CFD investigation of the atmospheric boundary layer under different thermal stability conditions

An accurate description of the atmospheric boundary layer (ABL) is a prerequisite for computational fluid dynamics (CFD) wind studies. This includes taking into account the thermal stability of the atmosphere, which can be stable, neutral or unstable, depending on the nature of the surface fluxes of momentum and heat. The surface fluxes and vertical profiles of wind velocity and temperature are described by Monin-Obukhov similarity theory. The implementation of this thermally stratified atmosphere into CFD has been examined in this study by using Reynolds-averaged Navier-Stokes (RANS) turbulence models, and the commercial code STAR-CCM+. The maintenance of the temperature, velocity and turbulence profiles along an extensive computational domain length was required, while simultaneously allowing for full variation in pressure and density through the ideal gas law. Effects of gravity and buoyancy were included, leading to the emergence of the hydrostatic pressure field and the resultant density changes expected in the real atmosphere. The CFD model was tested against measured data, from the literature, for the flow over a cosine hill in a wind tunnel. The standard k−ε and SST k−ω turbulence models represented the data most accurately. The flow over an idealised transverse dune immersed in the thermally stratified ABL was also investigated. It was found that the flow recovery was enhanced and re-attachment occurred earlier in unstable conditions, while flow recovery and re-attachment took longer in stable conditions. The effect of the dune on the flow higher up in the atmosphere was also felt at much higher distances for unstable conditions, through enhanced vertical velocities. Under stable conditions vertical velocities were reduced. This confirmed that the assumption of neutral conditions could lead to an incomplete picture of the flow conditions that influence any particular case of interest.