A detailed study is undertaken of the computational modelling of a sub-platform for floating offshore wind using the software Star-CCM+ with the application of the RANS approach. First, a mathematical introduction to the governing equations is carried out. Then, the computational grid is defined, and the grid-independence of the solution is verified. A time-dependent study is performed with the selected time-step. Finally, two examples of 3D decay tests in heave of the sub-platform without and with moorings are presented, accompanied by a damping factor study, with the aim of providing a better understanding of the hydrodynamic damping of the platform. Throughout the process, three degrees of freedom (DoFs) are locked due to the limitations imposed by the use of a symmetry plane; this implementation allowed us to reduce the computational cost of each simulation by 50%. Therefore, three DoFs (heave, surge and pitch) are considered. The coupling study, adding a mooring system in the decay tests and the regular wave tests, shows good agreement between the experimental and computational results. The first half-period of the simulations presents a greater discrepancy due to the fact that the damping of the platform is lower in the computational simulation. However, this does not imply that the hydrodynamic damping is underestimated but may be directly related to the lock of various DoFs associated with the hydrodynamic damping.

This study aimed to adjust the turbulence models to the real behavior of the numerical wave flume (NWF) and the future research that will be carried out on it, according to the turbulence model that best adjusts to each particular case study. The k-ε, k-ω and large-eddy simulation (LES) models, using the volume of fluid (VOF) method, were analyzed and compared respectively. The wavemaker theory was followed to faithfully reproduce the waves, which were measured in an experimental wave flume (EWF) and compared with the theory to validate each turbulence model. Besides, reflection was measured with the Mansard and Funke method, which has shown promising results when studying one of the most critical turbulent behaviors in the wave flume, called the breaking of the waves. The free surface displacement obtained with each turbulence model was compared with the recorded signals located at three points of the experimental wave flume, in the time domain of each run, respectively. Finally, the calculated reflection coefficients and the amplitudes of the reflected waves were compared, aiming to have a better understanding of the wave reflection process at the extinction zone. The research showed good agreement between all the experimental signals and the numerical outcomes for all the turbulence models analyzed.

A wave flume of 12.5 x 0.6 x 0.7 m (length x width x height) able to reproduce the ocean conditions of the most representative research facilities in the Basque Country (BiMEP-Biscay Marine Energy Platform and Mutriku Wave Energy Plant) has been recently installed at the laboratory of Fluid Mechanics of the Faculty of Engineering in Bilbao. This new facility has the capacity of producing a wide range of monochromatic and panchromatic waves by a piston-type wavemaker. Several ultrasonic wave probes measure the surface elevation, and the wave energy is dissipated in a passive parabolic beach in order to diminish significantly the reflection along the flume. A numerical model based on Reynolds Averaged Navier Stokes (RANS) equations has been developed to represent the turbulence and Eulerian Volume of Fluid (VOF) unsteady approach in STAR-CCM+ CFD code to track the evolution of the free surface. This numerical model has been validated with the corresponding experimental campaign covering a wide range of depths, wave heights, wavelengths and periods. Tests focused on the description of the basic hydrodynamic processes of wave generation and propagation, giving as a result the definition of the wavemaker to produce the required wave. Results were analysed together with the analytical solution based on potential flow theory. The experiments carried out in the present study establish the operational limits of the wave flume in terms of wave generation, propagation and extinction, defining the operational range of future experimental campaigns where wave interaction with floating structures, wave energy converters and mooring systems will be studied. Keywords: experimental wave flume, numerical wave flume validation, wave characterisation, BiMEP, renewable marine energy, CFD models