Specifying Radiation Properties at Boundaries and Interfaces

When modeling thermal radiation, inflow, outflow, and wall boundaries, that is, the open and closed boundaries, require the specification of surface materials. These surface materials are defined by the radiation properties emissivity, reflectivity, transmissivity, and optionally, reflection specularity. If you have interfaces within your domain, for example, solid-fluid interfaces, you specify the surface materials at the interface boundaries (not at the interfaces).

You have the possibility of adding your own surface materials to the surface materials database, so that you can re-use these materials across different simulations.

To specify radiation properties on boundaries and interfaces:

  1. Expand the Continua > [physics continuum] > Models > Surface Materials > Surface Materials node.
    By default, the surface material Default, with an emissivity of 0.8, a reflection specularity of 0.0, and a transmissivity of 0.0, is created. The reflectivity is calculated automatically according to Kirchhoff's law. You can modify these default radiation properties to suit your requirements.
  2. Edit the Default > Material Properties node and specify values for the following radiation properties:
    • Emissivity
    • Reflection Specularity
    • Reflectivity
    • Transmissivity
  3. If required, create additional surface materials:
    1. Right-click the Surface Materials > Surface Materials node and select Select Surface Materials.
    2. In the Select Surface Materials dialog, select a material. By default, only Cast Iron (Cast Iron) is available. You can save your own surface materials with individual radiation properties to the surface materials database and retrieve them here. For more information, see Step 5.
    3. Click Apply, then Close.
      A node for the selected material appears under the Surface Materials node.
    4. Edit the [surface material] > Material Properties node and specify values for emissivity, reflection specularity, reflectivity, and transmissivity.
  4. Associate the surface material(s) with individual boundaries or interface boundaries to apply the radiation properties:
    1. Select the Regions > [region] > Boundaries > [inflow boundary/outflow boundary/wall boundary/free stream boundary] > Surface Material node and set Surface Material to one of the surface materials that you created previously in the physics continuum. By default, Surface Material is assigned to Default.
    2. If you are modeling radiation at dual-sided or external boundaries and have set Regions > [region] > Physics Conditions > Radiation Transfer Option to either External or Internal and External, select the [region] > Boundaries > [inflow boundary/outflow boundary/wall boundary/free stream boundary] > External Surface Material node and set Surface Material to one of the surface materials that you created previously in the physics continuum.
  5. If you want to save a surface material to the surface materials database for re-use in the same simulation or other simulations:
    1. Right-click the Tools > Material Databases > Standard > SurfaceMaterials node and select New Surface Material.
    2. Select the Surface Material 1 (Surface Material 1) node and give Title and Symbol names that represent the material that you want to define.
    3. Right-click the [surface material (surface material)] node and select New Database Properties.
    4. In the Select Material Properties and Methods dialog, select which of the radiation properties—Emissivity, Reflection Specularity, Reflectivity, and Transmissivity—you want to define and then select their profile method.
    5. Click OK.
    6. Edit the [surface material (surface material)] node and specify values for the individual radiation properties.
    The surface material can now be selected in the physics continuum.
  6. At open boundaries such as pressure outlets or velocity inlets, select the Regions > [region] > Boundaries > [inflow/outflow boundary] > Physics Values > Radiation Temperature node and set Value to a temperature that has a total radiant emittance identical to that of a black body radiator.