Sound generated by the flow around an airfoil with an attached flap: From passive fluid–structure interaction to active control

Abstract

The sound generation by the flow around an airfoil at a mean angle of attack of 10° is numerically studied by using an immersed boundary method. The effect of a bio-inspired flap attached to the upper surface of the airfoil on the aerodynamics and acoustic outputs is studied. Both the stationary and small-amplitude (5°) pitching airfoils are considered. It is found that the flap increases the mean lift of the airfoil at both stationary and pitching conditions. Most importantly, it reduces the lift oscillation of the airfoil at stationary and low-frequency pitching conditions, which correlates to the sound power reduction of 60.3% and 25.3%, respectively, for the stationary and low-frequency pitching conditions, indicating that the pop-up flap used by birds possibly originates from the noise reduction point rather than the aerodynamic point. When the pitching frequency increases, the effect of the flap changes from reducing the sound to enhancing the sound, which implies that the stable wing position is important for adopting the pop-up flap strategy in noise reduction. It is also found that the flexibility of the flap increases the drag, decreases the lift, and enhances the acoustic outputs. The Fast Fourier Transfer of the sound signals shows that the sound is dominated by the vortex shedding and pitching for the stationary and pitching conditions, respectively. A further study of the active pitching flap shows that the out-of-phase pitching flap increases the mean lift and reduces the sound power, while, the in phase pitching has negative effects for both lift generation and sound reduction.