Fabio Pierella

Wind Turbines - wakes characterization

The conversion of wind energy to useful energy involves two processes: the primary process of extracting kinetic energy from wind and conversion to mechanical energy at the rotor axis, and the secondary process of the conversion into useful energy.

Wind turbine wakes have been a topic of research from the early start of the renewed interest in wind energy utilization in the late 1970s. From an outsider’s point of view, aerodynamics of wind turbines may seem quite simple. However, the description is complicated, by the fact that the inflow always is subject to stochastic wind fields and that, for machines that are not pitch-regulated, stall is an intrinsic part of the operational envelope. Indeed, in spite of the wind turbine being one of the oldest devices for exploiting the energy of the wind (after the sailing boat), some of the most basic aerodynamic mechanisms governing the power output are not yet fully understood. Several aspects that are unique for wind turbine aerodynamics:

• Although at present wind turbines are the biggest rotating machines on earth they operate in the lowest part of the earth boundary layer: this means that all the involved aerodynamic phenomena are essentially unsteady.
• Many wind turbines rely on stall for aerodynamic power limitation. Therefore a thorough understanding of unsteady (deep) stall is necessary.
• The flow in the blade tip and root region is three-dimensional: for example, due to centrifugal and Coriolis forces the flow in the boundary layer at the root is in spanwise direction, while the flow just outside the layer is chordwise. The relevance of two-dimensional data for wind turbine performance prediction is very limited.

Many times engineering rules-of-thumb have developed where a physical modeling and understanding of aerodynamics was not developed. A “Back to basics” approach, based on experiment in controlled conditions, is necessary to extend the applicability of these laws and to standardize the existing knowledge.

Purpose of this research is to perform wind tunnel experimental activity to work out a wide and sound data basis which will represent a reference for wind turbine wake models, with respect both to single turbine and wind farm wakes. A major focus will be held on the far wake, because it is believed to be more relevant for wind farm layout issues.