## 3 Results and Discussion
### Analysis of the Experimental Results  
Figure 17 shows the lift-drag coefficient and lift-drag ratio curves of the airfoil with and without VGs. Figure 17a demonstrates the lift coefficient curve. It can be seen that the lift coefficients of the airfoil with and without VGs were almost the same for an angle-of-attack (AoA) below \(8^{\circ}\). Above \(8^{\circ}\), the VGs could effectively increase the lift of the airfoil. In Table 4, it can be seen that when \(\lambda/H=9\), the stall AoA of the airfoil was \(16^{\circ}\), while under the other VG spacings, the stall AoA was \(18^{\circ}\). The maximum lift coefficient was obtained when \(\lambda/H=5\), which was increased by 48.77% compared to that of the clean airfoil. When \(\lambda/H=3\), \(\lambda/H=7\), and \(\lambda/H=9\), the maximum lift coefficient increased by 45.67%, 48.32%, and 30.20%, respectively. Therefore, the maximum lift coefficient of the airfoil was the largest when \(\lambda/H=5\). On the other hand, the drag coefficient was the worst when \(\lambda/H=9\). In Table 3, it can be seen that when \(\lambda/H=3\), the drag coefficient of the airfoil decreased by 82.06% compared to that of the clean airfoil at an AoA of \(18^{\circ}\). In addition, it decreased by 83.28% when \(\lambda/H=5\), by 78.43% when \(\lambda/H=7\), and by 13.06% when \(\lambda/H=9\). The lift-drag ratio of the airfoil increased by 741.03% compared to that of the clean airfoil at an AoA of \(18^{\circ}\) when \(\lambda/H=3\), by 821.86% when \(\lambda/H=5\), by 612.06% when \(\lambda/H=7\), and by 55.06% when \(\lambda/H=9\). Figure 16: Relationship between the pressure loss coefficient \(C_{\Delta P}\) and VGs spacings. Figure 15: Spanwise mean velocity profiles. In Figure 18, the surface pressure coefficient (\(C_{p}\)) distribution of the airfoil is presented. It can be seen that the curves of \(C_{p}\) distribution on the airfoil surface with and without VGs coincided at 0\({}^{\circ}\) and 5\({}^{\circ}\) AoA. At AoAs of 10\({}^{\circ}\), 15\({}^{\circ}\), and 18\({}^{\circ}\), the VGs postponed the pressure plateau to some extent. However, when \(\lambda/H=9\), the effect of the VGs flow control was the worst, and the pressure plateau appeared earlier than in the other three cases. It can be observed that the \(C_{p}\) curves of the other three VG spacings basically coincided. At an AoA of 20\({}^{\circ}\), the \(C_{p}\) curves and the position of the pressure platform of \(\lambda/H=9\) were similar to those of the clean airfoil, while the \(C_{p}\) curves of the other three VGs spacings were similar. At 22\({}^{\circ}\), the \(C_{p}\) curves of \(\lambda/H=3\) and \(\lambda/H=5\) basically coincided with the clean airfoil, while \(\lambda/H=7\) exhibited the best effect. When the AoA was equal to 25\({}^{\circ}\), the \(C_{p}\) curves of the airfoil with VGs coincided with those of the clean airfoil. Figure 19 shows the curve of the total pressure coefficient distribution at the wake rake. It can be seen that at AoAs of \(\alpha=0^{\circ}\), 5\({}^{\circ}\), and 10\({}^{\circ}\), for an airfoil with VGs, the total pressure loss at the wake rake was larger because the generators increased the thickness of the boundary layer and thus increased the viscous losses. When the AoA was 15\({}^{\circ}\) and 18\({}^{\circ}\), the total pressure loss of the airfoil with VGs was smaller than that of the clean airfoil. Among the four VG spacings, the total pressure loss of the airfoil with VGs was the largest when \(\lambda/H=9\), and under the other three spacings, the total pressure loss was similar. At \(\alpha=20^{\circ}\), the total pressure loss was the largest when \(\lambda/H=9\) and the smallest when \(\lambda/H=7\). At \(\alpha=22^{\circ}\), the total pressure loss was the largest when \(\lambda/H=7\), which was larger than that of the clean airfoil, while the total pressure loss of \(\lambda/H=7\) was the smallest. When \(\alpha=25^{\circ}\), the total pressure loss was the same, with or without VGs.