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

Pieterse, Jacobus Erasmus (2013-03)

Thesis (MScEng)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: An accurate description of the atmospheric boundary layer (ABL) is a prerequisite for computational fluid dynamic (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 diurnal variation between stable and unstable conditions in the Namib Desert interdune was measured and quantified using the wind velocity and temperature profiles that describe the thermally stratified atmosphere, as derived by Monin- Obukhov similarity theory. The implementation of this thermally stratified atmosphere into CFD has been examined in this study by using Reynoldsaveraged Navier-Stokes (RANS) turbulence models. 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. This included the implementation of zero heat transfer from the surface, through the boundary layer, under neutral conditions so that the adiabatic lapse rate could be sustained. Buoyancy effects were included by adding weight to the fluid, leading to the emergence of the hydrostatic pressure field and the resultant density changes expected in the real atmosphere. 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-ω turbulence models, modified for gravity effects, 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. It was also found that flow 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 at much higher distances for unstable conditions, through enhanced vertical velocities. Under stable conditions, vertical velocities were reduced, and the influence on the flow higher up in the atmosphere was much less than for unstable or neutral conditions. This showed that the assumption of neutral conditions could lead to an incomplete picture of the flow conditions that influence any particular case of interest.

AFRIKAANSE OPSOMMING: 'n Akkurate beskrywing van die atmosferiese grenslaag (ABL) is 'n voorvereiste vir wind studies met berekenings-vloeimeganika (CFD). Dit sluit in die inagneming van die termiese stabiliteit van die atmosfeer, wat stabiel, neutraal of onstabiel kan wees, afhangende van die aard van die oppervlak vloed van momentum en warmte. Die daaglikse variasie tussen stabiele en onstabiele toestande in die Namib Woestyn interduin is gemeet en gekwantifiseer deur gebruik te maak van die wind snelheid en temperatuur profiele wat die termies gestratifiseerde atmosfeer, soos afgelei deur Monin-Obukhov teorie, beskryf. Die implementering van hierdie termies gestratifiseerde atmosfeer in CFD is in hierdie studie aangespreek deur gebruik te maak van RANS turbulensie modelle. Die handhawing van die temperatuur, snelheid en turbulensie profiele in die lengte van 'n uitgebreide berekenings domein is nodig, en terselfdertyd moet toegelaat word vir volledige variasie in die druk en digtheid, deur die ideale gaswet. Dit sluit in die implementering van zero hitte-oordrag vanaf die grond onder neutrale toestande sodat die adiabatiese vervaltempo volgehou kan word. Drykrag effekte is ingesluit deur die toevoeging van gewig na die vloeistof, wat lei tot die ontwikkeling van die hidrostatiese druk veld, en die gevolglike digtheid veranderinge, wat in die werklike atmosfeer verwag word. Die CFD-model is gevalideer teen gemete data, vanaf die literatuur, vir die vloei oor 'n kosinus heuwel in 'n windtonnel. Die standaard k-ε en SST k-ω turbulensie modelle, met veranderinge vir swaartekrag effekte, het die data mees akkuraat voorgestel. Die vloei oor 'n geïdealiseerde transversale duin gedompel in die termies gestratifiseerde ABL is ook ondersoek. Daar is bevind dat die vloei herstel is versterk en terug-aanhegging het vroeër plaasgevind in onstabiele toestande, terwyl vloei herstel en terug-aanhegging langer gevat het in stabiele toestande. Daar is ook bevind dat vloei versnelling oor die kruin van die duin groter was onder onstabiele toestande. Die effek van die duin op die vloei hoër op in die atmosfeer is ook op hoër afstande onder onstabiele toestande gevoel, deur middel van verhoogte vertikale snelhede. Onder stabiele toestande, is vertikale snelhede verminder, en die invloed op die vloei hoër op in die atmosfeer was veel minder as vir onstabiel of neutrale toestande. Dit het getoon dat die aanname van neutrale toestande kan lei tot 'n onvolledige beeld van die vloei toestande wat 'n invloed op 'n bepaalde geval kan hê.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/80024
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