Design and Optimisation of a Low Reynolds Number Airfoil for Small Horizontal Axis Wind Turbines

Horizontal axis wind turbines (HAWTs) are scaled down to incorporate rotor blades that usually have a diameter ranging from two to four meters in length. A common misconception with regard to selection of airfoils followed by subsequent designing of rotor blades involves the use of NACA airfoils and other conventional high Reynolds number airfoils. Micro horizontal axis wind turbines usually operate at low Reynolds number conditions along the blade length. Conventional aerodynamic schemes cannot be applied to rotor blades operating under low Reynolds number conditions as compared to those occurring under high Reynolds number conditions since certain unusual aerodynamic phenomena predominate in the case of the former. The efficiency of a wind turbine is largely dependent on blade optimization, which is why airfoil selection of the rotor blade is of considerable importance. The difference between high and low Reynolds number operation is the onset of boundary layer transition. In the case of high Reynolds number operation, as is the case of aircraft propellers and other high speed turbines, boundary layer transition takes place before laminar separation, which is in direct contrast to low Reynolds number boundary layer phenomena, wherein laminar separation takes place before boundary layer transition. (paper)