Numerical investigation of tuna swimming: Kinematic performance and vortex dynamics under sinusoidal perturbations

This study presents a numerical investigation into the propulsion dynamics of tuna, with a focus on the effects of individual kinematic parameters and the introduction of sinusoidal perturbations on hydrodynamic performance and vortex field evolution. A precise morphological model and kinematic equations of tuna were established, and the swimming process was simulated using the immersed boundary method. Regarding the effect of a single kinematic parameter, the thrust coefficient increases monotonically with the Strouhal number (St). The propulsive efficiency, however, is non-monotonic, first increasing sharply to a peak value, maintaining a plateau, and then decreasing slightly with a further increase in St. In the case of motion perturbations, the hydrodynamic performance exhibits distinct characteristics. When amplitude-varying perturbations are introduced, the time-averaged thrust increases progressively with the disturbance amplitude. Furthermore, the propulsive efficiency not only improves but also consistently remains superior to the non-disturbance baseline. When frequency-varying perturbations are introduced, however, both the time-averaged thrust and propulsive efficiency demonstrate significant oscillatory behavior with increasing disturbance frequency. This research provides novel insights for the motion control of bio-inspired underwater vehicles.