PIV Measurements for Flow Fields around Wings of Butterflies and Dynamic Behaviors of Vortex Rings over the Wings

Abstract

Butterflies fly by combining wing flapping and gliding efficiently and have beautiful flight patterns. Moreover, the butterfly excels in rapid acceleration and turning. A number of studies on the mechanism of butterfly flight have been carried out in recent years. Moreover, a number of recent studies have examined the flow field around an insect wing. However, the dynamic behavior of the vortex formed on the insect wing and its growth process have not yet been clarified. The present authors conducted a flight observation experiment and clarified the behaviors of its wings in flight. Based on these results, we developed a flapping-wing robot without tail wings, which is similar to a real butterfly. The elastic deformation of the wings was found to be an important parameter for stable flight, and we focused on the flow field around the wings created by the flapping motion and its elastic deformation. The authors conducted a particle image velocimetry (PIV) measurement around the flapping butterfly wing of Idea leuconoe and investigated the vortex structure of the wing and its dynamic behavior. A vortex ring is formed over the butterfly wings when the wings flap downward to the bottom dead position. The vortex ring then passes over the butterfly completely and grows until reaching the wake at the bottom dead position. The vortex ring is formed over the wings regardless of the type of butterfly, although the scale of the vortex ring varies with the butterfly type.