A comparison of momentum transport models for numerical relativity

TL;DR

This paper compares two models of momentum transport in numerical relativity, analyzing their stability and impact on astrophysical simulations of neutron star mergers and accretion disks.

Contribution

It introduces and evaluates relativistic Navier-Stokes and turbulent stress models for subgrid effects in neutron star merger simulations.

Findings

Proper turbulent heating treatment prevents unphysical instabilities.

Star evolution and disk accretion rates are insensitive to the transport model.

Disk outflows are affected by the choice of momentum transport method.

Abstract

The main problems of nonvacuum numerical relativity, compact binary mergers and stellar collapse, involve hydromagnetic instabilities and turbulent flows, so that kinetic energy at small scales have mean effects at large scale that drive the secular evolution. Notable among these effects is momentum transport. We investigate two models of this transport effect, a relativistic Navier-Stokes system and a turbulent mean stress model, that are similar to all of the prescriptions that have been attempted to date for treating subgrid effects on binary neutron star mergers and their aftermath. Our investigation involves both stability analysis and numerical experimentation on star and disk systems. We also begin the investigation of the effects of particle and heat transport on post-merger simulations. We find that correct handling of turbulent heating can be important for avoiding unphysical instabilities. Given such appropriate handling, the evolution of a differentially rotating star and the accretion rate of a disk are reassuringly insensitive to the choice of prescription. However, disk outflows can be sensitive to the choice of method, even for the same effective viscous strength. We also consider the effects of eddy diffusion in the evolution of an accretion disk and show that it can interestingly affect the composition of outflows.