Effect of serrated trailing edges on aerofoil tonal noise

A wall-resolved large-eddy simulation of a symmetric Joukowski aerofoil with a 12 % thickness at a Reynolds number of 250 000, a Mach number of 0.4 and zero incidence angle is performed in order to investigate the effect of using a serrated trailing edge on the acoustic feedback event that generates a tonal noise. The acoustic feedback is investigated in detail to emphasise the interaction between the upstream travelling acoustic expansion wave and the laminar separation bubble. The simulation shows that the serrated trailing edges may result in a significant reduction of the tonal noise. This paper provides detailed investigations into the noise reduction mechanisms. The main finding is that the presence of a serrated trailing edge decreases the amplitude of the acoustic source pressure in the transitional region and gives rise to destructive phase interference in the wall pressure fluctuations in the vicinity of the trailing edge which weakens the acoustic feedback loop.