*3.2.1 S809 airfoil*

The S809 airfoil is a 21% thick profile, which specifically designed for horizontal-axis wind turbine applications. The S809 airfoil was tested in a low-turbulence wind tunnel (Tu = 0.2%) by Somers [34] at Re number of 2 million (based on chord length) and a Mach number of 0.15. Comparison of the numerical results by Langtry and Menter [11] γ-Reθ, Walters and Cokljat [14] k-kL-ω, and Medida [31] SA-γ-Reθ and B-C models [20] with the experimental data is given in **Figures 4**–**6**. In general, all transition models agree well with the experimental data until the stall angle. Although the lift and drag coefficients (**Figure 4**) are rather inaccurate after the stall angle, it is observed that the experimental measurements of the transition locations are quite successfully predicted by all models (**Figure 5**). Also, comparing the experimental and numerical pressure coefficient distributions on the S809 airfoil at 1̊ angle of attack, it is observed that the separation bubble is predicted quite well by all the models (**Figure 6**).
