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Fig. 10. Hydrodynamics of a single, flexible, flapping foil self-propelling at a
speed of 24 cm s–1. The white arrow shows the heave motion
(3.5 cm heave amplitude) of the rod that actuates the flexible foil, composed
of a plastic sheet of the same dimensions as the foils in
Fig. 9. Foil thickness is 0.32
mm, foil length=19 cm, foil height=6.8 cm, and the video sample rate is 250
Hz. The left panels show the flexible foil and water illuminated by a laser
light sheet from top to bottom; the flexible foil casts a shadow toward the
bottom of each image; these images have been contrast-enhanced. Large yellow
arrows in the left-hand panels show the direction of foil surface motion from
one panel to the next. The actuating rod to which the foil is attached and the
thin black foil itself have been enhanced by a white dot and line,
respectively, for clarity. In the right panels these images are analyzed to
quantify water flow velocities and vorticity around the flexible foil (vectors
could not be calculated in the fin shadows), as in the previous analysis of
two foil self-propulsion (Fig.
9). Note that an attached leading edge vortex (LEV) is visible at
0 ms as the foil leading edge nears the end of its downward motion and begins
to move up. This attached LEV persists throughout the duration of the
downstroke, until almost 930 ms (not shown). A distinct thrust wake is evident
behind the flexible foil, with a strong side component.
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