Hydrodynamic performance and vortex evolution mechanism of a bionic underwater robot with flexible fins of composite motion

Unmanned underwater robots have promising application prospects in marine exploration and complex tasks. In this study, a novel undulating-fin robot was designed based on the swimming mechanism observed in fish. The locomotion of the undulating fin is a combined movement involving fin rays oscillation and pitching along the propulsion direction. Furthermore, in-depth research was performed on the fluid dynamics mechanism of the combined movement. First, kinematic and dynamic models of the composite motion were established with thrust, oscillating and pitch parameters. Subsequently, the hydrodynamic performance of the robot was assessed and improved through the propulsive efficiency and dimensionless coefficients. Additionally, the fluid flow characteristics of robot propulsion were investigated by analyzing vortex generation and streamline distribution. Finally, the wake flow field of the undulating fin was reconstructed through dynamic mode decomposition (DMD) to explore the vortex evolution and thrust generation mechanism with dominant modes. The results reveal that the robot velocity is primarily determined by the undulating frequency. The propulsive efficiency reaches 28.8% with the frequency and pitch angle of 2.0 Hz and 10° in the best case within the tested parameters, respectively. The Strouhal number ( S t ) of the undulating fins with composite motion is between 0.21 and 0.27, leading to a degree of flow instability. In robot propulsion, a central jet forms with vortex shedding and triggers thrust generation. The mode decomposition of the wake flow field suggests that the alternating vortex structures is the crucial factor in thrust generation and efficiency improvement. Additionally, the dominant modes with high energy proportion demonstrate the spatiotemporal evolution and stability of the wake flow field. This research provides valuable insights into the propulsion mechanisms of underwater robots and the associated fluid flow characteristics.