## 5 Results and Discussions
### Comparison with 2D Wind Tunnel Data  
In order to minimize the 3D induction effect from blade root and tip, aerodynamic data of DU93-W-210 airfoil at \(\tau/\mathrm{R}=0.64\) of the blade was chosen to compare with the 2D wind tunnel data, where R is the radius of rotor. The Reynolds number was computed by Equation (11), where \(V\) is the velocity measured by the corresponding seven-hole probe, \(P\) and \(T\) are the atmospheric pressure and temperature measured by the corresponding sensor on the upstream anemometer mast, \(R\) is the ideal gas constant, \(\mu(T)\) is the kinetic viscosity calculated by Sutherland's law. The obvious fluctuations of Reynolds number can be observed in Figure 17. However, the mean Reynolds number is about \(2\times 10^{5}\) at the test cases. \[\mathrm{Re}=\frac{\rho VI}{\mu}=\frac{P}{RT}\cdot\frac{VI}{\mu(T)} \tag{11}\]  
Wind tunnel data of DU93-W-210 airfoil is from Mie University [32]. The measurements were performed in the low-speed wind tunnel with a maximum velocity of 52 m/s, and a measurement section size of 0.65 \(\times\) 0.65 \(\times\) 2.0 m. The turbulence intensity is less than 0.15% at the test section. Figure 16: Angle-averaged lift coefficients from each pitching angle case (**a**) and all the cases (**b**). Figure 17: Reynolds number range at different pitching angle cases. The experimental wind speed was at 30m/s corresponding to a Reynolds number of Re = 3.5 \(\times\) 10\({}^{5}\), which is the most proximate inflow conditions with the field conditions that can be found in the literatures. More detailed information about the wind tunnel measurements can be found in [PERSON]'s work [32]. Three methods were applied to investigate the effects of sideslip angle and induced velocity on the relationship between the lift coefficient and angle of attack. As is shown in Table 8, only sideslip angle correction for normalization of aerodynamic coefficients was taken into account in Method I. Only the induced velocity correction for the determination of the effective angle of attack was applied in Method II. In Method III, both corrections were applied. The relationship curves between lift coefficient and angle of attack with the three methods were plotted in Figure 18 and compared with wind tunnel data from Mie University [32]. It can be shown that good agreements are achieved between field data using Method III and wind tunnel data, especially at an angle of attack less than 8\({}^{\circ}\). However, remarkable deviations are observed when Method I was applied. The slope of relationship curve between lift coefficient and angle of attack is clearly lower. It means that induced velocity from lift circulation has appreciable impact on the directly measured velocity by leading-edge probes. When determining the angle of attack in the field, induced velocity correction must be taken into account. When Method II was applied, both Method II and Method III give almost similar results at angles of attack less than 10\({}^{\circ}\). It means that the sideslip angle has little impact on the relationship between the lift coefficient and the angle of attack. Furthermore, pressure coefficients of the field measurements, angle-averaged with the data of all the testing cases, were further compared with the wind tunnel data in detail. It is shown from Figure 19 that between two angles of attack, 6\({}^{\circ}\) and 12\({}^{\circ}\), the shapes of the pressure distributions are different, and at each angle of attack, the pressure distribution of the field measurement has good agreement with the wind tunnel data. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Method I** & **Method II** & **Method III** \\ \hline Sideslip angle correction & \(\bullet\) & \(\circ\) & \(\bullet\) \\ Induced velocity correction & \(\circ\) & \(\bullet\) & \(\bullet\) \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Computation methods of angle of attack. Figure 18: Comparison of lift coefficients of DU93-W-210 between the field test and 2D wind tunnel test.