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. The variation in wind speed with height is typically represented by a power law relating wind speed to height, as in wind turbine design guidelines IEC 61400-1, and is parameterised by the exponent of this power law. Wind shear that is adequately described by a power law may be considered simple. The use of a single wind shear exponent originates in historic wind measurement practice where data were acquired at a small number of heights and so a simple model was all that was possible to extrapolate these measured conditions to hub height and across the full rotor disc. However, richer datasets with measurements at many heights, such as those acquired with floating and scanning lidars, allow complex maritime wind shear phenomena such as low level jets (LLJs) to be studied. These complex shear phenomena have implications for offshore wind turbine fatigue loading and productivity. The simple power law currently used cannot describe complex shear phenomena. This presentation discusses an extension of the power law that enables characterisation of complex shear such as LLJs.  The wind shear exponent of the simple power law can be considered the slope of a log-log plot relating wind speed to height. An extended power law with more terms and additional exponents for different orders of term may be derived from a polynomial fit to the log-log plot. This is possible due to the availability of data for more heights (e.g., 10+ heights). It is found that including terms up to the third order allows a good fit to observed LLJ and other complex shear phenomena. An exponent for each order of term is derived, and the complex profile described by multiple exponents up to the third order, rather than a single exponent. The simple power law exponent is the first order exponent if all other orders of exponent are set to zero, and so the familiar simple power law is recovered as a special case. This presentation discusses the extended power law for wind shear and demonstrates it by fitting it to floating lidar measurements in which LLJs are seen to occur. Key parameters that emerge include deriving the intrinsic height scale of structures inherent in the complex wind shear profiles from the point of inflection of the 3rd order polynomial fit in the log-log plot. The height of an LLJ can be determined from the lower turning point of the polynomial. The evolution of wind shear profiles may be observed by plotting different orders of exponent and parameters such as the height scale against each other and how this varies over time. A real-world case study is provided by analysing wind shear observed in floating lidar data. This demonstrates the utility of the techniques described above for parameterising and tracking the evolution of complex wind shear phenomena.