Abstract
Flying organism and aquatic organism generate vortex ring on the wing, pectoral fin and caudal fin. It is noted that the vortex ring produce unsteady fluid force. However, the unsteady fluid force is produced by vortex ring is not clarified quantitatively. Therefore, a lot of researchers have suggested the method to estimate the unsteady fluid force indirectly. However, the scale and form of the vortex ring evaluated just qualitatively so the unsteady fluid force is not clarified quantitatively. Therefore, we need to clarify the scale and form of the vortex ring quantitatively. We clarified quantitatively the scale and form of the vortex ring generated on the butterfly wing for using PIV measurement. The sale of the vortex ring develops from leading edge toward trailing edge and the form of back of the vortex ring rises and elongates in downward flapping motion. In other words, the vortex ring generated on the butterfly wing changes the scale and form temporally and spatially. Therefore, the Lift produced by vortex ring generated on the butterfly wing is unsteady. Consequently, the unsteady lift produced by vortex ring depends on the change of the scale and form.
