Effects of flexibility and motion parameters on a flapping foil at zero freestream velocity

To investigate the influence of the flexibility and motion parameters of flapping foils on their hydrodynamic characteristics, this paper experimentally investigated five different flexibility models in a hydrostatic water tank over a wide range of motion parameters. With the help of a biomimetic flapping foil propulsion test platform, the combination of a pitching motion and heaving motion of the flapping foil is realized. Meanwhile, a six-axis force/moment sensor, a high-speed camera and particle image velocimetry (PIV) are used to record the mechanical parameters during the motion and visualize the flow field. The mechanical data are homogenized and normalized to obtain the variation of foils’ hydrodynamic properties with flexibility and motion parameters. The experimental images are processed to obtain the motion trajectories and the flow field information. The experimental results show that the highly flexible foil performs well at low frequencies; its vortices are a little stronger and slightly farther apart laterally but closer together longitudinally, and appear to persist farther downstream than those of the other models. Thus, flexibility plays an essential role in thrust stability and speed stability. Furthermore, the flap trajectory changes from haphazard to a Fig. 8 shape and ultimately into an arc as the stiffness increases.