Build the model in the preprocessor.

Select the appropriate set of solid elements to be flagged.

Apply any appropriate radiosity settings.

Specify decimation parameters for the selected solid elements via the RDEC command. Decimation allows you to use fewer radiation surface elements than there are underlying solid or shell element faces. Figure 5.4: Decimation illustrates this concept.

Figure 5.4: Decimation

Where different parts of the thermal model differ in size significantly, you should decimate these parts separately. Otherwise, smaller parts of the thermal model can be overdecimated.

You should estimate the number of radiosity surface elements on a decimated mesh before specifying the degree of decimation. The number should be enough to represent the original surface. For example, you would not want to represent a sphere using only five surface elements.

The goal of decimation is to reduce the time required for view factors calculation, as well as the heat flux calculation. For a small model with a small degree of decimation, the time saved for the view factors calculation could be offset by the amount of time required for the decimation calculations. Therefore, Ansys recommends using decimation only for sufficiently large models.

Specify symmetry options for the selected solid elements using the RSYMM command.

Use this command to specify either the plane of symmetry (POS) for planar reflection or the center of rotation (COR) for cyclic repetition. Note that POS reflection is NOT the same as COR repetition. Figure 5.5: Planar Reflection illustrates how the original sector is duplicated about a plane. Figure 5.6: Cyclic Repetition (Two Repetitions Shown) illustrates how the original sector is duplicated about a center point.

The figures below show the results of planar and cyclic repetition. Issue RSYMM,,,X for duplication around the X axis (Figure 5.5: Planar Reflection). Issue RSYMM,,,,n for a cyclic repetition (Figure 5.6: Cyclic Repetition (Two Repetitions Shown) uses RSYMM,,,,11. Only 2 repetitions are shown in the figure).

Figure 5.5: Planar Reflection

Figure 5.6: Cyclic Repetition (Two Repetitions Shown)

If you issue RSYMM more than once, each command will be processed in the order issued. For example, you can conduct a planar reflection about the global X axis, followed by a second planar reflection about the global Y axis:

rsym,,,x
rsym,,,y

Figure 5.7: Multiple RSYMM Commands

To achieve faster solution times and improved efficiency for symmetric models turn on view factor condensation (VFCO,,ENCL,1 or VFCO,,ENCL,2) to significantly reduce the view factor matrix (See View Factor Matrix Properties for a discussion on the underlying theory and governing equations.). It also reduces the problem to solving only for the independent radiosity flux (for details, see Radiosity Equations Simplified for Models with Symmetry). For an example problem illustrating the use of view factor condensation, see Example of a 3D Open Enclosure with Symmetry: Radiation Analysis with Condensed View Factor Calculation.

Generate the radiosity surface elements, SURF251/SURF252. Select the solid elements that you have flagged (using SF,,RDSF) and issue RSURF.

If you need to regenerate the surface mesh (for example, unsatisfactory degree of decimation, improper symmetry reflection, etc.), delete the unsatisfactory results (RSURF,clear,last), adjust your decimation or symmetry parameters, and reissue the RSURF command. All RSURF commands must be issued after the model is complete (that is, after all meshing operations are complete).

The RSURF command applies symmetry reflections only to radiosity surface elements created by the current RSURF command, even if other elements are selected. You must use RSURF to create the surface elements. You cannot create SURF251/SURF252 elements manually using the E, ESURF, or AMESH commands.

Solve the model, and postprocess as usual. You can postprocess radiation heat flux using the NMISC records in SURF251 and SURF252.

Resume your database or .cdb file using RESUME or CDREAD. The mapping information is automatically saved to an .rsm file if SURF251 and SURF252 elements are present in the model. The .rsm file will be located in the directory specified by the SAVE or CDWRITE command.

Solve the model, and postprocess as usual.