A Reynolds Averaged Navier-Stokes (RANS) Procedure for Predicting Skin Friction in Stable Shear Dominated Atmospheric Boundary Layers over Complex Terrain

Predicting the shear stress in the Atmospheric Boundary Layer over complex terrain using Computational Fluid Dynamics (CFD) has long been a delicate problem. Conventional Reynolds Averaged models can be of great use for producing shear stress figures over flat terrain, often falling within a known error margin when compared to empirical or experimental data. This known error margin can be attributed to mesh quality and numerical boundary conditions. The same models however, struggle to predict the Turbulent Kinetic Energy (TKE) of the wind when flowing over hills and slopes, which in turn influences the shear stress computation on top of the already existing error margin. Here, a framework to predict skin friction in boundary layers over complex terrain is developed using a RANS model. The topography of the authors’ home institution (the University of Cincinnati) is used as a case study due to its unique changes in terrain. The domain geometry was obtained through the CADMapper website. The SST k-ω RANS turbulence model is employed with a high-resolution mesh near the ground surface. A Law of the Wall velocity profile that was based off existing experimental measurements was used as the inlet velocity profile. The incoming flow was allowed to form into a fully developed turbulent boundary layer by letting it flow over a long flat plate until its skin friction values grew consistent with existing empirical correlations. The fully turbulent flow then entered the complex terrain region where measurements for the numerical shear stress were collected. Simulations were performed for both smooth and rough ground conditions. A collection of error percentages of numerically produced TKE results reported in previous, experimentally validated publications for flows over hills and other complex terrain was grouped together and used as reference for the results reported by this model. Final skin friction ranges are reported for different areas within the complex terrain. This procedure provides a streamlined method of predicting the range of shear stress and skin friction values over complex terrain while knowing the error margins for the reported results.