Impact of uncertainty in airfoil characteristics on wind turbine extreme loads

Wind tunnel test measurements to characterize the static lift and drag coefficients of airfoils used in wind turbine blades are shown to possess large uncertainties, which leads to uncertainties in the aerodynamic loads on the rotor. In this paper a rational stochastic model is proposed to quantify the uncertainty in airfoil static lift and drag coefficients based on field and wind tunnel data, aero-servo-elastic calculations and engineering judgment. The stochastic model is subsequently used to assess the effect of the uncertainty in airfoil static lift and drag coefficients on the prediction of extreme loads and structural reliability of large wind turbines. It is shown that the uncertainty in the static airfoil data has a significant impact on the prediction of extreme loads effects and structural reliability depending on the component, operating conditions (stand-still versus power production) and the correlations of aerodynamic variables along the span of the blades.