DIFFUSION SYNTHETIC ACCELERATION OF DISCONTINUOUS FINITE-ELEMENT TRANSPORT ITERATIONS

We present a discretization of the diffusion equation that can be used to accelerate transport iterations when the transport equation is spatially differenced by a discontinuous finite element (DFE) method. That is, we present a prescription for diffusion synthetic acceleration of DFE transport iterations. (The well-known linear discontinuous and bilinear discontinuous schemes are examples of DFE transport differencings.) We demonstrate that our diffusion discretization can be obtained in any coordinate system on any grid. We show that our diffusion discretization is not strictly consistent with the transport discretization in the usual sense. Nevertheless, we find that it yields a scheme with unconditional stability and rapid convergence. Further, we find that as the optical thickness of spatial cells becomes large, the spectral radius of the iteration scheme approaches zero (i.e., instant convergence). We give analysis results for one- and two-dimensional Cartesian geometries and numerical results for one-dimensional Cartesian and spherical geometries.