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

The seahorse, as a unique fish, swims with an upright posture. The dorsal fin of the seahorse exhibits two undulation amplitude envelopes, including the uniform sinusoidal (US) and decaying sinusoidal (DS) modes. The self-propelled swimming of the seahorse with the two modes is numerically studied when the inclination angle of its body is 85° and 30°. The simulation results show that the undulation amplitude envelope has a significant effect on the swimming speed, hydrodynamic force, pressure distribution and vortex structure. In comparison, the US mode is conducive to improving the swimming speed and hydrodynamic forces, particularly when the inclination angle is 30°. Additionally, five high- and low-pressure regions appear on the surface of the dorsal fin in the US mode, while only four pressure regions appear in the DS mode due to the decaying of undulation amplitude. For different undulating modes, the arrangement angle of the vortex in the flow field also changes, which may influence the thrust component. The vortex has been elongated to form a special strip structure in the wake, and the vortex intensity of the US mode is stronger. This work may provide insight into the design of the undulating patterns of the dorsal fin for seahorse-like robotic fish.