Radiation Group (Surface-to-Surface Radiation Interface)
This node enables you to specify radiation groups to speed up the radiation calculations and gather boundaries in a radiation problem that can see one another.
By default, all radiative boundaries (selected in a Diffuse Surface (Surface-to-Surface Radiation interface), Diffuse Mirror (Surface-to-Surface Radiation interface), or Prescribed Radiosity (Surface-to-Surface Radiation interface) node) belong to the same radiation group.
To change this, select the Use radiation groups check box under Radiation Settings to add a Radiation Group to a Surface-to-Surface Radiation (rad) interface.
When a node is added to another radiation group, it is overridden in the default group. Then this boundary can be added to other radiation groups without being overridden by the manually added radiation groups.
Be careful when grouping boundaries in axisymmetric geometries. The grouping cannot be based on which boundaries have a free view toward each other in the 2D geometry. Instead, consider the full 3D geometry, obtained by revolving the model geometry about the z axis, when defining groups.
For example, parallel vertical boundaries must typically belong to the same group in 2D axisymmetric components, but to different groups in a planar model using the same 2D geometry.
The figure below shows four examples of possible boundary groupings. On boundaries that have no number, the user has NOT set a node among the Diffuse Surface (Surface-to-Surface Radiation interface), Diffuse Mirror (Surface-to-Surface Radiation interface), and Prescribed Radiosity (Surface-to-Surface Radiation interface) nodes. These boundaries do not irradiate other boundaries, neither do other boundaries irradiate them.
On boundaries that belong to one or more radiation group, the user has set a node among the Diffuse Surface (Surface-to-Surface Radiation interface), Diffuse Mirror (Surface-to-Surface Radiation interface), and Prescribed Radiosity (Surface-to-Surface Radiation interface) nodes. The numbers on each boundary specify different groups to which the boundary belongs.
Figure 6-1: Examples of radiation group boundaries.
To obtain optimal computational performance, it is good practice to specify as many groups as possible as opposed to specifying few but large groups. For example, case B is more efficient than case D.
Boundary Selection
This section should contain any boundary that is selected in a Diffuse Surface, Diffuse Mirror, or Prescribed Radiosity node and that has a chance to see one of the boundary that is already selected in the Radiation Group.
Radiation Group
When the Wavelength dependence of surface properties is Constant, the radiation group is valid for all wavelengths, and all this section is then empty.
When the Wavelength dependence of surface properties is set to Solar and ambient or Multiple spectral bands, the radiation group is defined for all spectral bands by default. Clear Radiation group defined on spectral band i check boxes to remove the Bi spectral bands from these radiation groups.
Several settings for this node depend on the Wavelength dependence of surface properties setting, which is defined for the physics interface. See Radiation Settings.
If this node 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.
Theory for Surface-to-Surface Radiation
Location in User Interface
Context menus
If the Use radiation groups check box is selected under the Radiation Settings section:
 Surface-to-Surface Radiation>Radiation Group
Ribbon
Physics Tab with Surface-to-Surface Radiation selected:
Boundaries>Radiation Group