### 10.8.5. Thermal Radiation Control

The radiation modeling options based on ray tracing, Discrete Transfer and Monte Carlo, require additional controls. These controls are found on the Advanced Options tab of the Solver Control form.

Because these radiation models require considerable time, there are several ways to strike a balance between accuracy and computer time. This can be done by coarsening the fine mesh used for the flow field, or calculating the radiation field at a different frequency than the other transport equations.

#### 10.8.5.1. Iteration Interval

It sets the frequency of the radiation calculation respect to the flow solver. If left unset, radiation will be calculated at each flow solver iteration (that is, 1).

#### 10.8.5.2. Diagnostic Output Level

When performing the ray tracing calculation, either Discrete Transfer or Monte Carlo, several diagnostics can be written to the output file. The output is controlled as follows:

0 - Quiet. No output is reported to the output file even if minor problems have occurred. If the solver encounters a fatal error, it will stop automatically.

1 - Minimal. A diagnostic results file is written and warnings are reported. The diagnostic results file includes radiation quantities for each radiation element.

2 - Verbose. A diagnostic results file is written each radiation iteration and the solver will stop even for some warnings. This level is meant for debugging purposes only.

Information about the diagnostic results file is available. For details, see CFX Radiation File in the CFX-Solver Manager User's Guide.

#### 10.8.5.3. Ray Reflection Control

When modeling specular boundaries, a ray may be reflected multiple times. When the energy content of the original ray has dropped below the Ray Reflection Threshold, the tracing is halted.

Increasing the threshold may reduce accuracy. In this case, energy residuals and imbalances may not be smooth over a restart, in particular when the radiation properties have evolved during the initial run.

Decreasing the threshold will result in longer traces. Because the energy content of the original ray decays along the trace, after a certain run length its contribution becomes negligible. Reducing the threshold below a certain level, thus excessively prolonging the traces, will then have no significant influence on results.

#### 10.8.5.4. Coarsening Control

##### 10.8.5.4.1. Target Coarsening Rate

This represents the target ratio between elements in the fine mesh and radiation elements in the coarser mesh. The default value is 64; that is, the number of radiation elements is 64 times smaller than the number of elements in the fine mesh. The actual coarsening could be smaller than the target specified. A summary is presented in the Radiation Coarsening section of the output file.

##### 10.8.5.4.2. Minimum Blocking Factor

This represents the minimum number of elements within a coarser element at a given coarsening level.

##### 10.8.5.4.3. Maximum Blocking Factor

This represents the maximum number of elements within a coarser element at a given coarsening level.

##### 10.8.5.4.4. Small Coarse Grid Size

It is the minimum number of radiation elements in the coarser mesh. The coarsening algorithm will stop when either this value or the target coarsening rate is achieved.

##### 10.8.5.4.5. Diagnostic Output Level

The scope of radiation mesh coarsening diagnostic information written to the output file can be controlled by setting the diagnostic output level to 0, 1, or 2.

- 0 - Minimal.

  This is the default diagnostic output level. A basic summary of the radiation mesh coarsening information is written to the output file.

- 1 - Minimal.

  Same effect as output level 0.

- 2 - Verbose.

  The output file contains a detailed summary of the radiation mesh coarsening information. As well, additional radiation mesh coarsening information is written to the output file in the form of a table with the columns: `Crs Level`, `# Elems`, `Avg Nbrs`, `Avg Crs Rate` and `Tot Crs Rate`.

  - `Crs Level` is the progressive level of coarsening.

  - `# Elems` is the number of radiation elements at each progressive level.

  - `Avg Nbrs` is the average number of neighboring elements that each radiation element has.

  - `Avg Crs Rate` is the average coarsening rate achieved at each progressive level, calculated as the ratio of the number of radiation elements (found under `# Elems`) between the current and subsequent progressive levels.

  - `Tot Crs Rate` is the total coarsening rate achieved at each progressive level, calculated as the ratio of the number of radiation elements (found under `# Elems`) between the first and current progressive levels.

#### 10.8.5.5. Ray Tracing Control

This section only applies for the Discrete Transfer model.

##### 10.8.5.5.1. Iteration Interval

Sets the frequency for the calculation of the ray tracks. Because the ray tracing is time-consuming and disk-intensive, it is rarely used. The default value is zero; that is, the tracks are computed only once and stored permanently.

##### 10.8.5.5.2. Maximum Buffer Size

To minimize total memory usage and maximize disk throughput, information for the tracks is stored in a memory buffer before being written to disk. The Maximum Buffer Size parameter sets the maximum allowed buffer storage in words. The default value is 6000 words.

##### 10.8.5.5.3. Maximum Number of Track Segments

A single ray is made of tracks, the fraction of ray within a radiation element. In certain cases, highly specular surfaces with a small spacing, the rays may have infinite number of reflections before being totally absorbed. Whenever the number of tracks reaches this maximum, the tracing for this ray is halted, and the next ray is started. Default value is 9000.

##### 10.8.5.5.4. Maximum Number of Iterations

Limits the inner loop when there are reflecting boundaries; otherwise, the first iteration usually satisfies the iteration convergence criterion (1%).

##### 10.8.5.5.5. Iteration Convergence Criterion

Sets the maximum relative radiosity (emission plus reflection) change for convergence of the inner loop when there are reflecting boundaries.

##### 10.8.5.5.6. Ray Reflection Threshold

When modeling specular boundaries, a ray may get reflected multiple times. When the energy content of the original ray has dropped below this threshold, the tracing is halted.

##### 10.8.5.5.7. File Path

Specifies the location in which to store the track files. This is required only when the local disk space is not large enough to store the track files, or a slave in a parallel run does not have access to the run directory.

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