# Vanishing Phase Test

## Description

The case tests the stability of vanishing phases. This is particularly important
for light phases which are weakly coupled or uncoupled to other phases.

The test features a gas phase with zero volume fraction in water. The water is
driven by moving walls. There is no drag activated (phases uncoupled).

### Test 1: Without buoyancy

Without gravity, the gas phase is expected to stay at rest.

### Test 2: With buoyancy

With gravity, gas phase velocity becomes huge. The issue can be solved by
allowing buoyancy to scale down with alpha. The test deactivates the alpha
limiter for the buoyancy term. With this change, the gas phase is expected to
stay at rest.

## Technical Information

- 2D
- 9 cells
- no drag (phases uncoupled)

## Background

Velocity overshoots are likely for light phases which are uncoupled or weakly
coupled (empty regions, disperse gas). Such velocity overshoots may cause
unwanted computational effort (causing tiny time step sizes) or instabilities
(simulation crash). They have to be avoided.

The test proves, for example, that vanishing phases should not be exposed to the
pressure term (i.e. they should not be included in the projection)^[1].

Note that it has not been checked so far if turbulent normal stress can become
problematic in low phase fraction situations.

[1]: https://github.com/OpenFOAM/OpenFOAM-dev/commit/1455345ae0ad7f45d114a73eae04af17b9cd8b78
