# Phase fraction clipping fix

## Description

This case tests a fix related to the alpha clipping option (see explanations
below). It involves a constant mass transfer from `air1` to `air2` within a
stagnant water phase. The clipping becomes relevant because the transfer rate
is kept constant regardless of the remaining amount of `air1`, which is
inconsistent.

## Technical information

- 2D
- 3 phases
- constant mass transfer rate between 2 the two gas phases
- phase fraction values initialized as constants
- gravity points in third direction (no buoyancy, but still non-zero magnitude
  in case this is relevant for some closures)

## Post-Processing

The divergence at the reference point position is plotted over time.

## Additional Remarks

### Issue

Without the fix, divergence grows when ALL of the following conditions are met:

- `clip` is activated for alpha in `fvSolution`.
- The case runs parallel.
- There are at least three phases, locally coexisting with non-zero phase
  fraction.
- There is a phase fraction transfer between two phases. This can be a mass
  transfer, a morphology transfer (e.g. a transfer from `air1` to `air2`), or
  a volume transfer like applied in the degassing boundary condition.
- There is slip between the phases. The higher the relative velocity, the
  larger the divergence.

The divergence is created at the pressure reference point.

### Fix

Even for cases that are not perfectly consistent, serial and parallel runs
should yield identical results. To ensure this, an additional
`correctBoundaryConditions` call was introduced in `phaseSystemSolve`. This
ensures that the clipping is correctly communicated across processor boundaries
in parallel runs.
