.. _fluid_displacement

Fluid displacement
===================================

ComFLOW provides three methods for modeling the fluid displacement.
The attribute ``options/@free_surface`` can be set to ``"true"`` or ``"false"`` to indicate the presence or absence of a free surface, respectively.
If no free surface is present, all cells are assumed to be fully filled with liquid and no additional settings have to be provided.

In the presence of a free surface ComFLOW provides two categories of interface reconstruction:

1. simple line interface calculation (SLIC), see :numref:`fluid_displacement/slic`,
2. piecewise linear interface calculation (PLIC), see :numref:`fluid_displacement/plic`.

For most applications it is recommended to select a method from the second category since it generally yields more accurate results, in particular for wave simulations or simulations involving strong free-surface dynamics, e.g. sloshing, breaking waves.

The interface reconstruction method can be selected by setting ``numerical/vof/@method`` to either ``"SLIC_VOF"`` or ``"PLIC_VOF"``.
Depending on which method is selected, additional settings can be provided, which are described below. The input parameters are explained in more detail below. In ComFLOW several volume-of-fluid (VOF) methods are available which fall into two categories. The first category includes methods based on a piecewise constant reconstruction of the free-surface interface (SLIC-VOF).


.. _fluid_displacement/slic:

SLIC-VOF
---------------------------

When ``numerical/vof/@method="SLIC_VOF"`` a SLIC-VOF method is used which is based on the work of Hirt & Nichols.

.. _fluid_displacement/plic:

PLIC-VOF/-MOF
----------------------

When ``numerical/vof/@method="PLIC_VOF"`` a piecewise linear reconstruction is made of the free surface. In grid cells that contain a change of phase (liquid-air or liquid-vacuum) the free surface is described by a line piece (or plane section in 3-D), which is defined by a normal vector and line or constant (or plane constant in 3-D). Various methods are available for calculating the normal vector and line constant.
By means of the attribute ``numerical/vof/@plic_scheme`` the following methods can be selected:

====================            ===================================================================
``@plic_scheme``                description
====================            ===================================================================
``"PLIC/Youngs"``               Youngs' method
``"PLIC/Central"``              central method
``"PLIC/LSG "``                 **least-squares-gradient method**
``"4"``                         ELVIRA
====================            ===================================================================

The advection is discretized using a direction-split scheme. By means of the attribute ``numerical/vof/@advection``
the following options are available:

===================             ==================================================================================
``@advection``                  description
===================             ==================================================================================
``"SP"``                        Sussmann & Puckett scheme
``"MACHO"``                     **multi-dimensional advective conservative hybrid operator (MACHO)**
``"EILE"``                      Euler-implicit-Lagrangian-explicit (only available in 2-D)
``"COSMIC"``                    the COSMIS advection scheme
===================             ==================================================================================


Correction method
-------------------

All of the Volume-of-Fluid methods in ComFLOW can cause small over- and undershoots for the liquid fill ratio.
To ensure mass-conservation and to prevent artificial features, corrections have to be made.
The correction method can be enabled or disabled by setting ``numerical/vof/@correction`` to ``"basic"`` or ``"none"``, respectively.
It is not recommended to switch off the correction method.

.. ..rubric:: Footnotes

.. ..rubric:: References

.. ..bibliography:: references.bib