Three-dimensional flow field analysis of biomimetic undulating fins: A comparative study of fin morphologies

Fish and numerous aquatic organisms have undergone millions of years of evolution and natural selection, resulting in movement performance, energy efficiency, and maneuverability that surpass existing engineered propulsion technologies. Throughout biological evolution, fluid dynamic effects have played a significant role in shaping fish fin morphology, primarily through fin–flow interaction mechanisms. This study employs computational fluid dynamics to systematically compare and analyze the hydrodynamic performance and three-dimensional flow field characteristics of convex fins (modeled after the anal fin morphology of Apteronotus albifrons) and various geometric fin configurations. From a microscopic perspective, the research elucidates the optimized fin shape characteristics developed through biological evolution and their underlying flow field regulation mechanisms. The results demonstrate that the biologically derived convex fin exhibits streamlined characteristics and superior hydrodynamic performance compared to other fin geometries. Specifically, this fin shape efficiently directs fluid flow toward the posterior region of the fin surface, optimizing momentum transfer efficiency. Furthermore, the convex fin maintains a stable jet structure extending from the mid-section to the trailing edge, generating a thrust mechanism dominated by suction (negative pressure) due to pressure differentials. Additionally, the vortex structures distributed along the convex fin's length alternate both horizontally and vertically, ensuring multidirectional momentum exchange capabilities during fin oscillation and thereby enhancing maneuverability.