Hydrodynamics of jet-flapping combinatorial propulsion of a squid-like swimmer: Effects of stroke ratio and fin flapping frequency on thrust and efficiency

This study systematically investigates the hydrodynamic characteristics of a squid-inspired jet–fin combinatorial propulsion system using numerical simulations. A high-fidelity three-dimensional biomimetic squid model is established, integrating periodic mantle jetting with lateral fin flapping. On this basis, the mechanisms by which jetting and fin flapping generate thrust are thoroughly analyzed, and the effects of different maximum equivalent stroke ratios and fin flapping frequencies on thrust contribution, energy consumption, and wake structure are systematically compared. The results indicate that the momentum flux at the nozzle and the non-uniform axial pressure distribution within the cavity are key factors in thrust generation. Fin flapping primarily produces thrust through chordwise pressure differentials created by undulatory motions. Jet propulsion delivers strong thrust output and dominates overall thrust modulation, but its efficiency is constrained by significant energy consumption during the intake phase. Increasing the maximum equivalent stroke ratio markedly enhances jet thrust but also tends to induce trailing vortex structures, which further reduce propulsive efficiency. In contrast, fin flapping, as an effective auxiliary mechanism, provides more continuous thrust with lower energy expenditure. In summary, this study clarifies the respective advantages and synergistic effects of jetting and fin flapping in thrust modulation and efficiency optimization, offering theoretical guidance for the design and performance improvement of multimodal biomimetic underwater propulsion systems.