Numerical investigation of a bio-inspired underwater robot with skeleton-reinforced undulating fins

In this paper, the propulsion performance of a bio-inspired underwater robot with a pair of ray-supported undulating pectoral fins is numerically investigated with a fully coupled fluid-structure interaction model. In this model, the flexible fin rays are represented by nonlinear Euler-Bernoulli beams while the surrounding flow is simulated via solving the Navier-Stokes equations. Kinematically, each pectoral fin is activated independently via individually distributed time-varying forces along each fin ray, which imitate effects of tendons that can actively curve the fin rays. We find that the propulsion performance of the bio-inspired robot is closely associated with the phase difference between the leading edge ray and the trailing edge ray of the pectoral fin. The results show that with a symmetrical kinematics, the highest thrust is created when the phase difference is 90 degree while the point maximizing the propulsion efficiency varies with the motion frequency. It is also found that there is a minimum frequency of generating net thrust for a specific parameter setup, which rises as the increase of phase difference. Compared with the symmetrical kinematics, the non-symmetrical kinematics generates more complicated hydrodynamic forces and moments which may be beneficial for the turning maneuver.