# Improvement of Aerodynamic Performance of Savonius Wind Rotor Using Straight-Arc Curtain
## 4 Results and Discussion
### Static Performance
When the rotor position is \(\theta=150^{\circ}\), the _Cst_ value of the traditional Savonius wind rotor is about 0.0. The _Cst_ values of the wind rotor with the straight-arc curtain obtained from the numerical analysis are shown in Figure 10:0  
The results show that the _Cst_ values of the rotor with the curtain are always higher than 0.0, which indicates that all of the straight-arc curtains can effectively improve the positive moment of the concave blade so as to improve the static performance of the rotor. It also can be found that the rotor has the best performance with Curtain-a for curtain angles \(\alpha=15^{\circ}\) and \(\beta=30^{\circ}\), with Curtain-b for \(\alpha=15^{\circ}\) and \(\beta=30^{\circ}\), and with Curtain-c for \(\alpha=15^{\circ}\) and \(\beta=15^{\circ}\). This result once again indicates that the effects of the curtain angle and the length of the arc-shaped curtain on the performance of the rotor are not independent. The _Cst_ values obtained for Curtain-a, Curtain-b and Curtain-c for a curtain angle \(\alpha\) of \(15^{\circ}\) are shown in Figure 10:1  
Figure 10:2 Static torque coefficients of (**a**) Curtain-a, (**b**) Curtain-b and (**c**) Curtain-c (\(\theta=150^{\circ}\)). In Figure 15, 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, and Curtain-c are 0.121, 0.132, and 0.148, respectively. When the rotor position is \(\theta=165^{\circ}\), the \(Cst\) value of the traditional Savonius wind rotor is about \(-0.13\). The \(Cst\) values of the wind rotor with the straight-arc curtain obtained from the numerical analysis are shown in Figure 10:3  
Figure 10:4 Static torque coefficients of (**a**) Curtain-a, (**b**) Curtain-b and (**c**) Curtain-c (\(\theta=165^{\circ}\)). Figure 10:5 Static torque coefficients for different parameters (\(\theta=150^{\circ}\)). The results show that the \(Cst\) values obtained for the different curtains are always positive, which is not the case with the conventional rotor. This indicates that all of the straight-arc curtains can effectively decrease the negative moment of the convex blade and increase the positive moment of the concave blade. It can also be found that the best curtain angles for Curtain-a are \(\alpha=15^{\circ}\) and \(\beta=15^{\circ}\) and for Curtain-b and Curtain-c are \(\alpha=15^{\circ}\) and \(\beta=0^{\circ}\). The \(Cst\) values obtained for Curtain-a, Curtain-b, and Curtain-c for a curtain angle \(\alpha\) of \(15^{\circ}\) are shown in Figure 10:6  
Figure 17 shows that the \(Cst\) values obtained for Curtain-c from the numerical analysis are higher than the experimental ones for Curtain-a, Curtain-b, and Curtain-c at the same curtain angles. The maximum \(Cst\) values obtained for Curtain-a, Curtain-b, and Curtain-c are 0.081, 0.086, and 0.101, respectively. The above numerical analysis has shown that when the wind rotor is static, the straight-arc curtain can provide a better static performance than that achieved without the curtain. The extent of the enhancement is defined by the length of the arc-shaped curtain and the curtain angles. In addition, the effects of the curtain 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 optimal values can the wind rotor have the best aerodynamic performance. In all cases, the rotor with Curtain-c has the best performance. Hence, Curtain-c with \(\alpha=15^{\circ}\) and \(\beta=15^{\circ}\) is selected to explore the effect of the curtain on the dynamic performance of the Savonius wind rotor.