Browsing by Author "Calitz, Jacques"
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- ItemAnalysis of the Hydrodynamics of rectangular plunging Jets and the subsequent scouring in broken-up Rock Beds(Stellenbosch : Stellenbosch University, 2016-03) Calitz, Jacques; Bosman, Adèle ; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.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.