Semitransparent Surface (Radiation in Participating Media and Radiation in Absorbing-Scattering Media Interfaces)

Thisnode defines a boundary semitransparent to radiation. If no radiation is transmitted through the surface, use the Opaque Surface (Radiation in Participating Media and Radiation in Absorbing-Scattering Media Interfaces) node instead.

It prescribes incident intensities on a boundary and takes into account the net radiative heat flux absorbed by the surface, modeling emitted, reflected, and (diffusively and specularly) transmitted radiative intensity on the boundary.

When the is , the net radiative heat flux is defined as the difference between the incoming and outgoing radiative heat fluxes, and the incoming and outgoing radiative heat fluxes are defined from the weighted sums of the incident intensities. See Semitransparent Surface for more details.

When the is , the net radiative heat flux is defined from the incident radiation G. See Semitransparent Surface for more details.

At the internal boundaries, the net radiative heat fluxes, qr,net,d and qr,net,u on each side of the surface are defined. Specific radiative properties of the surface can be defined on each side of the boundary.

If this node is selected from the menu, choose the pair to apply this condition to. A pair must be created first. See Identity and Contact Pairs in the COMSOL Multiphysics Reference Manual for more details.

This section has fields and values that are inputs to expressions that define material properties. If such user-defined materials are added, the model inputs appear here.

There is one standard model input — the T, which is used in the expression of the blackbody radiative intensity.

	The definition of the boundary temperature may be different from that of the temperature in the adjacent domain.

This section is available when the is defined as or in the Radiation in Participating Media interface (see Participating Media Settings).

When the is , the fractional emissive power FEPk is automatically calculated for each spectral band as a function of the band endpoints and temperature.

When the is , define the ,FEPkfor each spectral band in the table displayed below. All fractional emissive powers are expected to be in [0,1] and their sum must equal 1. Select the check box to set specific and values in the table.

The radiation properties of the boundary and the external radiation intensity should be set in this section.

If the is :

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By default, the ε (dimensionless), τd (dimensionless), and τs (dimensionless) use values . These are properties of the material surface that depend both on the material itself and the structure of the surface. Make sure a material is defined at the boundary level (by default materials are defined at the domain level).

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For , set values or expressions. You can define temperature-dependent emissivity and transmissivities using the variable rad.T.

Select the check box to set specific values for each side. Select the and to have different material properties on each side. The boundary material specified is used only when , , , , , and are .

If the is or :

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By default, ε (dimensionless), τd (dimensionless), and τs (dimensionless) use values .

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When is , enter a value or expression for the ε. The wavelength is accessible through the rad.lambda variable. Any expression set for the emissivity is then averaged on each spectral band to obtain a piecewise constant emissivity. If the average value of the emissivity on each band is known, you can use instead the option to avoid the evaluation of the average.

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When is , enter a value or expression for the τd. The wavelength is accessible through the rad.lambda variable. Any expression set for the transmissivity is then averaged on each spectral band to obtain a piecewise constant transmissivity. If the average value of the transmissivity on each band is known, you can use instead the option to avoid the evaluation of the average.

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When is , enter a value or expression for the τs. The wavelength is accessible through the rad.lambda variable. Any expression set for the specular transmissivity is then averaged on each spectral band to obtain a piecewise constant specular transmissivity. If the average value of the specular transmissivity on each band is known, you can use instead the option to avoid the evaluation of the average.

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When is , enter a value for the for each spectral band. Within a spectral band, each value is assumed to be independent of wavelength. By default, the same emissivity is defined on both sides. Select the check box and fill the and columns of the table for a specific definition on each side.

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When is , enter a value for the for each spectral band. Within a spectral band, each value is assumed to be independent of wavelength. By default, the same transmissivity is defined on both sides. Select the check box and fill the and columns of the table for a specific definition on each side.

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Set the surface emissivity to a number between 0 and 1, where 0 represents a diffuse mirror and 1 is suitable for a perfect blackbody. The appropriate value for a physical material is somewhere in between and can be found in tables or measurements.

Set the surface diffuse transmissivity to a number between 0 and 1, where 0 applies to an opaque surface and 1 is appropriate for a fully transparent surface.

Set the surface specular transmissivity to a number between 0 and 1, where 0 applies to a perfect diffuse surface and 1 is appropriate when transmissivity is only specular.

	The Specular transmissivity is only needed when Discrete ordinates method is selected.

The external radiation intensity should be defined when the boundary condition is applied to an external boundary. This intensity is a power per unit solid angle and unit surface area projected onto the plane normal to the direction of radiation. It can be specified for each spectral band.

When the is set to , two options are available to define the external radiation intensity. Set to to define Iext (SI unit: W/m²/sr) directly. Alternatively, select to define Iext as Ib(Text) and set the Text (SI unit: K, default value 293.15 K) or select an to specify the temperature at which the blackbody radiation intensity Ib is evaluated.

When the is set to or to , the external radiation intensity, Iext,k is defined for each spectral band.

Set to and set the Text (SI unit: K, default value 293.15 K) or select an to define Iext,k from the blackbody intensity Ib(Text), the Text, and the fractional emissive power for each spectral band at external temperature FEPk(Text).

Set the to , to define Iext,k from user defined expression for each spectral band in the table displayed underneath.

Set the to , and define an expression for Iλ,ext(SI unit: W/m3/sr) to define Iext,k from a spectral distribution. In this case Iext,k is automatically computed per spectral band as the integral of the distribution over each spectral band. If this integral over each band is known, you can use instead the option to avoid the numerical overhead due to the evaluation of the average.

	When Discrete ordinates method is selected, the components of each discrete ordinate vector can be used in this expression. The syntax is name.sx, name.sy, and name.sz, where name is the name of the physics interface node. By default, the Radiation in Participating Media interface is rpm so rpm.sx, rpm.sy, and rpm.sz correspond to the components of discrete ordinate vectors.

Physics tab with or selected: