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Fig. 15. LEV formation at the start of the downstroke in Aeshna grandis in
counterstroking tethered flight. Yellow arrows point to the LEV throughout. In
(A), a separation bubble can be seen on the top surface of the wing early in
the phase of rotation (pronation) at the top of the upstroke prior to the
beginning of the downstroke. The separation bubble begins at the leading edge,
and flow reattaches at a point on the top surface between 1/4 and 1/2 of the
way to the trailing edge. In (B) the wing has rotated further and begun to
descend. The separation bubble is larger, with the separatrix reattaching on
the top surface about 3/4 of the way from the leading edge to the trailing
edge. In (C) the LEV has grown to cover the entire top surface of the wing,
and shear is apparent behind the trailing edge between the forwards moving
flow of the LEV and the backwards moving flow that has passed underneath the
wing. The LEV is fully formed in (D).
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