Optimization analysis of a bio-inspired robotic fish employing a crank-linkage propulsion system

Abstract Modern bio-inspired robotic fish design increasingly focuses on integrating biological inspiration with engineering-oriented structural solutions to enhance locomotion performance and meet practical application demands. Among these, the crank-linkage propulsion system presents a structurally efficient solution capable of delivering stable and effective thrust under high-frequency actuation. However, most existing numerical studies remain centered on fully biomimetic simulations, lacking systematic guidance for the engineering implementation of such transmission mechanisms. Starting from a tuna-inspired robotic fish model, this study systematically investigates the effects of crank length and caudal fin (CF) morphology on hydrodynamic performance and vortex dynamics. The influence of key flow parameters, namely the Reynolds number ( Re ) and Strouhal number ( St ), on propulsion characteristics is also considered. Results demonstrate that crank length significantly influences thrust generation by modulating interactions between the leading-edge vortex (LEV) and the posterior body vortex (PBV). For a tuna-inspired trunk and CF, a crank length of 0.28 L significantly enhances thrust generation through the synergistic interaction between PBV-induced LEV intensification and periodic vortex evolution, while maintaining nearly constant propulsive efficiency. Investigations on fin morphology reveal that, under constant chord length and fin area, propulsive efficiency generally decreases with increasing aspect ratio. Fins with aspect ratios close to 1 and area concentration near the trailing edge, such as the truncate type, enhance thrust generation by delaying LEV detachment and intensifying vorticity strength. Increased Re strengthens vortex interactions, while St affects wake structures. These findings offer theoretical insights for the optimized design of efficient, hybrid-driven robotic fish based on crank-linkage propulsion systems.