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Fig. 3. Free-flight flow visualization of the wake of the dragonfly Sympetrum
sanguineum in counterstroking flight with the smoke-plane close to the
right (far) wingtip. (A–J) Consecutive images from a 250 Hz high-speed
video recording. In contrast to the centreline flow shown in
Fig. 2, here the wingtip
vortices are clear (purple arrows), and form wake elements (green arrows) that
persist for several frames. However even here at the wingtips, where the wake
structure is at its most coherent, the wake elements lose their identity after
five frames (A–E, 1/50th of a second). The difference between the
apparent structure of the wake elements between the tip region and the
centreline region suggests that wake elements have a complex structure,
consistent with the lack of any defined starting vortex.
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