Mass Imbalance Correction Model Reference

The Mass Imbalance Correction model enforces mass conservation of a particular Eulerian phase in a VOF simulation. You can activate this model in one (at most) Eulerian phase in a VOF simulation.

In simulations where the flow rate of the phase through the system is very small compared to the volume of the phase in the system, any mass imbalance limits the time-step size that can be used. Every time-step must be converged to a very tight tolerance in order to conserve the mass of phases sufficiently well to avoid accruing error. This process can require large URFs for volume fraction and a small time-step size, which can increase the computational time to an impractical period.

The Mass Imbalance Correction model applies a mass compensation correction that preserves the mass of the fluid phases even if the system is not converged to a very tight tolerance. The mass conservation is enforced through an additional VOF source, which is applied in an explicit correction step of the Segregated VOF solver.

The correction is based on the mass conservation error that is recorded by the Mass Imbalance field. The excessive mass is redistributed among phase interface cells in a weighted manner. The correction is applied to one phase, and the remaining phases are normalized such that $\sum_{i} α_{i} = 1$ . This correction results in near-perfect mass conservation and ensures that $\sum_{T} Δ {\dot{m}}_{i, T} \approx 0$ for the specified phase $i$ , where $Δ {\dot{m}}_{i, T}$ denotes the mass imbalance of phase $i$ in cell $T$ .

Applying the Mass Imbalance Correction

The mass imbalance correction is applied to the relevant phase in the last inner iteration of each time step, provided that such an event can be detected reliably. For each Time model, the following conditions apply:

Implicit Unsteady

The following conditions must be satisfied:

The Maximum Inner Iterations stopping criterion is enabled.
The Maximum Inner Iterations property, Logical Rule, is set to Or.
No other stopping criteria controlling inner iterations are enabled, except (optionally) for the default Stop File criterion.

If any of these conditions is violated, the Mass Imbalance Correction model is deactivated and the corrections are not carried out.

PISO Unsteady

The mass correction is performed in the last PISO corrector step. This is independent of the Update Frequency that is specified in the Segregated VOF solver.

Table 1. Mass Imbalance Correction Model Reference
Provided By	[phase] > Models > Optional Models
Example Node Path	[phase] > Models > Mass Imbalance Correction
Requires	A VOF Multiphase simulation. A Eulerian phase.
Properties	None.