Www.Windfair.net - Closer look at Technology - Mitigating the impacts of uncertainties in wind flows

It's no surprise that changing wind flows significantly affect wind-turbine performance and lead to substantial uncertainties when assessing wind resources. That makes it important to understand the flow field over space and time. Techniques pioneered by a research group in a recently published paper can offer project engineers and developers with an understanding that allows addressing and mitigating the impacts of uncertainties in wind flows.

The paper, published in the Journal of Wind Engineering and Industrial Aerodynamics, was written by Claude Abiven, Senior Technical Manager of Natural Power France; Oisin Brady, Director of Natural Power France; and Dr. Jose Palma from the University of Porto. In the paper, High-Frequency Field Measurements and Time Dependant Computational Modeling for Wind Turbine Siting, spectral analyses of the simulations show a model accurately reproducing frequencies recorded at the mast. These frequencies were then linked to a physical phenomenon using a technique (Empirical Orthogonal Functions) not often used in the wind-power community. The method allows characterizing complex flow structures such as zones of recirculation based on a single-mast measurement.

The work was based on high-frequency wind data from a complex site on the west coast of Scotland. Spectral analyses carried out on this dataset and showed the existence of complex-flow phenomena including turbulence, vortex shedding, and direction veer that changed rapidly over short time scales. CFD (Computational Fluid Dynamics) modeling was carried out using the Ventos CFD wind-flow model in time-dependent mode, a feature the software developer says is not readily available in other CFD packages. CFD modeling showed the complex and transient flows the result of complex terrain. The software accurately mapped the flows across the site, away from the measurement location.

To give a sample of detail possible from the software, consider the situation the authors presented. A steady-state solution for northerly winds over the site contained two large vortices downwind of hill B in the accompanying four-panel image. The vorticies are likely to detach and be advected by the flow towards the mast. A frame-by-frame analysis in the accompanying image is of a 7,000 second, time-dependent calculation. It reveals that the north-east (top-right) vortex spreads south before a new and smaller vortex originates east of the site, with a lifetime of 500 to 700 sec. What remains of the original north-east vortex slowly regains strength before the same process starts again. A complete cycle takes about 1,000 sec.

Another steady-state formulation of atmospheric calculations suggests converge to one of the many possible solutions delivered by a time-dependent formulation (One is presented in the four-panel image) and therefore provides a largely limited view of the wind flow. The four-panel sequence is a situation of separated flow, a real-life 3D case in which the onset of separation cannot be determined with simple rule-of-thumb guidance, derived from linear models of wind flow over 2D hills.

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