## 2 Experimental Setup  
The experiments were carried out in a wind tunnel located at Yangzhou University. It was a close-loop type wind tunnel which contains two experimental sections. The measurements were conducted in the smaller section with the cross-section parameters of 3 m \(\times\) 1.5 m, and the length is 3 m. The operational wind speed range was 0\(\sim\)50 m/s and the calibrated maximum turbulence intensity in the free stream was 0.2%. The DTU-LN221 airfoil model [27], as shown in Figure 1, was adopted which has a chord length of 0.6 m and a span length of 1.5 m. The airfoil model was vertically placed in the test section. The bottom part of the airfoil section was connected by the rotating shaft, which was fixed on a rotational plate, as demonstrated in Figure 2. Therefore, the angle of attack can be remotely controlled via a shaft connection with a motor below the wind tunnel. The airfoil model was made of aluminum alloy where small taps were drilled on the surface of the middle section, with the location sketched in Figure 1. The arc length between the taps was approximately spaced every 2.5% of chord length starting from the leading edge to 90% of the trailing edge. The hollow plastic hoses were connected on the reverse side of the airfoil surface which has an outer diameter of 1.2 mm. Thus, a total of 77 pressure taps were arranged for the pressure measurement on the surface of the airfoil. The pressures on the airfoil surface were sensed by the electronic pressure acquisition system of PSI (Pressure Systems Inc., Hampton, VA, USA). The pressure system used in the experiment has a sampling frequency of 333.3 Hz, a measuring range of \(\pm\)2.5 kPa and a measuring accuracy of \(\pm\)0.05%. Figure 1 also shows the chord-wise position, so the lift and pitching-moment of the test airfoil were achieved by integrating the pressure and the position of the taps on the surface. The lift coefficient and pitching-moment coefficients were given by  
\[ \begin{split} C_{l}=C_{n}\cos\alpha-C_{t}\sin\alpha\\ C_{m}=\int\limits_{0}^{1}\bigl{(}C_{pl}-C_{pu}\bigr{)}(0.25-\hat{x} )d\hat{x}\end{split}  \tag{1}\]  
where  
\[ \begin{split} C_{n}=\int\limits_{0}^{1}\bigl{(}C_{pl}-C_{pu} \bigr{)}d\hat{x}\\ C_{t}=\int\limits_{\hat{y}_{l}\ max}^{\hat{y}_{u}\ max}\bigl{(}C_{p, be}-C_{p,nf}\bigr{)}d\hat{y}\end{split}  \tag{2}\]  
where \(C_{l}\) and \(C_{m}\) are the lift coefficient and pitching-moment coefficient, respectively; \(\alpha\) represents the angle of attack; \(\hat{x}\) is the relative chord length; \(\hat{y}\) is the thickness value relative to the chord length; \(C_{pu}\) and \(C_{pl}\) are the pressure coefficient on the suction and pressure sides of the airfoil, respectively; \(C_{n}\) and \(C_{t}\) are the normal force coefficient and tangential force coefficient, respectively. \(C_{p,be}\) and \(C_{praf}\) are the pressure coefficient before and after the maximum thickness of airfoil; \(\hat{y}_{u\ max}\) and \(\hat{y}_{l\ max}\) represent the maximum thickness values of the suction and pressure sides relative to chord length. At a small angle of attack, the aerodynamic drag largely consists of frictional drag, while the data measured by surface pressure taps cannot accurately represent the drag force, so the drag can be measured by the momentum method more precisely. As shown in Figure 2, a wake rake array was placed at 0.7 chord length behind the trailing edge of the airfoil and at the same vertical level as the pressure taps. The measurement range of the wake probes was 80.8 cm, 102 total pressure pipes (with an outer diameter of 1.2 mm) and 4 static pressure pipes (with an outer diameter of 2 mm) were averagely arranged, with an 20 cm apart for the static pressure pipes. To prevent air leakage, all pressure tubes were connected by plastic hoses to the pressure measurement device. Besides, two Pitot tubes were installed at 1.6 m measured from the downstream of grilles where the free stream velocity was recorded.