### 10.6.1. Fluid Models

Information on radiation modeling in multiple domains is available. For details, see Domain Considerations.

#### 10.6.1.1. Number of Rays

To determine the direction of the rays, the unitary hemisphere over the face of a parametric element is discretized using spherical coordinates. The span is divided into angles by the number of rays, and rays directions are computed to pass through the center of the angles. In total, the square of the number of rays is traced from an element surface. The default is set to 8.

#### 10.6.1.2. Transfer Mode

The Transfer Mode setting defines which radiative transfer mode is enabled. The default value is `Participating Media`; that is, the domain material emits, absorbs, and/or scatters radiation. The `Surface to Surface` option implies that volumetric emission, absorption and scattering are ignored regardless of the specified material properties.

Note: Solution time may be adversely affected when using the `Surface to Surface` option in a case involving a subdomain that only partially covers the whole domain. With the `Surface to Surface` option, the CFX-Solver tries to reduce the radiation mesh to a single element (because only surfaces are of interest) but the presence of the subdomain causes the number of elements to be at least two, triggering a potentially time-consuming ray trace. This problem applies to both the Discrete Transfer and Monte Carlo models.

The workaround is to use the `Participating Media` option instead of the `Surface to Surface` option and adjust the coarsening controls to produce a coarser radiation mesh than the default. Choosing a coarsening rate that produces a few hundred radiation elements should result in a solution similar to that under the `Surface to Surface` option, but with perhaps not the same resolution of the intensity field at the boundaries.

#### 10.6.1.3. Spectral Model

See Spectral Model.

#### 10.6.1.4. Scattering Model

See Scattering Model.

Release 18.2 - © ANSYS, Inc. All rights reserved.