## 5 Discussion  
Through the comparison of the aerodynamic performance of different airfoils with different turbulence intensities and Reynolds numbers, it is found that the flow field around the airfoil is very unstable under low Reynolds number conditions, and the flow phenomenon on the suction side of the airfoil is abundant. The effects of turbulence intensity mainly appeared to be that with the increase of  
Figure 19: The chordwise friction drag coefficient of airfoil at \(\alpha=8^{\circ}\) under the Reynolds number of (a) \(Re_{1}=26\),500; (b) \(Re_{2}=53\),000; (c) \(Re_{3}=212\),000. Figure 20: Airfoil aerodynamic performance at \(\alpha=5^{\circ}\), \(Re_{2}=53\),000. (a) Pressure distribution; (b) Coefficient of chordwise friction drag; (c) Effective shape of NACA0012 at \(Tu=0.6\%\). turbulence intensity, the aerodynamic performance of airfoil becomes more stable, the lift improves, the stalling angle increases, and the occurrence of an LSB or flow separation can be suppressed to a certain extent. The numerical results show that under low Reynolds number conditions, the level of the turbulence intensity determines the flow characteristics (The external factors such as surface roughness and noise are not considered in this paper). In the fully developed turbulent boundary layer, most of the velocity fluctuation originate from the average shear flow, which is reflected in the production term of the turbulent kinetic energy equation. If the freestream turbulence is strong, the higher velocity fluctuations outside the boundary layer diffuse into the boundary layer near the airfoil surface. The intensity of this diffusion effect directly determines the occurrence of the transition. If the freestream turbulence is weak, the occurrence of transition is hardly affected by external disturbances, but basically caused by the instability of its inner flow so that external disturbances are only as a trigger. [PERSON] [39] pointed out that when the freestream turbulence intensity is higher than 3%, the effect of pressure gradient on the transition can be ignored. Transition is mainly affected by external disturbances. When the turbulence intensity is less than 1%, the pressure gradient and shear flow dominate the stability of flow. And the occurrence of transition at turbulence intensity of 1%-3% is affected by both external and inner factors such as freestream turbulence intensity and pressure gradient. The increase of the Reynolds number also causes the transition to move towards the leading edge, so that the lift and stalling angle increase, but the mechanism of the earlier transition is different from that of turbulence intensity. The increase of turbulence intensity leads to an earlier transition because of the higher velocity fluctuations that penetrate within the boundary layer. However, the increase of the Reynolds number alters the state of the boundary layer at separation reducing the shear layer thickness and consequently its stability characteristics. [PERSON] [28] also concluded that the Reynolds number significantly influences the time-mean boundary layer structure at separation. This translates into higher growth rates and different dynamic properties of the shedding phenomenon. In addition, the evolution of the LSB structure on the suction side of the airfoil is closely related to the Reynolds number. Taking the medium angle of attack (\(\alpha=8^{\circ}\)) of the NACA0012 airfoil at \(Tu=0.6\%\) as an example, at \(Re_{1}=26\),500, an eddy similar to a long laminar separation bubble emerges on the upper surface of the airfoil, but fails to reattach to the wall. At \(Re_{2}=53\),000, a small short laminar separation bubble is generated near the leading edge. At \(Re_{3}=212\),000, an LSB occurs on the upper surface of the airfoil near the trailing edge. These phenomena show that the generation of separation bubbles is closely related to the Reynolds number if the Reynolds number is too small, the laminar flow is not able to reattach to the wall after the transition, and it is difficult to form an LSB.