Sensitivity to Aerodynamic Forces for the Accurate Modelling of Floating Offshore Wind Turbines

In order to accurately study floating offshore wind turbines model-scaled experimental campaigns are crucial. Parallel to it, numerical tools play an important role. They permit to: aid the preparation of the experiments, to make faster and more cost-effective design variations, and to obtain more physical insight on the flow. Within this context, in this paper the simulation of MARIN Stock Wind Turbine (MSWT) at model-scale (1/50) conditions has been addressed. Standard (BEMT, XFOIL) and advanced (CFD) methods have been used. Both the panel method code XFOIL and the BEMT tool FAST are engineering tools used successfully for the study of airfoils and wind turbines at typical full-scale conditions, e.g. Reynolds numbers > 1E6. However, for the current work at model-scale the characteristic Reynolds numbers are lower than 1E5. The two-dimensional input data for the BEMT code was calculated in the common way using XFOIL and also with the viscous-flow ReFRESCO CFD code. The aerodynamic characteristics of the flow over the airfoil are shown to be highly unsteady, with extensive regions of separation beyond moderate angles of attack. BEMT calculations showed a poor comparison of the CP and a better match in the CT. Three-dimensional CFD calculations of the MSWT at model and full-scale were performed, in which the flow over the blade at modelscale was seen to be highly separated with extensive cross-flow components, contrary to what is seen at computations at full-scale. The agreement of the 3D CFD calculations with the experimental results was remarkably good. These results show that these BEMT tools are very sensitive to the input provided, and approach their limits for flows at model-scale low-Reynolds number conditions.