Wind turbine loads during the evening transition period

The evening transition represents an interesting period lasting a few hours before the nocturnal stable boundary layer develops. It is also often a period of high energy demand. Large-eddy simulation is an efficient technique for the simulation of turbulent inflow fields that can be used in the computation of wind turbine loads. Unlike traditional methods (e.g., stochastic simulation), LES can be used to generate high-resolution turbulent flow fields in the ABL (atmospheric boundary layer) and to capture variations in key parameters related to wind turbulence and shear during the evening transition period. We conduct a number of idealized LES simulations to establish a database of four-dimensional flow fields with different combinations of atmospheric conditions (e.g., geostrophic wind) and surface boundary conditions (e.g., surface heat flux rate). A statistical analysis of turbine- scale variables (e.g., hub-height mean wind speed and standard deviation of wind speed in longitudinal direction, wind speed shear, etc.) reveals the high interrelations with each other. Further investigation shows that the surface heat flux rate plays an important role in the statistics of wind shear. Finally, we apply the database of inflow fields to the estimation of loads on a 5-MW wind turbine model. In particular, we study the variation in turbine load statistics during the evolution of the flow fields during the evening transition.