Computation of flapping wing sound

In the present study, unsteady flow and acoustic characteristics of the flapping wing are numerically investigated. The Reynolds number based on a maximum translational velocity of the wing and the wing chord length is Re=8800 and Mach number is M=0.0485. The flow around the flapping wing is predicted by solving the two-dimensional incompressible Navier- Stokes equations (INS), while the acoustic field is calculated by the linearized perturbed compressible equations (LPCE), both solved on the moving coordinates. The computational results show that the flapping wing sound is generated by the transverse and tangential motions of the wing with different sound generation mechanisms. A primary dipole tone at wing beat frequency is generated by the transverse motion, while other dipole tones at higher frequencies are produced by the vortex scattering at the trailing-edge of the wing during tangential motion. It is also found that the frequency composition of the primary tone changes with angle because of the torsional angle of the wing motion. This feature is only distinct for hovering, while at forward flight condition, the dipole tone at wing beat frequency is generated not only by the transverse motion but also by the wing-vortex interactions during upstroke. This wing-vortex interaction at forward flight also makes the far-field SPL spectrum more broadband.