# Aerodynamic Design and Performance Analysis of a Large-Scale Composite Blade for Wind Turbines
## 2 Aerodynamic Design and Analysis
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \(\mathbf{r/R}\) (Ratio) & \(\mathbf{r}\) (\(\mathbf{m}\)) & \(\mathbf{\lambda}\) & \(\mathbf{\theta}\) (\(\mathbf{rad}\)) & \(\mathbf{k}\) & \(\mathbf{h}\) & \(\mathbf{a}\) (\({}^{\circ}\)) & **Chord \(l\) (\(\mathbf{m}\))** \\ \hline
0.187 & 24.31 & 1.309 & 1.353 & 0.355 & 1.229 & 15.78 & 15.711 \\
0.252 & 32.76 & 1.764 & 1.399 & 0.347 & 1.133 & 14.09 & 11.976 \\
0.317 & 41.21 & 2.219 & 1.430 & 0.342 & 1.086 & 12.4 & 10.414 \\
0.382 & 49.66 & 2.674 & 1.452 & 0.340 & 1.060 & 10.71 & 9.321 \\
0.512 & 66.56 & 3.584 & 1.580 & 0.337 & 1.034 & 7.33 & 7.135 \\
0.577 & 75.01 & 4.039 & 1.490 & 0.336 & 1.027 & 5.64 & 6.426 \\
0.707 & 91.91 & 4.949 & 1.504 & 0.335 & 1.018 & 2.26 & 5.391 \\
0.772 & 100.36 & 5.404 & 1.510 & 0.335 & 1.015 & 0.53 & 5.064 \\
0.892 & 115.95 & 6.244 & 1.518 & 0.335 & 1.011 & 0.16 & 4.582 \\
1 & 130 & 7 & 1.523 & 0.334 & 1.009 & 0 & 4.267 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The aerodynamic design result of the 25 MW blade. Figure 1: Force and velocity vector on the section of the blade. Aerodynamic performance analysis was carried out to examine whether or not the performance of the designed blade satisfies the design requirements. The results of the aerodynamic design include the chord length and twisting angle for each cross-section and airfoil data. Aerodynamic performance analysis was conducted based on these design data. The aerodynamic performance analysis was carried out using the aerodynamic design result suggested in Table 2. The axial thrust, moment coefficients, and power coefficient were analyzed using the following equations. As a result of aerodynamic performance analysis, it was found that the performance coefficient had the maximum value of 7 for the tip speed ratio and decreased for other different tip speed ratios. Aerodynamic performance was theoretically calculated using the following equations, where \(R\) is rotor the radius and \(r\) means the length of the divided blade section. The power coefficient \(C_{P}\) was calculated using the following equation. The power coefficient calculated using the theoretical equation was compared with the power coefficient through a wind tunnel test. \(C_{F}\) means the axial thrust, \(C_{M}\) is the moment coefficient, and \(S\) is the area depending on the rotor diameter. \[C_{F}=2\int_{0}^{1}\Big{(}1-k^{2}\Big{)}\frac{r}{R}d\Big{(}\frac{r}{R}\Big{)} \tag{9}\]  
\[C_{M}=2\int_{0}^{1}\lambda(1+k)(h-1)\frac{r^{2}}{R^{2}}d\Big{(}\frac{r^{2}}{R ^{2}}\Big{)} \tag{10}\]  
\[C_{P}=C_{M}\times\lambda_{0} \tag{11}\]  
\[P=\frac{1}{2}\rho C_{P}SV^{3} \tag{12}\]  
In this study, the performance coefficients of similar blades were used for comparison to verify the conceptual design results. For similar blades, the performance coefficient was measured according to the blade's tip speed ratio determined in the wind tunnel tests. Figure 3 shows the data for the power coefficient and tip speed ratio determined in the wind tunnel test. At 12.5 m/s of rated wind speed, mechanical and electrical powers were 29.81 MW and 28.32 MW, respectively; thus, it was confirmed that the aerodynamic design requirements for blades were met. The calculation result, which showed that power changed depending on the blade's tip speed ratio, is presented in Table 3. Therefore, we chose a multipole ring generator of 30 MW capacity. It is automatically disconnected from the grid at 14 m/s wind speed and immediately stopped either by a mechanical brake or by feathering its blade for a variable-pitch wind rotor. Figure 2: The aerodynamic configuration and airfoil data of the 25 MW blade. Computational fluid dynamics (CFD) analysis was conducted to verify the theoretical result of the aerodynamic performance analysis. The aerodynamic design requirement was reflected to perform analysis modeling. A cylindrical area was set around the blade and set as the wind blew from left to right. Three blades were modeled to create a mesh around them.