A two-way coupled CFD-FEM framework for fluid-flexible structure interaction using OpenFOAM

Fluid interaction with flexible bodies is a complex hydrodynamic problem due to the moving and deforming fluid-structure interface, with widespread applications in naval architecture, coastal engineering, and renewable energy sector. This study presents a two-way coupled Computational Fluid Dynamics-Finite Element Method (CFD-FEM) model to simulate wave interaction with flexible structures. The framework combines for the first time the finite volume CFD solvers in OpenFOAM with finite element beam models in Project Chrono, an open-source multi-physics simulation engine, by dynamically linking precompiled libraries. A novel lumped point method is proposed to capture the fluid-structure interaction through a motion controller that manages lumped points and connectivity for each beam. The integrated hydrodynamic forces on each lumped point drive the Finite Element Analysis (FEA) model in Chrono to predict nodal displacements and rotations, which subsequently update the fluid mesh. To ensure stability, a strongly coupled scheme with dynamic under-relaxation is implemented, executing the fluid and structure solvers iteratively until convergence is achieved at each time step. The CFD-FEM model is applicable to both single- and two-phase flows for structures within a certain deformation range, and is inherently more efficient at 3D FSI simulations than the multi-region partitioned coupling toolbox, solids4foam, which requires a full 3D continuum representation of the solid. It compares well with classical benchmarks, as well as experimental data of the hydroelastic response of a flexible plate and a segmented barge under wave loadings. The model-data agreements demonstrate that the present CFD-FEM modelling framework is accurate in capturing nonlinear interactions between fluid and flexible structures. This high-fidelity, open-source modeling suite for fluid interaction with flexible structures is a step forward towards the ultimate goal of a unified framework for rigid, flexible, and hybrid structure systems with wide applications.