## 3 Experimental Results and Analysis
### Surface Pressure Characteristics  
To analyze the mechanism of turbulent inflow coupled with Gurney flap, Figure 16 illustrates the pressure coefficient distribution of the baseline airfoil and airfoil attached with the Gurney flap under different turbulent inflow when Reynolds number \(\mathrm{Re}=0.8\times 10^{6}\). The x-axis shows the chord-wise position x/c and the y-axis shows the pressure coefficients \(C_{p}\). The pressure coefficients \(C_{p}\) on the airfoil surface were given  
\[C_{\mathrm{p}}=\frac{p_{\mathrm{i}}-p_{0}}{0.5\rho L_{0}^{2}} \tag{7}\]  
where \(p_{\mathrm{i}}\) is the pressure at the pressure tap of i (i = 1:77), \(p_{0}\) is the free stream static pressure at the airfoil tested by the Pitot tube, \(\rho\) is the air density and \(U_{0}\) is the free stream velocity.  
Figure 16: _Cont._  
Figure 15: Variation of the aerodynamic characteristics with different thicknesses of Gurney flaps (T.I. of 19%, \(\mathrm{Re}:0.8\times 10^{6}\)). (**a**) Lift coefficient comparison; (**b**)drag coefficient comparison; (**c**) pitching-moment coefficient comparison; (**d**) lift-to-drag ratio comparison.  
Figure 16 illustrates that the pressure coefficients of the baseline airfoil under different turbulence conditions with small angles were similar to each other. However, with the increase of angle of attack, the differences become more pronounced at the leading edge. In contrast to the Gurney flaps, Figure 16a shows that at the angle \(=-9.6^{\circ}\), the differences between the flapped airfoils were small. With the increase of the angle of attack, as the angle of \(-3.6^{\circ}\), compared with the baseline airfoils, the intersection of the airfoil with Gurney flap moves to the leading edge under different conditions. Besides, the pressure coefficients (with the angle \(=-3.6^{\circ}\) and \(2.4^{\circ}\)), the pressure distribution indicate a larger the pressure difference in the range of \(0.8<\chi/c<0.9\). When the angle of attack was \(2.4^{\circ}\) and \(8.4^{\circ}\), both the pressure coefficient absolute values of the baseline airfoil and the Gurney flap tendency to increase with the increase of turbulence intensity, and the flapped airfoil becomes more obvious than the baseline airfoil. At a large angle of attack, due to the condition of stalling (Figure 16e,f), there was significant flow separation both on the suction side of flapped airfoil and baseline airfoil under uniform attack. The difference between the two cases  
inflow. The larger the turbulent inflow, the larger the peak value on the leading edge of the suction side, and also the larger pressure coefficient on the pressure side.  
As seen from the above, the influence of turbulent inflow coupled with Gurney flap on the pressure coefficients of airfoil was complicated. Turbulent inflow changes the flow around the airfoil surface and restrains the flow separation, after attaching the Gurney flap, the pressure distribution on both suction and pressure sides changed more significantly. Especially in the turbulence condition, the trailing edge pressure distributions were farther apart for the flapped airfoil. In these situations, the increased peak values of pressure coefficients at the leading edge increased the integral area of the pressure coefficients, so the lift force was therefore increased.