  This paper proposes an efficient potential and viscous flow decomposition
method for wave-structure interaction simulation with single-phase potential
flow wave models and two-phase Computational Fluid Dynamics (CFD) solvers. The
potential part - represents the incident waves - is solved with spectral wave
models; the viscous part - represents the complementary perturbation on the
incident waves - is solved with the CFD solver. This combination keeps the
efficiency and accuracy of potential theory on water waves and the advantage of
two-phase CFD solvers on complex flows (wave breaking, flow separation, etc.).
The decomposition strategy is called Spectral Wave Explicit Navier-Stokes
Equations (SWENSE), originally proposed for single-phase CFD solvers. Firstly,
this paper presents an extension of the SWENSE method for two-phase CFD
solvers. Secondly, an accurate and efficient method to interpolate potential
flow results obtained with the High Order Spectral (HOS) wave model on CFD mesh
is proposed. The method is able to reduce the divergence error of the
interpolated velocity field to meet the CFD solver's needs without
reprojection. Implemented within OpenFOAM, these methods are tested by three
convincing verification, validation and application cases, considering incident
wave propagation, high-order loads on a vertical cylinder in regular waves, and
a Catenary Anchor Leg Mooring (CALM) buoy in both regular and irregular waves.
Speed-ups between 1.71 and 4.28 are achieved with the test cases. The wave
models and the interpolation method are released open-source to the public.
