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Fig. 8. Wake of the pectoral fin measured on a parasagittal
(x–y) plane of the light sheet approximately parallel
with the fish body and intersecting the pectoral fin. The schematic drawing
(not to scale) at the top of the figure shows a lateral view of a swimming
fish during forward sinking. The average swimming velocity of the fish was
approximately 0.066 m s–1; the sinking angle was
approximately 27°; the body angle was approximately 20°. (A–C)
Instantaneous velocity fields of the wake for the down-stroke, stroke reversal
and up-stroke, respectively. The time was set to t=0 s for A; (B)
t=0.064 s; (C) t=0.224 s. The black vectors represent the
in-plane velocity in A–D. The color contour represents the magnitude of
the z-direction velocity W in A–C. (D–F)
Vorticity contours corresponding to the velocity fields in A–C,
respectively. The bold black arrows indicate the fluid jets generated by the
pectoral fin. White K1, K2 and
K3 denote vortices adjacent to the down-stroke and
up-stroke jets. The white and curved arrows indicate the directions of
rotation of vortices K1, K2 and
K3. A three-dimensional down-stroke jet (A) and an
up-stroke jet (C) were observed.
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