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Fig. 3. Hydrodynamic function of the pectoral fin in bluegill sunfish swimming at
0.5 L s–1, as seen in posterior view looking
upstream. A laser-generated sheet of light illuminates a thin slice of water
flow as well as the pectoral fin and body, which casts a shadow to the right.
Laser light penetrates the translucent fin, allowing flow between the fin and
the body to be quantified. Water flow in this figure is out of the page,
toward the reader. Images were obtained from 500 Hz digital video. (A–C)
Particle image velocimetry images showing the movement of the fin illuminated
by the laser light sheet in relation to the body and position of the other
fins. Duration of movement shown=0.48 s from panels A–C. Yellow arrows
show the key fin movements: note the cupped fin shape in A and B. (D–F)
Water flow patterns as a result of pectoral fin movement. This column is from
a different sequence than the frames in the left column. Yellow arrows
indicate water velocities (every other vector is shown), and the background
color scheme is coded so that black color indicates free stream flow velocity
(7 cm s–1), red color flow accelerated by the fin to greater
than free stream velocity, and blue color showing flow slowed below free
stream. Note that the pectoral fin accelerates flow on both the outstroke and
return stroke (red color in E and F).
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