# Aerodynamic Sensitivity Analysis for a Wind Turbine Airfoil in an Air-Particle Two-Phase Flow
## 5 Particle Effects on Other Two Airfoils  
For a further verification, the NACA0012 airfoil and the NH6 MW25 airfoil of a 6 MW wind turbine blade are introduced. The NH6 MW25 airfoil sketched in Figure 13 has a thickness of 25%.  
Figure 11: Particle concentration effects on moving distance of separation point on S809 airfoil at \(Re=1\times 10^{6}\).  
Figure 12: Particle concentration effects on lift-to-drag ratio of S809 airfoil at \(Re=1\times 10^{6}\).  
Figure 14 compares the lift coefficient curves at \(Re=1\times 10^{6}\) between the NACA0012 airfoil and two wind turbine airfoils of S809 and NH6 MW25. Different from the NACA0012 airfoil, the flow over a wind turbine airfoil separates at a smaller angle of attack and its typical lift curve consists of the linear, light stall and deep stall sections. As can be seen from Figure 14, for the S809 airfoil at \(Re=1\times 10^{6}\), the most sensitive angle of attack of \(9.1^{\circ}\) is just located in the light stall section.  
The air-particle flows over the NH6 MW25 and NACA0012 airfoils are simulated at \(m=1\%\) and \(Re=1\times 10^{6}\). Figures 15 and 16 show the lift and drag coefficient increments for the NH6 MW25 and NACA0012 airfoils, respectively. For the NH6 MW25 airfoil, the variation trends of aerodynamic coefficient increments with the angle of attack are similar to those of the S809 airfoil displayed in Figure 6, and the most sensitive angles of attack for lift and drag equal \(10^{\circ}\) and \(9^{\circ}\), respectively, which also locates in the light stall section (see Figure 14). However, for the NACA0012 airfoil, there is not a most sensitive angle of attack and the particle influences increase with increasing angle of attack due to the lack of light stall.  
Figure 17 compares the location and moving distance of separation point between three airfoils. Likewise, due to the light stall phenomenon, a maximum moving distance can only be observed for the S809 and NH6 MW25 airfoils. In Figures 15 and 17b, there is still an angle difference between the maximum increments of aerodynamic coefficients and the maximum moving distance of separation point for the NH6 MW25 airfoil. Figure 18 displays the pressure distributions over the S809 and NH6 MW25 airfoils at \(Re=1\times 10^{6}\) and \(m=0\%\). Due to the difference in the position of the highest point on the upper airfoil surface as well as the curvature difference between these two airfoils (see Figure 13), the adverse pressure gradient in the vicinity of the highest point on the upper surface of the S809 airfoil in Figure 18 locates farther downstream and is stronger compared with the NH6 MW25 airfoil. So, in Figure 17a, the separation point on the S809 airfoil moves upstream much faster while the angle of attack approaches the angle for maximum lift, and the curves for these two airfoils intersect.  
Figure 16: Lift and drag coefficient increments of NACA0012 at \(m=1\%\) and \(Re=1\times 10^{6}\).  
Figure 17: Location and moving distance of separation point for three airfoils at \(Re=1\times 10^{6}\): (a) Location of separation point; (b) moving distance of separation point.