From convection rolls to finger convection in double-diffusive turbulence

TL;DR

This study explores double-diffusive convection between horizontal plates, revealing a transition from convection rolls to salt fingers that enhances salinity transfer, and demonstrates the applicability of thermal convection theory to DDC.

Contribution

It uncovers the flow pattern transition in DDC and extends thermal convection theory to this complex flow regime.

Findings

Salinity flux increases with stabilizing temperature gradient initially.

Flow transitions from convection rolls to salt fingers.

Grossmann-Lohse theory applies to DDC across various parameters.

Abstract

Double diffusive convection (DDC), which is the buoyancy driven flow with fluid density depending on two scalar components, is ubiquitous in many natural and engineering enviroments. Of great interests are scalers transfer rate and flow structures. Here we systematically investigate DDC flow between two horizontal plates, driven by an unstable salinity gradient and stabilized by a temperature gradient. Counterintuitively, when increasing the stabilizing temperature gradient, the salinity flux first increases, even though the velocity monotonically decreases, before it finally breaks down to the purely diffusive value. The enhanced salinity transport is traced back to a transition in the overall flow pattern, namely from large scale convection rolls to well-organised vertically-oriented salt fingers. We also show and explain that the unifying theory of thermal convection originally developed by Grossmann and Lohse for Rayleigh-B\'{e}nard convection can be directly applied to DDC flow for a wide range of control parameters (Lewis number and density ratio), including those which cover the common values relevant for ocean flows.