Reynolds Number Extrapolation on High Thickness-Ratio Airfoil for Megawatt-Class Wind Turbine

A Reynolds number extrapolation method of determining aerodynamic loads acquired from a wind tunnel test on high thickness-ratio airfoils usually adopted in a megawatt-class wind turbine blade was investigated to obtain equivalent results to those of the field condition. For this purpose, the tendency of drag variation was identified depending on the Reynolds number and the thickness ratio based on the result from the wind tunnel test on airfoils. The linear-logarithmical relationship between the Reynolds number and the drag coefficient for the high thickness-ratio airfoil was then obtained. The present method was validated by a wind tunnel test for the 21% and the 30% thickness-ratio airfoils designed for a 5–10 megawatt-class wind turbine. The test was conducted within the chord-based Reynolds number ranging from 1.3 × 10⁶ to 2.6 × 10⁶, and extrapolated to the designed Reynolds number of 1.0 × 10⁷. The extrapolated aerodynamic loads were compared with the calculated results using XFOIL. It was shown that the wind tunnel test at low Reynolds numbers underestimates the loads compared to that at the designed Reynolds number, and that the XFOIL gives over-estimated results. However, the present method provides aerodynamic loads at the designed condition of the airfoil that are more realistic.