Large eddy simulations of stratified atmospheric boundary layers and wind farms

Large-eddy simulations (LES) were used to study atmospheric boundary layers (ABLs) and controlled numerical experiments were performed to understand the effect of atmospheric thermal stability on wind farm performance. We used LES to study most commonly observed conventionally neutral boundary layers (CNBLs). CNBLs are generally observed off-shore and are important for meteorological and wind energy applications. Most theoretical models till now have focused on understanding wind speed in the so-called inner layer, which extends up to about 100-150 meters. In part I of the thesis, we propose a novel approach for predicting the wind speeds in the outer region of a CNBL by combining ABL theory with classical perturbation method. This will be useful in developing turbulence models for engineering flow solvers and developing analytical wind turbine wake models for better wind energy forecasts. In part 2 of this thesis, the flow physics and the effect of stable boundary layer on wind farm power production is analysed by systematically increasing the stability of the boundary layer. Particularly we studied cases in which an interesting lower atmospheric phenomenon called low-level jets (LLJs) is observed. LLJs are the low-level maximum in the wind velocity profile and have been found to be highly energetic. We studied three scenarios when the LLJ is above, in the middle, or below the turbine rotor swept area. Wind farm LES of the above three scenarios shows that the wake recovery and the power production of the downstream turbines are reduced due to the higher thermal stability associated with the LLJs. However, an interesting phenomenon is observed when the hub-height of the turbines are greater than the LLJ height. Instead of the downward entrainment flux which is normally observed in a boundary layer, we observed an upward entrainment flux, which facilitates wake recovery. Therefore, in terms of wake recovery it is advisable to install turbines with heights slightly above the LLJ height. Otherwise, the wake recovery of the downstream turbines is severely affected increasing the so-called ‘wake losses’.
In this thesis, we focused mainly on cases in which the turbines the turbines are situated in simple setup such as horizontally homogeneous terrain. Future studies should focus on the interaction between atmospheric stability, complex terrain etc. for designing next generation wind farms.