Horizontal Axis Wind Turbine
In the rest of the chapter, the attention will be devoted to horizontal axis wind turbines (HAWTs) which represent, today, the main type of wind power machines that are developed and installed in all parts of the world. Much efforts have been done in understanding better the flow past HAWT both experimentally and analytically. A major wind tunnel campaign has been carried out by the National Renewable Energy Laboratory (NREL) at the NASA Ames Research Center large 80′ x 120′ wind tunnel facility . They used a two-bladed rotor of radius R = 5 m with blades equipped with the S809 profile. A picture of the wind turbine in the wind tunnel is shown in Fig. 10.5. Note in particular the well defined evolution of the tip vortex visualized with smoke emitted from the tip of one blade. One can count seven or eight turns with regular spacing, despite the dramatic distortion due to the encounter with the tower when the blade tip vortex passes in front of it.