Dynamics of mixed convective-stably-stratified fluids

L. A. Couston, D. Lecoanet, B. Favier, M. Le Bars

Research output: Contribution to journalArticlepeer-review

22 Scopus citations

Abstract

We study the dynamical regimes of a density-stratified fluid confined between isothermal no-slip top and bottom boundaries (at temperatures Tt and Tb) via direct numerical simulation. The thermal expansion coefficient of the fluid is temperature dependent and chosen such that the fluid density is maximum at the inversion temperature Tb>Ti>Tt. Thus, the lower layer of the fluid is convectively unstable while the upper layer is stably stratified. We show that the characteristics of the convection change significantly depending on the degree of stratification of the stable layer. For strong stable stratification, the convection zone coincides with the fraction of the fluid that is convectively unstable (i.e., where T>Ti), and convective motions consist of rising and sinking plumes of large density anomaly, as is the case in canonical Rayleigh-Bénard convection; internal gravity waves are generated by turbulent fluctuations in the convective layer and propagate in the upper layer. For weak stable stratification, we demonstrate that a large fraction of the stable fluid (i.e., with temperature T<Ti) is instead destabilized and entrained by buoyant plumes emitted from the bottom boundary. The convection thus mixes cold patches of low density-anomaly fluid with hot upward plumes and the end result is that the Ti isotherm sinks within the bottom boundary layer and that the convection is entrainment dominated. We provide a phenomenological description of the transition between the regimes of plume-dominated and entrainment-dominated convection through analysis of the differences in the heat transfer mechanisms, kinetic energy density spectra, and probability density functions for different stratification strengths. Importantly, we find that the effect of the stable layer on the convection decreases only weakly with increasing stratification strength, meaning that the dynamics of the stable layer and convection should be studied self-consistently in a wide range of applications.

Original languageEnglish (US)
Article number094804
JournalPhysical Review Fluids
Volume2
Issue number9
DOIs
StatePublished - Sep 2017
Externally publishedYes

ASJC Scopus subject areas

  • Computational Mechanics
  • Modeling and Simulation
  • Fluid Flow and Transfer Processes

Fingerprint

Dive into the research topics of 'Dynamics of mixed convective-stably-stratified fluids'. Together they form a unique fingerprint.

Cite this