Effects of the caudal fin morphology on the swimming performance of sharks

This study examined the impact of caudal fin morphology on shark swimming performance using a kinematic model derived from video data. Three shark species—the banded houndshark (Triakis scyllium) with a sickle-shaped caudal fin, the blue shark (Prionace glauca) with an asymmetrical crescent-shaped caudal fin, and the white shark (Carcharodon carcharias) with a relatively symmetrical crescent-shaped caudal fin—were selected for numerical simulation. The effects of caudal fin area (S) and caudal lobe asymmetry ratio (CLAR) on the swimming performance were analyzed. The results indicated that the white shark, with its symmetrical crescent-shaped caudal fin, exhibited superior propulsive performance compared to the other fin shapes. An increase in the caudal fin area enhanced the pressure difference, thereby improving thrust and lateral force coefficients. Specifically, as the caudal fin area increased from 0.0155 L2 to 0.0185 L2 and CLAR rose from 2.46 to 3.19, the thrust force increment gradually declined. Changes in CLAR influenced the caudal vortex shape and pressure difference, affecting propulsive and lateral force coefficients. A lower CLAR led to a higher thrust force, but excessively low CLAR reduced the pressure difference, decreasing thrust force by 1.7% when CLAR was 1.05 compared to 1.15. This study elucidates the relationship between caudal fin morphology and hydrodynamic characteristics, providing insights into fish behavioral studies related to caudal fin propulsion and serving as a reference for the optimized design of underwater bionic robot shapes.