The role of zonal flows in disc gravito-turbulence

TL;DR

This study investigates how zonal flows contribute to maintaining gravito-turbulence in accretion discs, revealing a cyclical process involving instabilities that sustain turbulence through energy extraction.

Contribution

It demonstrates the emergence and cyclical behavior of zonal flows in gravito-turbulence using a novel numerical approach under realistic disc conditions.

Findings

Zonal flows appear at the onset of gravito-turbulence.

A cycle of formation and destruction of zonal flows is driven by instabilities.

Zonal flows are crucial for the self-sustenance of turbulence.

Abstract

The work presented here focuses on the role of zonal flows in the self-sustenance of gravito-turbulence in accretion discs. The numerical analysis is conducted using a bespoke pseudo-spectral code in fully compressible, non-linear conditions. The disc in question, which is modelled using the shearing sheet approximation, is assumed to be self-gravitating, viscous, and thermally diffusive; a constant cooling timescale is also considered. Zonal flows are found to emerge at the onset of gravito-turbulence and they remain closely linked to the turbulent state. A cycle of zonal flow formation and destruction is established, mediated by a slow mode instability (which allows zonal flows to grow) and a non-axisymmetric instability (which disrupts the zonal flow), which is found to repeat numerous times. It is in fact the disruptive action of the non-axisymmetric instability to form new leading and trailing shearing waves, allowing energy to be extracted from the background flow and ensuring the self-sustenance of the gravito-turbulent regime.