Characteristics of Dynamic Forces Generated by a Flapping Butterfly and its Wake Structure

Abstract

A typical example of the flow field around a moving elastic body is that around butterfly wings. Butterflies fly by skillfully controlling this flow field, and vortices are generated around their bodies. The motion of their elastic wings produces dynamic fluid forces by manipulating the flow field. For this reason, there has been increased academic interest in the flow field and dynamic fluid forces produced by butterfly wings. A number of recent studies have qualitatively and quantitatively examined the flow field around insect wings. In some such previous studies, the vortex ring or vortex loop formed on the wing was visualized. However, the characteristics of dynamic forces generated by the flapping insect wing are not yet sufficiently understood. The purpose of the present study is to investigate the characteristics of dynamic lift and thrust produced by the flapping butterfly wing and the relationship between the dynamic lift and thrust and the flow field around the butterfly. We conducted the dynamic lift and thrust measurements of a fixed flapping butterfly, Idea leuconoe, using a six-axes sensor. Moreover, two-dimensional PIV measurement was conducted in the wake of the butterfly. The butterfly produced dynamic lift in downward flapping which became maximum at a flapping angle of approximately 0.0 deg. At the same time, the butterfly produced negative dynamic thrust during downward flapping. The negative dynamic thrust was not produced hydrodynamically by a flapping butterfly wing because a jet was not formed in front of the butterfly. The negative dynamic thrust was the kicking force for jumping and the maximum of this kicking force was about 6.0 times as large as the weight. On the other hand, the butterfly produced dynamic thrust in upward flapping which was approximately 6.0 times as large as the weight of the butterfly. However, the attacking force by the abdomen of the butterfly was included in the dynamic thrust and we have not yet clarified quantitatively the dynamic thrust produced by the butterfly wing.