## 3 Development of the Aerodynamic Field Measurement Platform  
### General Designation  
General designation determines what parameters needed to be measured and how to measure those parameters. For the field measurement platform in this work, aiming to investigate the effects of unsteady natural inflow on the aerodynamic behaviors of rotor and power generation of wind turbine, the most important parameters are inflow wind speed and direction, aerodynamic loads of blade, and operating data of wind turbine.  
The objective of inflow wind speed and direction measurement is to obtain the effective angle of attack that the blades suffered. Although an upstream anemometer mast was installed to measure inflow conditions of far field, natural wind is unsteady in time and un-uniform in space. The wind speed and direction measured at far field is not representative of the actual inflow conditions that the rotor suffered. Moreover, aerodynamic forces over the airfoils are very sensitive to angle of attack. Hence, the reliability of aerodynamic field measurement platform is determined by the measurement accuracy of angle of attack. In the general designation, it was planned that air probes are installed at the leading edge of blade to measure the relative inflow wind speed and direction that blades suffered directly. However, the inflow data measured by leading-edge air probes are still not the effective angle of attack. The effects of lift circulation on the flow direction at probe tip should be considered and corrected. A detailed description is shown in data reducing methods.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Span/m** & **Chord/m** & **Twist Angle/\({}^{\circ}\)** & **Airfoil** \\ \hline
0 & 0.42 & 14.99 & Circle \\
0.54 & 0.42 & 14.99 & Circle \\
0.79 & 0.53 & 14.99 & CAS W2-450 \\
2.79 & 0.67 & 8.40 & DU00-W-350 \\
3.79 & 0.55 & 5.69 & DU97-W-350 \\
4.79 & 0.47 & 3.84 & DU91-W2-250 \\
6.54 & 0.37 & 1.50 & DU93-W-210 \\
9.04 & 0.28 & \(-1.55\) & NACA 63418 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Chord and twist angle distributions of blades.  
Figure 2: Photo and parameters of the 100 kW wind turbine.  
Compared with concentrated aerodynamic force measurements, local flow patterns, such as flow transition and separation, can be deduced from pressure distribution measurement, which is suitable for fundamental research. Therefore, pressure distributions of five main airfoils, except NACA 63,418 in Table 3, were measured to evaluate aerodynamic loads over the rotor.  
Operating data of wind turbine, including power, rotor speed, pitch angle, yaw angle, and azimuth angle, are used to analyze the power performance and the blade attitude in cooperation with aerodynamic data of blade. Consequently, all the data, including inflow wind speed and direction from anemometer mast and air probes, pressure distributions, and operating data of the wind turbine, must be sampled synchronously.  
In summary, overall layout of field measurement platform is shown in Figure 3. Pressure taps were embedded in five airfoil sections, and five corresponding air probes were installed at the leading edge. Meanwhile, four strain gauges were pasted in the inner surface of blade root to monitor the structural response, and tufts were pasted on the suction side of another blade to assist flow visualization. Moreover, a synchronous data acquisition system was designed to ensure all the data sampled are correlative in time.