COMSOL 6.2 - Opaque Surface (Surface-to-Surface Radiation Interface)

This node is a variant of the Diffuse Surface (Surface-to-Surface Radiation Interface) node with specular reflectivity. It is available when the is set to in the settings of the Surface-to-Surface Radiation interface. It applies to glossy surfaces where specular radiation should be considered in addition to diffuse radiation. The node adds to its selection a radiosity shape function for each spectral band, and uses it as surface radiosity.

If only diffuse reflection is to be taken into account, use the Diffuse Surface (Surface-to-Surface Radiation Interface) node instead.

It is assumed that no radiation is transmitted through the surface. If radiative transmission is also to be taken into account, use the Semitransparent Surface (Surface-to-Surface Radiation Interface) node instead.

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 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 is . 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 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 button (

), and in the node under , set a value for the in the section.

Select a boundary system from the 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 node.

See Coordinate Systems in the COMSOL Multiphysics Reference Manual for more details.

These settings are the same as for the Diffuse Surface (Surface-to-Surface Radiation Interface) node.

By default, the of the surface emissivity is set to . The can be defined by an expression depending on other variables such as temperature or spatial coordinates. Switch to the or options to define the dependencies on the polar and azimuthal angles (θ and φ, respectively) by specifying a , fε. When using the option, select from the list any function of one argument already defined in the component, under the node. For the option, select from the functions of two arguments. By default, only the 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 expression, and used to define the total emissivity:

Select the check box to define specific directional dependencies of the surface emissivity on each side of the boundary, and set the and 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.

If the is :

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By default, ε (dimensionless) and ρd (dimensionless) use values . These are properties of the surface that depend both on the material itself and the structure of the surface. Make sure that 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 the emissivity and reflectivity as functions of temperature using the variable rad.T.

Select the check box to define 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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When is , enter a value or expression for ε. The wavelength is accessible through the rad.lambda variable. Any expression set for the emissivity is then averaged over 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 ρd. The wavelength is accessible through the rad.lambda variable. Any expression set for the reflectivity is then averaged on each spectral band to obtain a piecewise constant reflectivity. If the average value of the reflectivity on each band is known, you may 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 set 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 reflectivity is defined on both sides. Select the check box and fill the and 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 diffuse reflectivity of the surface to a number between 0 and 1, where 1 applies to a perfect diffuse surface and 0 is appropriate when the reflection is only specular.