Noise control of serrated trailing edge airfoil under small incidence angle

Inspired by the silent flight capability of owls, the serrated trailing edge design is considered as an effective method to reduce the turbulent boundary layer-trailing edge interference noise. In this study, the near-field flow and noise characteristics of a NACA 0012 airfoil with additional serrated trailing edges are investigated in detail using an implicit large eddy simulation approach with Reynolds number Re = $9.6 \times {10^4}$, far-field Mach number Ma = 0.1631, and angle of attack $\alpha = {4^ \circ }$. The simulation adopts unstructured grids with 70 million degrees of freedom. In this particular calculation, a small sawtooth-shaped rough strip is added to the airfoil surface to facilitate the fast transition to turbulence for both straight and serrated trailing edge cases. At an angle of attack of 4°, an increase in noise radiation is observed with respect to that at an angle of attack of 0°, with a deflection of the primary radiation direction and a noise reduction of about 2.5 dB in this direction. The flow analysis shows that the sawtooth induces the regularly distributed vortex pair structures at its sides, which facilitates noise reduction in the far-field. The analysis of the wall pressure fluctuation shows that the sawtooth mainly changes the space-time correlation properties near the trailing edges, and the space-time correlation properties of the pressure cannot be described by the existing velocity-based Taylor or elliptical correlation models. In addition, the sawtooth suppresses the noise radiation while causing some loss to the aerodynamic performance of the airfoil.