## 1 Introduction  
Wind turbines run in the atmospheric boundary layer, where wind shear [1; 2; 3; 4] and turbulence [5; 6; 7] are the main features that have important effects on the aerodynamic performance of blades, and affect the power generation. The use of 2D wind tunnel data in the present blade design method based on blade element momentum (BEM) [8] will introduce uncertainty for predicting aerodynamic performance of blades. The 2D wind tunnel data are usually obtained in steady and non-rotational inflow conditions, while wind shear, turbulence, yaw, and rotation are remarkable in the 3D field environment. The differences cause prominent deviation between actual power generation and predicted results by the design methods [9]. Although a number of empirical correction methods, such as stall delay [10; 11; 12], and dynamic stall [13; 14; 15], have been developed to reduce the differences between 3D field environment and 2D wind tunnel conditions, it is difficult to develop and validate the empirical correction methods due to lack of aerodynamic measurement method and data in the field. [PERSON] [16] has found larger deviation between actual wind turbine performance and predicting results in 1980s. To explore the source of deviation, several aerodynamic field measurements on rotors with diameter of 10-25 m were conducted from 1987 to 1998, which is listed in Table 1, such as Combined Experiment Phase I and II [17; 18; 19] and Unsteady Aerodynamics Experiment Phases III-IV [20; 21] by National Renewable Energy Laboratory (NREL), field tests by Technical University of Denmark (DTU) [22], Netherlands Energy Research Foundation (ECN) [23] and Delft University of Technology (DUT) [24]. Then a cooperative research was conducted in the IEA Annexes XIV [25] and XVIII [26], which improved the insight into 3D airfoil characteristics on rotors. Subsequently, a DAN-AERO MW [27] experiment in the field focusing on the aerodynamics and aeroacoustics of MW turbines was conducted from 2007 to 2010 by Riso DTU National Laboratory. In China, a field measurement platform was established based on a 33 kW wind turbine by Lanzhou University of Technology to investigate wind turbine aerodynamic and wake [28]. Actually, angle of attack, as the key parameter of aerodynamic performance of an airfoil or blade, is difficult to determine in the field. The angle of attack is defined as the geometrical angle between the undisturbed flow direction and the chord line in wind tunnel measurement. The inflow angle that blade suffered cannot be calculated by inflow data from a cup anemometer at the hub or the mast far away from wind turbine due to the unsteadiness and unevenness of the inflow. The methods to calculate the angle of attack in different test projects are also shown in Table 1. The details of each methods are described in the reports of IEA Annexes XVIII [26] and DAN-AERO MW experiment [27]. Additionally, no descriptions on angle of attack was found in the work of Lanzhou University of Technology [28]. Generally speaking, at low angles of attack, most methods agree well in terms of mean values. However, the angle of attack in power method differ more from the other methods. The frequency resolution of the inverse BEM method is poorer. Only one sample per revolution is considered. The stagnation method could not be used at high angles of attack due to the fact that, in stall, the stagnation point is not a unique function of the angle of attack. The wind tunnel method is based on 2D wind tunnel measurements, in which only the 2D upwash effects are corrected. The angle of attack in the probe method is made with the geometric angle of attack, without taking into account the effect of the wake induced velocity. Recently, [PERSON] [29] proposed two methods evaluating local bound circulation for determining the angle of attack on a section of a rotor blade with CFD computation data. [PERSON] [30] proposed a model that introduces two radially dependent interferences on the geometric angle with the analogy of 2D potential flows.