Plots of swirling strength combined with velocity vector at the wingspan of 0
The minimum distance between the leading edges of the two wings at 0
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Figure 12. 75R created by the flapping-wing system with the clap-and-fling effect (case A) and without the clap-and-fling effect (case B). LEV, leading edge vortex; TEV, trailing edge vortex; DW, downwash; IoA, influx of air; UW, upwash.
The minimum distance between the leading edges of the two wings at 0
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Figure 11 shows the snapshots of the swirling strengths and velocity vectors located at 0.25R of the two wings. In case A, when the wings approached each other to the end of the upstroke (t/T = 0.96–1.00), strong leading edge vortices (LEVs) and weak trailing edge vortices (TEVs) that remained attached to the wings were formed. 25R was approximately 0.5c in case A. The wings rotated about their leading edges to complete the clap of the wings at t/T = 1.02. At this period of time, the wing chords were nearly parallel but were not in contact with each other. As the wings drew closer to each other from the leading edge to the trailing edge, a downwash developed between the two trailing edges. Then, the wings rotated about their trailing edges to move away from each other (t/T = 1.04–1.12, fling phase). During this, the LEVs and TEVs were shed and diffused quickly. Subsequently, new weak LEVs and strong TEVs formed and grew in strength (t/T = 1.08–1.12).