Median fin function in bluegill sunfishLepomis macrochirus:streamwise vortex structure during steady swimming

Abstract

SUMMARY Fishes have an enormous diversity of body shapes and fin morphologies. From a hydrodynamic standpoint, the functional significance of this diversity is poorly understood, largely because the three-dimensional flow around swimming fish is almost completely unknown. Fully three-dimensional volumetric flow measurements are not currently feasible, but measurements in multiple transverse planes along the body can illuminate many of the important flow features. In this study, I analyze flow in the transverse plane 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 s–1 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. I propose a three-dimensional structure of the vortex wake in which these vortices from the caudal notch are elongated by the dorso-ventral cupping motion of the tail, producing a structure like a hairpin vortex in the caudal fin vortex ring. Vortices from the dorsal and anal fin persist into the wake, probably linking up with the caudal fin vortices. These dorsal and anal fin vortices 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 produce a total force that is comparable to that of the caudal fin. To provide additional detail on how different positions contribute to total force along the posterior body, the change in vortex circulation as flow passes down the body is also analyzed. The posterior half of the caudal fin and the dorsal and anal fins add vortex circulation to the flow, but circulation appears to decrease around the peduncle and anterior caudal fin. Kinematic measurements indicate that the tail is angled correctly to enhance thrust through this interaction. Finally, the degree to which the caudal fin acts like a idealized two-dimensional plate is examined: approximately 25% of the flow near the tail is accelerated up and down, rather than laterally, producing wasted momentum,a loss not present in ideal two-dimensional theories.

Learning from fish: Kinematics and experimental hydrodynamics for roboticists

“…Two-dimensional analyses ignore the threedimensional effects of fish shape on hydrodynamic patterns, and recent work has shown just how critical consideration of the three-dimensional shape and kinematic effects are [13,35,37,61,62] . Fig.…”

Volumetric imaging of fish locomotion

“…While these techniques have given us a great deal of insight into the fluid mechanics of fish locomotion, the current method of inferring three-dimensional wake structure from repeated two-dimensional PIV slices produces considerable room for error. A mechanism that instantaneously captures a three-dimensional wake structure is needed in order to understand fully the fluid interactions between fishes and their environment, and among the different fins used for propulsion [16,17].…”

Hydrodynamic function of dorsal fins in spiny dogfish and bamboo sharks during steady swimming

“…The thrust component of the second dorsal fin is also higher than the incident flow. Strong longitudinal components in the wake of the dorsal fin are also present in bluegill sunfish during steady swimming (Drucker and Lauder, 2001;Tytell, 2006). In bluegill, the vortices shed in the wake of the dorsal fin interact and have additive effects to the tail vortices, which could also be happening for these two fins in sharks.…”

“…Like the dorsal and anal fins in bluegill sunfish (Drucker and Lauder, 2001;Tytell, 2006), the vortices of the second dorsal fin in spiny dogfish appear to interact with the tail, although further research using three-dimensional vortex reconstruction (see Tytell, 2006) is necessary to fully demonstrate the extent to which three-dimensional vortices encounter the tail and modify flows generated there. In spiny dogfish, the tail vortices have been previously described as having a ring-within-a-ring vortex structure (see Flammang et al, 2011;Wilga and Lauder, 2004a).…”

“…Studies of flow visualization have been applied to pectoral fins Lauder, 2000, 2001;Lauder et al, 2006), caudal fins (Lauder, 2000;Drucker and Lauder, 2000;Wilga and Lauder, 2004a;Tytell and Lauder, 2004), dorsal fins Lauder, 2001, 2005;Tytell, 2006;Standen and Lauder, 2007) and pelvic fins (Standen, 2010) in a diversity of fishes, but the hydrodynamic function of dual shark dorsal fins has not yet been analyzed.…”