13.3.2 Setting Up the DTRM

Defining the Rays

When you select the Discrete Transfer model and click OK in the Radiation Model dialog box, the DTRM Rays dialog box (Figure  13.3.2) will open automatically. (Should you need to modify the current settings later in the problem setup or solution procedure, you can open this dialog box manually using the Define/DTRM Rays... menu item.)

Figure 13.3.2: The DTRM Rays Dialog Box

In this dialog box you will set parameters for and create the rays and clusters discussed in this section in the separate Theory Guide.

The procedure is as follows:

1.   To control the number of radiating surfaces and absorbing cells, set the Cells Per Volume Cluster and Faces Per Surface Cluster. (See the explanation below.)

2.   To control the number of rays being traced, set the number of Theta Divisions and Phi Divisions. (Guidelines are provided below.)

3.   When you click OK in the DTRM Rays dialog box, a Select File dialog box (see Section  2.1.6) will open prompting you for the name of the "ray file''. After you have specified the file name and chosen whether to write a binary ray file, ANSYS FLUENT will write the ray file and then read it afterward. During the write process the status of the DTRM ray tracing will be reported in the ANSYS FLUENT console. For example:

Completed 25% tracing of DTRM rays

Completed 50% tracing of DTRM rays

Completed 75% tracing of DTRM rays

Completed 100% tracing of DTRM rays

See below for details on DTRM Rays dialog box inputs.

If you cancel the DTRM Rays dialog box without writing and reading the ray file, the DTRM will be disabled.

Controlling the Clusters

Your inputs for Cells Per Volume Cluster and Faces Per Surface Cluster will control the number of radiating surfaces and absorbing cells. By default, each is set to 1, so the number of surface clusters (radiating surfaces) will be the number of boundary faces, and the number of volume clusters (absorbing cells) will be the number of cells in the domain. For small 2D problems, these are acceptable numbers, but for larger problems you will want to reduce the number of surface and/or volume clusters in order to reduce the ray-tracing expense. (See this section in the separate Theory Guide for details about clustering.)

Controlling the Rays

Your inputs for Theta Divisions and Phi Divisions will control the number of rays being traced from each surface cluster (radiating surface).

Theta Divisions defines the number of discrete divisions in the angle $\theta$ used to define the solid angle about a point $P$ on a surface. The solid angle is defined as $\theta$ varies from 0 to 90 degrees ( this figure in the separate Theory Guide), and the default setting of 1 for the number of discrete settings implies that there will be one ray traced from the surface.

Phi Divisions defines the number of discrete divisions in the angle $\phi$ used to define the solid angle about a point $P$ on a surface. The solid angle is defined as $\phi$ varies from 0 to 360 degrees ( this figure in the separate Theory Guide). The default setting of 4 implies that each ray traced from the surface will be located at a 90 $^\circ$ angle, and in combination with the default setting for Theta Divisions, above, implies that 4 rays will be traced from each surface control volume. In many cases, it is recommended that you at least double the number of divisions in $\theta$ and $\phi$.

Writing and Reading the DTRM Ray File

After you have activated the DTRM and defined all of the parameters controlling the ray tracing, you must create a ray file which will be read back in and used during the radiation calculation. The ray file contains a description of the ray traces (path lengths, cells traversed by each ray, etc.). This information is stored in the ray file, instead of being recomputed, in order to speed up the calculation process.

By default, a binary ray file will be written. You can also create text (formatted) ray files by turning off the Write Binary Files option in the Select File dialog box (see Section  2.1.6).

Do not write or read a compressed ray file, because ANSYS FLUENT will not be able to access the ray tracing information properly from a compressed ray file.

The ray filename must be specified to ANSYS FLUENT only once. Thereafter, the filename is stored in your case file and the ray file will be automatically read into ANSYS FLUENT whenever the case file is read. ANSYS FLUENT will remind you that it is reading the ray file after it finishes reading the rest of the case file by reporting its progress in the console.

Note that the ray filename stored in your case file may not contain the full name of the directory in which the ray file exists. The full directory name will be stored in the case file only if you initially read the ray file through the GUI (or if you typed in the directory name along with the filename when using the text interface). In the event that the full directory name is absent, the automatic reading of the ray file may fail (since ANSYS FLUENT does not know in which directory to look for the file), and you will need to manually specify the ray file, using the File/Read/DTRM Rays... menu item. The safest approaches are to use the GUI when you first read the ray file or to supply the full directory name when using the text interface.

You should recreate the ray file whenever you do anything that changes the mesh, such as: change the type of a boundary zone adapt or reorder the mesh scale the mesh You can open the DTRM Rays dialog box directly with the Define/DTRM Rays... menu item.

Displaying the Clusters

Once a ray file has been created or read in manually, you can click on the Display Clusters button in the DTRM Rays dialog box to graphically display the clusters in the domain. See Section  13.3.8 for additional information about displaying rays and clusters.