Dynamics of mixed convective--stably-stratified fluids
L.-A. Couston, D. Lecoanet, B. Favier and M. Le Bars
We investigate via direct numerical simulations (DNS) the turbulent dynamics of a convective fluid with and without rotation adjacent to a stable stratification. The fluid is confined between top and bottom boundaries with fixed temperatures (Ttop and Tbot), and we impose that the thermal expansion coefficient changes sign at temperature Tbot>Ti>Ttop. This leads to an unstable/stable density stratification of the lower/upper parts of the fluid, which as a result is expected to behave qualitatively similar to water close to its density maximum at Ti=4, and Earth’s outer core if a stable stratification exists close to the inner/outer core boundary and/or the core/mantle boundary (note that similar mixed dynamics are obtained in the atmosphere close to the troposphere/stratosphere boundary, and stars near the convective/radiative interface).
Influence of the convection on the waves in the stable zone and wave feedback on the convection
Results obtained from 51 two-dimensional DNS of Boussinesq equations without rotation have helped us understand the effect of the stratification strength of the stable layer on the system’s dynamics. For large stratifications, the convection is similar to the classical Rayleigh-Benard convection and is essentially unaffected by internal waves that propagate in the upper stable layer. For weak stratifications, however, the dynamical coupling is strong between the two layers and both high velocity buoyant plumes arising from the bottom boundary and slow entrained motions of the stable fluid contribute to a large convective heat flux. We reported the findings in a paper published in Physical Review Fluids. Several videos on the topic are available here.
ref : L.-A. Couston, D. Lecoanet, B. Favier and M. Le Bars, Dynamics of mixed convective--stably-stratified fluids, Phys. Rev. Fluids 2, 094804
Emergence of a mean horizontal flow in the wave region
The emergence of a mean horizontal flow in the stable layer changing direction over long time scales, hence similar to the Quasi-Biennial Oscillations of Earth’s stratosphere, and its effects on the convection beneath is of significant fundamental and applied interests. We demonstrated that this phenomenon is generic and can be obtained in DNS of convective--stably-stratified fluids. We explored the possibility to reproduce the mean flow dynamics in reduced models (akin to General Circulation Models) devoid of turbulent convection and using parameterized wave effects. We reported the findings in a paper published in Physical Review Letters. Several videos on the topic are available here.
ref : L.-A. Couston, D. Lecoanet, B. Favier and M. Le Bars, Order out of chaos : Slowly Reversing Mean Flows Emerge from Turbulently Generated Internal Waves, Phys. Rev. Lett. 120, 244505
physics focus story here
Wave excitation by turbulent convection in 3D
We have run 3D DNS of convective--stably-stratified fluids, which each need several Million cpu hours. Results from 3D DNS have allowed us to validate a theory that predicts the amount of energy that goes from the convection into the waves, as well as the spectral distribution of the wave energy in wave number-frequency space. The results are of significant interest for predicting how much wave power is available across geophysical and astrophysical fluids for driving wave-induced changes of the global dynamics. The results have been reported in a paper submitted to the Journal of Fluid Mechanics. Several videos on the topic are available here.
ref : L.-A. Couston, D. Lecoanet, B. Favier and M. Le Bars, The energy flux spectrum of internal waves generated by turbulent convection (submitted to the Journal of Fluid Mechanics)
The effect of rotation on the convection and internal waves in the stable layer is currently under investigation.
Open positions for this project: check out projects listed here under "Task 2: the fluid dynamics of heterogeneous core convection"
last updated : June 2018
