To define a surface radiative interaction:

1. From the main menu bar, select InteractionCreate.

   Tip:  You can also create a surface radiative interaction using the tool in the Interaction module toolbox.

2. In the Create Interaction dialog box that appears, do the following:

   • Name the interaction. For more information about naming objects, see “Using basic dialog box components,” Section 3.2.1.

   • Select the step. You can define radiation from a surface only during a heat transfer, coupled temperature-displacement, or coupled thermal-electrical step.

   • Select the Surface radiation type of interaction.

3. Click Continue to close the Create Interaction dialog box.

4. Use one of the following methods to select the surface:

   • Use an existing surface to define the region. On the right side of the prompt area, click Surfaces. Select an existing surface from the Region Selection dialog box that appears, and click Continue.

     Note:  The default selection method is based on the selection method you most recently employed. To revert to the other method, click Select in Viewport or Surfaces on the right side of the prompt area.

   • Use the mouse to select a region in the viewport. (For more information, see “Selecting objects within the current viewport,” Section 6.2.)

     If the model contains a combination of mesh and geometry, click one of the following from the prompt area:

     • Click Geometry if you want to select the surface or vertex from a geometry region.

     • Click Mesh if you want to select the surface or node from a native or orphan mesh selection.

     You can use the angle method to select a group of faces or edges from geometry or a group of element faces from a mesh. For more information, see “Using the angle and feature edge method to select multiple objects,” Section 6.2.3.

5. In the Edit Interaction dialog box that appears, select the Radiation type.

   • Select To ambient to specify heat transfer to the surrounding environment.

   • Select Cavity approximation (3D only) to approximate cavity radiation in three-dimensional models using uniform emissivity, a closed cavity, and an average cavity temperature.

6. If you selected To ambient in the previous step, complete the radiation definition as follows:

   1. Click the arrow to the right of the Emissivity distribution field, and select the option of your choice from the list that appears:

      • Select Uniform to define an emissivity that is uniform over the surface.

      • Select an analytical field to define a spatially varying emissivity. Only analytical fields that are valid for this interaction type are displayed in the selection list. Alternatively, you can click to create a new analytical field. (See Chapter 58, “The Analytical Field toolset,” for more information.)

   2. In the Emissivity field, enter the emissivity of the surface, .

   3. In the Ambient temperature field, enter the ambient temperature, .

   4. If you want to vary the ambient temperature with time, click the arrow to the right of the Ambient temperature amplitude field and select an amplitude from the list that appears. If desired, click to create a new amplitude; see “Selecting an amplitude type to define,” Section 57.3, for more information.

7. If you selected Cavity approximation (3D only) in Step 5, enter the emissivity of the surface, , in the Emissivity field.

8. Specify the absolute zero temperature, , on the temperature scale being used and the Stefan-Boltzmann constant, , in the Edit Model Attributes dialog box, as described in “Specifying model attributes,” Section 9.8.4.

9. Click OK to create the interaction and to close the editor.