Effect of fin geometry on the swimming performance of a bionic ocean sunfish robot with vertically arranged dual fins

The geometric design of fins is critical for the performance of bio-inspired underwater robots, yet the principles governing symmetrical, dual-fin propulsion systems remain underexplored. This study systematically investigates this issue using a novel robotic fish with vertically symmetric, actuated fins. We compare the swimming performance of ten fin shapes, constrained by identical swept area and height but distinguished by their edge-angle geometries. Experimental results demonstrate that fins with a 90° far-edge angle (type-p) increase forward speed by over 30% and reduce the cost of transport (CoT) by over 24% compared to 90° near-edge angle (type-b) counterparts. Furthermore, a U-shaped relationship between CoT and flapping frequency was identified for all fins, confirming a classic trade-off between maximum speed and optimal efficiency. Computational fluid dynamics simulations reveal the underlying mechanism for the performance difference: type-p fins generate a coherent, axially-aligned vortex that maximizes forward thrust, whereas type-b fins shed a wake with a significant vertical component. This vertical wake, while detrimental to forward efficiency, was successfully harnessed to achieve effective vertical maneuverability, a function the type-p fins could not perform. Collectively, these findings provide a clear hydrodynamic basis for task-specific fin design, demonstrating how fin edge geometry can be tuned to specialize a robot for either high-speed cruising or agile maneuvering. • Dual-fin robotic fish is developed to systematically evaluate fin geometry’s impact on swimming performance. • Fins with 90° far-edge angle boost speed by over 30% and reduce CoT by over 24% vs. fins with 90° near-edge angle. • Hydrodynamic analysis shows 90° far-edge fins create an axial vortex, 90° near-edge fins shed a vertical vortex. • Fin edge geometry can be tuned to prioritize propulsion or vertical maneuverability.