=================================
Restarting a simulation
=================================

The restart functionality of ComFLOW can be used to restart a simulation, for example after a
system failure or power breakdown. It can also be used ''by design" to restart a simulation with
modified simulation settings. One could for example start a simulation with large Poisson
tolerance ``linear_solver/stopping_criterion/outer_p`` (see :numref:`linear_solver/stopping_criterion`) for faster spin-up of the simulation and then continue with a smaller value for increased accuracy. Similarly one could
start-up a simulation with a large time step and then continue with a smaller one by reducing
the CFL tolerance (see :numref:`Chapter %s <time_integration>`).

It is possible to apply a restart with almost any change of simulation settings as long as it does
not concern the domain and grid setup or type of simulation (e.g. one or two-phase).
As a general rule, one should be careful with changing simulation settings after a restart.
The time step size as well as the end time of the simulation can be changed.
The time interval for post-processing output (snapshots, monitor points, etc.)
is identical to that of the original simulation unless the attribute ``@reset_counters~`` is set to ``"true"``.


The ComFLOW program includes the possibility to save your simulation and restart it at a later time.
Special snapshot files including additional information about the simulation status can be
written at a specified time interval. The settings related to the restart functionality are
included in the XML section ``/comflow/restart/``.
The following parameters can be specified:

* ``@tmin``, ``@tmax``, ``@dt``, ``@nr``:
  Output interval using the notation as explained in :numref:`xml_documentation/time_intervals`.
* ``@reset_counters``:
  If this parameter is set to ``"true"`` all the counters for post-processing output are reset upon restart, otherwise counting just continues where ComFLOW left off.

Upon restart ComFLOW reinterprets the main input file(s) hence it is possible to restart
a simulation with modified settings (e.g. a different convection scheme or a different CFL limit).

.. note::
   Be careful when changing simulation settings after a restart.
   Some settings such as the simulation type (e.g. one-phase/two-phase, with/without moving bodies, etc.) *cannot* be changed afterwards.

-------------------------------
Example
-------------------------------

The following code can be used to produce restart files at a given interval:

.. code-block:: xml

  <comflow>
     <restart dt="1.0" />
  </comflow>

The restart files will be stored at the locations ``${OUTPUT}/restart/autosave####.save``, where ``####`` is replaced by the file counter. It is recommended to copy or move important restart files to a new location outside the
subdirectory ``restart`` since this will prevent it from being overwritten by subsequent simulations.

To restart a simulation use the command-line option ``--restart RESTART`` where ``RESTART`` has to be replaced by the location
of the desired restart file. For more details, see :numref:`Chapter %s <commandline>`.


It is possible to restart the simulation with a modified input file. The option ``restart/@reset_counters`` causes the output intervals and counters of restart files and other post-processing output to be reset to the values specified in the *new* input file. Compare the following input section with that of the original input file listed above:

.. code-block:: xml

  <comflow>
     <restart dt="0.1" reset_counters="true"/>
  </comflow>




.. .. rubric:: Footnotes



.. .. rubric:: References

.. .. bibliography:: references.bib