Hydrodynamic synergy between undulating and caudal fins in a bionic underwater vehicle

This study investigates the hydrodynamic synergy of a biomimetic underwater vehicle propelled by coordinated undulating fins (dorsal and anal) and a caudal fin. Numerical simulations are conducted to analyze the effects of key kinematic parameters and three propulsion modes on swimming and hydrodynamic performance. Results indicate that higher frequencies and larger amplitudes increase thrust but amplify unsteady hydrodynamic loading, whereas a larger wavelength reduces roll fluctuations, representing a beneficial exception. Phase difference has a negligible influence on mean thrust but strongly affects loading steadiness. The coordinated multi-fin mode (Mode 3) achieves the highest forward speed with a favorable balance of lateral and yaw loading. A positive interaction force of 1.25 N confirms genuine nonlinear synergy, while the 22% circulation amplification and 73% momentum-flux enhancement relative to Mode 2 further support the proposed wake-coupling mechanism. These wake metrics, however, include the direct contribution of the additional active caudal fin and thus cannot alone isolate the nonlinear effect. These findings provide quantitative guidance for designing efficient multi-mode underwater vehicles.