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Fig. 8. Free-flight smoke visualization of Aeshna mixta in counterstroking
flight flying with increasing left roll and yaw and consequent side-slip.
There is a leading edge vortex near the midline (above the wing hinge), which
exhibits spanwise flow running from the wingtip towards the centreline.
(A–J) Consecutive images from a 250 Hz high-speed video recording. (A)
shows the end of the upstroke, the dragonfly is aligned with the flow, with
little roll or yaw, and the smoke streams form a vertical plane. In (B) the
dragonfly begins the downstroke and a LEV is formed (yellow arrow), the
dragonfly has also begun to roll and yaw to the left. In (C) the LEV grows,
and the vertical plane of the smoke streams is distorted so that the centre of
the LEV bulges towards the midline at the yellow arrow indicating a spanwise
flow towards the midline. In (E–H) as the yaw increases and the LEV
grows during the downstroke the bulge in the smokestreams caused by spanwise
flow towards the midline also increases. The LEV is still present at the end
of the downstroke in (I) and at the beginning of the upstroke in (J).
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