Quantitative Analysis of Fish-Like Propulsion Mechanisms Based on the Discrete Vortex Method

Fish actively control their bodies to interact with the surrounding vortex flow field and generate thrust. Current research on the propulsion mechanisms of robotic fish primarily involves qualitative comparisons of vortex structures from simulations and experiments, lacking quantitative analysis. This limitation hinders the design and optimization of robotic fish based on flow field characteristics. This paper uses a dynamic model based on the discrete vortex method to define vortices groups and their characteristics, such as center position, size, and wake velocity, to quantitatively describe the evolution of the wake flow. Then, a series of experiments are conducted to compare with the dynamic model and elucidate the propulsion mechanism. Flow field visualization experiments provide the changes in center position and size over time. As the center position moves further away, the impact of the vortices group on the robotic fish decreases. Clockwise and counterclockwise vortices groups approach the midline during formation, jointly creating a jet flow. Moreover, the increase in size over time indicates the dissipation process of the vortices group. A method to calculate the wake velocity distribution is proposed and a wake velocity measurement experiment is designed to compare the average near-distance speed behind the tail. Results show that at close distances from the robotic fish, a jet flow with a speed greater than the propulsion speed is generated. The simulation and experimental methods proposed in this paper effectively predict flow field characteristics.