Preliminary 3D CFD Simulations of a Single Baffle Hydraulic Flocculation Tank
Abstract
Extend the current 2D CFD models of flocculation tank in to 3D, will greatly enhance our understanding of the fluid dynamics of flow inside the flocculation tank. In this report, 4 sets of preliminary 3D computational simulations were presented and discussed: first investigated the difficulty of 3D models in converging to satisfactory residual; and then compared the assumption and validity of 2D and 3D models by imposing 2D assumption onto 3D model.
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 2D model was extended to 3D by minor modifications of its mesh and FLUENT parameters.
Procedures
Investigation of Convergence
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Click here for detailed procedure of mesh generation in Gambit and problem definition in FLUENT.
Results and Discussion
The solution data of FLUENT contains the numerical values of a complete set of fluid dynamics variables at all the nodes defined by the mesh. These values can be used to calculate all relevant characteristic parameters. Presented in the following discussions are the plots of residual as a function of iteration steps and contours of energy dissipation rate.
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Furthermore, the importance of z components could put the validity of 2D model as an approximation in question: in Case 4, even small components in z direction could make significant difference in results from 2D model, let alone in the real flow.
However, the above hypothesis must be further investigated, ruling out all other possible causes of differences. Particularly, the effect of the length of the period must be investigated by vary the width of the flocculator. Ideally, periodic repetition with infinitely small period length is equivalent to "uniform".
Conclusions
To build a well-conditioned model that converges to accurate numerical solution, the mesh density in all 3 dimensions must be in the same order of magnitude, and refined enough to resolve the region where fluid flows vary violently; "uniform" in z direction is not equivalent to no components in z direction, thus 3D model with periodic boundary condition may not be equivalent to 2D model, which also put the validity of 2D models in doubts.
Future Simulation Experiments
Possible future simulation experiments and research topics are:
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