Performance parameters analysis in 2D - preliminary simulation experiments
Objective
The effort began in Fall 2008 of trying to characterize flocculation tank performance using parameters calculated from numerical solutions from CFD simulations containing all values at each node of the mesh. The following sets of experiments are carried out to confirm the previous results and provide an exploratory basis for further investigation of the relationship between the performance parameters and geometry of the flocculation tank.
Methods and Procedures
h/b ratios of 5, 10 and 20 were tested with 5 baffles. The completed test are summarized in the following table:
Summary of completed tests
The series of geometries and meshes were created using the journal file, by varying the flocculator height. The boundary conditions and all the other FLUENT settings can be found in the report summaries: symmetry top boundary condition and wall top boundary condition
Results and Discussions
Click here for the results of the preliminary simulations, completely summarized in an Excel workbook. Only parts of the graphical results are given below.
Shown below are contours of energy dissipation rate for h/b of 5 and 10, with symmetry boundary condition at the water air interface and one case with no slip(i.e. wall) boundary conditions.
As shown in the Excel workbook of graphical and quantitative results, the energy dissipation pattern is very sensitive to convergence level. One order of magnitude difference in residual results in completely different shapes of energy dissipation region at the top of flocculator. It is also noted that using symmetric boundary condition at the water-air interface at top of the flocculator makes it more difficult to converge to lower residual levels. The original purpose of using symmetric boundary condition is to mimic the frictionless condition at the water-air interface, but the energy dissipation contour of h/b=20 may also suggest whether using wall or symmetry boundary condition at water-air interface may not make a significant difference, and both cases could produce similar results at a better convergence level, which is consistent with our physical intuitions.
Conclusions
- Energy dissipation rate contour and quantitative analysis show dependence of performance parameters as a function of h/b ratio, which suggest it worth further investigation.
- Results are very sensitive to convergence levels.
- Symmetric boundary condition makes it difficult to converge.
- Symmetric boundary condition and wall boundary condition may have similar results at good convergence levels (residual below e-6). We can use wall boundary condition to replace symmetry boundary condition to ensure accuracy of the results.