Prescribed Radiosity (Surface-to-Surface Radiation interface)
Use this node to specify radiosity on the boundary for each spectral band. Radiosity can be defined as blackbody or graybody radiation. A user-defined surface radiosity expression can also be defined.
Model Input
This section has fields and values that are inputs to expressions that define material properties. If such user-defined property groups have been added, the model inputs are included here.
There is one standard model input: the Temperature T is used in the expression of the blackbody radiation intensity and when multiple wavelength intervals are used, for the fractional emissive power. The temperature model input is also used to determine the variable that receives the radiative heat source. When the model input does not contain a dependent variable, the radiative heat source is ignored.
The default Temperature is User defined. When additional physics interfaces are added to the model, the temperature variables defined by these physics interfaces can also be selected from the list. The Common model input option corresponds to the minput.T variable, set to 293.15 [K] by default) and all temperature variables from the physics interfaces included in the model. To edit the minput.T variable, click the Go to Source button (), and in the Common Model Inputs node under Global Definitions, set a value for the Temperature in the Expression for remaining selection section.
Radiation Direction
When Wavelength dependence of surface properties is set to Constant select a Radiation direction based on the geometric normal (nx, ny, nz): Opacity controlled (the default), Negative normal direction, Positive normal direction, Both sides, or None.
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Opacity controlled requires that each boundary is adjacent to exactly one opaque domain. Opacity is controlled by the Opacity (Surface-to-Surface Radiation interface) condition.
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Select Negative normal direction to specify that the surface radiates in the negative normal direction (un vector direction).
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Select Positive normal direction if the surface radiates in the positive normal direction (dn vector direction).
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Select Both sides if the surface radiates on both sides.
When Wavelength dependence of surface properties is set to Solar and ambient or Multiple spectral bands select a Radiation direction for each spectral band: Opacity controlled (the default), Negative normal direction, Positive normal direction, or Both sides, or None. The Radiation direction defines the radiation direction for each spectral band similarly as when Wavelength dependence of surface properties is Constant.
If this feature is combined with heat transfer in 2D and 1D, the thickness is assumed to be infinite for the view factor computation. The user-defined value for dz is still used in the heat transfer equation.
Radiosity
Radiosity does not directly affect the boundary condition on the boundary where it is specified, but rather defines how that boundary affects others through radiation.
If Wavelength dependence of surface properties is Constant, select a Radiosity expression between Graybody radiation (the default), Blackbody radiation, or User defined.
If Wavelength dependence of surface properties is Solar and ambient or Multiple spectral bands, select a Radiosity expression between Graybody radiation (the default), Blackbody radiation, User defined for each band, or User defined.
Blackbody Radiation
When Blackbody radiation is selected it sets the surface radiosity expression corresponding to a blackbody.
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When Wavelength dependence of surface properties is set to Constant it defines J = eb(T) when radiation is defined on one side or Ju = eb(Tu) and Jd = eb(Td) when radiation is defined on both sides.
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When Wavelength dependence of surface properties is set to Solar and ambient or Multiple spectral bands, it defines for each spectral band Ji = FEPi(T)eb(T) when radiation is defined on one side or Ji, d = FEPi, d(Td)eb(Td) and Ju = FEPi, u(Tu)eb(Tu) when radiation is defined on both sides.
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When the temperature varies across a pair (for example when a Thin Layer (Heat Transfer interface) and Solid (Heat Transfer in Shells interface) condition is active on the same boundary), the temperature used to define the radiosity is evaluated on the side where the surface radiation is defined.
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The blackbody hemispherical total emissive power is defined by eb(T) = n2σT4
Graybody Radiation
When Graybody radiation is selected it sets the surface radiosity expression corresponding to a graybody.
By default, the Surface emissivity ε is defined From material. In this case, make sure that a material is defined at the boundary level (materials are defined by default at the domain level). If User defined is selected for the Surface emissivity, enter another value for ε. For User defined for each band option, set a value for each spectral band in the table.
If Wavelength dependence of surface properties is set to Constant:
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When radiation is defined on one side, define the Surface emissivity ε to set J = εeb(T), or
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When radiation is defined on both sides, define the Surface emissivity, upside εu and the Surface emissivity, downside εd on the upside and downside, respectively. The surface radiosity on upside and downside is then defined by Ju = εueb(Tu) and Jd = εdeb(Td) respectively.
If Wavelength dependence of surface properties is set to Solar and ambient or Multiple spectral bands, for all spectral bands:
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When radiation is defined on one side for Bi spectral band, define the Surface emissivity εi to set Ji = FEPiεieb(T), or
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When radiation is defined on both sides for Bi spectral band, define the Surface emissivity, upside εi, u and the Surface emissivity, downside εi, d on the upside and downside, respectively. The surface radiosity on upside and downside is then defined by Ju = FEPBi(Tu)εBi, ueb(Tu) and Jd = FEPBi(Td)εBi, deb(Td), respectively.
Set the surface emissivity  to a number between 0 and 1, where 0 represents diffuse mirror and 1 is appropriate for a perfect blackbody. The proper value for a physical material lies somewhere in-between and can be found from tables or measurements.
User Defined
If Wavelength dependence of surface properties is set to Constant and Radiosity expression is set to User defined, it sets the surface radiosity expression to J = J0, which specifies how the radiosity of a boundary is evaluated when that boundary is visible in the calculation of the irradiation onto another boundary in the model. Enter a value or expression for the Surface radiosity, J0.
Select the Define prescribed radiosity on each side check box to set specific values on each side, and define the surface Radiosity expression J0, u and J0, d on the upside and downside, respectively. The geometric normal points from the downside to the upside.
If Wavelength dependence of surface properties is Solar and ambient or Multiple spectral bands, similar settings are available for each spectral band.
User Defined on Each Band
The surface radiosity J0,i should be set for each spectral band by filling the table. Select the Define prescribed radiosity on each side check box to set specific values on each side, and set the Upside and Downside cells in the table.
Several settings for this node depend on the Wavelength dependence of surface properties setting, which is defined for the physics interface. See Radiation Settings.
Upside and downside settings can be visualized by plotting the global normal vector (nx, ny, nz), that always points from downside to upside. Note that the normal vector (ht.nx, ht.ny, ht.nz) may be oriented differently.
See Tangent and Normal Variables in the COMSOL Multiphysics Reference Manual.
To define temperature dependencies for the user inputs (surface emissivity for example), use the temperature variable ht.T, that corresponds to the appropriate variable (upside, downside, or average temperature of a layer, wall temperature with turbulence modeling), depending on the model configurations. See Boundary Wall Temperature for a thorough description of the boundary temperature variables.
Theory for Surface-to-Surface Radiation
Location in User Interface
Context menus
 Surface-to-Surface Radiation>Prescribed Radiosity
Ribbon
Physics Tab with Surface-to-Surface Radiation selected:
Boundaries>Prescribed Radiosity