Micro-air-vehicles (MAVs) and micro-flight robots that mimic the flight mechanisms of insects have attracted significant attention. From this reason, the flight mechanism of the butterflies and their flow fields also has attracted attention. A number of studies on the mechanism of butterfly flight have been carried out. Moreover, a number of recent studies have examined the flow field around insect wings. The present authors conducted a particle image velocimetry (PIV) measurement around the flapping wings of Cynthia cardui and Idea leuconoe and investigated the vortex structure and dynamic behavior produced. However, these results are for a flow field under a fixed condition. The vortex flow structure and the dynamic behavior generated by the wings of a butterfly in free flight are expected to be important for generating the aerodynamic forces required for flight. In the present study, we attempt to clarify the three-dimensional vortex structure around a butterfly in free flight by a scanning PIV measurement. The vortex ring formed by the front wings during the flapping downward grows without attenuation toward the wake. Moreover, during the flapping upward of the wings, a vortex rolls up from the wing, eventually forming a single vortex ring. This vortex ring forms in the vertical direction in contrast to vortex ring formed during the flapping downward, and we may anticipate that the two vortex rings interfere with each other as they advance toward the wake.

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