25. Radiative Transfer Unit

Figure 25.1: The organizational structure of the RadTrans unit.

The RadTrans unit is responsible for solving the radiative transfer equation

$$\frac{1}{c} \frac{\partial I}{\partial t} + \mathbf{\hat \Omega} \cdot \nabla I + \rho \kappa I = \eta,$$ (25.1)

where, $I(\mathbf{x},\mathbf{\hat\Omega},\nu,t)$ is the radiation intensity, $c$ is the speed of light, $\rho$ is the mass density, $\kappa(\mathbf{x}, \nu, t)$ is the opacity in units of $\textrm{cm}^2/\textrm{g}$, $\eta(\mathbf{x}, \nu, t)$ is the emissivity, $\nu$ is the radiation frequency, and $\mathbf{\hat\Omega}$ is the unit direction vector. This equation is coupled to the electron internal energy through

$$\frac{\partial u_e}{\partial t} = \int_0^\infty \textrm{d}\nu \int_{4\pi}\textrm{d}\mathbf{\hat\Omega} (\rho \kappa I - \eta),$$ (25.2)

where $u_e$ is the electron internal energy density.

The RadTrans unit is responsible for solving the radiative transfer equation and updating the electron energy using (25.2) over a single time step. This ensures that the total system energy is conserved. Radiation-hydrodynamics effects, such as work, are operator-split and handled by the hydrodynamics unit. Currently, there is only a single RadTrans solver, MGD, which uses a simple multigroup diffusion approximation and is described in Sec:MGD.