[Ahyar]

I've learned about courant number from various source, one of them is from https://www.idealsimulations.com/res...nt-number-cfd/ . as far as i know, the courant number in cfd always associated with velocity, but in multiphase simulation, the calculated entity is not only velocity, there are (for example) diameter distribution, phase fraction, etc. do these entities also affect the courant number during simulation? if it affects, how to set boundary conditions of these entities in order to make the simulation stable? since the courant number represent the stabilization of the simulation, right? thanks in advance

[geth03]

The courant number ist basically the residence time within a cell in the computed timestep in relative terms.
E.g. Co=2: flow will advance two cells within a timestep.
Co=0.5: in 2 timesteps flow will advance one cell.

In Openfoam the max Co will be computed from the max(Co_fromMixture, Co_maxFromAnyPhase) for multiphase solvers.
In the mixture the void fraction is a weighting factor.
Meanwhile, any phase with voidfraction=0 can generate high velocities, bc of numerical/modeling reasons.

So the way openfoam calculates Co, it will warn that something is odd, even if voidfraction is zero for that particular phase.

Hope it helps.