A computational fluid dynamics study of the near surface wind patterns over a desert dune and the effect on seed dispersion

Joubert, Eugene Christiaan (2010-03)

Thesis (MScEng)--Stellenbosch University, 2010.

Thesis

ENGLISH ABSTRACT: This project originated when a team of scientists at the Gobabeb training and research and centre observed seed accumulation sites on the slope of sand dunes in the Namib Desert. Seeds that accumulate on the slip face of a sand dune provide food for small desert creatures that in turn attract larger animals, resulting in a small ecosystem on the side of the dune. Since wind is the primary transport of seeds throughout the Namib Desert it is of interest to investigate wind patterns over the dune. In this project it is therefore desired to look at seed dynamics and deposition as a result of near surface wind patterns around a three-dimensional dune geometry using computational fluid dynamics. The project is a joint venture between the University of Stellenbosch and the University of Namibia. This document presents the South African MScEng thesis part. The literature review shows the dominant winds in the Namib Desert to be from the south to westerly direction. Previous studies on air flow over dunes focussed on sand movement and were often limited to simplified two-dimensional geometries and steady state simulations. From these studies the basic flow features associated with dunes can be identified. Lastly, factors that influence particle dynamics around dune geometries are looked at. These particle studies mostly involve the movement of sand rather than seeds but still provide valuable insight. The project methodology is explained and includes the equipment used, the considerations taken into account, the simplifications made as well as the procedure followed when conducting field work and simulations. A section of an actual Namibian linear dune is mapped in order to obtain a geometry for the simulations. Flow measurements are carried out with a wind mast to obtain velocity profile inlet conditions for the simulations. Furthermore, seed sampling is done by the collaborating Namibian team of which the data is used to obtain an effective seed particle model. Lastly, simulations are carried out using primarily OpenFOAM-1.5. The simulations look at general near surface wind patterns, time dependant flow features and particle movement and seed deposition around and on the linear dune. The results show different wind profiles for different wind direction. It is also possible to see how the profile changes as the flow accelerates up the dune slope. Two-dimensional results provide the opportunity to compare results with previous studies as well as to provide the basis for looking at aspects such as differencing schemes, turbulence models and parallel computing before three-dimensional simulations are carried out. The importance of higher order differencing schemes are confirmed in the two-dimensional results. The turbulence models, however, produce very similar results. The results from the two- and three-dimensional results show typical flow features associated with dunes. Transient flow features and separation vortex structures can be identified from time dependant simulations. Furthermore, particle simulations reveal how particles tend to be trapped in the recirculation regions. The conclusions explain how the project objectives were achieved and provide recommendations for future studies related to this project.

AFRIKAANSE OPSOMMING: Die projek het ontstaan toe naforsers areas van saad akumulasie op die hellings van duine in Namibë opgelet het. Hierdie akumulasie van plant materiaal verskaf die voedsel vir klein diere op die duin wat ‘n klein ekosisteem tot gevolg het. Aaangesien die primêre vervoermiddel vir sade in die woestyn wind is beoog die projek om deur die wind patrone oor die duin beter te verstaan die saad verspreiding te beskryf. Die doel is dus om saad verspreiding en akumulasie te beskryf deur die wind patrone te bekyk wat die verspreiding tot gevolg het deur gebruik te maak van numeriese vloei dinamika. Die projek is ‘n saamgestelde projek tussen die Universiteit van Stellenbosch en die Universiteit van Namibië. Hierdie dokument behels die Suid Afrikaanse MScIng gedeelte van die projek. Deur die hersiening van literatuur word daar gesien dat die domineerende wind rigtings as suid tot wes beskryf kan word. Vorige studies wat verband hou met wind vloei oor duine wys dat die meeste op twee-dimensionele eenvoudige geometrië gefokus het met tyd onafhankike simulasies. Dit is egter moontlik om die resultate te gebruik om karakteristieke vloei patrone te identifiseer wat met duin wind patrone geassosieer kan word. Laastens word daar gekyk na die faktore wat partikel beweeging beïnvloed, maar hierdie studies sluit hoofsaaklik sand partikels in eerder as sade. Die motodologie beskryf die toerusting, oorwegings en prosedures wat gevolg is tydens veld werk asook simulasies. Tydens die veld werk is ‘n gedeelte van ‘n Namib lineêre duin gemeet en so gebruik om ‘n geometrie te maak wat vir simulasie doeleindes gebruik kan word. Daar is ook wind meetings gedoen met ‘n wind mas om wind profiele vir inlaat kondisies vir die simulasies te kry. Verder het saad bestudeering die nodige data verskaf om ‘n voledige saad partikel model op te stel wat in die simulasies gebruik kan word. Laastens kyk die simulasies veral na algemene vloei patrone, tyd afhanklike vloei effekte en ook partikel beweging in die vloei veld. Die resultate wys hoe wind profiele verskil wat van verskillende rigtings af waai. Dit is ook moontlik on te wys hoe die wind profiele verander soos die wind versnel teen die duin op. Tweedimensionele simulasies verskaf die geleentheid om te kyk na die effek van verkillende numeriese modelle, turbulensie modelle en ook multi-prosesseerder verwerking. Tydens die twee-dimensionele simulasies is die belangrikheid van hoër orde numeriese metodes besef. Die verkillende turbulensie modelle het egter klein verkille gewys. Alby die twee- en driedimensionele resultate wys karakteristieke vloei patrone wat met duine geasosieer kan word. Verder het tyd afhanklike simulasies gewys hoe wind patrone verander met tyd. Die partikel simulasies wys ook die beweging van partikels deur die wind en hoe dit neig om te akumuleer in die hersirkulasie gebied agter die duin. Die gevolgtrekkings wys dat al die doelstellings bereik is en maak voorstelle vir toekomstige studies wat met hierdie studie verband hou.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/4357
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