TYTELL, E.D.; Harvard University: Median fin function in bluegill sunfish, Lepomis macrochirus: Streamwise vortex structure during steady swimming

Flow patterns in the transverse plane were examined at a range of positions around bluegill sunfish, Lepomis macrochirus, from the trailing edges of the dorsal and anal fins to the near wake. Simultaneous particle image velocimetry and kinematic measurements were performed during swimming at 1.2 body lengths per second to describe the streamwise vortex structure, to quantify the contributions of each fin to the vortex wake, and to assess the importance of three-dimensional flow effects in swimming. Sunfish produce streamwise vortices from at least eight distinct places, including both the dorsal and ventral margins of the soft dorsal and anal fins, and the tips and central notched region of the caudal fin. Vortices from the dorsal and anal fins persist into the wake, and do not differ significantly in circulation from the two caudal fin tip vortices. Because the circulations are equal and the length of the trailing edge of the caudal fin is approximately equal to the combined trailing edge length of the dorsal and anal fins, I argue that the two anterior median fins act like a second tail, producing a total force that is comparable to that of the caudal fin. Additionally, while the posterior half of the caudal fin and the dorsal and anal fins add vortex circulation to the flow, the peduncle and the anterior caudal fin appear to absorb it. Kinematic measurements indicate that the tail is angled correctly to enhance thrust through this interaction. Integrating these measurements, a three-dimensional structure of the vortex wake is proposed. Finally, the degree to which the caudal fin acts like a idealized two-dimensional plate is examined, and the possibility of substantial losses from tip vortices is described.