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Fig. 7. Analysis of flow field velocities around the hindwing using DPIV techniques
reveals the elaborate flow structures. The wing sections (white lines) are
viewed at 50% distance between the wing base and tip, and have a geometric
angle of attack of 45°. The direction of wing translation is from left to
right in the hindwing (lower wing in panels) and, where present in the images,
from right to left for the forewing (upper wing). The rotational axis for the
fore- and hindwing is 0.33 and 0.22 chord widths, respectively. The DPIV
images show wake velocity fields at two different times within the stroke
cycle (A-C, 15% of stroke cycle; D-F, 35% of stroke cycle) when the hindwing
flaps free from forewing downwash (A,D), when both wings flap but the forewing
(B,E) or the hindwing (C,F) leads wing motion by a quarter stroke cycle. White
open boxes in A-F indicate the region, located between the lower surface of
the wing and the free stream, from which downwash flow velocities and angles
were measured (see text for details). T, stroke cycle.
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