The dorsal and anal fins of fish interact with the tail fins to produce higher thrust and efficiency. We focused on thin elongated dorsal fins, like those of jackfish. We discovered that dorsal fins can act like the wing strakes of fighter jets, promoting flow attachment on a main lifting surface (wing/tail) by inducing spanwise flow and reducing the effective angle of attack. Beyond a critical sharpness, the effect is more costly than beneficial, meaning dorsal fins may be optimal when they are slightly blunted rather than razor sharp. See a video of the results here. (This work was done in collaboration with the Flow Simulation Research Group at the University of Virginia.)
Authors: Qiang Zhong, Haibo Dong, Daniel Quinn
Abstract: Multi-fin systems, like fish or fish-inspired vehicles, are governed by unsteady three-dimensional interactions between their multiple fins. In particular, dorsal/anal fins have received much attention because they are just upstream of the main thrust-producing fin: the caudal (tail) fin. We used a tuna-inspired fish model with variable fin sharpness to study the interaction between elongated dorsal/anal fins and caudal fins. We found that the performance enhancement is stronger than previously thought (15% increase in swimming speed and 50% increase in swimming economy) and is governed by a three-dimensional Dorsal Fin-induced Crossflow that lowers the angle of attack on the caudal fin and promotes spanwise flow. Both simulations and multi-layer Particle Image Velocimetry reveal that the crossflow stabilizes the Leading Edge Vortex on the caudal fin, similar to how wing strakes prevent stall during fixed-wing aircraft maneuvers. Unlike other fin-fin interactions, this mechanism is phase-insensitive and offers a simple, passive solution for flow control over oscillating propulsors. Our results therefore improve our understanding of multi-fin flow interactions and suggest new insights into dorsal/anal fin shape and placement in fish and fish-inspired vehicles.