For bubble columns, use a fraction of the bubble rise time for a total of one rise time, then increase the timestep to a fraction of the fluid circulation time. If species mass transfer is modeled, it can take a considerable amount of time for some species to come to equilibrium. Try setting the timestep size for the mass fraction equations to a large value if convergence is very slow for this equation class.

For dilute particle cases, use a fraction of the dynamical time scale for the momenta. This is essentially a fluid travel time through or around the region of interest, but is not always trivial. If volume fraction convergence is slow, then the time scale for these equations can be increased suitably.