Effect of caudal fin shape on swimming performance of a flying fish robot

The advancement of robotic fish necessitates examination of whether caudal fins should be specifically designed for individual body shapes or if generic, isometric fins can function effectively across multiple species. Addressing this knowledge gap is essential for optimizing performance and efficiency in robotic aquatic systems. This work examines the effect of caudal fin geometry on the swimming performance of a flying fish-inspired robot and proposes design guidelines for efficient robotic tails. Four fin designs were constructed based on the morphologies of flying fish, swordfish, whale sharks, and thresher shark. These designs was evaluated through computational fluid dynamics (CFD) and pool experiments. Key performance metrics, namely swimming speed, Strouhal number (St), Reynolds number (Re), and the cost of transport (COT), were assessed, with flow structures analyzed to interpret propulsive mechanisms. The flying fish caudal fin achieved the highest speed (up to 2.54 m s^-1) and the lowest COT (20.05-30.18). Moreover, St and Re values remained within biologically relevant ranges, suggesting superior propulsive efficiency. CFD results confirmed the flying fish caudal fin as the best-performing design, while over-elongated fins were found to be less efficient, requiring more power. Our results suggest that swimming efficiency can be improved through body-tail morphological compatibility, emphasizing the adaptation of tail designs to specific body platforms rather than cross-species transplantation. These findings provide actionable guidance for the design of high-efficiency, flying fish-inspired robotic caudal fins.