Model options
=============


The following inputs are defined using the ``mfix`` prefix.

+------------------------+----------------------------------------------------------------------------+----------+---------------------+
|                        | Description                                                                |   Type   | Default             |
+========================+============================================================================+==========+=====================+
| gravity                | Gravity vector [required]                                                  |   Reals  | 0 0 0               |
+------------------------+----------------------------------------------------------------------------+----------+---------------------+
| advect_density         | Flag to enable time evolution of fluid density.                            |   int    | 0                   |
+------------------------+----------------------------------------------------------------------------+----------+---------------------+
| advect_enthalpy        | Flag to enable time evolution of fluid temperature and enthalpy.           |   int    | 0                   |
+------------------------+----------------------------------------------------------------------------+----------+---------------------+
| solve_species          | Flag to enable the time evolution of fluid species mass                    |   int    | 0                   |
|                        | fractions.                                                                 |          |                     |
+------------------------+----------------------------------------------------------------------------+----------+---------------------+
| constraint             | Select low Mach number constraint.                                         | string   | IncompressibleFluid |
|                        |                                                                            |          |                     |
|                        | * ``IncompressibleFluid`` constraint is appropriate for cold flow systems  |          |                     |
|                        |   that do not have chemical reactions. Fluid temperature and species mass  |          |                     |
|                        |   fractions act as passive tracers if advected.                            |          |                     |
|                        |                                                                            |          |                     |
|                        | * ``IdealGasOpenSystem`` constraint is used if the physical domain         |          |                     |
|                        |   contains at least one pressure outflow boundary condition. The specified |          |                     |
|                        |   outflow (ambient) pressure is maintained so that the thermodynamic       |          |                     |
|                        |   pressure is constant.                                                    |          |                     |
|                        |                                                                            |          |                     |
|                        | * ``IdealGasClosedSystem`` constraint is used for systems that are fully   |          |                     |
|                        |   periodic or completely closed with no inflow / outflow boundaries.       |          |                     |
|                        |   The thermodynamic pressure evolves in time to balance processes like     |          |                     |
|                        |   chemical reactions.                                                      |          |                     |
|                        |                                                                            |          |                     |
+------------------------+----------------------------------------------------------------------------+----------+---------------------+

.. note::
   The thermodynamic pressure is a scalar value, not a scalar field. This is a consequence of
   the low Mach number assumption.


Fluid discretization
--------------------

The following inputs are defined using the ``mfix`` prefix.

+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| Key                             | Description                                                           |   Type      | Default      |
+=================================+=======================================================================+=============+==============+
| advection_type                  | Advection scheme                                                      |  string     |  Godunov     |
|                                 |                                                                       |             |              |
|                                 | * ``mol`` Predictor-Corrector Method of Lines. MAC velocities and     |             |              |
|                                 |   edge states are extrapolated in space.                              |             |              |
|                                 |                                                                       |             |              |
|                                 | * ``godunov`` Godunov. MAC velocities and edge states are             |             |              |
|                                 |   extrapolated in space and time using a second-order Taylor series   |             |              |
|                                 |   expansion.                                                          |             |              |
|                                 |                                                                       |             |              |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| redistribution_type             | Algorithms to address the '*small cell problem*' associated with      |   String    |  StateRedist |
|                                 | explicit cut-cell algorithms.                                         |             |              |
|                                 |                                                                       |             |              |
|                                 | * ``StateRedist`` :cite:p:`berger21,giuliani22`                       |             |              |
|                                 | * ``FluxRedist`` :cite:p:`chern87,colella06`                          |             |              |
|                                 | * ``NoRedist``: Do not redistribute fluxes.                           |             |              |
|                                 |                                                                       |             |              |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| redistribute_before_nodal_proj  | Redistribute the velocity field before the nodal projection           |   Bool      |  True        |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| redistribute_nodal_proj         | Redistribute the velocity field after the nodal projection            |   Bool      |  False       |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| use_drag_coeff_in_proj_gp       | Algebraically consistent p coeff in proj or (default) simplified form |   Bool      |  False       |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| use_drag_in_godunov             | Include a drag term in the Godunov flux or (default) not              |   Bool      |  False       |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| godunov_ppm                     | When using Godunov, use piecewise parabolic (PPM) instead of          |   Bool      |  False       |
|                                 | piecewise linear (PLM) reconstruction                                 |             |              |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| godunov_include_diff_in_forcing | When using Godunov, include viscous/diffusive terms in forcing terms  |   Bool      |  True        |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| godunov_use_forces_in_trans     | When using Godunov, add forcing terms in the construction of `trans`  |   Bool      |  False       |
|                                 | velocities                                                            |             |              |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| use_mac_phi_in_godunov          | When using Godunov, don't include the pressure gradient in the        |   Bool      |  False       |
|                                 | forcing term passed into the godunov routine; instead use gradient of |             |              |
|                                 | mac phi which contains the full pressure                              |             |              |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| correction_small_volfrac        | Threshold volume fraction for correcting small cell velocity          |             |              |
|                                 | at the end of the predictor and corrector                             |   Real      |  1.e-4       |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+



Additional available constraints
--------------------------------

Additional constraints may be imposed on problems which are under-determined such as particle settling
in a fully periodic domain. Currently, only particle constraints are supported.

The following inputs are defined using the ``particles`` prefix.

+---------------------+---------------------------------------------------------------------------+-------------+-----------+
|                     | Description                                                               |   Type      | Default   |
+=====================+===========================================================================+=============+===========+
| constraint          | Constraint type. Available options include:                               | String      | None      |
|                     |                                                                           |             |           |
|                     | * 'mean_velocity'                                                         |             |           |
|                     |                                                                           |             |           |
+---------------------+---------------------------------------------------------------------------+-------------+-----------+

For the `mean_velocity` constraint, the following inputs can be defined.

+------------------------+------------------------------------------------------------------------+-------------+-----------+
|                        | Description                                                            |   Type      | Default   |
+========================+========================================================================+=============+===========+
| mean_velocity_x        | mean particle velocity in dir=0                                        | Real        | Undefined |
+------------------------+------------------------------------------------------------------------+-------------+-----------+
| mean_velocity_y        | mean particle velocity in dir=1                                        | Real        | Undefined |
+------------------------+------------------------------------------------------------------------+-------------+-----------+
| mean_velocity_z        | mean particle velocity in dir=2                                        | Real        | Undefined |
+------------------------+------------------------------------------------------------------------+-------------+-----------+

Below is an example for zero mean particle velocity in all three directions.

.. code-block:: none

   particles.constraint = "mean_velocity"

   particles.constraint.mean_velocity_x = 0.
   particles.constraint.mean_velocity_y = 0.
   particles.constraint.mean_velocity_z = 0.

In the above example, at the end of each (fluid) time step, the global mean particle velocity is computed
in each direction, then subtracted from each particle's velocity vector so that the mean particle velocity
for the system is zero.


Deposition scheme
-----------------

The following inputs are defined using the ``mfix`` prefix.

+----------------------------+--------------------------------------------------------------------+-----------+---------------+
|                            | Description                                                        |   Type    | Default       |
+============================+====================================================================+===========+===============+
| deposition_scheme          | The algorithm used to transfer particle properties to the Eulerian |  String   | trilinear     |
|                            | grid. An overview of the schemes is provided                       |           |               |
|                            | :ref:`below<deposition_description>`.                              |           |               |
|                            |                                                                    |           |               |
|                            | * ``centroid``                                                     |           |               |
|                            | * ``trilinear``                                                    |           |               |
|                            | * ``true-dpvm``                                                    |           |               |
|                            | * ``trilinear-dpvm-square``                                        |           |               |
+----------------------------+--------------------------------------------------------------------+-----------+---------------+
| deposition_scale_factor    | The deposition scale factor.                                       |  Real     |   1.0         |
|                            |                                                                    |           |               |
|                            | Applicable only with `true-dpvm` and `trilinear-dpvm-square`       |           |               |
|                            | deposition schemes. The value must be in the intervarl             |           |               |
|                            | :math:`[0,\Delta x/2]` where :math:`\Delta x` is the mesh spacing. |           |               |
+----------------------------+--------------------------------------------------------------------+-----------+---------------+
| deposition_diffusion_coeff | If a positive value is set, a diffusion equation                   |  Real     |   -1.0        |
|                            | with this diffusion coefficient is solved to                       |           |               |
|                            | smooth deposited quantities.                                       |           |               |
+----------------------------+--------------------------------------------------------------------+-----------+---------------+


.. _deposition_description:

In the following subsections, the four deposition methods are briefly
described and illustrated.

Centroid
~~~~~~~~

The centroid deposition scheme transfers particle properties, for example
volume, to the Eulerian grid cell containing its center. Illustrated in
two-dimensions below, the particle center is in the upper-right cell; therefore,
the deposition weight for that cell is 1 and all other weights are zero.

.. _fig_centroid_deposition:

.. figure:: ./images/centroid.jpg
   :height: 2in
   :align: center
   :alt: visual demonstration of centroid deposition

   Example of centroid deposition.


Trilinear
~~~~~~~~~

Trilinear deposition transfers particle properties to the eight Eulerian grid
cells surrounding its center. First, low and high weights are calculated for each
direction, then they are multiplied together to form eight composite weights.
The following figure illustrates bilinear interpolation. The left image
shows weights computed in the *X*-direction, :math:`[0.15, 0.85]`, and the center
image shows weights computed in the *Y*-direction, :math:`[0.25,0.75]`. The right
image shows the composite weights for the four cells that comprise the 2D
neighborhood surrounding the particle.

.. _fig_trilinear_deposition:

.. figure:: ./images/trilinear.jpg
   :height: 2in
   :align: center
   :alt: visual demonstration of trilinear deposition

   Example of trilinear deposition.


True-DPVM
~~~~~~~~~

The "*true*" divided particle volume method (DPVM) calculates the actual volume
of a particle in each cell by computing the intersection between the grid cell
faces and the particle. The fractional volume (volume inside the cell
divided by total particle volume) is used as the deposition weight. If the
``deposition_scale_factor`` is defined, it multiplies the particle radius
increasing the effective area to transfer properties over.  The true-DPVM scheme is
illustrated below where the dashed line represents the scaled particle radius.
The *grown* particle intersects the top two and lower-right cells. Since the *grown*
particle does not intersect the lower-left cell, it has a deposition weight of zero.

.. figure:: ./images/dpvm.jpg
   :height: 2in
   :align: center
   :alt: visual demonstration of true divided particle volume deposition

   Example of true-dpvm deposition.



Trilinear DPVM square
~~~~~~~~~~~~~~~~~~~~~

The trilinear DPVM square method is similar to the trilinear scheme in that
low and high weights are computed for each direction, then multiplied together
to give a composite weight for each of the eight cells in the neighborhood.
The primary distinction is that the fractional volume cut by the grid cell
face for each direction is used to calculate the low and high weights. If the
``deposition_scale_factor`` is defined, it multiplies the particle radius
increasing the effective area to transfer properties over. In the below figure,
the left image shows the *X*-direction weights, and the center image shows
the *Y*-direction weights. The right image shows the resulting composite weights.


.. figure:: ./images/square_dpvm.jpg
   :height: 2in
   :align: center
   :alt: visual demonstration of true divided particle volume deposition

   Example of trilinear dpvm square deposition.



Deposition redistribution
-------------------------

The following inputs are defined using the ``mfix`` prefix.

+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| Key                             | Description                                                           |   Type      | Default      |
+=================================+=======================================================================+=============+==============+
| deposition_redist_type          | Algorithm used to redistribute excess solids to adjacent cells. This  |   String    |  MaxPack     |
|                                 | typically applies only to small cut-cells along the geometry.         |             |              |
|                                 |                                                                       |             |              |
|                                 | * ``MaxPack`` If the solids volume fraction exceeds the prescribed    |             |              |
|                                 |   ``max_solids_volume_fraction``, then the local average solids       |             |              |
|                                 |   volume is computed and solids volume exceeding the average is       |             |              |
|                                 |   reallocated to adjacent cells. The fraction of redistributed solids |             |              |
|                                 |   is applied to all transferred properties.                           |             |              |
|                                 |                                                                       |             |              |
|                                 | * ``StateRedist`` Use the state redistribution algorithm of           |             |              |
|                                 |   :cite:p:`berger21,giuliani22` to reallocate                         |             |              |
|                                 |   transferred quantities in small cells.                              |             |              |
|                                 |                                                                       |             |              |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| max_solids_volume_fraction      | Threshold solids volume fraction for `MaxPack` redistribution.        |  Real       |  0.6         |
|                                 | Redistribution of transferred quantities **only** occurs when the     |             |              |
|                                 | solids volume fraction of a cell exceeds this value.                  |             |              |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+
| deposition_redist_vfrac         | Threshold geometric volume fraction ``StateRedist`` redistribution.   |  Real       |  0.1         |
|                                 | Redistribution of transferred quantities **always** occurs for cells  |             |              |
|                                 | with a geometric volume fraction below this value.                    |             |              |
+---------------------------------+-----------------------------------------------------------------------+-------------+--------------+




Fluid-particle drag
-------------------

The following input is defined using the ``mfix`` prefix.

+--------------------------+-----------------------------------------------------------------------+-------------+-----------+
|                          | Description                                                           |   Type      | Default   |
+==========================+=======================================================================+=============+===========+
| model                    | Fluid-particle drag model.                                            | String      | None      |
|                          |                                                                       |             |           |
|                          | * ``WenYu`` :cite:p:`Wen66`                                           |             |           |
|                          | * ``Gidaspow`` :cite:p:`ding90,Lat00`                                 |             |           |
|                          | * ``BVK2`` :cite:p:`Bee07,Bee071,tenneti11,tang15`                    |             |           |
|                          | * ``SyamOBrien`` :cite:p:`syam88`                                     |             |           |
|                          | * ``UserDrag``                                                        |             |           |
|                          |                                                                       |             |           |
+--------------------------+-----------------------------------------------------------------------+-------------+-----------+
| model.SyamOBrien.c1      | Fitting parameters for ``SyamOBrien`` drag model                      | Real        | None      |
| model.SyamOBrien.d1      |                                                                       |             |           |
+--------------------------+-----------------------------------------------------------------------+-------------+-----------+
| include_divtau           | Interpolate the fluid shear stress to particles and include in the    | int         | 0         |
|                          | fluid-particle drag force. The force is applied to the fluid by       |             |           |
|                          | multiplying the shear stress by fluid volume fraction.                |             |           |
+--------------------------+-----------------------------------------------------------------------+-------------+-----------+


.. note::

  The ``UserDrag`` keyword is used to invoke a user defined drag model. This is accomplished
  by copying ``src/usr/usr_drag.cpp`` file into the build directory, implementing the desired
  drag model, and recompiling the code.  An example can be found in ``tests/DEM06-x``.


Heat transfer coefficients
--------------------------

The following input is defined using the ``mfix.convection`` prefix.

+-------------------+-----------------------------------------------------------------------+-------------+--------------+
|                   | Description                                                           |   Type      |  Default     |
+===================+=======================================================================+=============+==============+
| model             | Fluid-particle heat transfer coefficient model.                       | String      | RanzMarshall |
|                   |                                                                       |             |              |
|                   | * ``RanzMarshall`` :cite:p:`ranz52`                                   |             |              |
|                   | * ``Gunn`` :cite:p:`gunn78`                                           |             |              |
|                   | * ``NullConvection`` - disable fluid-particle heat transfer           |             |              |
|                   |                                                                       |             |              |
+-------------------+-----------------------------------------------------------------------+-------------+--------------+
