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Fig. 5. (A-L) Time sequence of the wake produced by a tandem dragonfly model wing
moving in a horizontal plane and highlighted by air bubbles in the mineral
oil. The bubbles are illuminated by conventional fiber optics that intersect
the wake at approximately 50% distance from the wing base and normal to the
wing surface at mid half stroke. The red lines in each graph indicate
inclination of the visible chordwise wing element as it appears on the video
images; the upper and lower lines show wing motion of the forewing and
hindwing, respectively. Leading wing edge is indicated by a red dot. (A-F)
Complete half stroke (upstroke) of the forewing moving from left to right.
(G-L) Complete half stroke (downstroke) of the forewing moving from right to
left. The time sequence shows the wake while the forewing leads hindwing
motion by a quarter stroke cycle. In all images yellow pictograms indicate the
location and spin of vortices either shed in the wake (vortex core is marked
by a cross) or attached to the wing (leading edge vortex). Only when clearly
visible in the fluid, the vortices' spin and location were reconstructed from
the video by eye and within the illuminated plane of the wake. Note: vortices
that were masked by other flow structures or moving outside the imaging plane
are not shown in this reconstruction. The stroke period for flapping motion is
0.96 s and stroke amplitude is 100°. See text for more details on stroke
kinematics. Images were taken using a conventional 50 Hz video camera (Sony,
TRV120E, Cologne, Germany).
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