Investigation Of Aerodynamic Characteristics Of Wind Turbine Rotor Under Control Process Based On Free-Vortex Method

The trend that wind turbine is larger and smarter is promoted by developments of low-wind-speed wind farm and the offshore wind farm. Moreover, the advanced design technology for wind turbine is required by wind power utilization. In operating process, aerodynamic loads of wind turbine are not only affected by unsteady wind conditions with shear, yaw, turbulence and gust wind conditions, among others, but also interacted with active pitch, yaw and variable-speed control, etc. In addition, aerodynamic characteristics of wind turbine are not only played an important role in structural and fatigue loads of blades and tower, but also affected the control strategy optimization and the power performance. Therefore, it is very important that the unsteady aerodynamic characteristics of wind turbine are investigated in the control process for understanding the rules of unsteady loads, optimizing the control strategy and improving design level of wind turbine. For this purpose, a numerical simulation method is developed for simulating the unsteady aerodynamic loads and vortex wakes of wind turbine based on the free-vortex wake method. In addition, a coupling model is built between the unsteady aerodynamic model of wind turbine rotor and other wind turbine models including structural dynamics model, generator model and controller model. The aerodynamic and aeroelastic characteristics of wind turbine rotor are investigated by the developed method and models not only in the condition with shear wind and yaw wind, but also in the pitching, yawing processes and in conditions of the grid voltage sag and the unsteady inflow.Three aerodynamic models are developed and compared including the Hess-Smith panel model, the Weissinger-L lifting surface model and the nonlinear lifting line model. To improve the convergence property in the nonlinear lifting line model, an iterative algorithm is developed based on the Newton-Raphson method. To simulate the unsteady characteristics of vortex wake, an improved vortex wake model, called hybrid wake model, is developed. The wake model solved by time-marching method consists of the vortex sheet model and the tip vortex filament model. Results show that the hybrid wake model has a higher computational efficiency.The aerodynamic loads of wind turbine rotor are computed in conditions of shear wind, yaw wind and yaw-shear wind during the wind turbine steady-state operating. Results show that the amplification of blade root flapwise moment is decreased at a positive yaw angle and increased at a negative yaw angle. Moreover, it is investigated that the aerodynamic loads of rotor are affected by the blade pitch error. Results show that amplifications of the tilt moment and yaw moment of rotor dramatically increased at blade pitch error and determined by the gap of pitch errors between different blades.The aerodynamic and aeroelastic characteristics of wind turbine rotor are investigated not only in the given pitching and the given yawing processes, but also in processes of the grid voltage sag and the unsteady inflow in dynamic control condition. Moreover, it is also investigated that the aerodynamic and aeroelastic characteristics of wind turbine rotor are interacted with active pitch and variable-speed adjusted by the control system.First of all, variations of aerodynamic loads of rotor are simulated in the given pitching and yawing processes. Results show that the overshoot of aerodynamic loads is mainly caused by delayed variations of trailing vortices and vortex circulation strength in pitching process. In addition, the aerodynamic loads of rotor are not only affected by the yaw angle but also affected by the yaw rate which could influence the relative velocity field around the rotor in yawing process.The second, for investigating the aerodynamic characteristic of rotor in the process of grid voltage sag, a coupling model consisting of the aerodynamic model, the generator model and the controller model are developed. The generator and controller model are established at the MATLAB/Simulink platform. Results show that the amplification of aerodynamic loads of rotor could generated by the voltage sag during very short time. Amplifications of blade thrust and blade root flapwise moment are dramatically larger than the amplification of blade root edgewise moment during the voltage sag.Finally, to research the aerodynamic and aeroelastic characteristics of wind turbine rotor in unsteady inflow conditions with the gust wind and the extreme shear wind, a coupling model is developed. The model consists of the aerodynamic model based on the free-vortex method and the structural dynamics model based on the FAST software. By the coupling model, it is investigated that the rotor loads are interacted with the active pitch and variable-speed control in conditions of the gust wind and the extreme shear wind. Results show that because of the pitch control delayed, the rotor pitch angle could be adjusted to incorrect angle during the gust wind and the extreme wind. It could make the aerodynamic loads of rotor dramatically increased and lead to larger deflections of blades and tower.