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Fig. 9. Hydrodynamics of the dual flapping foil robot, self-propelling at a speed
of 53 cm s–1. The distance between the foils is fixed at 0.5
chord lengths. The two foils have been programmed to move in sinusoidal motion
with a 140° phase lag difference between them and a period of 588 ms. The
front foil has a 20° pitch amplitude and a 2.5 cm heave distance, while
the rear foil moves with 30° pitch amplitude and a 3.5 cm heave distance.
These parameters are similar to those established in experimental and
computational studies of bluegill sunfish dorsal and anal fins
(Akhtar et al., 2007;
Drucker and Lauder, 2001). The
left panels show the foils and water illuminated by a laser light sheet from
top to bottom in these images; the foils cast shadows toward the bottom. Video
sample rate was 500 Hz. In the right panel these images are analyzed to show
water flow velocities and vorticity around the two foils (vectors were not
calculated in the fin shadows), as in the previous analysis of the sunfish
dorsal and anal fins (Fig. 4).
A distinct thrust wake is visible at 0 ms. Notice how vorticity from Foil 1
impacts Foil 2 as it moves inline with the first foil at 110 ms (B). An
attached leading edge vortex is visible on Foil 2 at 160 ms, enhanced by
incoming vorticity from Foil 1. Note also that water flow in the gap between
the two foils is nearly orthogonal to free stream flow at 0 and 160 ms,
similar to flow patterns observed between the dorsal fin and tail in sunfish
(Fig. 4). Every other vector is
shown for clarity in the right column; images on the left have been
contrast-enhanced.
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