Posters

Abstract

Wind shear is a key parameter when characterising wind conditions to determine if they are within the envelope of operational conditions for which a wind turbine specification is valid. Variation of wind speed with height is typically represented by a power law as in wind turbine design guidelines IEC 61400-1, parameterised by the power law exponent α. Shear that is adequately described α may be considered simple. The use of a single exponent originates in historic measurement practice where data were acquired at a small number of heights and so a simple model was all that was possible to extrapolate measured conditions to hub height. Richer datasets with measurements at many heights acquired with lidars allow complex shear phenomena such as low-level jets (LLJs) to be studied. This presentation discusses an extension of the power law that enables characterisation of complex shear such as, e.g., LLJs and intermediate boundary layers (IBLs). The extension achieves this with the addition of a single parameter, i.e., the extension is not a mathematical exercise that characterises data in a trivial manner by fitting a large number of parameters. The simple power requires the 1st order wind shear exponent α, hub height z0 and wind speed at hub height v0. Although the extended power law uses terms to the 3rd order, it is found that, when the reference height z0 is determined by the inherent structure of the shear profile, the extended power law requires only the additional 3rd order parameter, as the 0th and 2nd order terms are eliminated. A plot of the 3rd order against 1st order coefficients then shows the diurnal, seasonal and directional evolution of the wind shear profile due to variations in, e.g., atmospheric stability, as a cyclical process.