Influence of Atmospheric Stability on Wind Turbine Wakes

• Published in: Wind engineering 1994-01, Vol.18 (3), p.139-152
• Main Authors: Magnusson, Mikael, Smedman, Ann-Sofi
• Format: Article
• Language: English
• Subjects: Atmospherics; Meteorology; Turbines; Turbulence; Turbulent wakes; Velocity; Wind farms; Wind power; Wind turbines; Wind velocity
• ISSN: 0309-524X; 2048-402X

Data from a small wind farm on the island of Gotland in the Baltic Sea have been used to investigate the structure of wakes behind the turbines. Turbulence and wind speed were measured at several heights on two towers in the area. The turbines were placed in such configuration that the turbine wakes could be studied at 4.2 D, 6.1 D and 9.6 D (D = rotor diameter) during different atmospheric stability conditions. Both the 'velocity deficit' and the 'added turbulence' in the wake have been studied as functions of distance from the turbines and for different atmospheric conditions. The 'velocity deficit' decreases with increasing distance from the turbine and the centre line of the wake is somewhat lifted above hub height. The 'velocity deficit' is also a function of the atmospheric stratification with larger values in stable air. For stable stratification contour plots also show that the velocity deficit is tilted to the right with height. The increased turbulence in the wake has been expressed as 'added turbulence' At the distance 4.2 D two separated maxima at the blade tips are clearly seen both for unstable and stable stratification. The two maxima disappear more rapidly in unstable conditions than in stable because of more efficient mixing in the wake. At the largest distance, 9.6 D, both curves (stable and unstable) show aflat maximum in the centre.