Fluid model
===========

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

+------------------------------------------+------------------------------------------------------------+--------+----------+
|                                          | Description                                                |  Type  | Default  |
+==========================================+============================================================+========+==========+
| solve                                    | Specify the name of the fluid or set to None to disable    | String |  None    |
|                                          | the fluid solver. The name assigned to the fluid solver is |        |          |
|                                          | used to specify fluid initial and boundary conditions.     |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| viscosity                                | Specify which viscosity model to use for the fluid.        | String |  None    |
|                                          | Available options include:                                 |        |          |
|                                          |                                                            |        |          |
|                                          | * ``constant`` for constant viscosity model                |        |          |
|                                          |                                                            |        |          |
|                                          | A viscosity model is required if the fluid solver is       |        |          |
|                                          | enabled.                                                   |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| viscosity.constant                       | Constant fluid viscosity.                                  |  Real  |  0       |
|                                          | A value is required for ``constant`` viscosity model.      |        |          |
|                                          |                                                            |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| species                                  | Specify the species that constitute the fluid.             | String |  None    |
|                                          |                                                            |        |          |
|                                          | All listed species must be properly defined. See the       |        |          |
|                                          | :ref:`species definition documentation<InputsSpeciesDefs>` |        |          |
|                                          | for additional details.                                    |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| molecular_weight                         | Constant fluid molecular weight.                           | Real   |    0     |
|                                          |                                                            |        |          |
|                                          | A molecular weight should only be defined when using one   |        |          |
|                                          | of the *ideal gas* constraints and the species equations   |        |          |
|                                          | are not solved.                                            |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| specific_heat                            | Specify which specific heat model to use for the fluid.    | String |  None    |
|                                          | Available options include:                                 |        |          |
|                                          |                                                            |        |          |
|                                          | * ``constant`` - the fluid has a constant specific heat    |        |          |
|                                          |                                                            |        |          |
|                                          | * ``NASA7-poly`` the fluid specific heat is defined by a   |        |          |
|                                          |   low-temperature (T < 1000K) polynomial and a             |        |          |
|                                          |   high-temprature (T > 1000K) polynomial.                  |        |          |
|                                          |                                                            |        |          |
|                                          |   NASA7 polynomial format:                                 |        |          |
|                                          |                                                            |        |          |
|                                          |      :math:`c_p(T) = \sum_{i=0}^5 a_iT^i`                  |        |          |
|                                          |                                                            |        |          |
|                                          | * ``mixture`` - a *mixture* specific heat is computed      |        |          |
|                                          |   from :ref:`species specific heats<InputsSpeciesDefs>`    |        |          |
|                                          |   and local species mass fractions.                        |        |          |
|                                          |                                                            |        |          |
|                                          |      :math:`c_{p,\mathrm{mixture}} = \sum_n X_n c_{p,n}`   |        |          |
|                                          |                                                            |        |          |
|                                          | A specific heat model is required if advecting enthalpy.   |        |          |
|                                          | Furthermore, the model must be ``mixture`` if species      |        |          |
|                                          | equations are solved.                                      |        |          |
|                                          |                                                            |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| specific_heat.constant                   | Constant fluid specific heat                               | Real   |  0       |
|                                          | A value is required for ``constant`` specific heat model.  |        |          |
|                                          |                                                            |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| specific_heat.NASA7.a[i]                 | Specific heat polynomial coefficients. Polynomial          | Real   |  0.  0.  |
|                                          | coefficients are required if the fluid specific heat       |        |          |
|                                          | model is ``NASA7-poly``.                                   |        |          |
|                                          |                                                            |        |          |
|                                          | Each polynomial is defined by six coefficients (``a0``,    |        |          |
|                                          | ..., ``a5``), and two values are required for each         |        |          |
|                                          | coefficient. The first value is the low temperature        |        |          |
|                                          | polynomial coefficient (T < 1000K) and the second is the   |        |          |
|                                          | coefficient for the high temperature polynomial            |        |          |
|                                          | (T > 1000K). A total of twelve coefficients are required.  |        |          |
|                                          |                                                            |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| thermal_conductivity                     | Specify which thermal conductivity model to use for        | String |  None    |
|                                          | fluid. Available options include:                          |        |          |
|                                          |                                                            |        |          |
|                                          | * ``constant`` - the fluid has a constant thermal          |        |          |
|                                          |   conductivity model                                       |        |          |
|                                          |                                                            |        |          |
|                                          | A thermal conductivity model is required if advecting      |        |          |
|                                          | enthalpy.                                                  |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| thermal_conductivity.constant            | Constant fluid thermal conductivity.                       | Real   |  0       |
|                                          | A value is required for ``constant`` thermal conductivity  |        |          |
|                                          | model.                                                     |        |          |
|                                          |                                                            |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| thermodynamic_pressure                   | Thermodynamic pressure.                                    | Real   |  0       |
|                                          | A value is required when using the                         |        |          |
|                                          | ``IdealGasClosedSystem`` constraint. If using the          |        |          |
|                                          | ``IdealGasOpenSystem`` constraint, then the value must     |        |          |
|                                          | match all specified outflow pressures.                     |        |          |
|                                          |                                                            |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+
| reference_temperature                    | Reference temperature used to compute specific             | Real   | 298.     |
|                                          | enthalpy                                                   |        |          |
+------------------------------------------+------------------------------------------------------------+--------+----------+


Example inputs
--------------

Incompressible fluid
^^^^^^^^^^^^^^^^^^^^

The following setup shows how to define the most basic fluid model where the
only required physical property is fluid viscosity. We are using the ``IncompressibleFluid``
constraint; therefore, the *constant* fluid density is defined
by the initial and boundary conditions. The fluid energy and species equations
are not solved.

.. code-block:: bash
   :caption: Snippet of inputs defining a simple incompressible fluid. This is not a complete input file.

   mfix.constraint_type = IncompressibleFluid

   mfix.advect_density  = 0
   mfix.advect_enthalpy = 0
   mfix.solve_species   = 0

   # Fluid model settings
   # -----------------------------------------------------------------------
   fluid.solve = fluid0

   fluid.viscosity          = constant
   fluid.viscosity.constant = 1.8e-5


   # Initial Conditions
   # -----------------------------------------------------------------------
   ic.regions = full-domain

   ic.full-domain.fluid0.volfrac   =  1.0
   ic.full-domain.fluid0.density   =  1.0

   ic.full-domain.fluid0.velocity  =  0.0  0.0  0.0

   # Boundary Conditions
   # -----------------------------------------------------------------------
   bc.regions = inlet  outlet

   bc.inlet = mi
   bc.inlet.fluid0.volfrac  =  1.0
   bc.inlet.fluid0.density  =  1.0

   bc.inlet.fluid0.velocity =  1.0e-8  0.0  0.0

   bc.outlet = po
   bc.outlet.fluid0.pressure =  0.


Simple ideal gas
^^^^^^^^^^^^^^^^

In the following example, we use the ``IdealGasOpenSystem`` so that the fluid
density is calculated from the ideal gas equation of state.  In addition to
viscosity, we must also provide the fluid molecular weight. The fluid energy
and species equations are not solved, but the *constant* fluid temperature is
required in the initial and boundary conditions to fully specify the equation
of state. Lastly, the outflow boundary pressure is taken as the thermodynamic
pressure when evaluating the equation of state.


.. code-block:: bash
   :caption: Snippet of inputs defining a simple ideal gas. This is not a complete input file.

   mfix.constraint_type = IdealGasOpenSystem

   mfix.advect_density  = 1
   mfix.advect_enthalpy = 0
   mfix.solve_species   = 0


   # Fluid model settings
   # -----------------------------------------------------------------------
   fluid.solve = fluid0

   fluid.viscosity          = constant
   fluid.viscosity.constant = 1.8e-5

   fluid.molecular_weight = 29.0e-3


   # Initial Conditions
   # -----------------------------------------------------------------------
   ic.regions = full-domain

   ic.full-domain.fluid0.volfrac   =  1.0

   ic.full-domain.fluid0.velocity  =  0.0  0.0  0.0

   ic.full-domain.fluid0.temperature = 300.0


   # Boundary Conditions
   # -----------------------------------------------------------------------
   bc.regions = inlet  outlet

   bc.inlet = mi
   bc.inlet.fluid0.volfrac  =  1.0

   bc.inlet.fluid0.velocity =  1.0e-8  0.0  0.0

   bc.inlet.fluid0.temperature = 300.0

   bc.outlet = po
   bc.outlet.fluid0.pressure =  101325.



Ideal gas with energy
^^^^^^^^^^^^^^^^^^^^^

In the last example, we use the ``IdealGasOpenSystem``, however unlike the
previous demonstration, the energy equation is solved. Therefore, we must
define the fluid viscosity, molecular weight, thermal conductivity, and
specific heat. Again, the fluid temperature is required in the initial and
boundary conditions, and the outflow boundary pressure is taken as the
thermodynamic pressure for the system.


.. code-block:: bash
   :caption: Snippet of inputs defining an idea gas with energy. This is not a complete input file.

   mfix.constraint_type = IdealGasOpenSystem

   mfix.advect_density  = 1
   mfix.advect_enthalpy = 1
   mfix.solve_species   = 0

   # Fluid model settings
   # -----------------------------------------------------------------------
   fluid.solve = fluid0

   fluid.viscosity          = constant
   fluid.viscosity.constant = 2.0e-5

   fluid.thermal_conductivity = constant
   fluid.thermal_conductivity.constant = 0.026

   fluid.molecular_weight = 28.96518e-3

   fluid.specific_heat = NASA7-poly

   fluid.specific_heat.NASA7.a0 =  5.95960930E+00   3.08792717E+00
   fluid.specific_heat.NASA7.a1 =  3.56839620E+00   1.24597184E-03
   fluid.specific_heat.NASA7.a2 = -6.78729429E-04  -4.23718945E-07
   fluid.specific_heat.NASA7.a3 =  1.55371476E-06   6.74774789E-11
   fluid.specific_heat.NASA7.a4 = -3.29937060E-12  -3.97076972E-15
   fluid.specific_heat.NASA7.a5 = -4.66395387E-13  -9.95262755E+02


   # Initial Conditions
   # -----------------------------------------------------------------------
   ic.regions = full-domain

   ic.full-domain.fluid0.volfrac   =  1.0

   ic.full-domain.fluid0.velocity  =  0.0  0.0  0.0

   ic.full-domain.fluid0.temperature = 300.0

   # Boundary Conditions
   # -----------------------------------------------------------------------
   bc.regions = inlet  outlet

   bc.inlet = mi
   bc.inlet.fluid0.volfrac  =  1.0

   bc.inlet.fluid0.velocity =  1.0e-8  0.0  0.0

   bc.inlet.fluid0.temperature = 300.0

   bc.outlet = po
   bc.outlet.fluid0.pressure =  101325.


The following inputs are defined using the ``fluid`` prefix and control the convergence criteria
of the damped Newton solver used to compute temperature from enthalpy and specific heat.

+------------------------------------------+----------------------------------------------------------+--------+----------+
|                                          | Description                                              |  Type  | Default  |
+==========================================+==========================================================+========+==========+
| newton_solver.absolute_tol               | Define absolute tolerance for the Newton solver          |  Real  |  1.e-8   |
+------------------------------------------+----------------------------------------------------------+--------+----------+
| newton_solver.relative_tol               | Define relative tolerance for the Newton solver          |  Real  |  1.e-8   |
+------------------------------------------+----------------------------------------------------------+--------+----------+
| newton_solver.max_iterations             | Define max number of iterations for the Newton solver    |  int   |  500     |
+------------------------------------------+----------------------------------------------------------+--------+----------+
