Numerical analysis of combined inflow effects on fatigue loads of horizontal axis wind turbine rotor blades

A horizontal axis wind turbine installed in the open air experiences the wind field which is non -uniform in space and unsteady in time. The resulting yaw misalignment and wind shear bring about periodic fluctuation of aerodynamic loads on the rotor, and turbulence causes complicated load fluctuations. For the fatigue design of the rotor blades, it is imperative to predict the aerodynamic load fluctuation depending on various inflow conditions such as yaw misalignment, wind shear, and turbulence. The present paper describes a numerical method for calculating the aerodynamic loads for combined inflow condition and shows several numerical results. The turbulent wind field is simulated with the Mann model, which is based on a Fourier synthesis method. For the calculation of aerodynamic loads on the rotor blades, an inviscid aerodynamic model based on the asymptotic acceleration potential method was adopted. For the fatigue load analysis, each of the simulated time series of the aerodynamic load was processed with the Rainflow counting algorithm. The calculation results show that the effects of each inflow conditions on fatigue loads have nonlinear relations among each other. Therefore fatigue loads should be calculated on the combined inflow condition, by the model which can consider unsteadiness and non-uniformity of the each inflow condition, simultaneously.