Design and hydrodynamic analysis of controllable soft-body extension-driven flippers inspired by swimming frog flippers

Flippers are key tissues through which frogs swim in water, and their spreading and contracting state directly affects their ability to move while driving their hind limbs. First, inspired by movement mechanism of frog flippers, a controllable soft-body extension-driven frog-like flipper mechanism was designed by combining the large elasticity and strong ductility of soft-body materials. This mechanism is driven by single motor via linkage gear, enabling it to simulate the changing state of flipper extension precisely controlled by the frog as it swims. Second, nonlinear mechanical calculations of flipper deformation membrane were carried out, and deformation optimization model based on Ogden's super soft-body material was established. Subsequently, integral analysis of the deformed area of flippers was used to obtain a fluid dynamics model of the paddling performance. Theoretical calculations and computational fluid dynamics simulations have shown that flippers can instantaneously control changes in the flipper area to inhibit fluid separation while swimming, effectively reducing the water drag of flippers in water and thus increasing the swimming propulsive force. Finally, when the spreading area of flippers was the same, and the swimming efficiency increased by 1, 1.6, and 1.9 times compared with that of fixed, passive folding, and passive spring flippers, respectively.