## 4 Effect of Turbulence Intensity on Airfoil at Different Reynolds Numbers  
Considering the variability of the jet flow of TeDP aircraft affects the aerodynamic performance of the wing, the effect of turbulence intensity and Reynolds number on airfoil is studied in the following section. As stated earlier, there is quite a lot of research of for Reynolds number between 100,000 and 1200,000. Thus, \(Re_{1}\) = 26,500 and \(Re_{2}\) = 53,000 are chosen for further research, and the state of \(Re_{3}\) = 212,000 is calculated as a complement. In addition, since NACA0012 is a popular symmetric airfoil, the SD7037 airfoil is selected for comparison. The SD7037 airfoil has a relative thickness of \(\mathit{t}\)/\(\mathit{c}\) = 9.20% and a relative camber of \(\mathit{f}\)/\(\mathit{c}\) = 3.02%. It operates at a low Reynolds number and it is favored for R/C model sailplanes because of its working lift range (the lift range over the low drag region) that begins near a \(C_{L}\) of 0.2 and extends to near \(C_{L}\) of 1.0 where the drag begins to increase more rapidly [37]. During the calculation, the turbulence intensity at the leading edge of airfoils is guaranteed to be \(\mathit{T}\)/\(\mathit{t}\) = 6% and \(\mathit{T}\)/\(\mathit{t}\) = 0.6%. The lift and drag characteristics are shown in Figures 9 and 10. Generally, the flow around airfoils is complex in the range of \(Re\) = 10,000-100,000. There may be laminar boundary layer, laminar separation, transition, turbulent boundary layer and turbulent separation. It can be seen from Figures 9 and 10 that under \(\mathit{T}\)/\(\mathit{t}\) = 6% corresponding to states of \(Re_{1}\) = 26,500 and \(Re_{2}\) = 53,000, the lift coefficient increases linearly with small angle of attack. At \(Re_{2}\) = 53,000, the stalling angle of NACA0012 is 12\({}^{\circ}\), while that of SD7037 is 15\({}^{\circ}\). This shows that the SD7037 airfoil  
Figure 10: Aerodynamic characteristics of SD7037 at different Reynolds numbers and turbulence intensities. (**a**) \(C_{L}\)_vs \(\alpha\)_; (**b**) \(C_{D}\)_vs \(\alpha\)_.  
Figure 9: Aerodynamic characteristics of NACA0012 at different Reynolds numbers and turbulence intensities. (**a**) \(C_{L}\)_vs \(\alpha\)_; (**b**) \(C_{D}\)_vs \(\alpha\)_.  
has better lift characteristics. At \(Tu=0.6\%\), the lift coefficient is evidently nonlinear and the stalling angle is smaller for both of the airfoils. At \(Re_{1}=26\),500, the stalling angle of both airfoils decreases to only \(8^{\circ}\), and there is a sharp increase of drag around \(\alpha=10^{\circ}\). Figure 11 also indicates that the pressure drag of the airfoil suddenly increases at \(\alpha=10^{\circ}\), which may be the result of the development of the flow separation with the increase of the angle of attack. At \(Re_{3}=212\),000, the lift coefficient of both airfoils increases further, and the stalling phenomenon delays, which is more evident at \(Tu=0.6\%\). Notably, at \(\alpha=2^{\circ}\) and \(\alpha=5^{\circ}\), the lift coefficient of the airfoil at \(Tu=0.6\%\) is bigger than that at \(Tu=6\%\), which might have something to do with the generation of the LSB. In the aspect of drag, Figure 11a (in which \(C_{D,p}\) represents the coefficient of pressure drag, \(C_{D,f}\) the coefficient of friction drag) indicates that the NACA0012 airfoil has a small pressure drag at small angle of attack. With the angle of attack increasing to \(15^{\circ}\), the airfoil stalls and the pressure drag increases sharply at \(Tu=0.6\%\). Besides, in comparison with a high level of turbulence intensity, the frictional drag is smaller at \(Tu=0.6\%\). The variation of drag characteristics of the SD7037 airfoil is similar to NACA0012 (Figure 11b). Generally, as for a specific Reynolds number, the increasing freestream turbulence intensity results in a minor decrease in lift at small angle of attack, whereas at pre-stall angle of attack and higher turbulence intensity levels, lift increases and stall is delayed, as stated in [22].