1 Meteorology, Department of Wind Energy, Technical University of Denmark2 Department of Wind Energy, Technical University of Denmark
The largest wind turbines today often reach heights where traditional models of the wind speed and how it varies with height no longer can be expected to apply. For accurate assessment of wind energy resources and loads on wind turbines, there is a need for better understanding of the flow of air above the atmospheric surface layer. Continuous and detailed measurements of mean winds and turbulence above the surface layer are expensive and difficult to obtain. Computational fluid dynamics modelling of the atmospheric flow can be an attractive alternative or supplement to field experiments. In this study, the method of large-eddy simulation (LES) is applied to gain improved insight on the flow in the atmospheric boundary layer (ABL). The primary motivation behind the study has been to facilitate improvement of analytical wind profile models valid above the surface layer, however, the prospect of using LES more directly in applications such as short-term forecasting of the turbulent flow at e.g. wind farm sites is also considered. Two case studies based on measurements from the rural site of Høvsøre, Denmark and a suburban site in Hamburg, Germany demonstrate the need for accurate specification of the large-scale pressure forcing, when using LES for prediction of real-world wind profiles. In the Høvsøre case study, simulated wind speeds agree well with measurements throughout the ABL, but only when the applied forcing follows a height- and time-dependent pressure gradient estimated from continuous LIDAR measurements of the wind speed above the ABL. Including unsteadiness and baroclinic effects in the forcing also improves agreement with measurements in the Hamburg case study, but not as unambiguously as in the Høvsøre case study. It is concluded that the measurements available at and around the site in Hamburg are insufficient for accurate estimation of the driving pressure gradient, and that phenomena such as large-scale subsidence and advection also should be included in the LES for accurate wind profile prediction. A range of simulations of more idealized conditions are performed to study the influence of the free atmosphere Brunt Vaisala frequency and baroclinicity on the development and steady-state structure of neutral and near-neutral ABLs. It is found that an adjustment time of at least 16 hours is needed for the simulated flow to reach a quasi-steady state. The highly idealized conditions facilitate the formation of a super-geostrophic jet near the top of the ABL. It is considered to be a rare phenomena in the real-world ABL, and is not accounted for by the analytical models of the wind shear included in this study. It is furthermore shown that the considered wind profile model can be improved by appropriately accounting for the wind shear due to the free atmosphere Brunt Vaisala frequency and baroclinicity.
DTU Wind Energy PhD-0032; DTU-Wind-Energy-PhD-0032; DTU Wind Energy PhD-32