Propulsion performance of a cownose ray inspired robotic fish flapping pectoral fins near the ground

Cownose rays usually swim near the ground in the ocean and their swimming is inevitably affected by the ground effect. In this work, the hydrodynamic performance of cownose rays during near-ground swimming is investigated by experimental tests. The flapping of the pectoral fin of the robotic and numerical rays presents a high similarity in our kinematics comparison. We find a positive correlation between the flapping projected area in the direction of inflow and the thrust coefficient. The ground effect is found to have a great effect on the cownose rays swimming through our experiments and numerical simulations. The thrust and pitching moment increases as the near-substrate height decreases. Moreover, this ground effect is more prominent for symmetrical flapping compared with asymmetrical flapping. The flapping frequency and amplitude can also affect hydrodynamic performance significantly. This work may provide insight into the kinematics control of the pectoral fin flapping-based bio-inspired robots during near-substrate swimming.