CFD Simulation of a Floating Wind Turbine in OpenFOAM: an FSI approach based on the actuator line and relaxation zone methods

Abstract

Floating offshore wind turbines (FOWTs) have the potential to harness wind resources in deepwater, which is so far prohibitive for conventional approaches. This, however, comes at a cost: the platform’s extra degrees of freedom (DoFs) introduce complex aerodynamic and hydrodynamic behaviours. Therefore, FOWTs must be accurately modeled to reduce load uncertainties that ultimately prejudice their economic viability. This project implements a framework for the coupled, high-fidelity simulation of FOWTs in OpenFOAM. The tool is built upon two existing libraries: turbinesFoam [1] —for rotor modeling based on the actuator line method— and waves2Foam [2] —for wave-field generation and absorption based on the relaxation zone method. The multi-phase simulation uses the interFoam solver in combination with a morphing mesh technique and rigid-body model to represent the platform. The mooring restraints are computed with a quasi-steady, catenary model from waves2Foam. The turbinesFoam library, targeted at bottomfixed turbines, is modified so that it can accommodate arbitrary motions along the rigid-body DoFs. The platform-turbine FSI coupling follows a serial sub-iterating strategy based on the PIMPLE scheme. The simulation framework is built in a sequential style. First, the propagation of second-order waves in an empty tank is studied, followed by the decay oscillation of floating buoys from the experiments by Ito and Palm et al. [3, 4]. Then, the modified version of turbinesFoam is tested for the conditions from the OC6 Phase III campaign —a series of wind-tunnel tests carried out at Politecnico di Milano that analyzed the performance of a scaled 10-MW turbine under prescribed motions in pitch and surge [5]. Lastly, the coupled simulation of a 2-DoF (surge and pitch) semi-submersible FOWT under combined wind-wave conditions is achieved. The presented framework proved capable of modeling the aerodynamic performance of turbines under prescribed motion and produced plausible results for a semi-submersible FOWT under combined wind and wave conditions. Once carefully validated, this tool will have the potential to serve as a reliable technique for the advanced modeling of FOWTs.