The fluid-structure interaction problems of two side-by-side flexible plates with a finite aspect ratio in a three-dimensional (3D) uniform flow are numerically studied. The plates’ motions are entirely passive under the force of surrounding fluid. By changing the aspect ratio and transverse distance, the coupling motions, drag force and energy capture performance are analyzed. The mechanisms underlying the plates’ motion and flow characteristics are discussed systematically. The adopted algorithm is verified and validated by the simulation of flow past a square flexible plate. The results show that the plate’s passive flapping behavior contains transverse and spanwise deformation, and the flapping amplitude is proportional to the aspect ratio. In the side-by-side configuration, three distinct coupling modes of the plates’ motion are identified, including single-plate mode, symmetrical flapping mode and decoupled mode. The plate with a lower aspect ratio may suffer less drag force and capture less bending energy than in the isolated situation. The optimized selection for obtaining higher energy conversion efficiency is the plate flapping in single-plate mode, especially the plate with a higher aspect ratio. The findings of this work provide several new physical insights into the understanding of fish schooling and are expected to inspire the developments of underwater robots or energy harvesters.

A novel immersed boundary-lattice Boltzmann method(IB-LBM) is used to model the fluid-structure interaction problem with biologically flexible structure. In this algorithm, the fluid parameters are solved by LBM, while the boundary condition of structure are implemented by IB. Based on this framework, two tandem flexible plates pined in uniform viscous flow are simulated. The Reynolds number based on the incoming velocity and plate's length are fixed at 300 and 500, but the separation distance are varied in a wide range. Simulation results show that the flexible plates will flapping passively when the flow field reach its stability. And the motion characteristic of downstream plate is strongly influenced by the vortex shedding from the upstream one. Also, we find the hydrodynamics on the tandem plates are strongly depended on the separation distance. As the distance increases, drag on the upstream plate is always smaller than single one. However, the downstream one will show two opposite situations. The findings of this work would be benefit for designing novel underwater robots.