On the vertical structure of wind gusts

Abstract

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 100 m 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 10 m and 1.2 at 100 m 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 10 min 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 10 m level maximum gust was typically preceded by maxima at higher levels and vice versa: a 100 m gust was usually followed by a maximum at lower levels.

Weibull Wind-Speed Distribution Parameters Derived from a Combination of Wind-Lidar and Tall-Mast Measurements Over Land, Coastal and Marine Sites

“…The increase in wind-turbine height and the increase in area swept by the blades harvesting energy from airflow in the lower atmosphere have raised a need for better understanding of the structure of the vertical profile of the horizontal wind (Gryning et al 2007), gusts (Suomi et al 2015), and the monthly to annual long-term statistical distribution of boundary-layer winds ). Due to the strong non-linear relationship between wind speed and wind power, not only is an accurate description of the mean wind profile essential, but equally important is the description of its long-term variability for assessments of the wind-energy potential.…”

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“…The theoretical peak factor g t g ,theory can be derived from statistical considerations (Rice, 1944(Rice, , 1945Beljaars, 1987;Kristensen et al, 1991;Wichers Schreur and Geertsema, 2008;Suomi et al, 2015). The theoretical peak factor equation is g t g ,theory = 2 ln T τ…”

“…A thorough description of the site and its instrumentation is provided by Peña et al (2015). The long-term wind-gust conditions at this site have been investigated by Suomi et al (2015). Here, the study period covers 2 days, 10 and 11 October 2015, during which there were easterly winds of 4-10 m s −1 at the 100 m level and the surface layer stability underwent a clear diurnal cycle: unstable conditions during the day and stable conditions at night.…”

“…Brasseur, 2001;Woetmann Nielsen and Petersen, 2001;Wichers Schreur and Geertsema, 2008). However, the measured gustiness at the reference height is not always representative of the surrounding area because of the spatial variation in the aerodynamic roughness of the surrounding environment, which can have a significant impact on wind gusts (Wieringa, 1973;Suomi et al, 2013Suomi et al, , 2015. Hence, a fair comparison of model gust forecasts and observations is challenging, because the roughness in the model grid cell may differ from the conditions at the measurement site (e.g.…”

“…It typically takes a second or more to measure each line of sight with sufficient sensitivity and therefore the temporal resolution of the wind measurement is often of the order of tens of seconds, which is not sufficient for gusts (e.g. Suomi et al, 2015). However, Doppler lidars can also provide high-resolution turbulent measurements, in both the vertical direction (O'Connor et al, 2010) and, potentially, the horizontal direction (Vakkari et al, 2015).…”