Controlling Artificial Viscosity in SPH simulations of accretion disks

TL;DR

This paper evaluates existing methods for controlling artificial viscosity in SPH simulations of accretion disks and introduces a new, more effective switch based on neighbor orbit coherence to improve simulation accuracy.

Contribution

The paper identifies limitations of current AV switches and proposes a simple, neighbor-based algorithm that better activates AV only during true convergent flows.

Findings

Existing switches leave AV active in smooth shear flows.

Current methods do not prevent prograde particle alignments.

The new switch reduces unwanted particle alignments effectively.

Abstract

We test the operation of two methods for selective application of Artificial Viscosity (AV) in SPH simulations of Keplerian Accretion Disks, using a ring spreading test to quantify effective viscosity, and a correlation coefficient technique to measure the formation of unwanted prograde alignments of particles. Neither the Balsara Switch nor Time Dependent Viscosity work effectively, as they leave AV active in areas of smooth shearing flow, and do not eliminate the accumulation of alignments of particles in the prograde direction. The effect of both switches is periodic, the periodicity dependent on radius and unaffected by the density of particles. We demonstrate that a very simple algorithm activates AV only when truly convergent flow is detected and reduces the unwanted formation of prograde alignments. The new switch works by testing whether all the neighbours of a particle are in Keplerian orbit around the same point, rather than calculating the divergence of the velocity field, which is very strongly affected by Poisson noise in the positions of the SPH particles.