Turbulent mixing and layer formation in double-diffusive convection: 3D numerical simulations and theory

TL;DR

This study uses 3D numerical simulations and theory to explore double-diffusive convection, revealing layer formation, turbulent flux variations, and proposing new methods for understanding transport in astrophysical systems.

Contribution

It provides detailed 3D simulation analysis of double-diffusive convection, demonstrating layer formation and merging, and compares results with oceanographic theory, advancing astrophysical transport modeling.

Findings

Layer formation and merging observed in simulations.

Turbulent fluxes are much smaller than overturning convection.

Heat flux correlates with layer height.

Abstract

Double-diffusive convection, often referred to as semi-convection in astrophysics, occurs in thermally and compositionally stratified systems which are stable according to the Ledoux-criterion but unstable according to the Schwarzchild criterion. This process has been given relatively little attention so far, and its properties remain poorly constrained. In this paper, we present and analyze a set of three-dimensional simulations of this phenomenon in a Cartesian domain under the Boussinesq approximation. We find that in some cases the double-diffusive convection saturates into a state of homogeneous turbulence, but with turbulent fluxes several orders of magnitude smaller than those expected from direct overturning convection. In other cases the system rapidly and spontaneously develops closely-packed thermo-compositional layers, which later successively merge until a single layer is left. We compare the output of our simulations with an existing theory of layer formation in the oceanographic context, and find very good agreement between the model and our results. The thermal and compositional mixing rates increase significantly during layer formation, and increase even further with each merger. We find that the heat flux through the staircase is a simple function of the layer height. We conclude by proposing a new approach to studying transport by double-diffusive convection in astrophysics.