# Species definitions

The following inputs are defined using the prefix `species`:

| Description | Type | Default
--- | --- | --- | ---
solve | Specified name of the species or None to disable the species solver. The name assigned to the species solver is used to specify species inputs. | String | None
diffusivity | Fluid species diffusivity model. `constant` a constant diffusion coefficient is used for all fluid species. | String | None
diffusivity.constant | Constant species diffusivity – required for `constant` diffusivity model | Real | 0
specific_heat | Species specific heat model. This setting only applies if either `fluid.specific_heat = mixture` or `solids.specific_heat = mixture` `constant` a constant specific heat is defined for each species and a mixture specific heat is computed based on the fluid or particle composition. `NASA7-poly` the specific heat of each species is defined by one or more polynomials that are a function of temperature, then the mixture specific heat is computed based on the fluid or particle composition. NASA7 polynomial format: \(c_p(T)/R = \sum_{i=0}^4 a_{i}T^{i}\) | String | None
ignore_discontinuities | MFIX-Exa asserts that the low and high NASA-7 polynomials are continuous at the 1000K temperature split. However, some polynomials do not have valid high temperature coefficients, for example, liquid water. By setting this keyword to 1, the simulation will only warn that the polynomials are discontinuous. Simulations that use discontinuous polynomials and cross between the 1000K temperature split may fail when computing temperature from enthalpy. Therefore, this option should only be used when the simulation is not expected to cross over the transition temperature. | int | 0
viscosity.molecular | Molecular viscosity model of species. This setting only applies when `fluid.viscosity.molecular = mixture`. Options are `constant` or `Sutherland` or `Reid`. See viscosity model descriptions. | String | 0

The following inputs are defined for each species using the prefix `species.[species name]`:

| Description | Type | Default
--- | --- | --- | ---
molecular_weight | Molecular weight of species. Required for mixture molecular weight model and when including chemical reactions. | Real | 0
specific_heat.constant | Constant species specific heat. Required for all fluid (solids) species if the fluid (solids) `specific_heat = mixture` and the specific heat model is `constant` | Real | 0
specific_heat.NASA7.a[i] | Species specific heat polynomial coefficients. Required for all fluid (solids) species if the fluid (solids) `specific_heat = mixture` and the specific heat model is `NASA7-poly` Each polynomial is defined by six coefficients (`a0` … `a5`). Coefficients `a0` through `a4` are use for computing specific heat while all six coefficients are used to compute enthalpy. The seventh coefficient, `a6`, may be provided for completeness but is not used as MFIX-Exa presently does not need to compute entropy. By default, two sets of coefficients are required. The coefficients define two polynomials used to compute the specific heat and enthalpy across low and high temperatures. The split temperature (Tsplit) defines the transition from low-to-high polynomials. A single polynomial can be used for all temperatures by setting the split temperature to -1. Three or more polynomials can be used by defining multiple split temperatures. | Real | None
specific_heat.NASA7.Tsplit | Defines the temperature transition between NASA-7 polynomials. Setting `Tsplit = -1` signifies that a single polynomial defines the specific heat and enthalpy for all temperatures. Specifying `N` split temperatures where `N>1`, signifies `N+1` polynomials are used to define the specific heat and enthalpy. | Real |
enthalpy_of_formation | Enthalpy of formation of species. This input is required only when the specific heat model is `constant` and chemical reactions are defined. | Real | 0
viscosity.molecular.constant | Constant species fluid viscosity. A value is required for `constant` species viscosity model. | Real | 0
viscosity.molecular.Sutherland.T_ref | Sutherland model reference temperature for species. A value is required for `Sutherland` species viscosity model. | Real | 0
viscosity.molecular.Sutherland.mu_ref | Sutherland model reference viscosity at T_ref for species. A value is required for `Sutherland` species viscosity model. | Real | 0
viscosity.molecular.Sutherland.S | Sutherland model temperature for species. A value is required for `Sutherland` species viscosity model. | Real | 0
viscosity.molecular.Reid.A viscosity.molecular.Reid.B viscosity.molecular.Reid.C viscosity.molecular.Reid.D | Reid model constants for species. Values are required for `Reid` species viscosity model. | Real | 0

## Example inputs

### Fluid species as passive scalars

In the following example, two species are defined and assigned to the fluid. We are required to define the species diffusivity and initial and boundary conditions. The `IncompressibleFluid` constraint is used, and the fluid density and species mass fractions are defined in the initial and boundary conditions. The fluid energy equation is not solved, and because we are not updating fluid density, the local species concentrations do not affect the fluid.

Listing 8 Snippet of inputs defining species as passive tracers. This is not a complete input file.

    mfix.constraint = IncompressibleFluid

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

    # Species model settings
    # -----------------------------------------------------------------------
    species.solve  =  N2  O2

    species.diffusivity = constant
    species.diffusivity.constant  = 1.9e-5

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

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

    fluid.species =  N2  O2

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

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

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

    ic.full-domain.fluid.species.N2   =  0.77
    ic.full-domain.fluid.species.O2   =  0.23

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

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

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

    bc.inlet.fluid.species.N2   =  0.77
    bc.inlet.fluid.species.O2   =  0.23

    bc.outlet = po
    bc.outlet.fluid.pressure =  0.

### Fluid as mixture

In this example, the fluid is treated as a mixture of two species and the `IdealGasOpenSystem` constraint is used. Because we are solving the energy equation, the fluid thermal conductivity is provided and the specific heats for both species are needed. The species specific heat model is set to `NASA7-poly`; therefore 12 coefficients (6 low-temperature range coefficients and 6 high-temperature range coefficients) are defined for each species. Additionally, species molecular weights are provided to compute the mixture molecular weights needed to evaluate the ideal-gas equation of state.

Fluid temperature is specified by the initial and boundary conditions, and unlike the previous example, density is no longer provided. Instead, the ideal gas equation of state is used to compute density from temperature, fluid composition, and the thermodynamic pressure. The thermodynamic pressure is taken as the outflow boundary condition pressure which is set here to 1 atmosphere.

Listing 9 Snippet of inputs defining fluid as a mixture with `NASA7-poly` specific heat model. This is not a complete input file.

    mfix.constraint = IdealGasOpenSystem

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

    # Species model settings
    # -----------------------------------------------------------------------
    species.solve  =  N2  O2

    species.diffusivity = constant
    species.diffusivity.constant  = 1.9e-5

    species.specific_heat = NASA7-poly

    # Oxygen
    species.O2.molecular_weight = 31.99880e-3
    species.O2.specific_heat.NASA7.a0    =  3.78245636E+00    3.66096065E+00
    species.O2.specific_heat.NASA7.a1    = -2.99673416E-03    6.56365811E-04
    species.O2.specific_heat.NASA7.a2    =  9.84730201E-06   -1.41149627E-07
    species.O2.specific_heat.NASA7.a3    = -9.68129509E-09    2.05797935E-11
    species.O2.specific_heat.NASA7.a4    =  3.24372837E-12   -1.29913436E-15
    species.O2.specific_heat.NASA7.a5    = -1.06394356E+03   -1.21597718E+03

    # Nitrogen
    species.N2.molecular_weight = 28.01340e-3
    species.N2.specific_heat.NASA7.a0    =  3.53100528E+00    2.95257637E+00
    species.N2.specific_heat.NASA7.a1    = -1.23660988E-04    1.39690040E-03
    species.N2.specific_heat.NASA7.a2    = -5.02999433E-07   -4.92631603E-07
    species.N2.specific_heat.NASA7.a3    =  2.43530612E-09    7.86010195E-11
    species.N2.specific_heat.NASA7.a4    = -1.40881235E-12   -4.60755204E-15
    species.N2.specific_heat.NASA7.a5    = -1.04697628E+03   -9.23948688E+02

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

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

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

    fluid.specific_heat = mixture

    fluid.species =  N2  O2

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

    ic.full-domain.fluid.volfrac   =  1.0

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

    ic.full-domain.fluid.species.N2  =  0.77
    ic.full-domain.fluid.species.O2  =  0.23

    ic.full-domain.fluid.temperature = 300.0

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

    bc.inlet = mi
    bc.inlet.fluid.volfrac  =  1.0

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

    bc.inlet.fluid.species.N2   = 0.77
    bc.inlet.fluid.species.O2   = 0.23

    bc.inlet.fluid.temperature = 300.0

    bc.outlet = po
    bc.outlet.fluid.pressure =  101325.