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Fig. 5. Free-flight smoke visualization of the flow around the wings of Aeshna
mixta executing a pitch-down manouver. (A–F) Consecutive images
from a 250 Hz high-speed video recording. Rotation of the hindwing at the end
of the downstroke causes a rapid increase in angle of attack, and initially
attached flow over the hindwing separates to form a large leading edge vortex.
In (A) the flow is still attached over the hindwing (yellow arrow), but in
(B), as the wing rotates, increasing angle of attack, the flow separates
(yellow arrow) forming a small separation bubble. This increases in size in
(C), and in (D) the stagnation point where the separatrix touches down on the
top surface of the hindwing is visualised (blue arrow). The LEV continues to
grow as angle of attack increases in (E), and still has not been shed in (F),
at the beginning of the upstroke. There is no evidence of any spanwise
flow.
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