
The process of solving a multiphase system is inherently difficult, and you may encounter some stability or convergence problems. If a timedependent problem is being solved, and patched fields are used for the initial conditions, it is recommended that you perform a few iterations with a small time step, at least an order of magnitude smaller than the characteristic time of the flow. You can increase the size of the time step after performing a few time steps. For steady solutions it is recommended that you start with a small underrelaxation factor for the volume fraction, it is also recommended not to start with a patch of volume fraction equal to zero. Another option is to start with a mixture multiphase calculation, and then switch to the Eulerian multiphase model.
Stratified flows of immiscible fluids should be solved with the VOF model (see Section 23.3). Some problems involving small volume fractions can be solved more efficiently with the Lagrangian discrete phase model (see Chapter 22).
Many stability and convergence problems can be minimized if care is taken during the setup and solution processes (see Section 23.14.4).