An improved direct forcing immersed boundary method for floating body simulations in waves

This paper introduces a novel direct forcing immersed boundary method tailored to simulate nonlinear interactions between ocean waves and arbitrarily complex free-floating structures. Within an open-source hydrodynamic framework, we couple the fluid–structure interaction (FSI) algorithm with the two-phase flow solver through forcing at the fluid–solid interface. We substantially enhance this coupling process by altering the density interpolation method, significantly reducing the interface smearing region, which improves both the stability and accuracy of the fluid flow in the vicinity of the floating objects. The tracking of the fluid–solid interface in the Eulerian domain is based on a level set function, thus avoiding the need for dynamically moving or overset meshes and greatly simplifying the mesh generation process. Rigid body dynamics are implemented using Euler parameters and Hamiltonian mechanics, allowing for arbitrarily large motions of the floating body. The presented approach is tested and validated with several 2D and 3D problems, including a full-scale simulation of a floating semi-submersible offshore wind turbine in waves. All numerical results demonstrate the accuracy and robustness of the new method, highlighting its potential as an outstanding alternative to existing numerical approaches for realistic floating-body simulations in waves.