Semitransparent Surface (Radiation in Participating Media and Radiation in Absorbing-Scattering Media Interfaces)
This node 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 Radiation discretization method is Discrete ordinates method, 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 Radiation discretization method is P1 approximation, 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.
Pair Selection
If this node is selected from the Pairs menu, choose the pair to apply this condition to. A pair must to be created first. See Identity and Contact Pairs in the COMSOL Multiphysics Reference Manual for more details.
Models Input
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 Temperature T, which is used in the expression of the blackbody radiative intensity.
Fractional Emissive Power
This section is available when the Wavelength dependence of radiative properties is defined as Solar and ambient or Multiple spectral bands in the Radiation in Participating Media interface (see Participating Media Settings).
When the Fractional emissive power is Blackbody/Graybody, the fractional emissive power FEPk is automatically calculated for each spectral band as a function of the band endpoints and temperature.
When the Fractional emissive power is User defined for each band, define the Fractional emissive power, FEPk for 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 Define fractional emissive power on each side check box to set specific Upside and Downwside values in the table.
Surface Radiative Properties
The radiation properties of the boundary and the external radiation intensity should be set in this section.
Emissivity, Diffuse Transmissivity, and Specular Transmissivity
If the Wavelength dependence of radiative properties is Constant:
By default, the Emissivity ε (dimensionless), Diffuse transmissivity τd (dimensionless), and Specular transmissivity τs (dimensionless) use values From material. 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).
For User defined, set values or expressions. You can define temperature-dependent emissivity and transmissivities using the variable rad.T.
Select the Define properties on each side check box to set specific values for each side. Select the Boundary material, upside and Boundary material, downside to have different material properties on each side. The boundary material specified is used only when Emissivity, upside, Emissivity, downside, Diffuse transmissivity, upside, Diffuse transmissivity, downside, Specular transmissivity, upside, and Specular transmissivity, downside are From material.
If the Wavelength dependence of radiative properties is Solar and ambient or Multiple spectral bands:
By default, Emissivity ε (dimensionless), Diffuse transmissivity τd (dimensionless), and Specular transmissivity τs (dimensionless) use values From material.
When Emissivity is User defined, enter a value or expression for the Emissivity 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 User defined for each band option to avoid the evaluation of the average.
When Diffuse transmissivity is User defined, enter a value or expression for the Diffuse transmissivity 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 User defined for each band option to avoid the evaluation of the average.
When Specular transmissivity is User defined, enter a value or expression for the Specular transmissivity 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 User defined for each band option to avoid the evaluation of the average.
When Emissivity is User defined for each band, enter a value for the Emissivity 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 Define properties on each side check box and fill the Upside and Downside columns of the table for a specific definition on each side.
When Diffuse transmissivity is User defined for each band, enter a value for the Diffuse transmissivity 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 Define properties on each side check box and fill the Upside and Downside columns of the table for a specific definition on each side.
When Specular transmissivity is User defined for each band, enter a value for the Specular transmissivity 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 Define properties on each side check box and fill the Upside and Downside columns of the table for a specific definition on each side.
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.
External Radiation Intensity
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 Wavelength dependence of radiative properties is set to Constant, two options are available to define the external radiation intensity. Set for External radiation intensity to User defined to define Iext (SI unit: W/m²/sr) directly. Alternatively, select Blackbody radiation to define Iext as Ib(Text) and set the External temperature Text (SI unit: K, default value 293.15K) or select an Ambient temperature to specify the temperature at which the blackbody radiation intensity Ib is evaluated.
When the Wavelength dependence of radiative properties is set to Solar and ambient or to Multiple spectral bands, the external radiation intensity, Iext,k is defined for each spectral band.
Set External radiation intensity to Blackbody radiation and set the External temperature Text (SI unit: K, default value 293.15K) or select an Ambient temperature to define Iext,k from the blackbody intensity Ib(Text), the External temperature Text, and the fractional emissive power for each spectral band at external temperature FEPk(Text).
Set the External radiation intensity to User defined for each band, to define Iext,k from user defined expression for each spectral band in the table displayed underneath.
Set the External radiation intensity to User defined distribution, 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 User defined for each band 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.
Location in User Interface
Context Menus
Ribbon
Physics tab with Radiation in Participating Media or Radiation in Absorbing-Scattering Media selected: