Self-Propulsion Hydrodynamics of a Hybrid Underwater Robot Combining Oscillating-Fin and Pulsed-Jet Actuation

This study investigates the self-propulsion hydrodynamics of an underwater robot combining four oscillating-fin units with pulsed-jet actuation. A numerical framework couples three-dimensional unsteady computational fluid dynamics, dynamic meshing, and longitudinal self-propulsion dynamics for parallel (PS) and circular (CS) fin-motion arrangements. Fin-only propulsion provides the baseline for evaluating amplitude effects and jet assistance. Increasing the oscillation amplitude raises the stable swimming velocity Ustable in both arrangements, with CS maintaining a higher baseline speed. Under hybrid propulsion, the central jet entrains and stretches the fin wakes, reorganises vortical structures, and redistributes the pressure field. In CS mode, the mean swimming-speed enhancement was approximately 145.5%, 92.1%, and 65.3% at oscillation amplitudes of 20°, 25°, and 30°, respectively, demonstrating an amplitude-dependent benefit of pulsed-jet assistance. Averaged across the investigated amplitudes over the acceleration interval, positive streamwise impulse increases by approximately 116.9% and 117.6% in PS and CS modes, respectively, while propulsive impulse effectiveness increases by 2.9 and 1.6 percentage points. These impulse measures describe hydrodynamic loading rather than energetic efficiency. The results are consistent with direct jet momentum input accompanied by unsteady fin–jet interactions and provide a hydrodynamic basis for the configuration design and parameter selection of hybrid underwater propulsion systems.