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

The seahorse exhibits a special upright posture during swimming, which is different from the horizontal propulsion of other fish. Seahorses can swim simultaneously in both horizontal and vertical directions and change their movement path in the vertical plane by adjusting the inclination angle of the body. Thus, the two-degree-of-freedom (2-DoF) swimming of the seahorse is numerically analyzed to reveal its propulsive mechanism when the inclination angles α of its body are 85° and 30°. The research results indicate that hydrodynamic force can be decomposed into horizontal and vertical components on the vertical plane, and changes in the inclination angle can cause adjustments for both components, ultimately affecting their swimming speed and trajectory. Although the resultant speeds are nearly identical, it is more conducive to upward swimming at α = 85°, while it is more effective for horizontal motion at α = 30°. Unlike the backward diffusion appearing in the wake of other fish, the strip vortex generated by the seahorse dorsal fin diffuses simultaneously in both horizontal and vertical directions, indicating the generation of forward and heaving forces. Compared to the one-degree-of-freedom propulsion, the diffusion direction of the strip vortex and the position of the attachment vortex in the 2-DoF swimming have changed, especially at α = 85°. This study is beneficial for expanding the design of the propulsion mode of innovative robotic fish.