Numerical Simulations Of Steady And Unsteady Aerodynamics Of Wind Turbines

The development of wind energy is a long-term strategic task in China. Systemic investigations and good understanding of the energy transform to ensure the reliability of wind turbine and to improve wind turbine performance are keystones of the sustainable development of wind energy industry. Due to the importance of CFD applications in wind turbine aerodynamics, a very important issue is the accuracy of CFD simulations. However, there exist many types of transition which has considerable impact on the development of boundary layer, separation flow and the aerodynamic performance. The investigation on transition and the development of transition models may improve the accuracy of CFD simulations and the design of wind turbine. In view of the dramatic impact of the wind shear and tower shadow effect on wind turbine performance, a numerical model close to the actual wind turbine, which contains the rotor, nacelle as well as the tower, should be used to analysis the influence of laws and mechanism systematically. The analysis results could provide detailed guidance and reference for wind farm micro-siting, turbine design and type selection as well as wind power prediction. Therefore, the present thesis is devoted to the investigation of the steady flow field under uniform wind speed, the unsteady influence of wind shear on rotor performance, and further the unsteady influence of tower shadow effect on turbine performance under uniform and shear wind speeds, by using commercial CFD software Fine/TurboTM.The steady simulations on wind turbine blade under uniform wind speed focus on the computational domain scale, grid size, turbulence and transition models, as well as the influence of turbine nacelle. Firstly, the influences of computational domain scale and grid size on the aerodynamic performance and load distributions of steady simulations under high wind speed is investigated, where the stall-controlled wind turbine NORDTANK 500/41 is taken as the test case. Detailed analysis on the boundary layer, performance and details of the flow field of the FFA-W3-241 airfoil and NORDTANK 500/41 wind turbine blade are performed using full turbulence Spalart-Allmaras (S-A) model,k-ωSST model, as well as the transition model Abu-Ghannam & Shaw (AGS) in S-A. The influence of nacelle on the aerodynamic performance of the blade are analysised using the model of NREL Phase VI wind turbine. The simulation results show that the simulated aerodynamic performance is independent to the computational domain scale when the computational domain is larger than 10 rotor radii. When the performance convergence history of steady computations has large periodic variations, the result averaged of a period is more reliable than one of the certain time step. Full turbulence S-A model and transition AGS model have good performance on the predication of aerodynamic performance and flow details. The nacelle has influence on the separated flow and vortex structure at the hub of blade, as well as the spanwise development of radial flow, especially the flow in the separation region. Taking the influence of nacelle into account will improve the accuracy of CFD simulations。A quasi-three dimensional unsteady numerical module is imposed and validated on the wind shear influence investigation of rotor performance. Based on this model, the influence law and mechanism of different wind profiles and the wind shear effects on different spanwise locations are analysised. The results are further validated by full three-dimensional unsteady simulations. The test case used above is NREL Phase VI wind turbine. The comparison shows that the quasi-three dimensional model proposed can capture the aerodynamic characteristics, which can be used in the aerodynamic analysis of wind shear at specific spanwise position without boundary layer separation or with small separation region. The wind shear increases the fluctuations of wind turbine performance and loads, as well as the flow parameters, with their variations along azimuth angle is of periodic approximately in cosine function, and the amplitude increases with the wind shear exponent and along the span, which cause an asymmetric feature in the rotating plane and phase lagged compared with the peak of wind speed. Nonetheless, The effect of wind shear is relatively small on the flow parameters compared with that of linear velocity increase along span, which will not change the load distribution along span, and do harmless impact on the blade structural stability.An upwind wind turbine DF90 is used as the simulation model in the unsteady simulations on the tower shadow effect. The analysis focus on the variation of total performance parameter, distribution of pressure, limiting streamline close to the wall and section streamline patterns. The results show that the block of tower will cause dramatic changes of blade load when the wind speed is uniform, showing a sudden increase before the tower and then decrease rapidly and slow recovery afterwards. Income flow deflection always exists due to the rotating of turbine, which results in the unbalance of forces at both sides of tower and 3p vortex shedding after tower. The periodic load under the impact of tower easily leads to the dynamic response of blade and tower, which will do harm to the operation and life of machine. Under the wind shear, the increase of the load amplitude is nearly two times than that of the uniform wind speed, with the fluctuation increases along span. This results in imbalance load distributions of the rotor plane, where the upper half is larger then that of the lower. Meanwhile, the shear winds reduce the load dramatic variation acceleration near the tower, and the mean value and variation of load before and after the wind shear effect change little due to the phase difference of three blades.The present thesis investigates systematically on both steady and unsteady flows of wind turbine based on the model from single blade to rotor, and finally the turbine contains tower. The complicated wind shear and nonlinear effect of tower also investigated, which prove a more accurate model close to the real physical one. The simulated results can be used as the guidance and reference for the load analysis and aerodynamic design from airfoil to blade by using CFD simulations, which also give the basis of understanding of the complicated unsteady aerodynamic features of wind shear and tower shadow effects, so as to the support to the reliable and high performance design of wind turbine.