Characteristics of wind turbines

The accurate characterization of the near-surface wind speed profile (up to altitudes of about 200 m) is crucial for a variety of wind energy applications, including wind energy resource assessment and forecasting, and estimating wind shear across turbine blades from field data or real-time hub-height wind speed estimations upwind of a wind farm. The simpler models used by the industry lack some physics in comparison to the more advanced Computational fluid dynamics (CFD) models.

There are many cases where faster models can accomplish tasks that would simply be too time consuming with slower models (e.g. layout optimization, power forecasting). However, there is still much to learn from the physics of the Computational fluid dynamics (CFD) models in order to develop the faster engineering The wind turbine wake effect is a major issue during the calculation of life expectancy and power output of a wind farm.

During the designing phase, the wind farm developers want to have a excellent estimate of how to locate the wind turbines to achieve the most power and the least fatigue loads. In this procedure, the wind farm wake models can be used to optimize the wind farm layout supported by the different economical factors (e.g. wind farm lifetime, power production, type of soils, cables cost, and water depth). Accurate wind farm wake models may, therefore, augment the cost profits of the wind farm.

However, wind turbines generate power by removing kinetic energy from the wind. From this concept of extracting energy, it is clear that if kinetic energy is removed from the wind, the kinetic energy left downstream of a wind turbine is reduced compared to the kinetic energy upstream of the wind turbine. Background information on the characteristics of the wakes are essentialy to model the wind turbine wake in order to estimate how much energy is left for the downstream wind turbines and how much power the wind turbines can produce.

There is, in some patterns, the atmospheric turbulence scale, characterizing the motion of large atmospheric eddies, which increases linearly with height in the surface layer and can be up to several hundred meters. Then, there is the wake interface turbulence scale, characterizing the mixing of the wake boundaries with the atmospheric flow, which eventually evolves to be roughly of the same order of magnitude than the wind turbine rotor diameter. And finally, there is the turbulence length-scale of the blade induced vortex structures, which are related to

the dimension of the blade chord.

Because of these different scales, which are more or less important at different locations in the wake, a natural simplification is to speak about close wake and far wake regions. The close wake region (close to the rotor) is in the direct vicinity of the wind turbine, where the vortices (rotating flow structures) caused by the tips and roots of the blades are present. In this region, wakes are characterized by a high turbulence intensity and large velocity gradients.

The far wake (further downstream) region is located at a larger distance downstream, where the flow velocity profile has evened out more, and the effect of the rotor is only seen through more large-scale effects of velocity deficit, and increased turbulence intensity. While models have presented for these three forms of scales for about a century, little is known about the interaction between them.