The Effects of Spanwise Stiffness on Thrust Generation in Flexible Pitching Fins

Abstract

Prior numerical studies suggest that stiffness gradients in bio-inspired fish fins can enhance thrust through favorable fluid-structure interactions, but physical validation remains limited. As such, this study experimentally investigates whether spanwise stiffness variations in additively manufactured fins produce measurable differences in thrust and thereby propulsive performance. Four fins with identical geometry, but differing stiffness profiles were tested: a baseline uniform configuration without variable stiffness, as well as a cupping, W-shaped, and heterocercal distribution. Each fin was driven in a prescribed pitching motion within a water tank and the resulting thrust forces were measured using a low-friction force measurement system. Signal processing procedures were then used to mitigate experimental noise and obtain reliable mean thrust values for comparing fin performance. It was determined that at lower pitching frequencies (0.70–0.80 Hz), mean thrust differences between different distributions were small and comparable to the experimental uncertainty, indicating that thrust production was governed largely by the flapping motion. At higher frequencies (0.85–0.90 Hz), clear performance separation emerged. All variable-stiffness fins produced greater thrust than the uniform configuration, with the W-shaped distribution generating the highest mean thrust. At 0.90 Hz, the maximum difference between configurations exceeded the experimental uncertainty by more than twenty-five times. These results demonstrate that stiffness gradients significantly influence thrust under hydrodynamic loading and can inform the design of more efficient passive propulsion systems.