Add porous media input section (!104)

docs/source_docs/user_guide/inputs/porous_media_defs.rst

+.. _InputsPorousMediaDefs:
+
+Porous media
+============
+
+Homogeneous porous media are modeled by the addition of a momentum
+source term, :math:`\boldsymbol{S}_{pm}`, to the fluid momentum equation.
+
+.. math::
+
+   \boldsymbol{S}_{pm} =-\frac{\mu_f}{C_1} \boldsymbol{u}_f - C_2\frac{1}{2}\rho_f\lvert \boldsymbol{u}_f \rvert \boldsymbol{u}_f
+
+:math:`\mu_f`, :math:`\rho_f`, and :math:`\boldsymbol{u}_f` are the fluid viscosity, density, and velocity and
+:math:`C_1` and :math:`C_2` are user defined constants.
+
+The following inputs are defined using the ``pm`` prefix.
+
++--------------------------+------------------------------------------------------------------------------+----------+-----------+
+|                          | Description                                                                  |   Type   | Default   |
++==========================+==============================================================================+==========+===========+
+| regions                  | Regions used to define porous media.                                         | String   |  None     |
++--------------------------+------------------------------------------------------------------------------+----------+-----------+
+| [region].fluid.volfrac   | Fluid volume fraction of the porous media region.                            |  Real    |  None     |
++--------------------------+------------------------------------------------------------------------------+----------+-----------+
+| [region].c1              | Porous media permeabilty constant for the source term in fluid               |  Real    |  None     |
+|                          | momentum equation.                                                           |          |           |
++--------------------------+------------------------------------------------------------------------------+----------+-----------+
+| [region].c2              | Porous media inertial resistance factor for the source term in fluid         |  Real    |  None     |
+|                          | momentum equation.                                                           |          |           |
++--------------------------+------------------------------------------------------------------------------+----------+-----------+

docs/source_docs/user_guide/run-time_inputs.rst

@@ -50,6 +50,7 @@ keywords such as ``mfix``, ``amr``, ``geometry``, ``nodal_proj`` etc.
    inputs/fluid_model
    inputs/solids_model
    inputs/species_defs
+   inputs/porous_media_defs
    inputs/chemical_reactions
    inputs/regions_defs
    inputs/initial_conditions