Labyrinth weir hydraulics : validation of CFD modelling

Robertson, Guy Kinloch (2014-04)

Thesis (MScEng)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: The use of computational fluid dynamics (CFD) as a design tool is becoming increasingly popular in the water resources field. This thesis aims to extend the knowledge of CFD and determine the usefulness of current CFD programs as a modelling tool. This thesis also seeks to determine the accuracy of CFD modelling when compared to physical modelling, the more established form of model testing. It is important that research is conducted on the validation of CFD because with an increase in computer power, processing speed and continual development in the programs used to generate the models, CFD could become an essential tool for the hydraulic engineer. A current key difficulty faced by CFD programs is the mapping of the free surface level of a body of fluid in a two-phase (water and air) flow condition. This is further complicated by the existence of three-dimensional flow over a labyrinth weir and a fluctuating nappe, which at times requires a free surface level to be mapped both above and below the nappe. This thesis begins by detailing the design methods and actual design of a typical labyrinth weir. It then describes the construction of a 1:20 scale physical model, testing procedures, goals, and the results of the physical model tests. Following the physical model study, the thesis discusses the development of a three-dimensional CFD model, designed in a way that matched the physical model. Simulation results obtained from the CFD model are then compared to those from the physical model study and the accuracy and suitability of CFD modelling as a design tool are evaluated. This evaluation considers the surcharge upstream of the weir and transient pressures on the weir. The thesis concludes with recommendations for further research in this field. The results achieved show that the CFD model was able to accurately map the movement of particles within the domain, to fully develop a flow profile, and to accurately predict the water surface level. The pressure readings obtained during CFD modelling were in the same order as those obtained during physical modelling. However, the CFD modelling pressure readings did not often accurately correspond with the physical modelling data, with the average error being 92%. These results indicate that there is still further development required in CFD before it can be relied upon as a design tool independent of other experimental methods. The difficulty and the length of time taken to generate the results also indicate that, at this stage and in this particular scenario, the engineer would be better served through the use of a physical model.

AFRIKAANSE OPSOMMING: Die gebruik van gerekenariseerde vloeidinamika (CFD) as ’n ontwerpinstrument het toenemend gewild begin raak op die gebied van waterhulpbronne. Die doel van hierdie verslag is om kennis van CFD uit te brei en die nut van huidige CFD-programme as ’n modelleringsinstrument te bepaal. Daar word voorts ook gepoog om die akkuraatheid van CFD-modellering te bepaal in vergelyking met fisiese modellering – die meer gevestigde vorm van modeltoetsing. Dit is noodsaaklik dat navorsing gedoen word oor die bekragtiging van CFD, want met ’n toename in rekenaarkrag, verwerkingsnelheid en deurlopende ontwikkeling in die programme wat gebruik word om die modelle te genereer, sal CFD ’n noodsaaklike instrument vir die hidroulika-ingenieur word. ’n Belangrike probleem wat CFD-programme tans inhou, is die kartering van die vry oppervlak van ’n liggaam vloeistof in ’n tweefasse vloeitoestand (water en lug). Dit word verder bemoeilik deur die bestaan van driedimensionele vloei oor ’n labirint-stuwal en ’n skommelende “nappe”, wat by tye vereis dat ’n vry oppervlak sowel bo as onder die “nappe” gekarteer met word. Die verslag begin met ’n uiteensetting van die ontwerpmetodes en fisiese ontwerp van ’n tipiese labirintstuwal. Die bou van ’n 1:20-skaal- fisiese model, toetsprosedures, doelwitte en die resultate van die toetse op die fisiese model word dan beskryf. Ná die studie van die fisiese model, word die ontwikkeling van ’n driedimensionele CFD-model bespreek, wat ontwerp is om by die fisiese model te pas. Die simulasie-resultate van die CFD-model word dan vergelyk met dié van die studie van die fisiese model en die akkuraatheid en geskiktheid van CFD-modellering as ’n ontwerpinstrument word geëvalueer. In hierdie evaluering word die opdamming stroomop van die stuwal en druk op die stuwal ondersoek. Die verslag word afgesluit met aanbevelings vir verdere navorsing op hierdie gebied. Die resultate toon dat die CFD-model die beweging van partikels in die domein akkuraat kon karteer ten einde ’n volledige vloeiprofiel te ontwikkel en die watervlak akkuraat te voorspel. Die drukke wat tydens CFD-modellering verkry is, stem egter nie ooreen met die lesings wat tydens fisiese modellering verkry is nie. Die gemiddelde fout is 92%. Hierdie resultate toon dat verdere ontwikkeling in CFD nodig is voordat daarop staat gemaak kan word as ’n ontwerpinstrument wat onafhanklik van ander eksperimentele metodes gebruik kan word. Die moeilikheidsgraad en die lang tydsduur betrokke by die generering van resultate is ook ’n aanduiding dat die gebruik van ’n fisiese model die ingenieur op hierdie stadium en in hierdie spesifieke scenario beter tot diens sal wees.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/86277
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