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Stably stratified turbulent flows over an unbounded, smooth, planar sloping surface at high Grashof numbers are examined using direct numerical simulations (DNS). Four sloping angles (α = 15°; 30°; 60° and 90°) and three Grashof numbers (Gr = 5 × 1010; 1 × 1011 and 2:1 × 1011) are considered. Variations in mean flow, second-order statistics and budgets of mean-(MKE) and turbulent-kinetic energy (TKE) are evaluated as a function of α and Gr at fixed molecular Prandtl number .Pr = 1/. Dynamic and energy identities are highlighted, which diagnose the convergence of the averaging operation applied to the DNS results. Turbulent anabatic (upward moving warm fluid along the slope) and katabatic (downward moving cold fluid along the slope) regimes are identical for the vertical wall set-up (up to the sign of the along-slope velocity), but undergo a different transition in the mechanisms sustaining turbulence as the sloping angle decreases, resulting in stark differences at low α. In addition, budget equations show how MKE is fed into the system through the imposed surface buoyancy, and turbulent fluctuations redistribute it from the low-level jet (LLJ) nose towards the boundary and outer flow regions. Analysis of the TKE budget equation suggests a subdivision of the boundary layer of anabatic and katabatic flows into four distinct thermodynamical regions: (i) an outer layer, corresponding approximately to the return flow region, where turbulent transport is the main source of TKE and balances dissipation; (ii) an intermediate layer, bounded below by the LLJ and capped above by the outer layer, where the sum of shear and buoyant production overcomes dissipation, and where turbulent and pressure transport terms are a sink of TKE; (iii) a buffer layer, located at 5 ≲ z+≲ 30, where TKE is provided by turbulent and pressure transport terms, to balance viscous diffusion and dissipation; and (iv) a laminar sublayer, corresponding to z+ ≲ 5, where the influence of viscosity is significant. (·)+ denotes a quantity rescaled in inner units. Interestingly, a zone of global backscatter (energy transfer from the turbulent eddies to the mean flow) is consistently found in a thin layer below the LLJ in both anabatic and katabatic regimes. [ABSTRACT FROM AUTHOR]

Most large-eddy simulation studies related to wind energy have been carried out either by using a fixed pressure gradient to ensure that mean wind direction is perpendicular to the wind turbine rotor disk or by forcing the flow with a geostrophic wind and timely readjusting the turbines' orientation. This has not allowed for the study of wind farm characteristics with a time-varying wind vector. In this paper, a new time-adaptive wind turbine model for the large-eddy simulation framework is introduced. The new algorithm enables the wind turbines to dynamically realign with the incoming wind vector and self-adjust the yaw orientation with the incoming wind vector similar to real wind turbines. The performance of the new model is tested first with a neutrally stratified atmospheric flow forced with a time-varying geostrophic wind vector. A posteriori, the new model is used to further explore the interaction between a synthetic time-changing thermal atmospheric boundary layer and an embedded wind farm. Results show that there is significant potential power to be harvested during the unstable time periods at the cost of designing wind turbines capable of adapting to the enhanced variance of these periods. Stable periods provide less power but are more constant over time with an enhanced lateral shear induced by an increased change in wind direction with height. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Here we present unique evidence of a k-1 scaling behaviour in the atmospheric boundary layer and its connection to large scale coherent structures within the boundary layer. Wind lidar measurements were conducted above a lake under cold atmospheric conditions. The large coherent structures could be visually observed over Lake Geneva in Switzerland when cold air met the relatively warm water. Proper orthogonal decomposition of the experimental data acquired with the wind lidar clearly reveals coherent oscillations of both the fluctuating velocity field and the water aerosol field over the surface of the lake. Precise identification of the large coherent structures propagating in the flow allows for detailed analysis of their contribution to the total spectral budget. Additionally, it is shown that the experimental data agree well with recent theoretical predictions. [ABSTRACT FROM AUTHOR]