Limitation of Global Courant Number (VOF Model)
The default value of Global Courant Number is given 2 in Ansys Fluent, But how much this value can be changed? What is optimum value? What will be impact on results by increasing and decreasing this value? I am modeling a tank drainage problem and monitoring the interface.
Global Courant number could be experimented in the range of 1-5.
Ideally, Interface should not cross more than 1 cells in 1 time step, which is stability criterion for explicit scheme. In that way, optimal value should always be lesser than or equal to 1.
However, other equations should not be affected by the time restriction imposed on the volume fraction tracking. Fluent internally breaks the actual time step into sub-time steps, so that each sub-time step satisfies the Courant based criterion.
There is another entry for Courant number on the multiphase panel, by default 0.25.
Therefore the number of internal sub-time steps = Global Courant number devided by Local Courant number
For Global Courant Number = 2, sub-time-steps = 8
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