Coherence of Turbulent Wind Under Neutral Wind Conditions at FINO1

Current wind turbine design standards allow for two different approaches to model the wind field used for engineering estimates; The Mann turbulence model and the Kaimal wind spectra combined with a coherence function. The point wind spectra generated, using the recommended parameters, are similar, but there are differences in the spatial distribution of the turbulence. Especially at larger separation distances the two models will have different coherence values. As the offshore wind turbines continue to increase in rotor size, it is getting more important to model the correct spatial distribution across the large rotor areas in order to calculate the fatigue damage. There are currently few measurements of the marine boundary layer, and one of the few offshore metrological masts is located at FINO1. In this study, the measurements of the wind velocities at three different heights at FINO1 have been used to study the vertical coherence. The study is limited to January 2008, and only neutral atmospheric conditions are considered. Based on the measurements, one can see that there are large differences between the measured values and the coherence estimated by the recommended values. However, the Mann turbulence model shows a trend that is more similar to the measured coherence values, than the coherence function used with the Kaimal spectra.