| The unsteady aerodynamics and three-dimensional (3-D) rotational effect are the most challenging aspects in the wind turbine performance calculation. From the two points of view, this paper gives a calculation method for the unsteady aerodynamic characteristics of the wind turbines including the 3-D rotational effects. The classical momentum-blade theory is modified and its results provide aerodynamic inputs to the unsteady aerodynamic and dynamic stall model. Using the Leishman-Beddoes model, this paper represents three parts of flow behaviour for dynamic airfoil including attached flow, separated flow and dynamic stall. The flow separation is delayed due to rotation, which changes the blade aerodynamic characteristics greatly compared to its non-rotating counterpart. Based on the blade boundary layer theory and its results, this paper gives an analytical relationship between the 3-D rotational effect and the flow separation point, which efficiently improves calculation of the 3-D rotational effect. Together with consideration of likely chordwise loading distribution, the 3-D rotational effect model is coupled to the unsteady aerodynamic and dynamic stall model, which can be used to calculate the aerodynamic characteristics of wind turbine blades, such as normal force, chord force and pitching moment etc. in the unsteady cases. Finally, the calculated results are compared with experimental data and discussed in detail at different wind velocities and yawed angles of the wind turbine in this paper. The results illustrate that the calculation with the 3-D rotational effect improves the performance calculation and coincides with the experiment well to some extent. The method applied in this paper is simple and efficient; so much so that it can be used not only to calculate the unsteady aerodynamic characteristics but also to be couple with calculation of structural dynamics and aeronautics for the wind turbines, providing reasonable aerodynamic inputs. |
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