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Fig. 6. Hydrodynamic function of the dorsal fin in yellow perch (Perca
flavescens, 18.0 cm total length, L) to illustrate fin positions
(left panels) and wake flows (right panels) during steady swimming at 0.5
L s–1. The spiny and soft dorsal fins and the dorsal
lobe of the caudal fin have been illuminated with a horizontal laser light
sheet for particle image velocimetry. (A) Fin positions and wake flows at a
time when the soft dorsal fin has reached maximum excursion to the right; (B)
Fin positions 292 ms later when the soft dorsal fin is at its maximal
excursion to the left side of the perch. Note the shear layer resulting from
the spiny and soft dorsal fins, evident as the elongated regions of red and
blue vorticity in the right-hand panels. The soft dorsal fin also sheds a drag
wake with regions of vorticity visible in the gap between the soft dorsal fin
and the caudal fin. Images on the left have been contrast-enhanced to better
reveal the laser-illuminated particles. Shadows where vectors were not
calculated represent regions where the laser light sheet was blocked by the
fish body or fins.
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