In this paper, the hybrid model, qaleFOAM is used to numerically simulate the interaction between the focusing wave and a seabed mounted cylinder. The qaleFOAM couples the fully nonlinear potential theory (FNPT) based quasi Lagrangian Eulerian Finite Element Method (QALE-FEM) and the two-phase incompressible Navier-Stokes (NS) solver, OpenFOAM/InterDyMFoam, using the domain-decomposition approach. In the qaleFOAM, the computational domain is split into two, i.e. one small domain near the cylinder (NS domain) and the rest (FNPT domain) with a transitional zone between two sub-domains. The wave is generated by a wavemaker in the FNPT domain by using the pre-specified time histories of the paddle motion and propagates into the NS domain through the transitional zone. In the NS domain, the Reynolds Averaged Navier-Stokes (RANS) without turbulent viscosity (laminar model) is employed and the outlet of the NS domain is equipped with an effective passive wave absorber. Two wave conditions considered in this paper are specified by the ISOPE 2020 Comparative Study session. The numerical results are compared with the experimental data and satisfactory results have been achieved.
The environmental conditions in the offshore structure are complex, and there are extreme waves, which have enormous wave heights and concentrated energy, resulting in potentially destructive damages to offshore structures. The extreme wave has a significant nonlinearity, which invalids the linear wave theory and second order wave theory for analyzing the related mechanisms, basic characteristics, evolution process and effects on the structure. Furthermore, to accurately model the generation and propagation of the extreme waves, a sufficiently large domain is required, allowing fully development of the nonlinearity (Wang et al, 2016). On the other hand, the fixed cylinder is a popular structure in hydraulic, marine, coastal and offshore engineering, e.g. as the foundations for oil/gas platform and offshore wind turbines. In many scenarios, the size of the cylinder is considerably smaller than the characteristic wavelength. This implies a considerable viscous effect (Ma and Patel, 2002), which cannot be directly imposed by the potential theory unless introducing an artificial viscosity, which needs to be calibrated using experimental data or other reliable numerical results (Yan and Ma, 2007). Due to these facts, the computational fluid dynamics (CFD) solutions, based on the Navier- Stokes (NS) model, are sought for evaluation of the survivability of the structures or exploring viscosity/turbulence dominated phenomenon, e.g. the vortex induced vibrations. However, the CFD software packages are often time-consuming for modelling wave-structure interactions, as demonstrated by recent comparatives studies, e.g. Ransley et al (2019, 2020) and Yan et al (2019), in particular when the computational domain is large in order to capture the nonlinear evolution of the extreme waves.