## 3 Experimental Results and Analysis
### Wake Profile Characteristics  
Figure 17 presents the distribution of wake velocity measured by the wake rake array under uniform inflow. As can be observed, when the angle of attack was \(8.4^{\circ}\), which was before the stall angle of attack, the wake of airfoil with the Gurney flap was inclined to the pressure side of the airfoil. When the angle of attack was \(11.4^{\circ}\), which was after the stall angle, the Gurney flap weaken the ability of the wake position deflection. Moreover, the wake velocity deficit was significantly increased, and the Gurney flap began to have side effects on the aerodynamic characteristics of the airfoil. The higher the Gurney flap is, the larger the wake velocity deficit is, so it indicates a higher mean drag.  
By comparing the flows in the near trailing edge region of the airfoil with Gurney flap. The resultant velocity distribution of the baseline airfoil and the airfoil attached with Gurney flap (9 mm in height and 4.5 mm in thickness) at the angle of attack \(8.4^{\circ}\) (before the stall angle) and \(11.4^{\circ}\) (after the stall angle) under uniform inflow were depicted in Figure 18. The vertical direction Y denoted the forward distance from the hot-wire probe to the trailing edge, and the horizontal direction X denoted the lateral distance from the hot-wire probe to the trailing edge. As shown in Figure 18, the black solid spots were the actual measurement positions by using the electronically controlled traverser. As shown, the wake flows shift to the pressure side in both the two situations. When the angle of attack was \(8.4^{\circ}\), the airfoil with Gurney flap had a larger velocity deficit than the baseline airfoil, which suggests an increased drag. When the angle of attack was \(11.4^{\circ}\), the velocity deficit of the baseline airfoil was also less than the Gurney flap, but there was no obvious difference between the offset direction caused by the flapped airfoil and the baseline airfoil which were being similar to the far field results measured by the wake rake array.  
Figure 17: The wake velocity measured by the wake rake array (uniform inflow, Re: \(0.8\times 10^{6}\)). (**a**) \(\alpha=8.4^{\circ}\); (**b**) \(\alpha=11.4^{\circ}\).  
Figure 19 presents the power density spectrum measured by the region 1 cm directly behind the trailing edge under uniform inflow. As can be observed, at the angle of attack \(8.4^{\circ}\), both along-wind and across-wind display an obvious peak in the power density spectrum, meanwhile the across-wind energy increased in the flapped airfoil. It indicates there are counter rotating vortices generated by the Gurney flap, while the wake of baseline airfoil does not show the existence of vortex shedding [33]. Gurney flaps also have a wake structure similar to the Karman vortex street behind the cylinder at bigger Reynolds number (i.e., of \(0.8\times 10^{6}\)), the flow oscillations were caused by the separating of the shear layer on the pressure side and suction side of the trailing edge. The Karman vortex street both can be found in uniform flow and turbulent flow, so the increased lift produced by the Gurney flap seems can be explained in turbulent flow condition, either. Moreover, from the lift coefficients shown above, the size of structures was affected by turbulence level. Besides, as compared with the reference data under similar condition [33], with the increase of the Reynolds number, from \(\text{Re}=1\times 10^{5}\) to \(0.8\times 10^{6}\), the Strouhal number decreased from 0.151 (Re: \(1\times 10^{5}\), 2% chord length) to 0.128 (Re: \(2\times 10^{5}\), 2% chord length) and to 0.088 (Re: \(0.8\times 10^{6}\), 1.5% chord length).  
Figure 18: Velocity distribution of the region behind trailing edge (uniform inflow, Re: \(0.8\times 10^{6}\)). (**a**) \(\alpha=8.4^{\circ}\), baseline airfoil; (**b**) \(\alpha=8.4^{\circ}\), Gurney flap 9 mm \(\times\) 4.5 mm; (**c**) \(\alpha=11.4^{\circ}\), baseline airfoil; (**d**) \(\alpha=11.4^{\circ}\), Gurney flap 9 mm \(\times\) 4.5 mm.