Aerodynamic force and vortex structures of flapping flexible hawkmoth-like wings

Aerodynamic characteristics were determined and flow visualizations were carried out in order to interpret the effects of wing flexibility for hawkmoth-like wings on aerodynamic force generation. The flexibility varied according to wing thickness: Case 1 used a 3-mm thick rigid wing, while Cases 2, 3, and 4 used 0.8-, 0.5-, and 0.35-mm thick flexible wings, respectively. The wings were constrained to a sinusoidal flapping motion in a water tank, and digital particle image velocimetry captured three sections of each wing model by fixing the position of the laser, shooting at 30%, 50%, and 70% of the rigid wing length. The flexible wings had phase delays in the stroke motion, which had influence on vortex generation, particularly the leading-edge vortex (LEV). As the wing became flexible, the vorticity of the LEV and the corresponded lift decreased. However, Case 2 had greater aerodynamic force than other cases due to the behavior of the new LEV after the wing reversal. In particular, the new LEV around the wingtip was delayed in its dispersal due to part of the LEV generated during the previous stroke. The encounter between the new LEV and the LEV residue induced the flow over the leading-edge, preventing the new LEV dispersal. Along with the higher stroke velocity after a specific time, the delayed LEV dispersal helped the flexible wing have higher lift. Cases 3 and 4, on the other hand, showed large wing deformations during flapping, which caused the vortex structures around the flapping wing to become relatively unstable. Accordingly, they had much less aerodynamic force than the previous cases. These results help to explain how flexible wings obtain more or less aerodynamic force, and they also suggest the importance of setting a specific range of flexibility to enable greater aerodynamic force for insect-inspired flapping micro aerial vehicles (MAVs).