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Danish Center for Computational Wind Turbine Aerodynamics and Atmospheric Turbulence

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Title : Statistics of local extremes.
Authors : Larsen, G.C.; Gierbooms, W.; Hansen, K.S.,

Summary :
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 difference may cause substantial differences in the load patterns of a wind turbine when a gust event is imposed. The Newgust method is a rational way of taking wind speed gust situations into account in a design process. A corner stone in the method is a cogent algorithm to embed a wind speed gust of a prescribed appearance in a stochastic wind field [Dragt, 1996]. However, dealing with wind turbine design, not only detailed knowledge on the spatial/time structure of the gust event is required. The probability of occurrence of a gust event with a given wind speed amplitude/magnitude is equally important. This theme is addressed in the present report. A theoretical expression for the probability density function associated with local extremes of a stochastic process is presented. The expression is basically based on the lower four statistical moments and a bandwidth parameter. The theoretical expression is subsequently verified by comparison with simulated synthetic wave/wind fields as well as with measured wind fields covering a broad range of mean wind speed situations and terrain conditions. The simulated wave fields represents narrow band processes, whereas the wind fields represent broad-banded processes. 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. Full text version is available from Ris° Library as a pdf-document.

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