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Fig. 9. Free-flight smoke visualization of the flow over the wings of Aeshna
grandis flapping in-phase in level flight, but with a slight yaw to the
left. The process of leading edge vortex formation is visualised, and the LEV
has spanwise flow from the centreline towards the wingtip. (A–J)
Consecutive images from a 250 Hz high-speed video recording. The leading edge
vortex forms over the forewing in image sequence (A–C) at the start of
the downstroke. Although the fore-wing moves upwards between images B and C,
the wing rotates in a nose-down sense about an axis of rotation close to the
mid-chord. This must cause a local increase in angle of attack at the leading
edge, and the separation bubble that develops into the LEV forms during this
phase of motion (yellow arrows). The smoke streams at the centre of the LEV
are distorted in (D), bulging out towards the wingtip, which shows that there
is a spanwise flow from centreline towards the wingtip – the opposite
direction to that seen in Fig.
6. The bulge in the leading edge vortex is still present in (E),
but decreases in (F) and is no longer apparent in (G–J), indicating that
there is no longer a spanwise flow within the leading edge vortex as the wings
approach the end of the downstroke and the LEV expands to cover both fore- and
hindwings. The shear layer (secondary vortices?) within the leading edge
vortex is apparent in (H–J), and the LEV has lifted off from the leading
edge of the forewing in (J) as indicated by the presence of a smoke
bifurcation at the point of the yellow arrow.
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