Aerodynamic performance detecting for floating offshore wind turbine using RANS-BEMT approach

Nowadays the demands for floating offshore wind turbine (FOWT) have exceeded 5MW with the rapidly growing wind market. The aerodynamic environment of FOWT is more complex than onshore or fixed offshore wind turbine due to larger motions of the floating platform. However, a limited simulation and load estimation capability make aerodynamic analysis a challenge. It is questionable whether some industry aerodynamic analysis codes like conventional Blade Element Momentum theory (BEMT) is accurate. Results suggest that current methods for predicting the aerodynamic loads are lacking. This paper presents the unsteady aerodynamic performance of National Renewable Energy Laboratory (NREL) 5MW FOWT experiencing a periodic pitch and surge motions using commercial multi-purpose CFD solver STAT CCM+ 9.02 compared with FAST v7.0. Some discrepancies are found. 3D results are used to estimate 2D airfoil characteristics to modify two important parameters in BEMT codes: the axial and the tangential induction factors by applying the reduced axial velocity method to get the local angle of attack of CFD solutions. As shown in the results, BEMT method cannot predict aerodynamic performance accurately, especially in the motion condition. Therefore, a new modified RANS (Reynolds-Average Navier-Stokes)-BEMT approach is raised. Corrected BEMT method using the CFD airfoil results is an interesting thing to find if it can modify the accuracy of the results.