When you have activated one of the radiation models (except for the surface-to-surface model, which requires no additional properties), there will be additional properties for you to set in the Create/Edit Materials Dialog Box:
For the P-1 model, you must set the radiation Absorption Coefficient and Scattering Coefficient (
and 
in Equation 5–57 in the Theory Guide).For the Rosseland model, set the Absorption Coefficient and Scattering Coefficient (
and 
in Equation 5–58 in the
Theory Guide).For the DTRM, only the Absorption Coefficient is required (
in Equation 5–91 in the Theory Guide).For the DO model, set the Absorption Coefficient and the Scattering Coefficient (
and 
in Equation 5–98 in the
Theory Guide). In addition, if you
are modeling semi-transparent media, specify the Refractive Index
(
or 
in Equation 5–117 in the Theory Guide). With the DO model, you can
specify radiation properties for solid materials, to be used when semi-transparent media
are modeled.For the MC model, set the Absorption Coefficient and the Scattering Coefficient (
and in Equation 5–98 in the
Theory Guide). In addition, if you
are modeling semi-transparent media, specify the Refractive Index
( or in Equation 5–117 in the Theory Guide). With the MC model, you can
specify radiation properties for solid materials, to be used when semi-transparent media
are modeled.
For additional information, see the following sections:
To define the absorption coefficient, you can specify a constant value, a temperature-dependent function (see Defining Properties Using Temperature-Dependent Functions), a composition-dependent function, or a user-defined function. The absorbing and emitting parts of the radiative transfer equation (RTE), Equation 5–56 in the Theory Guide, is a function of the absorption coefficient. The absorbing or emitting effects depend on the chosen radiation model. If there are only absorption effects, then Lambert’s Law of absorption applies
 | (8–69) |
where 
is the radiation intensity, 
is the absorption coefficient,
and 
is the distance through the material.
If you are modeling non-gray radiation with the P-1, DO, or MC radiation models, you also have
the option to specify a constant absorption coefficient in each of the gray bands. The
absorption coefficient is requested in units of 1/length. Along with the scattering
coefficient, it describes the change in radiation intensity per unit length along the path
through the fluid medium. Absorption coefficients can be computed using tables of emissivity
for 
and 
O, which are generally available in textbooks on radiation heat
transfer.
To define a constant absorption coefficient, simply enter the value in the field next to Absorption Coefficient in the Create/Edit Materials Dialog Box. Select constant in the drop-down list first if it is not already selected.
Ansys Fluent also allows you to input a composition-dependent
absorption coefficient, where the local value of 
is a function of the local
mass fractions of water vapor and carbon dioxide. This modeling option
can be useful for the simulation of radiation in combustion applications.
The variable-absorption-coefficient model used by Ansys Fluent is
the weighted-sum-of-gray-gases model (WSGGM) described in Radiation in Combusting Flows in the Theory Guide. To activate
it, first enable the species calculation and make sure that 
and 
O are present
in the mixture. Next, select wsggm-domain-based, wsggm-user-specified, or user-defined-wsggm in the drop-down list to the right of Absorption
Coefficient in the Create/Edit Materials dialog box. If you select user-defined-wsggm, the User-Defined Functions dialog box will
open, allowing you to select a previously loaded compiled UDF library
or a previously interpreted UDF (see Hooking DEFINE_WSGGM_ABS_COEFF UDFs in the Fluent Customization Manual). The WSGGM options
differ in the method used to compute the path length, as described
in the section that follows.
When the WSGGM is used to compute the absorption coefficient,
you can choose how path length, 
, is defined for Equation 5–145 in the Theory Guide. See Radiation in Combusting Flows in the Theory Guide to determine which method
is appropriate for your case.
You will select the path length method when you choose the property input method for Absorption Coefficient, as described previously.
If you choose wsggm-domain-based,
is set equal to a mean beam length calculated by Ansys Fluent according
to Equation 5–146 in the Theory Guide, which is an average dimension of the domain; no further inputs
are required.If you choose wsggm-user-specified,
is set equal to a mean beam length that you enter
for Path Length in the WSGGM User Specified Dialog Box.If you choose user-defined-wsggm, Ansys Fluent will initially compute the absorption coefficient in the same manner as described for the wsggm-domain-based option; however, you have the option of writing a user-defined function that customizes this calculated value. If the soot model is enabled, you can also use the UDF to customize the soot absorption coefficient computed by Ansys Fluent. See
DEFINE_WSGGM_ABS_COEFFin the Fluent Customization Manual for further details.
If you are using the non-gray DO model (see The DO Model Equations of the Theory Guide and Defining Non-Gray Radiation for the DO Model), the non-gray P-1 model (see The P-1 Model Equations of the Theory Guide and Setting Up the P-1 Model with Non-Gray Radiation), or the non-gray MC model (see Monte Carlo (MC) Radiation Model Theory), you can specify a different constant absorption coefficient for each of the bands used by the gray-band model. Select gray-band from the Absorption Coefficient drop-down list in the Create/Edit Materials dialog box and then define the absorption coefficient for each band in the Gray-Band Absorption Coefficient Dialog Box. (Note that you must complete this dialog box in order to proceed.)
Ansys Fluent will include the effect of particles on the absorption coefficient if you have turned on the Particle Radiation Interaction option in the Discrete Phase Model Dialog Box (only for the P-1 and DO radiation models).
If you are modeling soot formation and you want to include the effect of soot formation on the absorption coefficient, turn on the Soot-Radiation Interaction in the Soot Model Dialog Box. The soot effects can be included for any of the radiation models, as long as you are using the WSGGM to compute a composition-dependent absorption coefficient. Note that you can use the user-defined-wsggm option to customize the soot absorption coefficient calculated by Ansys Fluent, as described previously.
The scattering coefficient is, by default, set to zero, and it is assumed to be isotropic. You can specify a constant value, a temperature-dependent function (see Defining Properties Using Temperature-Dependent Functions), or a user-defined function. You can also specify a non-isotropic phase function.
The scattering coefficient is requested in units of 1/length. Along with the absorption coefficient, it describes the change in radiation intensity per unit length along the path through the fluid medium. You may want to increase the scattering coefficient in combustion systems, where particulates may be present.
To define a constant scattering coefficient, simply enter the value in the field next to Scattering Coefficient in the Create/Edit Materials Dialog Box. (Select constant in the drop-down list first if it is not already selected.)
Scattering is assumed to be isotropic, by default, but you can also specify a linear-anisotropic scattering function. If you are using the DO model, Delta-Eddington and user-defined scattering functions are also available.
To model isotropic scattering, select isotropic in the Scattering Phase Function drop-down list. No further inputs are necessary. This is the default setting in Ansys Fluent.
To model anisotropic scattering, select linear-anisotropic in the Scattering Phase Function drop-down
list and set the value of the phase function coefficient (
in Equation 5–58 in the Theory Guide).
To use a Delta-Eddington phase function, select delta-eddington in the Scattering Phase Function drop-down
list. This will open the Delta-Eddington Scattering Function Dialog Box, in which you
can specify the Forward Scattering Factor and Asymmetry Factor (
and 
in Equation 5–107 in the Theory Guide). Since
this is a modal dialog box, you must tend to it immediately.
To use a user-defined phase function, select user-defined in the Scattering Phase Function drop-down
list. The user-defined function will contain specifications for 
and 
in Equation 5–108 in the Theory Guide. More information
about user-defined functions can be found in the Fluent Customization Manual.
The refractive index is the ratio of speed of light in the medium to the speed of light in vacuum. It is by default set to 1. You can specify a constant value in the field next to Refractive Index.
If you are using the non-gray DO model (see The DO Model Equations of the Theory Guide and Defining Non-Gray Radiation for the DO Model), the non-gray P-1 model (see The P-1 Model Equations of the Theory Guide and Setting Up the P-1 Model with Non-Gray Radiation), or the non-gray MC model (see Monte Carlo (MC) Radiation Model Theory), you can specify a different constant refractive index for each of the bands used by the gray-band model. Select refractive-band from the Refractive Index drop-down list in the Create/Edit Materials dialog box and then define the refractive index for each band in the Gray-Band Refractive Index Dialog Box. Note that because this is a modal dialog box, you must tend to it immediately.
You can display the computed local values for 
and 
using the Absorption Coefficient and Scattering Coefficient items in the Radiation... category of the variable
selection drop-down list that appears in postprocessing dialog
boxes. You will also find the Refractive Index in the Radiation... category.