Local three-dimensional simulations of the convective overstability in protoplanetary discs

TL;DR

This study uses 3D simulations to explore how convective overstability in protoplanetary discs creates structures like vortices and zonal flows, affecting solid material concentration.

Contribution

It extends previous axisymmetric models by performing 3D simulations to analyze the nonlinear states and their dependence on parameters like Reynolds and Peclet numbers.

Findings

3D simulations show similar weakly nonlinear states at low Re.

Burst cycles with zonal flows and vortices occur at higher Re and low Pe.

Persistent zonal flows and elongated vortices appear at larger Pe.

Abstract

At certain radii protoplanetary discs may sustain a form of oscillatory convection (`convective overstability'; COS) due to localised adverse entropy gradients. The resulting hydrodynamical activity can produce coherent structures, such as zonal flows and vortices, that may concentrate solid material and aid their further coagulation. In this paper we extend previous axisymmetric runs by performing local three-dimensional simulations of the COS, using the code SNOOPY. As parameters are varied, we characterise how the various axisymmetric COS saturated states are transformed in 3D, while also tracking their interrelationship with the subcritical baroclinic instability. In particular, at low Reynolds number (Re) our 3D simulations exhibit similar weakly nonlinear and wave turbulent states to our earlier axisymmetic runs. At higher Re, but low Peclet number (Pe), we obtain bursty cycles involving the creation of zonal flows, the subsequent development of planar vortices, and their destruction by elliptical instability. For larger Pe, however, zonal flows can persist, alongside weaker more elongated vortices. These results further reveal the diversity of the COS's behaviour, and show that solid accumulation via COS-induced vortices may not be straightforward.