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3. When you click OK in the DTRM Rays dialog box, a Select File dialog box 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.

(b) 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 Section 5.3.5: Clustering in the
separate Theory Guide for details about clustering.)

(c) 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 θ used to define the solid angle
about a point P on a surface. The solid angle is defined as θ varies from 0 to 90 degrees
Phi Divisions defines the number of discrete divisions in the angle φ used to define the solid angle
about a point P on a surface. The solid angle is defined as φ varies from 0 to 360 degrees. The default
setting of 4 implies that each ray traced from the surface will be located at a 90◦ 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 θ and φ.

(d) 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.

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