The aerodynamic characteristics of wind turbines are strongly influenced by the chosen airfoil. In addition, for wind turbines with rotor diameters in the order of 10m or less low-Reynolds number effects associated with laminar separation can become a concern. The ensuing deviation from the high Reynolds number airfoil characteristics is of interest to the designer and can often not be predicted accurately with airfoil design methods. While wind tunnel measurements in quiet tunnels can provide needed aerodynamic data they do not offer the detailed insight into the time-dependent flow physics that direct numerical simulations can provide. We employed direct numerical simulations for investigating the low-Reynolds number aerodynamics of 4 different airfoil sections. The S809 profile, which was designed for a Reynolds number of 2 million, is compared with the S822 and the Wortmann FX63-137 low-Reynolds number profiles and the symmetric NACA0025 airfoil. Simulations were carried out for chord based Reynolds numbers of 55,000 and Re = 100,000. The numerical results are compared with XFoil predictions and wind tunnel data. Based on instantaneous flow visualizations aspects of the transitional flow are discussed. Much more affordable two-dimensional calculations are shown to provide correct lift data only when the extent of flow separation is small.
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