### 1.4.8. Radiation Properties

Radiation properties are required if a thermal radiation model is used. The Absorption Coefficient and Scattering Coefficient should be positive values or zero for transparent media. If you are using either the P1 or Rosseland radiation models, and at least one of the coefficients must be nonzero. The Refractive Index is a value greater than or equal to 1; it is approximately equal to 1 for most gases.

For a Fixed or Variable Composition Mixture, the radiation properties are determined by a mass fraction weighted arithmetic average. This is often not the most appropriate averaging method for these quantities. You can set radiation properties for the mixture in the Materials details view instead, if you want. For details, see Mixture Properties Tab in the CFX-Pre User's Guide.

The radiation properties specified for a mixture are ignored when using the `Weighted Sum of Gray Gases` or `Multigray` spectral models.

Important: Explicitly setting radiation properties for multicomponent flows is recommended. When a large number of components are involved, it takes a significant amount of solver CPU time to calculate these mixture properties. Often, the radiation properties are the same for all components, so computing radiation mixture properties results in wasted effort. You can avoid the radiation mixture property calculation by providing values for the mixture when it is created. An example of this is available.

For further details, see Material Properties for Radiation.

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