Analysis Of Aerodynamic Performance Of Floating Wind Turbines Using CFD And BEMT Methods

Scaling effects caused by applying Froude-scaling to both wind and waves during model-testing of floating offshore wind turbines (FOWTs) results in poor model-scale aerodynamic performance of geometrically scaled turbines. This led to the “performance-scaled” MARIN Stock Wind Turbine (MSWT) which showed to be successful in obtaining the correct thrust loads at model-scale conditions. Additionally it was found that conventional blade-element-momentum-theory-based (BEMT) modelling tools are not suitable for model-scale conditions.
Recent research in which these problems have been addressed are presented in this paper. First a 3D CFD study was performed in which the behaviour of the flow over the commonly studied NREL 5MW baseline turbine and the MSWT geometries was performed. Both model and full-scale conditions were studied for a fixed non-moving platform and rotor-only turbine. It was found that scaling effects are indeed significant and a highly three-dimensional and additionally separated flow was observed. Based on these findings two methods were proposed to expand the applicability of BEMT-based tools to off-design and modelscale conditions.
First, instead of using commonly used 2D XFOIL data, 2D CFD RANS data was used. The use of purely 2D data from 2D CFD RANS computations did however not result in the desired improvements when compared to XFOIL-based results. The second proposed method is based on the use of 2D airfoil data obtained by post-processing of 3D flow data coming from 3D CFD computations. This new approach was shown to be successful and can therefore be extremely useful for future model-scale FOWT testing campaigns to do preliminary performance predictions. All BEMT-based and CFD results presented in this paper were compared to model-scale experimental data of the NREL 5MW turbine and the MSWT over the full range of TSR.