VOF Guidelines

This section describes the set-up and analysis procedures applicable to Volume Of Fluid (VOF) simulations.

See VOF Waves Guidelines for recommendations specific to wave simulations.

Meshing Considerations

Use the following meshing guidelines, where applicable:

Make the mesh fine enough to resolve all geometry of interest and all important flow features.
Refine the mesh locally to resolve flow separation and shed vortices.
Either use a trimmed cell mesh and align the mesh with the free surface, or use polyhedral cells and inspect the results to make sure that they resolve the free surface properly.
Set mesh size to meet the requirements for any wall treatment models.

To estimate the size that is needed for near-wall cells, calculate the value for y+ as:

y^{+} = \frac{u^{*} y}{v}

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where $y$ is the distance from the wall, $v$ is the fluid velocity and $u^{*}$ is the reference velocity:

u^{*} \approx \sqrt{\frac{τ_{w}}{ρ}}

where $ρ$ is the fluid density and $τ_{w}$ is the wall shear stress. $τ_{w}$ is related to the skin friction coefficient ${\bar{C}}_{f}$ :

τ_{w} = \frac{1}{2} {\bar{C}}_{f} ρ U_{\infty}^{2}

where $U_{\infty}$ is the free stream velocity.

Physics Models

When selecting physics models, use the following guidelines where applicable:

Remember to select the Gravity model.
Remember that the Cavitation and Surface Tension models are available with VOF.
Define Eulerian Phases for the fluids in the system.
For internal flows, use K-Epsilon models.
For external flows with little or no separation, use Spalart-Allmaras models, or K-Omega models as a second choice.
For external flows with more severe separation, use K-Omega models.
For flows with anisotropic turbulence, use Reynolds Stress Transport models.
Use the All-y+ Wall Treatment whenever it is compatible with the selected model.
Use the Interface Momentum Dissipation model in cases of:
- Low-speed flows that are dominated by surface tension.
- Wave models with unrealistically high velocities in the gas phase near free surface.
Set reference densities to zero or to the density of the lightest phase. See Variable Density Flows.

Analysis Controls

Use the following analysis guidelines, where applicable:

Use double precision if the single-precision results are unsatisfactory, or if the difference in density between phases is large.
Set stopping criteria using key variables rather than a specific number of iterations.
In most cases, run the analysis as transient, even if the final solution must be steady-state.
For steady solutions, set the lower and upper Courant numbers CFL_l and CFL_u to large values, for example, CFL_l = 50, CFL_u = 100.
This practice ensures that the HRIC scheme is always used at free surfaces.

See Volume of Fluid (VOF) Properties.
Use the default second-order space differencing scheme unless poor grid quality or other reasons require the use of a less accurate but more robust first-order discretization.
Use the default first-order time differencing scheme when expecting a steady-state or weakly transient solution. Use the second-order scheme when accurate wave propagation or similar phenomena must be simulated.
To estimate time-step, monitor the Courant number at the free surface. Target a Courant-number less than 0.7 for the first-order time discretization. Target a Courant-number less than 0.5 for the second-order time discretization. These targets apply for any cell face in the free-surface zone:
$C = \frac{U Δ t}{Δ x} < 0.7$ (1st-order) or $< 0.5$ (2nd-order)

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where $U$ is a characteristic velocity with one of these definitions:
- For non-naval applications, $U$ is the group velocity of a wave:
  $U = \frac{1}{2} \sqrt{\frac{g λ}{2 π}}$
  
  where $λ$ is the wavelength.
- For naval applications, see VOF Waves Guidelines.

Convergence

To promote convergence, use the following guidelines where applicable:

For multi-component phases, you should always specify initial values of mass fraction or molar fraction for every phase, including any phases that are empty initially (that is, the initial volume fraction of the phase is 0.0). For a phase that is empty initially, it is recommended that you set the initial mass fraction to the mass fraction values that the phase has when it enters the solution domain.
For naval applications, allow enough time for the flow to cross 5–10 vessel lengths.
For transient cases, make sure that the solution is converging to reasonable tolerances at each time-step.
Monitor overall quantities of interest, such as resistance, lift, sinkage, and trim. For steady simulations, these quantities converge to constant values when converged.
Under normal conditions, you need not use the sharpening factor for free surfaces; the HRIC convection discretization scheme produces sharp interfaces by itself (typically within one cell). Use the sharpening factor only when the interface becomes smeared by splashing against walls or by wave overturning (such as in severe sloshing). Interface smearing occurs when the mesh is sufficiently fine to resolve all characteristic length scales of the free surface. Examples are when small droplets are produced by splashing or when the time-step is too large and the simulation cannot properly resolve the motion of the free surface. Typical values of sharpening factor for such cases are around 0.2. Raising the sharpening factor increases the runtime necessary to preserve volume conservation.