Hydrodynamic mechanisms implicit in the transition from surface standing-and-walking to standing-and-turning behavior in robotic dolphins

In order to overcome the operational challenges faced by traditional underwater robots at the water surface and improve their surface maneuverability, this study adopts the dolphin as a biological model to investigate its hydrodynamic performance during the transition from standing-and-walking (SAW) to standing-and-turning (SAT) behaviors on the water surface. The research leverages the compound motion of the body, caudal fin, and pectoral fins in a three-dimensional dolphin physical model to realize three novel SAT modes based on the SAW, respectively: different amplitude (DA), different frequency (DF), and different phase (DP). Through a series of numerical simulations, the time-varying patterns of key parameters during the transition process were described, and the mapping relationships between kinematic parameters and hydrodynamic performance for each mode were quantitatively analyzed, revealing the transient evolution of the flow field around the dolphin model. The results showed that the proposed SAT modes could simultaneously generate vertical and horizontal thrusts, along with torque around the body's longitudinal axis, enabling the transition from SAW to SAT behavior. Among the three turning modes, the DF mode exhibited the most stable maneuverability. Notably, under specific conditions, the turning radius of the DP mode can reach 0.014 m, effectively achieving in-place SAT behavior, which is challenging for traditional underwater robots. This work provides a novel approach to addressing the surface operation challenges faced by traditional underwater robots, significantly enhancing their maneuverability on the water surface.