Hydrodynamic analysis of the upright swimming of seahorse

Abstract

The seahorse is the only creature in the ocean that can maintain an upright posture while swimming. This paper mainly discusses the hydrodynamic characteristics and the flow field structure of the seahorse when it swims upright. Using a three-dimensional seahorse model, numerical simulations of self-propelled swimming are conducted by establishing the kinematic equations of its dorsal fin. The focus is on elucidating the effects of the undulation frequency and the inclination angle on swimming performance. The results indicate that a higher undulation frequency of the dorsal fin leads to better acceleration performance, or in other words, greater hydrodynamic forces. The inclination angle of the seahorse's body also directly affects its hydrodynamics and the flow field structure. Unlike other fish that swim horizontally, the seahorse generates forward and upward thrust as the flow field simultaneously spreads backward and downward. Since the upright posture makes the forward thrust much smaller than the upward one, the seahorse has low efficiency in forward propulsion when swimming upright. As the inclination angle decreases, the forward thrust gradually increases and exceeds the upward force, which allows for a rapid improvement in the swimming velocity. The simulation findings of this study are consistent with previous experimental observations.