Thesis (PhD)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT:This work is concerned with the numerical simulation of the aeolian snow transportation
process (drifting or wind blown snow) and especially the snow deposition and erosion
phenomenon (snow drift). The research work is interested in modelling the atmospheric
boundary layer wind flow and its associated snow drifting processes around threedimensional
obstacles by means of computational fluid dynamics (CFD).
A modelling method is required to predict and evaluate the snow drifting phenomenon
surrounding the SANAE IV research station in Antarctica. This station is of an elevated
design to ensure that wind blown snow may travel around the structure relatively undisturbed
and without deposition near the structure. This design is partly successful but localised drifts
are formed especially leeward of the interconnecting structures that join the main building
sections together.
The theoretical and numerical description to describe the turbulent transport of the two-phase
mixture of air and snow particles is investigated. This theory is subsequently employed to
describe the snow deposition and erosion process and two models are developed to determine
the deposition flux onto the snow surface. These models presented and discussed are a
threshold based approach and a conservative based approach. The first model is dependent on
a threshold shear velocity to determine the onset of either erosion or deposition. The second
model determines the deposition or erosion flux based on the conservation of the snow mass
transport in the near surface control volume. A numerical scheme that evaluates the snow
deposition flux at the surface and forces a temporal surface adaptation during the simulation is
established and implemented in a commercial CFD software code by means of user
subroutines.
Various test cases for which observed snow drift data are available are numerically modelled
to validate the snow drift schemes presented in this work. These tests include the wind driven
snow accumulation around a three-dimensional cube, around two adjacent three-dimensional
cubes and near a typical porous snow fence. The results indicate that both methods can predict
realistic snow drifts for a variety of wind flow conditions but also show that the conservative
approach is superior to the threshold based approach in describing the snow drift process
around obstacles. This model allows drifts to form not only in areas of low flow velocities but
also under high shear conditions. The theoretical investigation and the development and
validation of the conservatively based snow drift scheme shows that drift formation depends
strongly on the near surface flow divergence and secondary flow structures. To resolve the
snow drift formation under a variety of flow conditions a three-dimensional field solution is
required to determine velocity and snow concentration gradients and include the effects of
near surface convective and turbulent entrainment.
The model is applied to numerically simulate and predict snow drifting around the SANAE
IV base for a moderate as well as a high wind speed event. The predicted snow drift around
the base agrees favourably with the observed drifts at the station. Further numerical
simulations are carried out to evaluate the effects a few design modifications may have on the
snow deposition. These results suggest that a simple baffle plate installation near the bottom
of the interconnecting link structures may minimise the snow accumulation leeward of that
area.
This study shows that to achieve realistic numerical snow drift predictions around, on or near
obstacles, a conservative based snow drift scheme should be considered using some form of
temporal terrain adaptation strategy. Only then does one include a sufficient level of
important flow effects such as deposition along near surface boundaries of strong flow
divergence which plays as an important role as vertical settling and entrainment in
determining deposition rates. / AFRIKAANSE OPSOMMING:Hierdie studie behels die numeriese simulasie van windgedrewe sneeubeweging asook die
daarmee gepaardgaande sneeu neerslag en erosie eienskappe. Die navorsing het verder
belang in die berekening van die atmosferiese grenslaag vloei en die simulasie van sneeu
neerslag naby drie-dimensionele strukture deur gebruik te maak van berekeningsvloeimeganika
(BVM).
‘n Berekeningsmetodiek is nodig om die eienskappe van die sneeu neerslag rondom die
SANAE IV navorsingsstasie in Antarktika te voorspel en te evalueer. Die bogrondse struktuur
is spesifiek so ontwerp om te verseker dat wind gedrewe sneeu hoofsaaklik onversteurd verby
die struktuur kan beweeg sonder neerslag teenaan die struktuur. Die ontwerp is grotendeels
suksesvol alhoewel sneeu neerslag wel lokaal plaasvind, wind af vanaf die aansluitings
strukture tussen die hoof geboue.
Die teoretiese en numeriese beskrywing van die twee-fase lug- en sneeumengsel beweging
word ondersoek en gebruik om die sneeu neerslag en erosie einskappe te beskryf. Twee
modelle wat hierdie verskynsel beskryf word beskryf en bespreek naamlik ‘n drumpel
gebaseerde benadering en ‘n konserwatief gebaseerde benadering. Die eerste model is
afhanklik van ‘n drumpel skuifsnelheid om die aanvang van of erosie of neerslag te bereken.
Die tweede model bereken die neerslag eerder gebaseer op die behoud van die sneeu massa
vloei in die kontrole volume naby aan die oppervlak. ‘n Numeriese metode is ontwikkel en
geimplementeer in ‘n kommersiële BVM sagteware pakket deur van gebruikerssubroetines
gebruik te maak. Die ontwikkelde kode evalueer die sneeu neerslag vloed by die oppervlak en
forseer ‘n tydafhanklike oppervlak aanpassing gedurende die simulasie.
Die sneeu neerslag metode wat beskryf word in hierdie studie word ge-evalueer teen verskeie
toetsgevalle waarvoor daar waargenome sneeu neerslag resultate beskikbaar is. Hierdie toetse
sluit in die wind gedrewe sneeu neerslag rondom ‘n drie-dimensionele kubus, rondom twee
naby geleë kubusse en naby ‘n tipiese poruese sneeu heining. Die resultate dui aan dat beide
die metodes realistiese sneeu neerslag voorspel vir verskeie wind toestande. Die studie wys
ook dat die konserwatief gebaseerde benadering vir die beskrywing van die sneeu neerslag
proses meer akkuraat is as die drumpel gebaseerde benadering aangesien die neerslagvoorspel kan word nie net alleenlik in gebiede met lae vloeisnelhede nie, maar ook in gebiede
waar hoë skuifsnelhede teenwoordig is. Die teoretiese ondersoek, ontwikkeling en toepassing
van die konserwatief gebaseerde model dui daarop dat die neerslag afhanklik is van die
divergensie van die vloeiveld asook van die sekondêre vloei patrone naby die oppervlak. Ten
einde die sneeu neerslag vir verskeie toestande op te los is dit nodig om snelheids- en
sneeukonsentrasie gradiënte te kan bereken in ‘n drie-dimensionele vloei veld om sodoende
die invloed van naby-oppervlak konveksie en turbulente verspreiding in ag te neem.
Die metode word toegepas deur die sneeu neerslag rondom die SANAE IV navorsingsstasie te
voorspel vir ‘n gematigde asook ‘n hoë wind snelheid toestand. Die sneeu neerslag
voorspelling stem gunstig ooreen met die waargenome neerslag by die struktuur. Verdere
numeriese simulasies is uitgevoer om die invloed van ontwerpsverandering op die neerslag te
evalueer. Uit hierdie resultate blyk dit dat ‘n eenvoudige plaat struktuur onder die
aansluitingsstrukture die sneeu neerslag wind af mag verminder.
Hierdie navorsingsstudie dui daarop dat ‘n tydafhanklike terrein aanpassing strategie saam
met die konserwatiewe neerslag model noodsaaklik is ten einde realistiese resultate te behaal
vir die sneeu opbou rondom of naby strukture. Sodoende word genoegsame vlakke van
belangrike vloei verskynsels, soos die invloed van vloei divergensie, in ag geneem wat net so
‘n belangrik rol in neerslag speel soos vertikale afsetting.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/15935 |
| Date | 12 1900 |
| Creators | Beyers, Johannes Hendricus Meiring |
| Contributors | Harms, T. M., University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xxii, 254 leaves : ill. |
| Rights | University of Stellenbosch |
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