Black hole-neutron star post-merger evolution using viscous relativistic hydrodynamics

The post-merger evolution of black hole-neutron star and neutron star-neutron star systems is driven by magnetohydrodynamic turbulence. ~Such multiscale problems are very costly to simulate. ~One approach is to use artificially large seed magnetic fields to resolve the magnetorotational instability. ~Another is to add some model of subgrid-scale effects, with subgrid angular momentum transport often being modeled as a shear viscosity. ~We simulate the disk from a black hole-neutron star model in both ways and compare results. ~We present a new implementation of the relativistic Navier-Stokes equations in the Spectral Einstein Code, with accompanying star and accretion torus tests. ~For the post-merger system, we analyze the combination of shocks and turbulent/viscous dissipation acting to heat the disk in the early post-merger phase.