  To simulate the interaction of ocean waves with marine structures, coupling
approaches between a potential flow model and a viscous model are investigated.
  The first model is a fully nonlinear potential flow (FNPF) model based on the
Harmonic Polynomial Cell (HPC) method, which is highly accurate and best suited
for representing long distance wave propagation. The second model is a CFD
code, solving the Reynolds-Averaged Navier-Stokes (RANS) equations within the
\openfoam toolkit, more suited to represent viscous and turbulent effects at
local scale in the body vicinity. Two one-way coupling strategies are developed
and compared in two dimensions, considering fully submerged and fixed
structures. A domain decomposition (DD) strategy is first considered,
introducing a refined mesh in the body vicinity on which the RANS equations are
solved. Boundary conditions and interpolation operators from the FNPF results
are developed in order to enforce values at its outer boundary. The second
coupling strategy considers a decomposition of variables (functional
decomposition, FD) on the local grid. As the FNPF simulation provides fields of
variables satisfying the irrotational Euler equations, complementary velocity
and pressure components are introduced as the difference between the total flow
variables and the potential ones. Those complementary variables are solutions
of modified RANS equations. Comparisons are presented for nonlinear waves
interacting with a horizontal cylinder of rectangular cross-section. The loads
exerted on the body computed from the four simulation methods (standalone FNPF,
standalone CFD, DD and FD coupling schemes) are compared with experimental
data. It is shown that both coupling approaches produce an accurate
representation of the loads and associated hydrodynamic coefficients over a
large range of incident wave steepness and Keulegan-Carpenter number.
