Hydrodynamic effect and Fluid-Structure coupled vibration of underwater flexible caudal fin actuated by Macro fiber composites

Underwater bionic robotics or vehicles using oscillating flexible structures as power sources are attracting considerable attention. In this study, the hydrodynamic effect and fluid–structure coupled vibration of a flexible caudal fin actuated by macro fiber composites(MFC) are investigated. A fluid–structure coupled dynamic equation of the MFC-actuated caudal fin with a variable cross-section is established, and both the internal moment generated by the MFC and the complex-valued hydrodynamic forces are considered. Distributions and evolutions of the flow velocity and pressure in the vicinity of the oscillating caudal fin with different parameters are visualized through comprehensive parametric CFD simulations. Simulation results show that the presence of the gap introduces significant flow reshaping effects to the flow distribution and hydrodynamics. Distributions of the flow velocities and concentrated pressure regions depend highly on the gap width of the forked caudal fin. Then, a revised hydrodynamic function depending on three nondimensional governing parameters is developed, this is, the characteristic oscillating frequency, the width ratio, and the gap-to-width ratio. Formulas for the real and -imaginary parts of the complex-valued hydrodynamic function are developed respectively through the comprehensive CFD framework. Theoretical and experimental results show that the revised hydrodynamic function and developed model are capable of estimating the hydrodynamic effect and underwater dynamic response of the MFC-actuated caudal fin. Compared with the classical hydrodynamic function for the rectangular beam, the expression for the beam overestimates the added mass effect exerted on the caudal fin, and underestimates the hydrodynamic damping effect, thus leading to a smaller estimated resonant frequency and bandwidth, but a higher predicted response peak. These findings are useful for the robotic fish or autonomous underwater vehicles which are propelled by oscillating foils actuated by smart actuators.