Due to the influence of the scale effect, the thrust of model wind turbines mismatch the target value under the Froude-scale condition in floating offshore wind turbine model tests. This needs to redesign model blades to satisfy the similarity requirements of the force. This paper presents the design methodology of multiple airfoils blade based on the blade element momentum theory and the map of the lift and drag coefficients. Then, compared with the single airfoil blade in numerical simulation, the thrust value of the multiple airfoils blade is closer to the target value.
National research institutions and scholars carry out different model tests of floating offshore wind turbines to study the performance and improve the efficiency of floating offshore wind turbines. The model tests of Hywind and Windfloat are the most representative among the model tests of floating offshore wind turbines. The Hywind model test was carried out in Marintek, in which the power of wind turbine prototype is 5MW and the scale ratio is 1/47 (Nielsen, Hanson and Skaare, 2006). The Windfloat model test was carried out in the UC Berkeley wave tank, in which the power of wind turbine prototype is 5MW and the scale ratio is 1/105 (Roddier, Cermelli, Aubault and Weinstein, 2010).
The thrust of the wind turbines under the Froude scaled wind is less than the target thrust value because of the scale effect in the floating offshore wind turbine model tests. To ensure the accuracy of the test, blades are redesigned to meet the target value. For example, NACA4412 is used as the airfoil to form blades in the model test carried out in the ocean engineering basin of Institute of Industrial Science, the University of Tokyo (Nihei and Fujioka, 2011). However, due to the pitch control system installed in the model of wind turbines, the cross section of the blade root zone changes, which means that the airfoil of the root zone is no longer the single airfoil. It causes the lift and drag coefficients calculated from single airfoil are not suitable. In the multiple blade, the lift and drag coefficients of different blade elements are calculated from different airfoils which reduce the effect of this problem.
