## 4 Effect of Turbulence Intensity on Airfoil at Different Reynolds Numbers
### The Influence of Turbulence Intensity Upon Airfoils at \(Re_{1}=26\),500  
At \(Re_{1}=26\),500, the aerodynamic characteristics of airfoils were analyzed at angles of attack of \(5^{\circ}\), \(8^{\circ}\) and \(10^{\circ}\). The pressure distribution and chordwise friction drag coefficient (\(C_{f,X}\)) of airfoils in varied turbulence intensities is shown in Figures 12 and 13. Analyzing the pressure coefficient and friction coefficient under different turbulence intensities, together with the details of the corresponding flow field (Figures 14-17), it can be seen that:  
Figure 11: The component of the drag coefficient of airfoils varies with the angle of attack at different Reynolds numbers and turbulence intensities. (**a**) NACA0012; (**b**) SD7037. Figure 12: Airfoil pressure distribution affected by different turbulence intensities at three angles of attack. (**a**) \(\alpha=5^{\circ}\); (**b**) \(\alpha=8^{\circ}\); (**c**) \(\alpha=10^{\circ}\). Figure 13: Airfoil chordwise skin friction drag coefficient affected by different turbulence intensities at three angles of attack. (**a**) \(\alpha=5^{\circ}\); (**b**) \(\alpha=8^{\circ}\); (**c**) \(\alpha=10^{\circ}\). Figure 14: The NACA0012 airfoil characteristics such as turbulent kinetic energy and streamline shape at \(Tu=0.6\%\); (**a**) \(\alpha=5^{\circ}\); (**b**) \(\alpha=8^{\circ}\); (**c**) \(\alpha=10^{\circ}\). At \(\alpha=5^{\circ}\), the surface pressure distribution curve of the NACA0012 airfoil is smooth at \(Tu=6\%\), indicating that the flow is stable and the streamline adheres well (Figures 12a and 14a). While the boundary layer on the upper surface of SD7037 airfoil begins to separate at about 82%c and reattached near the trailing edge, forming a separation bubble (Figure 16a). However, because the separation bubble is small, the airfoil pressure distribution does not change significantly. At \(Tu=0.6\%\), the pressure distribution of the NACA0012 airfoil around 30%-60%c on the upper surface forms a pressure platform. The \(C_{f,X}\) becomes negative at 18%c, which indicates that the flow is separated (Figures 13a and 15a). The streamline around NACA0012 wraps a slender eddy due to the laminar separation (Figure 15a). The SD7037 airfoil also experiences flow separation, but the separation point is slightly delayed, at about 25%c (Figure 17a). When the angle of attack of the airfoils increases to \(8^{\circ}\), flow separation occurs near the trailing edge of the NACA0012 airfoil at \(Tu=6\%\), and a separation bubble forms near the trailing edge of the SD7037 airfoil (Figures 13b and 16b). At \(Tu=0.6\%\), flow separation occurs near 4%c on the upper surface of NACA0012 airfoil, and the separation area extends to the trailing edge (Figure 15b). Similarly, flow separation occurs near 14%c on the upper surface of SD7037 airfoil (Figure 17b). With the angle of attack increasing to \(10^{\circ}\), at \(Tu=6\%\), a slender separation bubble is generated around 3%-16%c on the upper surface of the NACA0012 airfoil, and the flow separation area near the trailing edge is expanded (Figure 14c). The location of trailing edge separation bubble on the SD7037 airfoil is slightly advanced, and a longer separation bubble is formed near 8%-20%c. At \(Tu=0.6\%\), the pressure distribution platform on the upper surface of the two airfoils appears earlier. The flow separation has the characteristics of separation bubbles in terms of pressure distribution. From the evolution of flow separation on the upper surface of the airfoils at \(Tu=0.6\%\) can be concluded that the increasing angle of attack results in the increase of adverse pressure gradient around the trailing edge, contributing to the separation of the boundary layer. Then the separation point  
Figure 16: The SD7037 airfoil characteristic such as turbulent kinetic energy and streamline shape at \(Tu\) = 6%, (**a**) \(\alpha=5^{\circ}\); (**b**) \(\alpha=8^{\circ}\); (**c**) \(\alpha=10^{\circ}\).