Roles of wing flexibility and kinematics in flapping wing aerodynamics

Abstract

Insects are excellent fliers because of their ability to generate precise wing kinematics and deform their wings for the sufficient production of unsteady aerodynamic forces. Most of the previous studies on the effects of wing kinematics on the aerodynamics have been limited to the use of rigid wings, and leaves the contribution of wing flexibility unknown. Here, an experiment was conducted to investigate the effect of varied sweep duration and timing of rotation on the unsteady aerodynamic characteristics of hovering flexible and rigid wings at Re of 104. This study found that the forces generated by the flexible wing showed a conspicuous phase delay, which was more sensitive to the change in sweep duration than the timing of rotation. The transient negative lift associated with rigid wings undergoing delayed and advanced wing rotations totally disappeared in the flexible case. A digital particle image velocimetry (DPIV) measurement at the middle of stroke revealed a slight difference in the vortical structures surrounding the two wings in terms of proximity to the shed trailing-edge vortices (TEVs). Also, the linearly twisted nature of the flexible wing caused the coherent leading-edge vortex (LEV) to be stabilized throughout the wingspan. This increased the radial limit of the delayed stall from 3.6c in the rigid wing to 4.8c in the flexible wing. In general, the flexible wing with symmetric and delayed wing rotations generated the higher efficiency. The corresponding net force vectors were tilted in an almost vertical direction in comparison to the rigid wing. This is an indication that natural fliers adopt specific wing kinematics in addition to their wing deformation for the upward titling of their net force vector, which will significantly enhance their aerodynamic performance.