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To extend the current 2D CFD models of flocculation tank in to 3D, will greatly enhance our understanding of the fluid dynamics of flow dynamics inside the flocculation tank. In this report, 4 four sets of preliminary 3D computational simulations were presented and discussed: first investigated on investigating the difficulty of 3D models in converging to satisfactory residual; and then compared on comparing the assumption and validity of 2D and 3D models by imposing 2D assumption onto 3D modelrunning 3D simulations with the model on which 2D assumptions were imposed.
Introduction
Compared with 2D models, CFD simulations in 3D provide closer numerical approximation to the real fluid flows in hydraulic flocculators, which enable better quantitatively investigation of performance parameters of flocculation and thus lead to well-studied guidelines for the design, construction and operation of hydraulic flocculators of AguaClara treatment plants.
To extend the current 2D model into 3D, preliminary simulations were essential for evaluate the scope of the project before more conclusive tests. The goals were: to get familiar with using Gambit and FLUENT in 3D settings; to come up with effective approaches for 3D simulations; and to investigate whether adding the 3rd dimension has significant effect on the simulation results. The evaluate the relative validity of 2D models vs 3D models. In this report, the 2D model was extended to 3D by minor modifications of its mesh and FLUENT parameters.
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2D models approximate the real 3D flows by assuming all variables of fluid flows are uniform along the 3rd dimension, i.e. the z direction. Thus it was expected that the results from 2D models could be duplicated if imposing this "uniform" condition on the 3D model. In Case 4, this imposition was incorporated in the 3D model by applying periodic boundary condition to the walls in the 2D plane, i.e. the x-y plane, assuming that periodic repetition along the z direction was equivalent to "uniform":
- Case 4: Periodic boundary condition was defined at the walls along the x-y plane, 1st order solver with convergence criteria of 10^-6 2nd order solver with convergence criteria of 10^-6 (solution obtained from the 1st order solver was used as an initial guess for the 2nd order solver).
Click here for more detailed procedure procedures of mesh generation in Gambit and problem definition in FLUENT.
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