Influences of serrated trailing edge on the aerodynamic and aeroacoustic performance of a flapping wing during hovering flight

The influences of serrated trailing edge on the aerodynamic and aeroacoustic performance of a flapping wing during hovering flight are investigated using a hybrid framework of an immersed boundary Navier-Stokes solver for the flow field and the Ffowcs Williams-Hawkings (FW-H) analogy for the sound field. A rigid rectangular wing with an aspect ratio of 2 undergoes pitching and stroke motions at a Reynolds number (Re) of 310 and a Mach number (M) of 0.012. Simulations are conducted by varying the dimensionless wavenumber k * from 2 pi to 10 pi and wave amplitude 2 h * from 0.25 to 1.0. We find that at k * = 8 pi and 2 h * = 1.5 (D4), the average sound power level is reduced by up to 6.8 dB within the Strouhal number (St) between 2.0 and 4.0 compared to that of a plain trailing edge while the lift coefficient is maintained. The directivity at St = 0.2, St = 0.4, St = 2.2, and St = 2.4 is discussed. It is found that the serrations of D4 do not affect the directivity for the first two frequencies and significantly reduce the magnitude of the directivity for the last two frequencies. The serrations of D4 considerably alter the flow field near the wing surface and reduced the surface pressure fluctuations near the wing tip, leading to the noise reduction. The lift coefficient of D4 is not significantly changed, because the reduction in the pressure-contributed lift is compensated by an increase in the shear stress-contributed lift. The serrations with higher 2 h * and k * have larger shear stress-contributed lift.