.. _initialization:

Initial conditions
===================================

This chapter describes options for initialization of the flow solution.


Initial liquid distribution
------------------------------

There are several methods for specifying the initial liquid distribution:

* No initialization (all cells are empty).
* Initialization by the built-in wave model as specified in the section ``waves/``. This option is enabled
  when ``waves/model/@model`` is set to anything but ``"none"``.
  The attribute ``@start_with_still_water`` can be set to ``"true"`` or ``"false"`` to start with still water or a fully developed wave field. The attributes ``@initialize_fs`` must be set to ``"true"``.
* Initialization by a plug-in as specified in the section ``coupling/external_solution/``. This option is only applied
  when the attribute ``external_solution/@specialbc_initialize`` is set to ``"true"``.

The above methods cannot be combined in one simulation.
However, it is possible to add additional liquid sections by means of the options described in sections :numref:`%s <initial_conditions/liquid_boxes>`, :numref:`%s <initial_conditions/liquid_objects>` and :numref:`%s <initial_conditions/greenwater>`.

* basic geometric shapes by means of the ``liquid/box/`` construct,
* detailed configurations by means of the ``liquid/object/`` construct, which contains geometrical building blocks, similar to those used for the geometry definition (this option was formerly implemented by means of the program LIQDEF),
* a dedicated option for greenwater simulations, located in the section ``liquid/greenwater/``.


.. _initial_conditions/liquid_boxes:

Liquid boxes
..............

In many situations only a simple initial liquid configuration is required.
Using the ``box/`` construct one or more bounding boxes can be filled with liquid.

A frequently encountered example is the "still water" box which defines the half-space :math:`z\leq0`:

.. code-block:: xml

      <initial_conditions>

           <liquid>

                <box shape="rectangular" mask="0">-Inf Inf   -Inf Inf   -Inf 0.</box>

           </liquid>

      </initial_conditions>



.. _initial_conditions/liquid_objects:

Liquid building blocks
...........................

In the following example the simulation is initialized with still water and
a spherical droplet of water with a radius of 5 cm is included at the location (0,0,1) using a spherical building block, which is documented in :numref:`Chapter %s <geometry_definition>`:

.. code-block:: xml

      <initial_conditions>

           <liquid>

                <!-- still water -->
                <box shape="rectangular" mask="0">-Inf Inf   -Inf Inf   -Inf 0.</box>

                <!-- spherical droplet -->
                <object>
                    <element shape="4">
                        0.   0.   1.
                        0.05 0.05 0.05
                    </element>
                </object>

           </liquid>

      </initial_conditions>

In the following example the initial liquid configuration is stored in
an external file using the ``file_reader/`` construct as explained in :numref:`Chapter %s <geometry_definition>`:

.. code-block:: xml

      <initial_conditions>

           <liquid>

                <!-- still water -->
                <box shape="rectangular" mask="0">-Inf Inf   -Inf Inf   -Inf 0.</box>

                <!-- add extra liquid as specified in external file -->
                <object>
                     <file_reader geodef_legacy_file="${INPUT}/liquid.in" filter="all"/>
                </object>

           </liquid>

      </initial_conditions>

The file ``${INPUT}/liquid.in`` contains the same information as would be read by the pre-processing program LIQDEF, which was included in previous ComFLOW. In versions 4.0 and later pre-processing is no longer necessary.


.. _initial_conditions/greenwater:


Green water box
...............................

*This option is currently not officially supported.*


..
  .. code-block:: none

    ------------------------------------------------------------------------
    -- green water parameters ----------------------------------------------
    grnwtr
    1
    ------------------------------------------------------------------------
    high    low     length
    12.0    -5.0    30.0
    ------------------------------------------------------------------------
    width   a       b
    47.0    -0.056  6.0
    ------------------------------------------------------------------------


  * ``grnwtr``: this variable is an integer with value 0 or 1.
    When ``grnwtr`` equals 1, the initial liquid configuration is a
    dam of water described by the parameters
    ``high``, ``low`` and ``length`` and the form of the deck.
    Remark: in versions 3.1.2 and earlier, the XZ-walls of the domain were made free slip automatically if ``grnwtr=1``.
    In later versions this is no longer the case and any free-slip boundaries need to be specified manually (see \cref  {section:comflowin:inoutflow} as well as the example ``greenfixedbow`` in the examples directory).}

    Outflow boundaries can be defined using the input lines below (\cref{section:comflowin:inoutflow}).
    When ``grnwtr`` equals 0, the initial liquid configuration is given
    at the line of the 'definition initial liquid configuration'
    (\cref{section:comflowin:liquid}) or by the parameter ``wave`` (see
    next lines of ``comflow.in``, \cref{section:comflowin:waves}). The
    geometry of the ship should be defined using the pre-processor
    GEODEF.
  * ``high``, ``low`` and ``length``: the initial water distribution (the
    wall of water) is described by these parameters; ``high`` is the maximum
    height of the wall at the front of the bow, ``low`` is the minimum height
    of the wall achieved at ``length`` meter behind the bow.
    Between ``low`` and ``high`` a linear interpolation is used. The situation
    achieved by setting ``high=12.0``, ``low=-5.0`` and
    \param{length}{30.0} is given in \cref{fluiddistr} (side-view in
    XZ-plane and top view).
  * ``width``: this variable stands for the width of the deck.
  * ``a`` and ``b``: the form of the bow shape in the dam of
    water is given by a parabola of the form $x = \verb|a| y^2 +
    \verb|b|$. The values for the default water dam configuration are:
   ``a=0.056}, ``b=-6.0`` (see \cref{fluiddistr}).


Initial velocity field
------------------------------

The velocity field is initialized to zero, unless one of the following holds:

* a wave model is active,
* a plug-in is used.

.. ..rubric:: Footnotes

.. ..rubric:: References

.. ..bibliography:: references.bib