### 10.8.4. Sources

Non-thermal radiation sources can be set when using either the Discrete Transfer or Monte Carlo radiation models.

These non-thermal radiation sources (fluxes at boundaries) are divided into 2 groups: isotropic and directional. The Directional Radiation Source and Directional Radiation Flux is only supported when using the Monte Carlo model.

The strength of the source or flux can be a function of the spectral variables: frequency, wavelength in vacuum, or wavenumber in vacuum when using the Multiband spectral modeling option.

#### 10.8.4.1. Directional Radiation Source

It allows the specification of the source strength and its direction. The direction can be specified by either Cartesian Components or Cylindrical Components using a local axis.

#### 10.8.4.2. Isotropic Radiation Source

This allows the setting of a single source strength when the source strength is uniform in all directions.

#### 10.8.4.3. Directional Radiation Flux

Specifies collimated non-thermal radiation flux at boundaries. The direction can be set by using Cartesian Components, Cylindrical Components using a local axis, or Normal to Boundary. The external refractive index can be set. If the value is set, the refraction at the boundary is calculated automatically in a manner similar to that for radiation through domain interfaces; see Radiation Through Domain Interfaces in the CFX-Solver Theory Guide. If the external refractive index is left unspecified it is assumed to have the same value as the material in the domain and the radiation will pass through the boundary without any refraction or reflection.

#### 10.8.4.4. Combining Radiation Sources

Multiple radiation sources can, in general, be defined on any given locator that is eligible to have a radiation source. However, when using the Monte Carlo and Discrete Transfer models, the following limitations apply:

- For the `Gray` and `Weighted Sum of Gray Gases` spectral model options, multiple radiation sources cannot be defined on the same locator unless they are all isotropic.

- For the `Multiband` spectral model option, a given locator may have multiple radiation sources if the strengths and directions do not overlap within the same spectral band. Otherwise, the results will be incorrect.

  To help prevent overlapping spectral bands on the same source locator, you can:

  - Combine multiple isotropic sources into a single source definition (where possible).

  - Combine multiple frequency (or wavelength) dependent sources into a single source using CEL for the strength and, if relevant, direction.

#### 10.8.4.5. Isotropic Radiation Flux

Specifies a directionally uniform, non-thermal radiation flux.

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