NUMERICAL MODELLING OF OGEE SPILLWAY HYDRUALICS USING CFD AND ITS VALIDATION THROUGH PHYSICAL MODEL TESTS

Abstract

“As a part of the design process for hydroelectric generating stations hydraulic engineers typically conduct some form of model testing. The desired outcome from the testing can vary considerably depending on the specific situation, but often characteristics such as velocity patterns, discharge rating curves, water surface profiles, and pressures at various locations are measured. Due to recent advances in computational power and numerical techniques, it is now possible to obtain much of this information through numerical modeling.” “Modern Computational Fluid Dynamics (CFD) modeling are becoming common design and analysis tools in the engineering field. Nowadays, project designs involve the use of CFD techniques along with physical scale modeling to analyze the complex rapidly varied and turbulent flows which would not be easily analyzed by physical modeling. In particular, the consideration and/or use of CFD modeling in the Hydraulic Engineering field remains on the increase. Apart from being used for comparison with other design techniques, CFD may in future become a standalone modeling technique in hydraulic structures design.” “This research aims to use CFD models to validate the simulation of the flow over a ogee dam spillway which was installed in the Hydraulic Laboratory of Stellenbosch University. To achieve this simulation of the flow which involves an interaction between water and air, the flow behavior has been mapped by the Volume of Fluid (VOF) and the realizable "k-ε" turbulence numerical models. The Volume of Fluid (VOF) and the realizable "k-ε" models simulate the free surface of two-phase flow and the flow turbulence, respectively. Firstly, it subsequently presents the geometry and dimensions of the physical models, the testing procedure and the experimental test results achieved from this modeling exercise. For CFD modeling, a commercially available Computational Fluid Dynamics (CFD) package, AnsysFluent, was used. To model the physical model, the use of Reynolds-averaged Navier-Stokes equations in combination with the realizable k-ε eddy-viscosity closure model was adopted. The process of CFD model development and the underlying theory of it are discussed in this VI

thesis. In order to determine the required mesh size, the mesh sensitivity tests were conducted on the 2 dimensional models.” “Finally, the pressure readings and water levels along with the water surcharge produced by numerical models are discussed through a validation process by comparing the CFD model in 2D and 3D results with the results obtained from physical models. The outcome proved that CFD models are able to map the behavior of both flow phases since they exhibited a close correlation to those achieved in the physical models. Even though some slight differences in values were revealed, the graphical trend remains reasonably similar for all test results.”