Wind fields during the evening transition period and their influence on wind turbine loads

The late afternoon hours in the diurnal cycle precede the development of the nocturnal stable boundary layer. This so-called evening transition (ET) period is often when energy demands peak. To capture physical characteristics of the wind fields in the atmospheric boundary layer (ABL)-particularly the interplay of shear and turbulence-during this evening transition period, stochastic simulation approaches though inadequate are commonly used to assess wind turbine loads because they are more tractable. Large-eddy simulation (LES), on the other hand, may be used to generate high-resolution ABL turbulent flow fields. We present a suite of idealized LES 4-dimensional flow fields that define a database representing different combinations of large-scale atmospheric conditions (characterized by, for instance, geostrophic wind) and surface boundary conditions (characterized by surface heat flux). Our ultimate objective is to evaluate the performance of wind turbines during the ET period. Accordingly, we conduct a statistical analysis of turbine-scale variables (including hub-height mean wind speed, standard deviation of wind speed in the streamwise direction, and wind speed shear) and discuss patterns of inter-relationships among them. Surface heat fluxes are seen to greatly influence wind shear. Finally, we apply the database of LES-based inflow wind fields in aeroelastic simulation of a 5-MW wind turbine model. We discuss how turbine loads change as the wind field evolves during the evening transition. We also discuss extreme and fatigue loads on the rotor and tower that result from different ABL conditions.