# Improvement of Aerodynamic Performance of Savonius Wind Rotor Using Straight-Arc Curtain
## 4 Results and Discussion
### Pressure and Velocity Contours Around the Rotor  
Pressure contours generated from the numerical simulation of the wind rotor with and without Curtain-c for two different angle-of-attacks are shown in Figure 19. In all cases, higher pressure values are found at the concave blade of the Savonius rotor model. A negative pressure region develops from the convex side of the blade. By comparing Figure 19a,b, it can be found that the pressure around arrow 1 is significantly lower than the pressure around arrow 2. This means that placing Curtain-c in front of the wind rotor can effectively weaken wind flow onto the convex blade and reduce the pressure at the convex side of the convex blade so as to improve the positive static torque of the convex blade. By comparing Figure 19c,d it can be found that the pressure around arrow 3 is significantly higher than that around arrow 4 and the pressure around arrow 5 is significantly lower than that around arrow 6. This means that Curtain-c can effectively promote the wind flow to the concave blade, increase the pressure at the concave side of the concave blade so as to improve the positive static torque of the concave blade. In addition, it is noteworthy that the pressure around arrow 6 is higher than that around arrow 4 and the pressure around arrow 3 is higher than that around arrow 5. This result can be used to explain why the \(Cst\) value of the wind rotor without the curtain is negative whereas with Curtain-c it is positive when the wind position is 165\({}^{\circ}\).  
Figure 18: Variation of the power coefficient with the TSR for (**a**) Curtain-a, (**b**) Curtain-b (**c**) Curtain-c as well as the (**d**) comparison of the three curtains (\(U\) = 10 m/s).  
The velocity contours generated from the numerical simulation of the wind rotor with and without Curtain-c when the wind position is \(45^{\circ}\) are shown in Figure 20a-d. The first thing that can be noticed in all the Figures is the recirculation zone on the convex side (arrow 1) of the convex blade. This means that Curtain-c can change the direction of the wind and prevent wind flow to the convex blade so as to reduce the negative torque on the convex blade. Another result present in the wind rotor with Curtain-c is the wind velocity around the wind blades, which is increased in area 3. The velocity increase in this area suggests that the positive contribution of the concave and convex blades to the overall torque is significantly increased. In addition, it should be noted that Curtain-c can harvest more of the wind that flows to the wind rotor.  
Velocity contours of the wind rotor with and without Curtain-c when the wind position is \(165^{\circ}\) are shown in Figure 21.  
Figure 21: Comparison of the velocity contours for the wind rotor (**a**) with Curtain-c, (**b**) without the curtain and their corresponding local images (**c,d**) at \(\theta=165^{\circ}\).  
Figure 20: Comparison of the velocity contours for the wind rotor (**a**) with Curtain-c, (**b**) without the curtain and their corresponding local images (**c,d**) at \(\theta=45^{\circ}\).  
By comparing Figure 21a,b, it can be found that Curtain-c can effectively prevent wind flow to the concave blade (area 1 and area 2). This can reduce the negative torque on the convex side of the concave blade. By comparing Figure 21c,d, it can be found that Curtain-c can effectively increase the wind velocity around the concave side (area 3) of the convex blade and the concave side (area 5) of the concave blade. The velocity increase in this area suggests that the positive contribution of the concave and convex blades to the overall torque is significantly increased.