Research On The Calculation Methods Of Wind Turbine Blade’s Aerodynamic Load

The aerodynamic characteristics and aerodynamic loads of wind turbines have a significant impact on wind turbines’safety and economics. With the development of wind power technology, many researchers at this field have focused on how to design the blades which have better aerodynamic characteristics and load performance. Because wind turbines are very big and they are working in the natural, the wind farm operation is very complex and it is difficult to accurately assess the blade load. In the study of wind turbine blades’ aerodynamic performance, both domestic and foreign scholars have used and improved the mature theories and methods in other field, such as wind tunnel experiments, Blade Element Momentum theory, CFD methods and so on. However, all of the analytical methods have certain limitations. It is difficult to meet the engineering need to analyze and evaluate the variety of complex condition accurately and rapidly. In this paper, I have studied the aerodynamic load of the wind turbine blade from two-dimensional airfoil to three-dimensional blade, and compared and analyzed the calculation results. My work provided a foundation to establish and select the accurate methods to calculate the wind turbine blades loads.In this paper, based on the U.S. National Renewable Energy Laboratory (NREL) Phase Ⅵ wind turbine, I have discussed the characteristics of the several frequently-used methods, and compared the calculation results with the wind tunnel experiment results.Firstly, the two-dimensional CFD method was adopted to study the aerodynamic performance and aerodynamic loads on the condition of the steady and unsteady inflow. By comparing with the existing experimental results, it was pointed out that in the attached flow, two-dimensional calculation results were larger than the experimental results in the front of the blade suction side. But trailing vortex and centrifugal force resulting from the blade rotation would "smooth out" the pressure sophisticated distribution of the blade leading edge on the suction side. The two-dimensional airfoil model in the flow field would cause a reaction force to the flow field which would make the model aerodynamic force and the flow velocity appeare out of sync. For the rotation blade, because of the counter-acting force, after the wind turbine the air would get a velocity on the circumference direction. But compared with the change in the axial velocity, it was very small.Secondly, using the Blade Element Momentum theory to analyze the blade aerodynamic loads, we could find that the aerodynamic loads mainly distributed on the central and the tip part of the blades, and the tangential load in the wind turbine almost didn’t change. But blade normal load increased linearly from the middle part to the tip part, which would greatly increase the blade root bear moment. With the increase of the blade length, the blade root structural design would have higher requirements. The increase in blade length would put forward higher requirements for the blade root’s structure design. Compared with the two-dimensional CFD method, the surface load of the blade calculated by the Blade Element Momentum theory method had a little different but they had the same trend along the radial direction of the blade.After the BEM theory, the three-dimensional CFD method was used to get the blade loads. Compared with three-dimensional CFD calculations, the reason of the two-dimensional CFD calculation method’s error was:two-dimensional calculation method could not examine the impact of the trailing vortex; three-dimensional rotational effect would also affect the flow field around the blade. When the flow was attached, the two-dimensional pressure distribution of the airfoil S809didn’t match with the pressure distribution obtained by the three-dimensional CFD calculation, especially in the front of the suction side. This was mainly due to the suction side pressure distribution of the airfoil S809, which was more complex than the previous airfoil model. The suction side pressure gradient was large. So we could not ignore the affect of the centrifugal force on the blade aerodynamic characteristics even when the flow was attached.By comparing the various calculation methods, it was recommended that at the beginning of the blade design to choose the2D-CFD method to determine the airfoil. When we are certain of the blade geometry, if the airfoils’pressure gradient distribution changes little and the control strategy makes the blade mainly run in the un-stall condition, we can apply the2D-CFD method and BEM theory to calculate the aerodynamic loads. For the large separated flow, it is recommended to use3D-CFD methods to determine its aerodynamic loads.