Analysis of the Hydrodynamics of rectangular plunging Jets and the subsequent scouring in broken-up Rock Beds

Calitz, Jacques (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The scour mechanisms of rock are highly variable, equally so the geotechnical and hydrodynamic conditions of each study area, which complicate the prediction of scouring. Cases such as Kariba Dam and Ricobayo Dam are examples where, due to inadequate scour prediction methods, scouring caused major damage to the downstream riverbed. Rock scouring due to plunging jets is thus an important area of study. Various studies have been done to predict the scour hole depth and its extent using physical laboratory models and their subsequent empirical formulas. A classification method, the Erodibility Index method (EIM) by Annandale (1995), was also developed to give an indication of the depth of scouring. Physically based methods, such as the Comprehensive Scour Method (CSM) including the Quasi-steady impulsion method (QSI) by Bollaert (2002 and 2014), tried to incorporate several of the rock scour mechanisms, to determine both the depth and extent of the scour hole. The current study focused on using a 1:40 physical laboratory model to ascertain the applicability of using PVC blocks to replicate rock blocks and the subsequent scouring thereof. The drop height, as well as the tailwater, varied between the different tests. The PVC blocks were able to replicate the scour hole to a relatively good extent and could sustain steep slopes replicating the repose angle of rock. The subsequent scour holes from the physical laboratory models were compared to the depths calculated using empirical formulas and the classification method by Annandale (1995) (EIM). The methods overestimated the depths, and the EIM was found to be very sensitive to both the hydrodynamic and geotechnical boundary conditions. The physically based method proposed by Bollaert (2002) (CSM) overestimated the scour hole extent, as compared to the physical laboratory model scour hole, but the scour hole profile (shape and depth), was in agreement. The use of Computation Fluid Dynamics (CFD) to simulate hydraulic problems has become more viable, due to advancements in computational power. The hydrodynamic characteristics of the jet in the air and in the plunge pool was modelled in 2D using FLUENT, as the scour mechanisms of rock cannot presently be modelled using commercial CFD codes. The hydrodynamic conditions in the air were modelled with good comparison to both that of the physical laboratory model, as well as the calculated conditions of Stellenbosch University https://scholar.sun.ac.za -iiithe jet in the air, while the hydrodynamic conditions (velocity and pressure) in the plunge pool were overestimated in comparison to current methods available, as used in the EIM as well as the CSM and QSI methods, due to possible flow confinement and deflection effects. In conclusion, the study firstly confirmed the applicability of using PVC blocks to model scouring due to plunging jets in rock beds, and secondly it established the use of current scour prediction methods in validating small scale scour hole profiles (shape and depth). The use of 2D CFD modelling, in predicting the hydrodynamic conditions of the plunging jet in the air and plunge pool, was also introduced with relative success.

AFRIKAANSE OPSOMMING: Die voorspelling van erosie is gekompliseerd as gevolg van erosie meganismes van rots wat baie wisselend is, asook die unieke geotegniese en hidrodinamiese kondisies van elke area wat bestudeer word. Gevalle soos Kariba Dam en Ricobayo Dam is goeie voorbeelde waar, as gevolg van onvoldoende erosie voorspellings metodes, die rots erosie grootskaalse skade aan die stroom-af rivierbedding veroorsaak het. Rots erosie wat deur vallende water strale teweeggebring is, is dus ‘n baie belangrike area en vereis deeglike navorsing. Verskeie studies is al gedoen om die diepte en omvang van die resulterende erosiegat te bepaal deur gebruik te maak van fisiese modelle en die daaropvolgende emperiese formules. ‘n Klassifiserende metode, die Erodibility Index method (EIM) deur Annandale (1995), is ontwikkel om ‘n indikasie van die erosie diepte te gee. Fisies gebaseerde metodes, soos die Comprehensive Scour Method (CSM) insluitend die Quasi-steady impulsion method (QSI) deur Bollaert (2002 en 2014), het soveel moontlik gepoog om van die rots erosie meganismes te inkorporeer, om beide die diepte asook die omvang van erosie van die erosie gat te bepaal. Die huidige studie is toegespits op die gebruik van ‘n 1:40 fisiese model om die toepaslikheid van die gebruik van PVC blokke om rots blokke na te boots en die daaropvolgende erosie vas te stel. Die valhoogte, asook die plons dompel waterhoogte, is verander tussen toetse. Die PVC blokke was instaat om die erosie gat redelik goed na te boots en kon ‘n steil gradiënt volhou aan die sykante van die erosie gat. Die daaropvolgende erosie gate van die fisiese model is vergelyk met die dieptes soos bepaal deur die emperiese fomules, asook die dieptes van die klassifiserende metode deur Annandale (1995) (EIM). Die metodes het die dieptes oorskat en die EIM metode was baie sensitief in terme van die hidrodinamika en geotegniese grens kondisies. Die fisiese gebaseerde metodes, soos ontwikkel deur Bollaert (2002) (CSM), het die erosie gat se omvang oorskat in vergelyking met die fisiese model. Die erosie gat profiel het wel ooreengestem met die profiel van die fisiese model. Die gebruik van Computation Fluid Dynamics (CFD) om hydroliese probleme te simuleer is deesdae meer uitvoerbaar, as gevolg van die vooruitgang in rekenaartegnologie en kapasiteit. Die hidrodinamiese Stellenbosch University https://scholar.sun.ac.za -veienskappe van die water straal in die lug, asook in die dompel poel, is gemodelleer in 2D, deur gebruik te maak van FLUENT. Die erosie meganismes kan huidiglik nie deur kommersiele CFD sagteware gemodelleer word nie. Die gemodelleerde hidrodinamiese toestand van die water straal in die lug het goed ooreengestem met beide die fisiese model asook die berekende eienskappe. Die hidrodinamiese aspekte in die dompel poel (druk en snelheid), is oorskat in vergelyking met die huidig beskikbare metodes om die hidrodinamika van die water straal te bepaal soos gebruik in die CSM en EIM metodes. Die geskatte waarde van die hidrodinamiese toestande was groter – dit kan toegeskryf word aan die moontlike effek van defleksie en beperking van watervloei. Ter opsomming, die studie het eerstens vasgestel wat die toepaslikheid van PVC blokke is om rots erosie as gevolg van water strale te modelleer en tweedens het die studie vasgestel dat huidige erosie vooruitskattings metodes gebruik kan word vir klein skaalse modelle. Die gebruik van 2D CFD modelle, om die hidrodinamiese kondisies van water strale in die lug en dompel poel te beraam, is met redelike sukses toegepas.

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