Thesis (MScEng)--Stellenbosch University, 2013. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/80024 |
| Date | 03 1900 |
| Creators | Pieterse, Jacobus Erasmus |
| Contributors | Harms, T. M., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 120 p. : ill. |
| Rights | Stellenbosch University |
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