The composite mechanism of damping and elasticity for a fish-like flexible propeller

Fish in nature adjust their swimming modes by regulating muscle stiffness under different ocean conditions. Peduncle stiffness plays a significant role in propulsive performance. However, flexible structures designed with single-mechanism elastic components produce optimal propulsive performance only at specific frequencies. Existing structures were difficult to achieve effective propulsion over a wide range of frequencies. To this end, this article firstly proposed a flexible fish-like propeller based on the composite mechanism of damping and elasticity. Combining with the compliance characteristics of the elastic structure at a specific frequency and damping characteristics that the damping force raised rapidly with frequency, the biological propeller maintained a reasonable phase and flapping amplitude in a wide frequency range, ensuring that the passive flexible tail promoted propulsive performance over a wide frequency range. Secondly, based on the blade element theory, fluid forces exerted on the biological propeller were calculated by drag forces and added mass forces. A hydrodynamic-twisting-damping torque (HTDT) model was established to analyze the motion of the passive flexible joint. Finally, the propulsive performance was tested over a grid of torsional stiffness and dynamic damping viscosity. The results showed that torsional stiffness and dynamic damping viscosity significantly influenced the phase and flapping amplitude of the caudal fin. The experimental results further found the optimized combination with the optimal torsional stiffness and dynamic damping viscosity. This work further offers valuable insights into the development and optimization of the fish-like propeller.