## 3 Experimental Results and Analysis
### Experimental Results of the Gurney Flap under Uniform Inflow  
This subsection starts to investigate the aerodynamic characteristics of the effect of Gurney flap in low turbulent inflow condition, the Reynolds number was decreased to \(0.8\times 10^{6}\). Seen from the Figure 10, three lift and drag curves were compared with the original at the angle of attack ranged from \(-9.6^{\circ}\) to \(14.4^{\circ}\). As shown, the lift coefficients increased with the increase of the height of the Gurney flap before the stall angle, while it was also realized that the effect of the Gurney flap was limited at a high angle of attack. The lift coefficient slopes and the stall angles of the flapped airfoil remain essentially unchanged compared to the baseline airfoil, but the flapped airfoil caused a leftward shift compared to the baseline airfoil. In order to show the changes of aerodynamic characteristics clearly, the following lift and drag differences were defined:  
\[\Delta cl=\frac{\left|cl_{\text{bs}}-cl_{\text{gf}}\right|}{cl_{\text{bs}}} \times\,100\% \tag{4}\]  
\[\Delta cd=\frac{\left|cd_{\text{bs}}-cd_{\text{gf}}\right|}{cd_{\text{bs}}} \times\,100\% \tag{5}\]  
\[\Delta cld=\frac{\left|cld_{\text{bs}}-cl_{\text{gf}}\right|}{cld_{\text{bs}}} \times\,100\% \tag{6}\]  
where \(cl_{\text{bs}}\) and \(cd_{\text{bs}}\) are the lift and drag coefficients obtained from the baseline airfoil, respectively. \(cl_{\text{gf}}\) and \(cd_{\text{gf}}\) are the lift and drag coefficients obtained from the flapped airfoil. \(cld_{\text{bs}}\) and \(cld_{\text{bs}}\) are the lift-to-drag ratio obtained from the baseline airfoil and flapped airfoil. Figure 10: Variation of the aerodynamic characteristics with different heights of Gurney flaps (uniform flow, Re: \(0.8\times 10^{6}\)). (**a**) Lift coefficient comparison; (**b**) drag coefficient comparison; (**c**) pitching-moment coefficient comparison; (**d**) lift-to-drag ratio comparison. When the flap height changes to 6 mm, 9 mm and 12 mm, the maximum lift coefficients were increased by 8.47%, 9.56% and 13.50% at 9.4 degrees, respectively. Looking into the drag coefficients shown in Figure 10b, within the range of -9.6\(\sim\)10.4\({}^{\circ}\), the drag coefficients show regular change where the frictional drag dominates as expected. The drag coefficients rise rapidly in the stall state, at this time the change in drag coefficient was dominated by pressure drag. For example, at an angle of attack of 12.4\({}^{\circ}\), with the flap heights of 6 mm, 9 mm, 12 mm the drag coefficients were increased by 39.67%, 59.92% and 113.43%, respectively. According to Figure 10c, the Gurney flap generated a prominent increase in the pitching-moment coefficient compared to the baseline airfoil, the value increased with the heights of the Gurney flaps and reached negative peak values at approximately 4.4\({}^{\circ}\). Based on Figure 10d, the lift-to-drag ratios were compared. When the angle of attack was less than 8.4\({}^{\circ}\), the lift-to-drag ratios were all larger than the baseline airfoil. However, the maximum lift-to-drag ratios of the Gurney flap at three heights were smaller than the baseline airfoil. When the flap heights were 6 mm, 9 mm and 12 mm, the corresponding maximum lift-to-drag ratios were decreased by 17.16%, 22.79% and 24.47%, respectively. In the range after the stall angle, the presence of the Gurney flap reduces the lift-to-drag ratios and the higher the flap height, the more the lift efficiency decreases. When the angle of attack was 12.4\({}^{\circ}\), for example, the lift-to-drag ratios of the flap with heights of 6 mm, 9 mm and 12 mm were decreased by 26.55%, 38.11% and 52.20%, respectively. Meanwhile, we observed the same trend from the other thicknesses of the Gurney flaps (i.e., of 1.5 mm), When the flap height changes to 6 mm, 9 mm and 12 mm, the maximum lift coefficients were increased by 9.27%, 9.78% and 14.08% at 9.4 degrees. The following study in this section will be focused on the effect of the thickness of the Gurney flap.