On the vertical structure of wind gusts

The increasing size of wind turbines, their height and the area swept by their blades have revised the need for understanding the vertical structure of wind gusts. Information is needed for the whole profile. In this study, we analyzed turbulence measurements from a 100m high meteorological mast at the Danish National Test Station for Large Wind Turbines at Høvsøre in Denmark.

The site represents flat, homogeneous grassland with an average gust factor of 1.4 at 10m and 1.2 at 100m level. In a typical surface-layer gust parametrization, the gust factor is composed of two components, the peak factor and the turbulence intensity, of which the turbulence intensity was found to dominate over the peak factor in determining the effects of stability and height above the surface on the gust factor.

The peak factor only explained 15% or less of the vertical decrease of the gust factor, but determined the effect of gust duration on the gust factor. The statistical method to estimate the peak factor did not reproduce the observed vertical decrease in near-neutral and stable conditions and near-constant situation in unstable conditions.

Despite this inconsistency, the theoretical method provides estimates for the peak factor when comparing gust durations of 1 and 3 s with averaging period lengths of 10min and 1 h. A new technique to study the timing of maxima at different levels relative to the maximum gust at some level was developed.

Results showed that a 10m level maximum gust was typically preceded by maxima at higher levels and vice versa: a 100m gust was usually followed by a maximum at lower levels.