title: From convection rolls to finger convection in double-diffusive turbulence.
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 environments. Of great interests are scalars' 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-organized vertically oriented salt fingers. We also show and explain that the unifying theory of thermal convection originally developed by Grossmann and Lohse for Rayleigh-Bénard 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.
authors.affiliation: Physics of Fluids Group, MESA+ Institute and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands; yantao.yang@utwente.nl.
authors.forename: Yantao
authors.identifier: http://orcid.org/0000-0001-5065-2769
authors.initials: Y
authors.lastname: Yang
authors.affiliation: Physics of Fluids Group, MESA+ Institute and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands; Dipartimento di Ingegneria Industriale, University of Rome Tor Vergata, Rome 00133, Italy;
authors.forename: Roberto
authors.initials: R
authors.lastname: Verzicco
authors.affiliation: Physics of Fluids Group, MESA+ Institute and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands; Max-Planck Institute for Dynamics and Self-Organization, 37077 Göttingen, Germany.
authors.forename: Detlef
authors.initials: D
authors.lastname: Lohse
keywords: buoyancy-driven flow; double-diffusive convection; thermohaline convection