the design of the innovative trailing-edge serrations is contributed to utilize the fluid physics behind it to implement a broader sound suppression strategy.