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Danish Center for Computational Wind Turbine Aerodynamics and Atmospheric Turbulence

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Title : SPATIAL COHERENS OF THE LONGITUDINAL TURBULENCE COMPONENT
Authors : Gunner Chr. Larsen and Kurt S. Hansen

Summary :
The design fatigue loading of wind turbine structures is traditionally determined from aeroelastic simulations, where a structural dynamic model is exposed to suitable synthetic turbulence wind fields. The wind fields are usually generated according to the recommendations given in the IEC 61400-1 standard, in which the turbulence is described combining the Kaimal spectrum with an exponential coherence model or, alternatively, by the Mann uniform shear spectral tensor. Recent investigations have shown that significant differences in simulated wind turbine tower fatigue loading may occur using the two suggested turbulence models. The differences in load predictions have been identified mainly to differences in the descriptions of the spatial coherence in the two turbulence models. In order to calibrate the coherence descriptions given in the IEC 61400-1 standard, a substantial amount of full scale wind field data, extracted from "Database on Wind Characteristics" (http://www.winddata.com/), have been analysed. The analysis is restricted to spatial coherence of the longitudinal turbulence component, but both vertical and horizontal spatial separations have been investigated. Terrain- and atmospheric stability classifications have been merged into a classification according to the turbulence integral length scale. The measured coherences are thus parameterised in terms of turbulence integral length scale, measuring heights, (vertical/horizontal) spatial spacing, frequency and reduced frequency. Finally, a revised expression for the spatial coherence, associated with vertical separation, is compared with the measured coherences as well as with coherence predictions from the IEC 61400-1 standard as well as from the Mann model, and recommendations arising from this comparison are given.

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