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.
Coordinate System Selection
Select a boundary system from the Coordinate system list to define the reference vector for the azimuthal angle. The first component of the boundary system selected is the reference vector used for directional surface properties. The default is the boundary system of the model, and the list contains any additional boundary system added under the Definitions node.
Directional dependence of radiative properties is available when the Surface-to-surface radiation method is set to Ray shooting in the settings of the Surface-to-Surface Radiation interface.
See Coordinate Systems in the COMSOL Multiphysics Reference Manual for more details.
Radiation Direction
These settings are the same as for the Diffuse Surface (Surface-to-Surface Radiation Interface) node.
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 radiative properties is Constant, select a Radiosity expression between Graybody radiation (default), Blackbody radiation, or User defined.
If Wavelength dependence of radiative properties is Solar and ambient or Multiple spectral bands, select a Radiosity expression between Graybody radiation (default), Blackbody radiation, User defined for each band, or User defined.
Blackbody Radiation
When Blackbody radiation is selected it defines the surface radiosity expression corresponding to a blackbody. Define the External temperature Text (SI unit: K, default value 293.15 K) to specify the temperature at which the blackbody radiation intensity eb is evaluated. Alternatively, select an Ambient temperature defined in an Ambient Properties node under Definitions.
•
When Wavelength dependence of radiative properties is set to Constant it defines J = eb(Text) when radiation is defined on one side or Ju = eb(Text) and Jd = eb(Text) when radiation is defined on both sides.
•
When Wavelength dependence of radiative properties is set to Solar and ambient or Multiple spectral bands, it defines for each spectral band Ji = FEPi(Text)eb(Text) when radiation is defined on one side or Ji, d = FEPi, d(Text)eb(Text) and Ju = FEPi, u(Text)eb(Text) when radiation is defined on both sides.
•
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 with a temperature Text.
Directional dependence of radiative properties is available when the Surface-to-surface radiation method is set to Ray shooting in the settings of the Surface-to-Surface Radiation interface. By default, the Directional dependence of the surface emissivity is set to Constant. The Emissivity can be defined by an expression depending on other variables such as temperature or spatial coordinates. Switch to the Polar or Polar and azimuthal options to define the dependencies on the polar and azimuthal angles, θ and φ, by specifying a Directional emissivity function, fε. When using the Polar option, select from the Directional emissivity function list any function of one argument already defined in the component, under the Definitions node. For the Polar and azimuthal option, select from the functions of two arguments. By default, only the Zero function is available in the list, defining fε(θ) = 0 or fε(θ, φ) = 0.
The other dependencies (temperature T, spatial coordinates x) are treated separately in the Emissivity expression, and used to define the total emissivity:
Select the Define properties on each side check box to define specific directional dependencies of the surface emissivity on each side of the boundary, and set the Directional dependence, upside and Directional dependence, downside accordingly.
The reference vector for the azimuthal angle is given by the first component of the Coordinate System selected in the Coordinate System Selection section.
By default, the 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 Emissivity, enter another value for ε. For the User defined for each band option, set a value for each spectral band in the table. Within a spectral band, each value is assumed to be independent of wavelength.
If Wavelength dependence of radiative properties is set to Constant:
•
When radiation is defined on one side, define the Emissivity ε and the External temperature Text to set J = εeb(Text), or
•
When radiation is defined on both sides, define the Emissivity, upside εu, Emissivity, downside εd and the External temperature Text. The surface radiosity on upside and downside is then defined by Ju = εueb(Text) and Jd = εdeb(Text), respectively.
If Wavelength dependence of radiative properties is set to Solar and ambient or Multiple spectral bands, for all spectral bands:
•
When radiation is defined on one side for the Bi spectral band, define the Emissivity εi and the External temperature Text to set Ji = FEPiεieb(Text), or
•
When radiation is defined on both sides for Bi spectral band, define the Emissivity, upside εu, Emissivity, downside εd and the External temperature Text. The surface radiosity on upside and downside is then defined by Ju = FEPBi(Text)εBi, ueb(Text) and Jd = FEPBi(Text)εBi, deb(Text), 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 radiative 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 radiative 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 radiative 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