Study on the effect of caudal fin damage on swimming performance of fish

The caudal fin of fish, as the primary organ for locomotion, significantly influences their swimming performances, yet caudal fin damage is a common occurrence in natural environments. This study systematically investigates the impact of different shapes and degrees of caudal fin damage, as well as incoming flow velocities, on the swimming performance of crucian carp (Carassius carassius) through a combination of experimental and numerical simulation methods. The experimental results indicate that caudal fin damage significantly reduces the thrust and lateral force coefficients in crucian carp. Fish with intact caudal fins exhibit optimal swimming performance, with an average thrust coefficient of 1.1214. In contrast, fish with different shapes of damaged caudal fins, under the same damage area, show a decrease in the average thrust coefficient by 14.961% (lower lobe damage), 14.768% (upper lobe damage), 15.734% (bilateral damage), and 23.938% (central damage), respectively. Flow field analysis reveals that caudal fin damage alters the vortex street structure in the wake region, with positive and negative high-pressure areas diminishing as damage severity increases, thereby reducing the contribution of vortex-induced thrust. Furthermore, an increase in the incoming flow velocity exacerbates the adverse effects of caudal fin damage on swimming performance. When the flow velocity reaches 0.8 l/s, the thrust coefficients become negative, indicating that the fish cannot generate sufficient propulsive force. This study aims to advance research in fish locomotion physiology and provide a reference for evaluating motion dysfunction in underwater biomimetic fish vehicles following partial structural loss.