Hybrid control of aerofoil self-noise by coupling air blowing and trailing-edge serration

Active and passive control methods have been extensively studied and employed for reducing turbulent-boundary-layer (TBL) trailing-edge (TE) noise of aerofoils, for example air blowing and TE serration. To further explore the potential of noise reduction, this paper proposes a hybrid method coupling blowing and serration at the trailing edge, and investigates the effects of air blowing, TE serration, and the hybrid blowing-serration (HBS) on the TBL TE noise of a NACA0012 aerofoil at a Reynolds number of Re=4×10. Large eddy simulation and Ffowcs Williams–Hawkings acoustic analogy were used to analyse these effects and the respective noise reduction mechanisms. The numerical results show that the HBS method exhibits superior acoustic performance to the blowing and serration methods, with a maximum reduction of 20.5 dB in broadband noise and 5.7 dB in overall sound pressure level. It is revealed that the HBS method functions primarily through three noise reduction mechanisms. Firstly, blowing eliminates the outward flow motion at the serration root and tip, resulting in a forced flow alignment. Secondly, it weakens the turbulence energy in the near wake as for air blowing. Finally, this method mitigates spanwise coherence at the trailing edge, thereby causing destructive interference of noise sources, similar to the effect of TE serration.