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Fig. 4. Visualization of the dorsal fin wake during steady swimming at 0.5
L s–1 by rainbow trout. The frontal-plane wake
velocity field (Fig. 1A, plane
2) is shown as a matrix of yellow vectors on either side of the body; vectors
overlying the body have been deleted. The trailing edge of the dorsal fin (D)
is visible at the left of each panel, with white arrows indicating the
direction of its movement. The adipose fin (Ad) and caudal fin (C) are also
illuminated by the light sheet. Flow fields are shown at two times
corresponding to the end of dorsal fin movement to the right (A) and the end
of the next half-beat to the left (B). Vortical wake structures generated by
the dorsal fin are numbered 1–4 in order of their appearance within the
frontal plane. Structure 1 is the oldest, having been generated by the dorsal
fin stroke to the right preceding that illustrated in A. The paired vortex
morphology characteristic of younger wake elements is no longer evident in
structure 1. Structure 2, formed during the stroke to the left preceding that
shown in B, is a well-developed vortex pair with central jet flow. Structure
3, formed by the fin motions shown in A and B, has the same morphology as
structure 2 but is situated on the opposite side of the body. Structure 4 is
the youngest wake element, part of an incipient vortex pair and produced by
the dorsal fin stroke to the left illustrated in B. Note that the dorsal fin
wake at this low swimming speed is comprised of vortex centers located on each
side of the body and that wake jets are oriented posterolaterally. Free-stream
velocity of 8.5 cm s–1 has been subtracted from the vector
fields to highlight vortices. Yellow scale arrow, 10 cm s–1;
white scale bar, 1 cm.
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