The wing motion of a flying insect such as a butterfly produces fluid force by manipulating the flow field around the wing. These forces enable a butterfly to rapidly accelerate, turn, and hover. A number of recent studies have examined the flow field around insect wings. In recent years, quantitative flow visualization techniques, such as PIV measurement, have been advanced rapidly, and the study of the flow field around insect wings using PIV has been actively performed. As a result, the two- and three-dimensional vortex structures and their dynamic behaviors have been investigated quantitatively. However, the dynamic behaviors of these vortex structures have not been related to the dynamic force characteristics. The purpose of the present study is to clarify the relationship between the dynamic lift generated by the flapping butterfly wing and the dynamic behavior of the vortex ring rolled up from the butterfly wing as well as to investigate the role of the vortex ring. We conducted a dynamic force measurement of a flapping Cynthia cardui using a six-axes sensor and three kinds of shafts: a straight shaft, a short L-shaped shaft, and a long L-shaped shaft. Moreover, a two-dimensional PIV measurement was conducted in the wake of the butterfly.

The butterfly is given not only negative dynamic lift but also the reactive force (positive lift) due to the jet flow induced by the vortex ring in the upward flapping. As a result, the butterfly produces dynamic lift in the downward flapping and produces not only negative dynamic lift but also dynamic lift in the upward flapping. Based on these results, it is considered that the vortex ring released into the wake contributes to the dynamic lift generated furing flight. That is, it was concluded that the vortex ring rolled up from the flapping wing has an important role in flight even after being released into the wake.

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