.. _fluidtutorial:

Fluid Tutorial
==============

Introduction
------------

In this tutorial we want to simulate the incompressible flow past a
circular cylinder. For further details and references we refer the reader to:
[Wall99]_


.. figure:: /_assets/tut_fluid_problem.jpg
   :alt: Problem definition and geometrical setup (with friendly permission ;-))
   :width: 95%
   :align: center

   Problem definition and geometrical setup (with friendly permission ;-))


Preprocessing
-------------

Creating the Geometry with Cubit
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We will use Cubit for creating the geometry and the mesh.
Within Cubit, open the Journal-Editor (*Tools* :math:`\to` *Journal
Editor*), paste the text below and press *play*.
After successful geometry and mesh creation, export everything to an
EXODUS file of your choice via *File* :math:`\to`\ *Export...* and
set the dimension explicitly to 2D.

.. literalinclude::  /tutorial_fluid.jou
   :linenos:

The generated mesh should look like this:

.. figure:: /_assets/tut_fluid_mesh.jpg
   :alt: Mesh for a flow past a circular cylinder.
   :name: fig_domainDecomposition
   :width: 95.0%
   :align: center

   Mesh for a flow past a circular cylinder.

Working with |FOURC|
--------------------

General Procedure of Creating a Valid |FOURC| Input File
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The execution of |FOURC| is controlled by an input file. Let us create a file
``tutorial_fluid.4C.yaml`` and enter the essential parameters:

.. code-block:: yaml

    PROBLEM TYPE:
      PROBLEMTYPE: Fluid
    FLUID DYNAMIC:
      LINEAR_SOLVER: 1
      PREDICTOR: explicit_second_order_midpoint
      NUMSTEP: 10
      RESTARTEVERY: 1
    SOLVER 1:
      SOLVER: UMFPACK
    MATERIALS:
    - MAT: 1
      MAT_fluid:
        DYNVISCOSITY: 0.004
        DENSITY: 1
    FUNCT1:
    - SYMBOLIC_FUNCTION_OF_SPACE_TIME: 0.5*(sin((t*pi/0.1)-(pi/2)))+0.5
    




The mesh is read from the EXODUS file created in the previous step. We can read the mesh as
follows:

.. code-block:: yaml

    FLUID GEOMETRY:
      FILE: tutorial_fluid.e
      ELEMENT_BLOCKS:
      - ID: 1
        ELEMENT_NAME: FLUID
        ELEMENT_DATA: MAT 1 NA Euler
    




This tells |FOURC| to read the mesh as the fluid geometry and assign corresponding
elements.

Setting the boundary conditions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The boundary conditions are set as follows:

.. code-block:: yaml

    DESIGN LINE DIRICH CONDITIONS:
    - E: 1
      ENTITY_TYPE: node_set_id
      NUMDOF: 3
      ONOFF:
      - 1
      - 1
      - 0
      VAL:
      - 1.0
      - 0.0
      - 0.0
      FUNCT:
      - 1
      - null
      - null
    - E: 2
      ENTITY_TYPE: node_set_id
      NUMDOF: 3
      ONOFF:
      - 0
      - 1
      - 0
      VAL:
      - 0.0
      - 0.0
      - 0.0
      FUNCT:
      - null
      - null
      - null
    - E: 3
      ENTITY_TYPE: node_set_id
      NUMDOF: 3
      ONOFF:
      - 0
      - 1
      - 0
      VAL:
      - 0.0
      - 0.0
      - 0.0
      FUNCT:
      - null
      - null
      - null
    - E: 4
      ENTITY_TYPE: node_set_id
      NUMDOF: 3
      ONOFF:
      - 1
      - 1
      - 0
      VAL:
      - 0.0
      - 0.0
      - 0.0
      FUNCT:
      - null
      - null
      - null
    DESIGN POINT DIRICH CONDITIONS:
    - E: 5
      ENTITY_TYPE: node_set_id
      NUMDOF: 3
      ONOFF:
      - 1
      - 1
      - 0
      VAL:
      - 1.0
      - 0.0
      - 0.0
      FUNCT:
      - null
      - null
      - null
    





Running a Simulation with |FOURC|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Execute 4C as usual:

.. container:: center

   ``./4C <input_directory>/<input_file_name> <output_directory>/output_prefix``

The prefix that you choose will be applied to all output files that |FOURC| generates.

Postprocessing
--------------

You can postprocess your results with any visualization software you
like. In this tutorial, we choose *Paraview*.

Filtering result data
~~~~~~~~~~~~~~~~~~~~~

-  Before you can admire your results, you have to generate a filter
   which converts the generic binary |FOURC| output to the desired format.
   Starting from the ``build-release`` directory, execute
   ``make post_drt_ensight``.

-  The filter should now be available in the ``build-release`` folder.
   Filter your results with the following call inside the
   ``build-release`` folder:

   .. container:: center

      ``./post_drt_ensight - -file=<outputdirectory>/outputprefix``

-  Further options of the filter program are made visible by the command
   ``./post_drt_ensight –help``

Visualize your results in Paraview
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-  After the filtering process is finished open *paraview* by typing

::

      paraview &

-  *File :math:`\to` Open* and select the filtered ``*\*.case*`` file:
   *outputprefix_fluid.case*

-  Press *Apply* to activate the display.

-  Set the time step in the top menu bar to :math:`19` (:math:`=0.2`).

-  In the *Color* section you can now choose between *pressure* and
   *velocity*. Select velocity and pick the :math:`X`-component from the
   adjacent drop-down menu. Then press the *Rescale* button and the
   *Show* button. You receive a visualization of the :math:`X`-velocity
   field, which should look similar to this figure:

.. figure:: /_assets/tut_fluid_xvel.png
   :alt: X-velocity for a flow past a circular cylinder
   :name: fig_FlowPastCylinder_x-velocity
   :width: 95.0%
   :align: center

   :math:`X`-velocity for a flow past a circular cylinder