Mean gust shapes

The gust events described in the IEC-standard are formulated as coherent gusts of an inherent deterministic character, whereas the gusts experienced in real situation are of a stochastic nature with a limited spatial extension. This conceptual differencemay cause substantial differences in the load patterns of a wind turbine when a gust event is imposed. Methods exist to embed a gust of a prescribed appearance in a stochastic wind field. The present report deals with a method to derive realistic gustshapes based only on a few stochastic features of the relevant turbulence field. The investigation is limited to investigation of the longitudinal turbulence component, and consequently no attention is paid to wind direction gusts. A theoreticalexpression, based on level crossing statistics, is proposed for the description of a mean wind speed gust shape. The description also allows for information on the spatial structure of the wind speed gust and relies only on conventional wind fieldparameters. The theoretical expression is verified by comparison with simulated wind fields as well as with measured wind fields covering a broad range of mean wind speed situations and terrain conditions. The work reported makes part of the project“Modelling of Extreme Gusts for Design Calculations ” (NEWGUST), which is co-funded through JOULEIII on contract no. JOR3-CT98-0239.