How dorsal fin sharpness affects swimming speed and efficiency

How well fish swim depends on complex hydrodynamic interactions between their multiple fins. It has been shown, for example, that the wakes of anal fins, dorsal fins, and finlets can boost the thrust produced by the caudal fin. What fin shapes and thicknesses maximize this boost are unknown.  Here we show that dorsal/anal fins only increase caudal fin thrust when the tips of their cross sections are sufficiently sharp. We quantified fin tip sharpness by parameterizing fin shapes using Bezier curves. Particle Image Velocimetry reveals that beyond a critical fin sharpness, the wake of the dorsal/anal fin promotes flow attachment along the caudal fin, resulting in higher thrust. We show that under certain conditions, dorsal/anal fins function like leading edge slats and slots, redirecting and stabilizing the incoming flow as it continues on to the main lifting surface. Our results demonstrate that cross-sectional shape is a key parameter in determining whether a dorsal fin will indeed increase swimming performance. Specifically, we show that sharpness is a critical constraint for effective dorsal/anal fin design – both in fish and in fish-inspired Unmanned Underwater Vehicles.