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Fig. 1. Schematic reconstruction of wake pattern during wake–wing interaction
in fruit fly and dragonfly model wings. (A–D) Wake capture mechanism at
dorsal and ventral stroke reversal. At the beginning of each half stroke, the
leading and trailing edge vortex system generates an inter-vortex stream
towards the wing. (E–H) Schematic reconstruction of vortex generation
and shedding during clap-and-fling maneuver using a generic
Drosophila stroke kinematics
(Lehmann et al., 2005).
Chordwise wing segments at the end of the upstroke (F), during the clap phase
(G), and during the fling phase (H) before the two wings separate for the
downstroke. Leading and trailing edge vortices are shed into the wake at the
end of each half stroke. The low-pressure region evolving between the wings
during the fling pulls fluid around the leading and the trailing wing edge
into the opening cleft. (I–K) Reconstruction of vortices and local flow
conditions at maximum transient lift production in tandem model wings of a
dragonfly. (I) Wing kinematics follows a generic pattern, as found in various
species (Maybury and Lehmann,
2004). (J,K) Flow characteristics at 0.35 fraction of the stroke
cycle at which either the fore- (upper wing; J) or the hindwing (lower wing;
K) leads wing motion by a quarter stroke cycle. Local flow vector (black) in
the vector diagrams is calculated from the velocity and angle of the combined
fore–hindwing downwash (green vector) determined in a region below the
hindwing's surface and the translational velocity of the hind wing section
(gray vector). Effective angle of attack for the hind wing section (left
value) and local flow velocity (right value) are shown, respectively, in
parentheses below the vector diagram. Open arrows indicate the direction of
wing motion. Vortical circulation in the hindwing's leading edge vortex (LEV)
is shown in parentheses. The different strengths of starting and leading edge
vortices are indicated approximately by the size of the plotted vortices. Blue
and red arrows represent normalized vectors of total force attenuation and
enhancement, respectively, compared to a wing flapping free from mirror-
(F–H) and forewing (J,K) downwash. The exact inclination of the force
vectors slightly differs from the orientation normal to the wing's surface, as
shown in the schematics, because of shear forces in the fluid.
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