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Fig. 7. Free-flight smoke visualization of the flow around the wings of
Sympetrum sanguineum accelerating vertically with the wings stroking
in-phase. A leading edge vortex (yellow arrows) forms and grows to extend over
both sets of wings. (A–H) Consecutive images from a 250 Hz high-speed
video recording. The dragonfly is moving from left to right through the smoke
plane and the smoke is approximately 1/4 wing-length in (A) and coincident
with the wing hinge in (H). (A) The end of the upstroke. (B) During the
forewing rotation prior to the downstroke, there is some evidence of the start
of LEV formation. In (C) the LEV is already clearly formed (yellow arrow). In
(D–F) the LEV rapidly grows, the smoke streams within the LEV are
thinned by the increased velocities in that region making it darker, and the
stagnation point where the separatrix touches down moves aft from the forewing
onto the hindwing. In (F–H), as the downstroke ends and the wing
rotates, the LEV is shed into the wake. There is a saddle-point (red arrows)
in the wake where smoke-streams bifurcate in the shear layer between the
current LEV and the wake-vortex representing the LEV shed from the previous
wake. There is no evidence of any spanwise flow.
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