.. _two_phase

Two-phase flow
===================================

This chapter describes various aspects of the two-phase flow model in ComFLOW.


General
---------------------------

The following solution modes can be set in the attribute ``options/@multi_phase``:


.. list-table::
    :widths: 25 75
    :header-rows: 1

    * - ``"@multi_phase"``
      -
    * - ``"0"``
      - incompressible one-phase flow
    * - ``"1"``
      - incompressible/compressible two-phase flow (phase 1 is incompressible, phase 2 is compressible)
    * - ``"2"``
      - incompressible two-phase flow (both phases are incompressible)


When using the two-phase model of ComFLOW, the density is discretized in the cell centers (the locations of the pressures) and an averaging method has to be used to determine the density on the cell faces (the locations of the velocities).
The density averaging method can be selected with the attribute ``numerical/interpolations/@density_averaging``.
The following methods are available (recommended in bold):

.. list-table::
    :widths: 25 75
    :header-rows: 1

    * - ``"@density_averaging"``
      -
    * - ``"weighted"``
      - a basic weighted average
    * - ``"gravity_consistent"``
      - a gravity consistent averaging method as described in  :cite:`wemmenhove2008numerical`, that accounts for the local fluid distribution
    * - ``"improved"``
      - **a gravity consistent averaging method based on an improved reconstruction of the free-surface interface**

Fluid properties
---------------------

For both phases physical properties can be specified. These settings can be provided in the XML sections
``physical/liquid_phase`` and ``physical/gas_phase``.
The following phase attributes can be set:

.. list-table::

    * - ``density``
      - The density expressed in ``kg m^-3``
    * - ``viscosity``
      - The dynamic viscosity expressed in ``kg/m.s``
    * - ``gamma``
      - The expansion coefficient in the equation of state. For air :math:`\gamma=1.4` is frequently used. This attribute is ignored for the liquid phase (phase 1).

Note that for a one-phase simulation the settings in ``physical/gas_phase/`` are ignored.

The atmospheric pressure can be specified with attribute ``physical/@atmospheric_pressure``.
A typical value for the atmospheric pressure is 10,000 Pa.


Surface tension
--------------------

*The surface tension model of ComFLOW is currently undergoing significant changes, more information will follow.*

..
    * ``sigma`` $[N/m]$ is the kinematic surface tension ($\sigma/\rho1$) of
      the fluid, which is only needed when the simulation has a free surface,
      so when \param{slosh}{1}. Without a free surface this value is irrelevant.
    * ``theta`` denotes the contact angle between the free surface and the
      solid boundary, given in degrees (so $0\leq$ {\tt theta}$\leq 180$); only used
      when \param{slosh}{1} and \param[>]{sigma}{0.0}.




.. ..rubric:: Footnotes

.. ..rubric:: References

.. ..bibliography:: references.bib