In numerical simulations, wind boundary layers can be simulated similarly to a wind tunnel using long fetches with ground roughness. However, to reduce computational effort, it is more efficient to define a suitable profile directly in the input. It is important to note that the prescribed distribution of velocities and turbulence parameters may change before reaching the actual area of interest. Most turbulence models can only depict isotropic apparent viscosity. For the k- ε model, and similarly for the k-ω model, an analytical solution exists that can represent the desired stable flow with only two parameters, provided the boundary conditions at the upper boundary and ground level (z0) are properly formulated. However, a more accurate approach would be to avoid defining an inflow in the lowest region up to z0 and to shift the logarithmic profile by the offset d0.
For RANS calculations, steady-state boundary conditions are usually applied at the inflow. LES simulations, which often require a time-dependent inflow, have various approaches available. One option is a separate simulation of the boundary layer over a long roughness field. Alternatively, periodic boundary conditions with a shortened fetch can be used.
A more efficient approach is the generation of synthetic, turbulent wind inlet conditions. Here, a mean flow profile is mathematically superimposed with temporal fluctuations to create a realistic boundary layer profile. Various methods exist, including generating random fluctuations based on given turbulence intensities or using measurement data.
Particularly effective methods utilize statistical properties of the turbulent flow, such as spectra, correlation functions, or length and time scales, to generate three-dimensional, unsteady inflow. It should be noted that the representation of very large and very small vortex structures may be limited by the extent of the study domain or the filtering used in LES.
It is crucial to consider that a predefined wind profile changes within the study area. This must be taken into account in defining the wall function on the ground, ensuring its applicability and reasonableness are verified.