A Numerical Study Of Aeroelasticityof Large-scale Wind Turbines

The large-scale horizontal wind turbine is a complex multi-body dynamic system composed of nacelle, hub, transmission system, blades, tower and so on. With the development of large-scale wind turbine, blades are becoming more flexible, at the same time the natural frequencies of slender tower are becoming lower. The blades rotating around the low speed shaft, it is easy to couple the flexible blade and tower, resulting in dynamic coupling effects. Large wind turbines are running at complicated situations and suffering the unsteady aerodynamic loads. Unsteady aerodynamic loads and flexible blades are easily coupled, resulting in aeroelastic effects. It is the key for safety of wind turbine to properly take account of the dynamic coupling effects and aeroelastic effects, which caused by bodies coupling effects and structure-fluid coupling effects. It is the prerequisite for solving this problem that multi-body system dynamics responses and steady/unsteady aerodynamic loads are calculated accurately. The multi-body dynamic method is used to simulate the responses of wind turbines. The steady/unsteady loads of wind turbines are predicted through free vortex wake method, and more accurate unsteady loads are predicted through the CFD method. A fast aeroelastic simulation platform is established by coupling the free vortex wake method with the multi-body dynamic method, and a more accurate aeroelastic simulation platform is established by coupling the CFD method with the multi-body dynamic method. The more detail and more accurate aeroelastic responses are computed, and the aeroelastic responses are fast calculated.In the aspect of the dynamic response simulation of wind turbines, composite laminate model is used to represent the turbine blade structure, and a thin plate model is used to represent the tower structure. The shell elements are used to discrete the blade and tower model, and dynamic analysis is accounted. The elastic deformations of hub are much small, thus the hub is simplified as a rigid body. Each part of the wind turbine is not independent, which is constrained through different types of joints. A type of fixed constraint and a type of motion constraint model are built. A fixed constraint is used to connect the tower and the base at the bottom of the tower. Another fixed constraint is used to connect the tower and the nacelle at the top of the tower. The transmission system is represented by a motion constrain connecting the rotor and nacelle. Floating frame of reference is used to represent the total motion of flexible bodies. Rigid body motions are represented by the motions of the floating frame and modal shapes represent the elastic motions. Multi-body system’s governing equations are derived using Lagrange equation of the first kind, where constraint equations are composed of point and vector equations derived from revolute joints and motion joints. The resulting differential-algebraic equations are transformed to algebraic equations through Taylor predictor and BDF Corrector methods. The nonlinear algebraic equations are solved through Broyden method, thus a multi-body numerical method is established.A free vortex wake method(FVW) is established to rapidly calculate aerodynamic performance which considers the structure effect. The blade is simplified as a lifting line in the model, which is arranged at the 1/4 chord through the vortex wake method. The wake vortexes spiral from the blade boundary and down to the downstream, which are attached at the rotating blade. The curved vortex lines are represented with the straight-line vortex element. Viscous vortex core model is introduced to eliminate the singularity in the vortex core, and the Biot-Savart’s law is used to calculate the induced velocity. The unsteady aerodynamic characteristics of airfoil are calculated under the Leishman-Beddoes dynamic stall model. And the three dimensional rotational effect caused by the rotating blades is considered. The time marching method is established to solve the wind turbine wake geometry and aerodynamic performances. Deformations of large flexible blade affect the wake geometry and aerodynamic characteristics. The structure deformations and vibration velocities are feeding back to wake geometry and aerodynamic calculations. Thus a new free wake calculation method considering structural effects is established.A computational fluid dynamics method is established to simulate the flow field of wind turbine, and the compressible unsteady N-S equations are solved through finite volume method. Center difference method is used in spatial discretization, and the Runge-Kutta method is used for explicit time marching. S-A turbulence model is used. The wind speed is always low, which will cause numerical instability. Precondition method is used to fix the instability and parallel method is used to improve the efficiency. A rigid-elastic dynamic grid method based on the elastic deformation method is presented. The rotational motions, flexible deformations of the blade, and flexible deformations of the tower are calculated with the previous dynamic grid method. The previous methods are verified by Phase VI model.The multi-body dynamics method is coupled with the aerodynamic calculation method, and the transfer of data between fluid and structure is performed by interpolation methods. The unsteady loads are transmitted to the blade structure at each time step, which are calculated by the vortex wake method. The deformations and velocities of blades are transmitted to the wake calculations, which are calculated with multi-body dynamic system method. The loads composed of surface pressures and frictions are transferred to the blade structure when the CFD method is performed. The blade deformations are transferred to the surface grid, and then dynamic grids of wind turbine are generated through rigid-elastic dynamic mesh method. Thus the CFD/MBD method is established.