Hydrodynamic characteristics of an undulatory fin robot during mode switching

Most existing studies on undulatory fin robots focus on steady swimming with fixed gaits, while the control continuity and hydrodynamic response during mode switching processes have not been systematically investigated. To address this limitation, a unified control-hydrodynamic solution framework is proposed, which integrates an improved central pattern generator (CPG) with a constraint immersed boundary method (cIBM) to capture the entire mode switching process. This framework ensures the generation of smooth control signals and enables a systematic hydrodynamic evaluation throughout the mode switching process. Based on this framework, several representative mode switching scenarios are investigated: straight motion acceleration and deceleration, straight motion to differential turning, straight motion to oblique motion, forward–backward reversal, in-place turning reversal, and vertical motion reversal. Simulation results indicate that the proposed CPG generates smooth, continuous control commands that balance body stability and fast response. The stability metric remains between 0.85 and 3.09, while the response metric spans 0.16–6.43, ensuring adaptability to diverse maneuvering requirements. The wake flow exhibits a continuous and stable evolution, revealing the hydrodynamic mechanisms underlying the suppression of abrupt velocity variations and body instability during switching. These results establish a hydrodynamic foundation for mode switching control and enhanced maneuverability in complex underwater environments.