## 4 Conclusions  
In summary, this paper presented a study of the aerodynamic performance of airfoils with/without the Gurney flap under different turbulence conditions by means of wind tunnel experiments. The experimental observations were as follows:  
The Gurney flap deflects the wake position from the pressure side of the airfoil, increasing the vertical distance between the airfoil chord and the middle arc. The surface pressure characteristics revealed that the pressure difference of the trailing edge was larger for the flapped airfoil in the turbulence condition. Moreover, due to the special wake structure, the Gurney flap also indicates the vortex generated at the trailing edge, which results in a greater lift of the airfoil. The lift increment obtained from Gurney flap was influenced by the turbulence intensity, meanwhile a large increase in drag was observed as well. Under a medium range of turbulence intensity (i.e., of 10.5%), the maximum lift-to-drag ratios increased from 14.35% (flap 6 mm \(\times\) 4.5 mm, a = 12.4 deg) to 14.47% (flap 6 mm \(\times\) 1.5 mm, a = 12.4 deg), the 1.0% Gurney flap case has the best performance, that's probably because the decrease of boundary layer thickness under turbulent conditions, so the height of 1.0% was optimal. However, under a much larger turbulence inflow (i.e., of 19.0%), all experimental data have shown that the impact was negligibly small. The drag increment was found to surpass the benefit of the increase in lift, resulting in a worse lift-to-drag ratio than the baseline airfoil. It was of importance to note that there are two kinds of measurement uncertainties could affect the acquired results. One was stochastic uncertainties caused by the discrete pressure data which can be eliminated by multiple measurements. The other one was systematic uncertainties caused by measuring instruments which can be reduced by calibration of the instruments before the experiment. Based on the observations from the present measurements, it was found that under very high turbulence level, e.g., with 19% of turbulence intensity, the Gurney flap has very small influence on the airfoil aerodynamic performance. The measurement results show the Gurney flap may be used in areas with slightly lower turbulence. Nevertheless, wind turbines in real life were running in the turbulent atmospheric boundary layer, as shown above, the effects of Gurney flap are highly dependent on turbulent inflow, it seems that the Gurney flap would be installed as a scalable form and preferred to have smaller size, such that it would get more potential benefits at all periods of rotation. In addition, it should be noted that the incoming flow is often a combination of rotational wake from the previous wind turbines. Research on the lift increment of Gurney flap was not isolated, it always interacted with such highly unsteady aerodynamic issues. Therefore, to analyze more precisely the effects of combining such turbulence levels with Gurney flap, further experiments should be carried out for the rotor blades in highly turbulent flows. This was in progress on a wind turbine blade composed of several DTU-LN221 wing sections. **Author Contributions:** Conceptualization, J.Y. ; conducted the data collection, J.Y. and Y.Y. ; funding acquisition, H.Y. ; supervision, N.L., H.Y. and W.Z. ; formal analysis, J.Y., H.Y., W.Z. ; writing--original draft preparation, J.Y. ; writing--review and editing, H.Y., W.Z. ; All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by Postgraduate Research & Practice Innovation Program of Jiangsu Province under grant number KYCX19_2104 and Youth Fund Project of Jiangsu Natural Science Foundation under grant number BK20170510. **Acknowledgments:** The authors wish to express acknowledgement to the Postgraduate Research &Practice Innovation Program of Jiangsu Province under grant number KYCX19_2104 and Youth Fund Project of Jiangsu Natural Science Foundation under grant number BK20170510.