The effects of undulating patterns of the dorsal fin on seahorse swimming performance.

This article investigates the effects of two undulation amplitude envelopes—uniform sinusoidal (US) and decaying sinusoidal (DS) modes—of the seahorse dorsal fin on swimming performance using numerical simulations at body inclination angles of 85° and 30°. Results indicate that the US mode enhances swimming speed and hydrodynamic forces compared to the DS mode, with speed increases of about 12.5% at 85° and 48.36% at 30°, and corresponding thrust improvements of 25.59% and 101.72%, respectively. Pressure distribution analyses show alternating high- and low-pressure regions on the dorsal fin surface, with the number and size of these regions varying according to the amplitude envelope. The study also reveals that both modes generate elongated strip vortices rather than typical vortex rings, with differences in vortex intensity and arrangement angle influencing thrust components. These findings provide insights for designing undulating patterns in bionic seahorse robotic fish, suggesting the DS mode favors energy-efficient, stable upright swimming, while the US mode is preferable for faster, more horizontal swimming.