Prescribed Radiosity
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 Inputs
These settings are the same as for Diffuse Mirror.
Radiation Direction
When Wavelength dependence of emissivity 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, or Both sides.
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Opacity controlled requires that each boundary is adjacent to exactly one opaque domain. Opacity is controlled by the Opacity condition.
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Select Negative normal direction to specify that the surface radiates in the negative normal direction.
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Select Positive normal direction if the surface radiates in the positive normal direction.
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Select Both sides if the surface radiates on both sides.
When Wavelength dependence of emissivity 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 emissivity 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 d 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.
Select a Radiosity expression: Graybody radiation (the default), Blackbody radiation, 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 emissivity 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 emissivity is set to Solar and ambient or Multiple spectral bands, it defines for each spectral band JBi = FEPBi(T)eb(T) when radiation is defined on one side or JBi, d = FEPBi, d(Td)eb(Td) and Ju = FEPBi, 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 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.
If Wavelength dependence of emissivity 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 Material on upside, the Surface emissivity εu, Material on downside and the Surface emissivity 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 emissivity 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 εBi to set JBi = FEPBiεBieb(T), or
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When radiation is defined on both sides for Bi spectral band, define the Material on upside, the Surface emissivity εBi, u, Material on downside and the Surface emissivity εBi, 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 emissivity 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 Surface radiosity expression, J0.
When the Radiation direction is set to Both sides (under Radiation Settings) also define the surface Radiosity expression J0i, u and J0i, d on the upside and downside, respectively. The geometric normal points from the downside to the upside.
If Wavelength dependence of emissivity is Solar and ambient or Multiple spectral bands, similar settings are available for each spectral band.
Several settings for this node depend on the Wavelength dependence of emissivity 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
 Heat Transfer with Surface-to-Surface Radiation>Radiation>Prescribed Radiosity
 Surface-to-Surface Radiation>Prescribed Radiosity
More locations are available if the Surface-to-surface radiation check box is selected under the Physical Model section. For example:
Heat Transfer in Solids>Radiation>Prescribed Radiosity
Ribbon
Physics Tab with Surface-to-Surface Radiation selected:
Boundaries>Surface-to-Surface Radiation>Prescribed Radiosity
Physics Tab with Heat Transfer, Heat Transfer in Solids, Heat Transfer in Fluids, Heat Transfer in Porous Media, Heat Transfer in Building Materials, Bioheat Transfer, Heat Transfer with Surface-to-Surface Radiation or Heat Transfer with Radiation in Participating Media selected:
Boundaries>Radiation>Prescribed Radiosity