# Application of Simultaneous Symmetric and Cambered Airfoils in Novel Vertical Axis Wind Turbines
## 4 Results and Discussion
### Dynamic Stall Phenomena  
For NSA-1, when the NACA0018 is in front of the wind, the vorticity contours are shown in the first column of Figure 9. In this condition, the first leading edge vortex (LEV) is developing at \(\Delta\theta=140^{\circ}\). Before \(\Delta\theta=170^{\circ}\), the first LEV is fully developed and is separated from the pressure side of the airfoil. After the first LEV separation, at \(\Delta\theta=170^{\circ}\), the first trailing edge vortex (TEV) is visible, which helps to decrease lift coefficient while the separated first LEV sheds to the downstream. At \(\Delta\theta=180^{\circ}\), the first TEV is separating, and the second LEV is developed. This second LEV is ready to separate when \(\Delta\theta=190^{\circ}\). After that, small vortices generate and shed rapidly downstream, which is visible at \(\Delta\theta=260^{\circ}\). These small vortices do not have enough time to develop, and then they do not cause significant load changes. For TSR of 1.5, Figure 6a shows that the NACA0018 airfoil in NSA-1 results in a C\({}_{\rm p}\) increase after \(\Delta\theta=110^{\circ}\) and then after the LEV generation, a sudden C\({}_{\rm p}\) decrease is observed after \(\Delta\theta=140^{\circ}\). This sudden C\({}_{\rm p}\) fall affects the overall performance of NSA-1. For the NSA-1 turbine, when the S815 airfoil is located in front of the wind, the vorticity contours are shown in the second column of Figure 9. The S815 airfoil is an airfoil with a high camber. The first LEV is generated at a low angle of \(\Delta\theta=30^{\circ}\). The first LEV is developed at \(\Delta\theta=50^{\circ}\) when the second LEV is ready to complete. The LEV is a low-pressure vortex that causes lift production. The first LEV delay for NACA0018 results in a strong vortex. Although dynamic stall is observed for S815, it did not cause significant power reduction after LEV separation (Figure 6a). Thus, the average C\({}_{\rm p}\) does not vary significantly when S815 is in front of the wind. It can be concluded that since S815 is not a  
Figure 8: Averaged power coefficient: (**a**) based on all TSRs, (**b**) based on all VAWTs.  
Figure 7: VAWT power coefficients vs. TSRs.  
power extraction airfoil, the dynamic stall related to S815 does not change the domain of power coefficient variation significantly.  
Figure 9: \(Cont.\)  
The vertical structures of NACA0018 and S815 for different arrangements in the NSA-2 turbine are shown in the last two columns of Figure 9. The interesting point is that their  
Figure 9: (**a**) Vorticity contours of NSA-1 and NSA-2 for zero rotational angle (\(\Delta\theta\) = 0\({}^{\circ}\), see Figure 1) at TSR = 1.5; column 1: NSA-1, NACA0018; column 2: NSA-1, S815; column 3: NSA-2, NACA0018; column 4: NSA-2, S815. (**b**) Continued.  
vortical structure around airfoils is very close to that of NSA-1. Changing the location of NACA0018 results in varying the maximum and minimum C\({}_{\rm P}\) values for each turbine, as shown in Figure 9. The combination of the loads on each blade results in different power extraction. According to Figures 6 and 9, dynamic stall phenomena are more effective in lower TSRs which agrees with the work of [PERSON] and [PERSON] [45].