Energy Extraction Performance of a Flapping Wing with Active Elastic Airbag Deformation at the Leading Edge

To promote energy extraction efficiency, an active deformation method to implant an elastic airbag at the leading edge of flapping wings is proposed in this study. The charging and discharging processes of the airbag are accomplished by sinusoidally adjusting the local deformation angle. We investigated numerically the effect of both the reduced frequency f∗ and the local deformation amplitude α on the energy extraction efficiency with the chord based Reynolds number Rec=5×105. Our results show that as the reduced frequency f∗ increases, the energy extraction efficiency of flapping wings with various local deformation α first rises and then drops with a maximum energy efficiency around 0.4. Energy extraction efficiency via local deformation of the leading edge can be enhanced up to 25%, compared with the non-deformed flapping wing in the lower reduced frequency range. Analysis on flow field and the working coefficients indicates, due to the airbag deformation, the leading edge vortex is more closely attached to the suction side of the flapping wing with larger local deformation elastic airbag (α =20∘ and 25∘), inducing more preferable pressure distribution and higher power coefficients with less oscillation throughout the motion cycle.