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

[摘要] ENGLISH ABSTRACT: This project originated when a team of scientists at the Gobabeb training and research and centreobserved seed accumulation sites on the slope of sand dunes in the Namib Desert. Seeds thataccumulate on the slip face of a sand dune provide food for small desert creatures that in turnattract larger animals, resulting in a small ecosystem on the side of the dune. Since wind is theprimary transport of seeds throughout the Namib Desert it is of interest to investigate windpatterns over the dune. In this project it is therefore desired to look at seed dynamics anddeposition as a result of near surface wind patterns around a three-dimensional dune geometryusing computational fluid dynamics. The project is a joint venture between the University ofStellenbosch and the University of Namibia. This document presents the South African MScEngthesis part.The literature review shows the dominant winds in the Namib Desert to be from the south towesterly direction. Previous studies on air flow over dunes focussed on sand movement and wereoften limited to simplified two-dimensional geometries and steady state simulations. From thesestudies the basic flow features associated with dunes can be identified. Lastly, factors thatinfluence particle dynamics around dune geometries are looked at. These particle studies mostlyinvolve the movement of sand rather than seeds but still provide valuable insight.The project methodology is explained and includes the equipment used, the considerations takeninto account, the simplifications made as well as the procedure followed when conducting fieldwork and simulations. A section of an actual Namibian linear dune is mapped in order to obtaina geometry for the simulations. Flow measurements are carried out with a wind mast to obtainvelocity profile inlet conditions for the simulations. Furthermore, seed sampling is done by thecollaborating 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 atgeneral near surface wind patterns, time dependant flow features and particle movement and seeddeposition around and on the linear dune.The results show different wind profiles for different wind direction. It is also possible to seehow the profile changes as the flow accelerates up the dune slope. Two-dimensional resultsprovide the opportunity to compare results with previous studies as well as to provide the basisfor looking at aspects such as differencing schemes, turbulence models and parallel computingbefore three-dimensional simulations are carried out. The importance of higher orderdifferencing schemes are confirmed in the two-dimensional results. The turbulence models,however, produce very similar results. The results from the two- and three-dimensional resultsshow typical flow features associated with dunes. Transient flow features and separation vortexstructures can be identified from time dependant simulations. Furthermore, particle simulationsreveal how particles tend to be trapped in the recirculation regions.The conclusions explain how the project objectives were achieved and provide recommendationsfor future studies related to this project.

[发布机构] Stellenbosch University