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

[摘要] ENGLISH ABSTRACT: An accurate description of the atmospheric boundary layer (ABL) is a prerequisitefor computational fluid dynamic (CFD) wind studies. This includes taking intoaccount the thermal stability of the atmosphere, which can be stable, neutral orunstable, depending on the nature of the surface fluxes of momentum and heat.The diurnal variation between stable and unstable conditions in the Namib Desertinterdune was measured and quantified using the wind velocity and temperatureprofiles that describe the thermally stratified atmosphere, as derived by Monin-Obukhov similarity theory. The implementation of this thermally stratifiedatmosphere into CFD has been examined in this study by using ReynoldsaveragedNavier-Stokes (RANS) turbulence models. The maintenance of thetemperature, velocity and turbulence profiles along an extensive computationaldomain length was required, while simultaneously allowing for full variation inpressure and density through the ideal gas law. This included the implementationof zero heat transfer from the surface, through the boundary layer, under neutralconditions so that the adiabatic lapse rate could be sustained. Buoyancy effectswere included by adding weight to the fluid, leading to the emergence of thehydrostatic pressure field and the resultant density changes expected in the realatmosphere. The CFD model was validated against measured data, from literature,for the flow over a cosine hill in a wind tunnel. The standard k-ε and SST k-ωturbulence models, modified for gravity effects, represented the data mostaccurately. The flow over an idealised transverse dune immersed in the thermallystratified ABL was also investigated. It was found that the flow recovery wasenhanced and re-attachment occurred earlier in unstable conditions, while flowrecovery and re-attachment took longer in stable conditions. It was also found thatflow acceleration over the crest of the dune was greater under unstable conditions.The effect of the dune on the flow higher up in the atmosphere was also felt atmuch higher distances for unstable conditions, through enhanced verticalvelocities. Under stable conditions, vertical velocities were reduced, and theinfluence on the flow higher up in the atmosphere was much less than for unstableor neutral conditions. This showed that the assumption of neutral conditions couldlead to an incomplete picture of the flow conditions that influence any particular case of interest.

[发布机构] Stellenbosch University