On the Vertical Structure of Wind Gusts

Wind gusts, defined as short duration (typically 3 s) wind velocity maxima, are usually observed and modeled at the 10 m reference height. However, the recent increasing trend in wind turbine size has established needs for understanding the vertical structure of gust events in a deeper layer. In this study, we introduce a new technique that aims in evaluating the timing of gust events at different heights. The analyses are based on turbulence measurements from a meteorological mast at the Danish National Test Station for Large Wind Turbines located in Høvsøre in northwest Denmark. Analyses were made for 15 periods, each of several (2.5-10) hours duration, all representing strong wind conditions. The cases cover different stability conditions and include both stationary and increasing/decreasing wind speed conditions. Results show that a maximum gust at the 10 m level is typically preceded by a local maximum at higher levels, and there is on average a linear tilt in the time lag between the levels. Between the 10 and 100 m levels the lag is about 5-20 s, which corresponds to about 60 to 250 m horizontal distance.