
Several recommendations for improving the accuracy and convergence of the VOF solution are presented here.
Setting the Reference Pressure Location
The site of the reference pressure can be moved to a location that will result in less roundoff in the pressure calculation. By default, the reference pressure location is the center of the cell at or closest to the point (0,0,0). You can move this location by specifying a new Reference Pressure Location in the Operating Conditions panel.
Define Operating Conditions...
The position that you choose should be in a region that will always contain the least dense of the fluids (e.g., the gas phase, if you have a gas phase and one or more liquid phases). This is because variations in the static pressure are larger in a more dense fluid than in a less dense fluid, given the same velocity distribution. If the zero of the relative pressure field is in a region where the pressure variations are small, less roundoff will occur than if the variations occur in a field of large nonzero values. Thus in systems containing air and water, for example, it is important that the reference pressure location be in the portion of the domain filled with air rather than that filled with water.
Pressure Interpolation Scheme
For all VOF calculations, you should use the bodyforceweighted pressure interpolation scheme or the PRESTO! scheme.
Solve Controls Solution...
Discretization Scheme Selection for the Implicit and Explicit Formulations
When the implicit scheme is used, the available options for Volume Fraction Discretization are
When the explicit scheme is used, the available options for Volume Fraction Discretization are
When using the explicit scheme, First Order Upwind, Second Order upwind, and DonorAcceptor can be made available under Volume Fraction Discretization by using the following text command:
solve set expert
You will be asked a series of questions, one of which is
Allow selection of all applicable discretization schemes? [no] 
to which you will respond yes.

You are encouraged to use the CICSAM scheme, as it gives a sharper interface than the modified HRIC scheme.


In VOF modeling, using a highorder discretization scheme for the momentum transport equations may reduce the stability of the solution compared to cases using firstorder discretization. In such situations, it is recommended to use a loworder variant of RhieChow face flux interpolation, which can be turned on using the text command:
solve set numerics
When asked to disable high order RhieChow flux?[no], enter yes.

PressureVelocity Coupling and UnderRelaxation for the Timedependent Formulations
Another change that you should make to the solver settings is in the pressurevelocity coupling scheme and underrelaxation factors that you use. The PISO scheme is recommended for transient calculations in general. Using PISO allows for increased values on all underrelaxation factors, without a loss of solution stability. You can generally increase the underrelaxation factors for all variables to 1 and expect stability and a rapid rate of convergence (in the form of few iterations required per time step). For calculations on tetrahedral or triangular meshes, an underrelaxation factor of 0.70.8 for pressure is recommended for improved stability with the PISO scheme.
Solve Controls Solution...
As with any FLUENT simulation, the underrelaxation factors will need to be decreased if the solution exhibits unstable, divergent behavior with the underrelaxation factors set to 1. Reducing the time step is another way to improve the stability.
UnderRelaxation for the SteadyState Formulation
If you are using the steadystate implicit VOF scheme, the underrelaxation factors for all variables should be set to values between 0.2 and 0.5 for improved stability.