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FLUENT 6 - Converging a VOF mold filling simulation
Converging VOF solutions may be challenging when the ratio of viscosity of the two fluids is far from 1.0.
Mold filling is a typical example of this problem. The injected material may have a viscosity a few orders of magnitude greater than that of the fluid being displaced (air, for example).
Suggested strategies are provided in the resolution below.
1) Increase the air viscosity such that ratio is about 1e+3 to 1e+4. This is an approximation and should be done with a great care.
Due to high viscosity and density of mold as compared to air, the effect of this change in property of air would not significantly affect the mold flow which is of primary interest. The improvement of convergence is significant while the approximation typically results in negligible loss of fidelity.
2) Mark one or two cells at the inlet and patch the fill material VOF to 1.0 in these cells. This provides the fill material with an easy entry to the mold and will help the simulation get off to a stable start.
3) If step 2 does not resolve the convergence problem at the start, then try enabling "Solve VOF every iteration" from the Multiphase Model panel and keep this enabled for a few time steps. Once the case shows good convergence, disable this selection.
4) A low time step is necessary at the start of the simulation. This can be increased gradually to a value where solution does not go unstable.
5) Set PISO for pressure - velocity coupling. Use PRESTO! for pressure discretization.
6) With PISO scheme, higher URFs can be used. However, for this class of problems, momentum URF should be less than 0.5. A value 0.2 to 0.4 can be selected based on the stability of solution.