The Fluence Rate Calculation node is used to compute the fluence rate within selected domains. Fluence rate is defined as the total radiant power that would hit a small sphere located within the domain, divided by the cross sectional area of the sphere. The sphere is assumed to be much smaller than any geometric details but much larger than the wavelength of the radiation.

The variable for fluence rate is <phys>.E0, where <phys> is the name of the Surface-to-Surface Radiation interface, for example rad.E0. When several spectral bands are defined, the fluence rate can be evaluated per wavelength with <phys>.E0_band<k>, where <k> is the band number. This variable is precomputed and its values are stored at Gauss points on the mesh, which makes it very fast to evaluate. The Gauss points order is defined in the discretization section of the physics interface node.

Another variable, <phys>.fr_band<k>, can also be used to visualize the fluence rate. The difference is that this variable is evaluated using an operator at the exact position where it is requested (instead of interpolating from the Gauss point values). As these values are not cached, the evaluation time is much more important compared to <phys>.E0.

See View Factor Evaluation for more details on the underlying theory.

Physics tab with Surface-to-Surface Radiation selected in the model tree: