# Improvement of Aerodynamic Performance of Savonius Wind Rotor Using Straight-Arc Curtain
## 4 Results and Discussion  
### Static Performance  
When the rotor position is \(\theta\) = 45\({}^{\circ}\), the static torque coefficient (\(Cst\)) of the traditional Savonius wind rotor is about 0.32. The \(Cst\) values of the wind rotor with the straight-arc curtain obtained from the numerical analysis are shown in Figure 10.  
Figure 10: \(Cont.\)  
Figure 9: Variation of the power coefficient with the TSR. The results show that straight-arc curtains with different parameters can improve the _Cst_ value of the wind rotor, and the extent of the enhancement relies on the geometric parameters of the straight-arc curtain. The _Cst_ values obtained for Curtain-a, Curtain-b, and Curtain-c when \(\alpha=10^{\circ}\) are higher than those obtained when \(\alpha=15^{\circ}\) and \(\alpha=20^{\circ}\). This indicates that the best angle for the straight curtain is \(10^{\circ}\). When \(\alpha\) is fixed, Curtain-a with \(\beta=15^{\circ}\) has better performance than Curtain-a with \(\beta=0^{\circ}\) and \(\beta=30^{\circ}\), and Curtain-b and Curtain-c with \(\beta=0^{\circ}\) have better performance than both curtains with \(\beta=15^{\circ}\) and \(\beta=30^{\circ}\). Therefore, the best angle for the arc-shaped curtain of Curtain-a is \(15^{\circ}\), and the best angle for the arc-shaped curtain of Curtain-b and Curtain-c is \(0^{\circ}\). This result indicates that the effects of the angle and the length of the arc-shaped curtain on the performance of the rotor are not independent. Only when the angle and the length have their optimal values can the wind rotor have the best aerodynamic performance. The _Cst_ values obtained for Curtain-a, Curtain-b, and Curtain-c when the curtain angle \(\alpha\) is \(10^{\circ}\) are shown in Figure 11.  
Figure 11 shows that Curtain-c has a better performance than Curtain-a and Curtain-b under the same conditions. The maximum _Cst_ values obtained for Curtain-a, Curtain-b, and Curtain-c are 0.421, 0.454, and 0.479, respectively. The maximum _Cst_ value obtained for Curtain-c is about 50% higher than that of the rotor without the curtain. When the rotor position is \(\theta=60^{\circ}\), the Cst value of the traditional Savonius wind rotor is about 0.28. The _Cst_ values of the wind rotor with the straight-arc curtain obtained from the numerical analysis are shown in Figure 12.  
Figure 11: Static torque coefficients for different curtains (\(\theta=45^{\circ}\)). Figure 10: Static torque coefficients of (**a**) Curtain-a, (**b**) Curtain-b and (**c**) Curtain-c (\(\theta=45^{\circ}\)). Figure 12 shows that the _Cst_ values of the rotor with the curtain are always higher than 0.28, which indicates that all of the straight-arc curtains can effectively improve the static performance of the rotor. It can also be seen that the variation trend of _Cst_ for the rotor with the curtain angle is similar in the three Figures. The highest _Cst_ values for the three curtain groups are found at around \(\alpha=15^{\circ}\) and \(\beta=0^{\circ}\) and the lowest ones at around \(\alpha=20^{\circ}\) and \(\beta=30^{\circ}\). This indicates that the best angle for the arc-shaped curtains of Curtain-a, Curtain-b, and Curtain-c is \(0^{\circ}\) and the best angle for the straight curtains of Curtain-a, Curtain-b, and Curtain-c is \(15^{\circ}\). The _Cst_ values obtained for Curtain-a, Curtain-b, and Curtain-c for a curtain angle \(\alpha\) of \(15^{\circ}\) are shown in Figure 13.  
As can be seen from Figure 13, Curtain-c still has a better performance than Curtain-a and Curtain-b under the same conditions. The maximum _Cst_ values obtained for Curtain-a, Curtain-b,  
Figure 12: Static torque coefficients of (**a**) Curtain-a, (**b**) Curtain-b and (**c**) Curtain-c (\(\theta=60^{\circ}\)). Figure 13: Static torque coefficients for different curtains (\(\theta=60^{\circ}\)). and Curtain-c are 0.415, 0.487, and 0.544 respectively. The maximum _Cst_ obtained for Curtain-c is about 90% higher than that of the rotor without the curtain.