Two-degree-of-freedom swimming of the seahorse in a vertical plane.

This article focuses on the numerical analysis of the two-degree-of-freedom (2-DoF) swimming mechanism of seahorses, which uniquely swim with an upright posture allowing simultaneous horizontal and vertical propulsion. By simulating seahorse swimming at inclination angles of 85° and 30°, the study reveals that changes in body inclination adjust the decomposition of hydrodynamic forces into horizontal and vertical components, influencing swimming speed and trajectory: a higher vertical thrust occurs at 85°, while horizontal thrust dominates at 30°. The dorsal fin generates strip vortices that diffuse in both directions, producing forward and heaving forces, with vortex diffusion direction and wake structure varying between 2-DoF and one-degree-of-freedom (1-DoF) swimming modes. These findings enhance understanding of seahorse locomotion and offer theoretical insights for designing robotic fish capable of multi-dimensional propulsion.