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23.14.2 VOF Model

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 round-off 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 $\rightarrow$ 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 round-off 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 body-force-weighted pressure interpolation scheme or the PRESTO! scheme.

Solve $\rightarrow$ Controls $\rightarrow$ 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 Donor-Acceptor can be made available under Volume Fraction Discretization by using the following text command:

solve $\rightarrow$ set $\rightarrow$ 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.

figure   

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

figure   

In VOF modeling, using a high-order discretization scheme for the momentum transport equations may reduce the stability of the solution compared to cases using first-order discretization. In such situations, it is recommended to use a low-order variant of Rhie-Chow face flux interpolation, which can be turned on using the text command:

solve $\rightarrow$ set $\rightarrow$ numerics

When asked to disable high order Rhie-Chow flux?[no], enter yes.



Pressure-Velocity Coupling and Under-Relaxation for the Time-dependent Formulations


Another change that you should make to the solver settings is in the pressure-velocity coupling scheme and under-relaxation factors that you use. The PISO scheme is recommended for transient calculations in general. Using PISO allows for increased values on all under-relaxation factors, without a loss of solution stability. You can generally increase the under-relaxation 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 under-relaxation factor of 0.7-0.8 for pressure is recommended for improved stability with the PISO scheme.

Solve $\rightarrow$ Controls $\rightarrow$ Solution...

As with any FLUENT simulation, the under-relaxation factors will need to be decreased if the solution exhibits unstable, divergent behavior with the under-relaxation factors set to 1. Reducing the time step is another way to improve the stability.



Under-Relaxation for the Steady-State Formulation


If you are using the steady-state implicit VOF scheme, the under-relaxation factors for all variables should be set to values between 0.2 and 0.5 for improved stability.


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Next: 23.14.3 Mixture Model
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