Role of Trailing-Edge Vortices on the Hawkmothlike Flapping Wing

A time-course force measurement and time-resolved particle image velocimetry study were conducted to investigate the unsteady characteristics of an insect wing. In most cases, the tendencies of the aerodynamic forces in the stroke phase were extremely similar to the stroke velocity profiles, which indicated the appositeness of the steady aerodynamic model. The time-course forces showed that the wing-wake interaction appeared in temporally and spatially restricted sections right after the stroke reversal. The time-resolved particle image velocimetry taken near the stroke reversal demonstrated the vortex-dominated flowfields including the leading-edge vortex and the trailing-edge vortices. This was in contrast to the middle of the stroke, which only had a stable leading-edge vortex. The results showed that the unsteadiness was highly associated with the trailing-edge vortex structures. In particular, the wing-wake interaction were substantially influenced by the behavior of the number 2 trailing-edge vortex, which became the element of the counter-rotating vortex pair. The leading-edge vortex of the next stroke was rarely connected to such unsteadiness. This finding implies that the wing kinematics that determines the characteristics of the trailing-edge vortices were the key to the unsteadiness and the wing-wake interaction on the flapping wing.