In the presentation, we review the use of Large Eddy Simulations (LES) to study turbulent boundary layers flowing over surfaces with and without arrays of wind turbines. Specifically, the talk will focus on the use of LES results to develop tools and concepts that are simple enough to be used in applied, engineering contexts. Two such applications will be described. First, we study the combination of wakes and boundary layer structure in wind farms. This leads to the formulation of the coupled wake-boundary layer model for wind farm design and optimization. Second, we focus on the spatio-temporal properties of boundary layers, and propose an analytically tractable model for the frequency wavenumber spectrum of streamwise velocity based on the random sweeping hypothesis. This work is done in collaborations with Richard Stevens, Michael Wilczek and Dennice Gayme and the main financial support is provided by the National Science Foundation (the WINDINSPIRE project).