Why is phase volume or mass not conserved in a VOF simulation?

Problem: the phase amount changes unexpectedly

You may observe a liquid level that slowly falls in a nominally closed tank, integrated phase volume that drifts over time, apparent liquid loss at outlets, or disagreement between total mass conservation and phase-specific conservation.

Total mixture mass can be balanced while the reported phase inventory is still wrong if phase fraction, density or report definitions are inconsistent. Treat mixture and phase balances separately.

Build the complete phase balance

For a phase with density ρ and volume fraction α, the inventory is based on the domain integral of αρ. For incompressible constant-density liquid, phase volume is often an equally useful diagnostic.

Integrate the phase-specific flux at every inlet, outlet, opening and moving boundary. For a moving mesh or changing domain volume, account for the formulation used by the solver rather than applying a stationary control-volume assumption blindly.

Open boundaries are a common source of apparent phase loss

Before calling the loss numerical, visualize phase flux direction on the boundaries and compare it with the physical expectation throughout the transient, not only at one instant.

Then investigate timestep and interface transport

If the physical balance is incomplete, reduce timestep and inspect local interface Courant number. Excessive interface motion per step can increase clipping, boundedness corrections or numerical diffusion depending on the solver formulation.

• Map the smallest cells intersected by the moving interface.
• Use local interface speed rather than only inlet velocity in the timestep estimate.
• Check whether interface-compression settings are too aggressive for the mesh.
• Inspect phase-fraction boundedness and any solver warnings.
• Repeat the balance with a smaller timestep before changing multiple controls.

Check whether the report is measuring what you think

Phase volume, phase mass, mixture mass and volume integral of α are different quantities when density varies or compressibility is enabled. Make sure the field function and integration domain are appropriate.

• Confirm density units and whether density is phase-specific or mixture-based.
• Do not multiply by density twice in a custom report.
• Check that all regions containing the phase are included.
• Use the same reference state and time sampling across cases.
• Distinguish physical phase transfer, evaporation/condensation or source terms from numerical loss.

Fix in a diagnostic order

1. Verify report definitions and units.

   Make sure the observed drift is real.

2. Close the physical phase balance.

   Account for all phase fluxes and sources.

3. Inspect boundary phase behavior.

   Especially backflow, openings and wave outlets.

4. Reduce timestep.

   Test whether the unexplained imbalance decreases.

5. Refine the interface region if needed.

   Keep spatial and temporal resolution coupled.

6. Only then tune interface numerics.

   Change one transport control at a time and retain conservation evidence.