Research On The Unsteady Aerodynamic Characteristics Of Wind Turbine Blades

Wind turbine is large rotating machinery of external flow, and the flow around the wind turbine blade is quite complex. In the operating environment, due to wind shear, tower shadow effect, gusts, pitch control and yaw regulation, the aerodynamic characteristics of the wind turbine are highly unsteady. So far, the understanding about the unsteady flow mechanism of the wind turbine is very limited. Engineering methods mainly rely on the results of uniform inflow plus empirical correction models to determine the load in the unsteady process. In this way, the error could be large; therefore a higher load safety factor is needed. Using high-precision numerical model, carrying out detailed study of unsteady flow mechanism and load variation process, can guide the design and optimization process of wind turbine blades, also it can provide a theoretical basis to improve the empirical correction models in engineering methods.In this thesis, two numerical simulation methods are selected. They are full CFD method which modeling the geometrical profile of blade, and the body force source item substitution method such as actuator disc/line model, respectively. The influence of models and parameters of above methods on the simulation accuracy is detailed analyzed, and then a systematic study of wind turbine aerodynamic characteristics during uniform inflow, shear inflow and fast pitching process is carried out.In CFD method, solving viscous flow of blade boundary layer is very important; therefore detailed assessment of the accuracy of different turbulence models in aerodynamic prediction is executed at first. The Spalart-Allmaras model, the low Reynolds number k-ε model, the k-ω SST model and the Transition SST model which considers flow transition are selected to simulate the aerodynamic characteristics of S809airfoil and NREL Phase VI experimental wind turbine under uniform inflow condition. By comparing the simulation results with the experimental data, it shows that:in case of attached flow, Transition SST model is very accurate in predicting the location of transition zone and pressure distribution; but in case of separated flow, the convergence is slow and non-physical pressure peaks exist in the leading edge of suction surface. The k-ω SST model converges better, and gives more accurate predictions in sectional pressure distribution and integrated load than the other two models. The one-equation S-A model in some cases fails to capture the flow details around the blade. Meanwhile, in case of flow separation, there are always large deviations between predictions using turbulence model and the experimental data.Then unsteady aerodynamic characteristics of NREL Phase VI wind turbine under wind shear inflow condition are simulated with CFD method, and wind shear exponent variation is considered to represent the wind profile in different wind farms. The numerical results show that: during shear inflow, the integrated load and sectional aerodynamic force fluctuate in a cosine curve style, and the values of wind shear exponent have great influence on the fluctuation amplitude of aerodynamic load. Also, there is a phase lag between the sectional aerodynamic force extremum and the wind speed extremum, which is especially obvious in the vicinity of blade root. This phenomenon is explained in the paper.Owing to the high computational resources occupying and time-consuming of the full CFD method in unsteady simulations, the body force source item substitution method is used in the following studies. With this method, it is unnecessary to model blade geometry and resolve the boundary layer, which has simple grid structure and can reduce computational resources consumption.Considering wind turbine unsteady dynamic inflow phenomenon, an actuator disc numerical computational code is developed to simulate the fast pitching process of NREL experimental wind turbine. Through comparing the simulation results with relevant experiment data, it shows that actuator disc method can well simulate the process of load overshot and gradually recovering to the balance. The characteristic time of dynamic inflow is related to thrust coefficient and spanwise location. During downward pitching step, the time required to reach equilibrium state is longer than the upward pitching step, because of the larger thrust coefficient and smaller flow speed in the wake. Comparing with full CFD method, the actuator disc method greatly reduces the consumption of resources and time in the simulation process.The actuator line method, which takes the actual number of blades and rotation of blades in rotor plane into account, can handle more general unsteady aerodynamic process. In this paper, an actuator line numerical computational code is developed, and the values of the parameters in the model are discussed. Through simulating flow field and load of NREL experimental wind turbine under uniform inflow condition, the results show this method has approximate accuracy with full CFD method. Then, the actuator line code is used to study the unsteady aerodynamic characteristics of NREL wind turbine under wind shear inflow condition. Two hub height wind speeds of7m/s and10m/s are selected. During the rotation of the rotor, the blade integrated load fluctuating curves under the two inflow speed have obvious differences in shape and relative amplitude. The differences in fluctuating amplitude are quantitatively compared, and an explanation is given by showing the sectional angle of attack and coefficient of lift variation curve with azimuth angle. Compared with full CFD method, the actuator line method saves computational resources during unsteady cases, and can give details of flow field in wake.