Spatio-temporal analysis of shear traction dynamics on fish body surface

Fish swimming is a prominent research focus in biomimetic hydrodynamics, and studying the surface stress dynamic of fish contributes to the shape and structural design of robotic fish as well as the energy-efficient control of schooling robotic fish. Traditional research primarily relies on conventional numerical simulations that involve mesh generation and testing. However, the deformation of a swimming fish significantly impacts local grids, and the uncertainties in mesh structure reduce simulation accuracy. To address these challenges, this study employs the open-source computational fluid dynamics solver Palabos, which is based on the lattice Boltzmann method (LBM), to investigate the stress dynamics on fish surfaces. The immersed boundary method algorithm is coupled with LBM and enhanced with a multi-direct forcing scheme to improve precision and effectively handle fluid–solid interactions. Simulations reveal the spatial variations of shear traction on the fish surface. Furthermore, dynamic mode decomposition is applied to analyze the temporal and spatial evolution of shear traction distribution. The results indicate that under steady undulation, high-speed regions and areas of high vorticity occur where the fish's head and tail make contact with the water surface. Despite changes in tail-beating frequency, the overall flow field and vortex shedding pattern remain consistent. Further stress analysis reveals that shear traction oscillates regularly, with its amplitude varying along the fish's body surface. During swimming, shear traction is concentrated at the tail tip and the fish's head edge, regions that also feature large vortices in the fluid field. Based on the shear traction distribution, it can be inferred that the primary source of propulsion originates from the fish's back. The findings provide a theoretical foundation and reference points for the design of bionic fish exoskeletons, strategies for reducing surface drag, and fluid field optimization. This study elucidates the mechanisms underlying surface shear traction distribution on fish, offering valuable insight for related research.