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Buoyant mixing

french version

Members: J. Znaien, Y. Tanino, F. Moisy, D. Salin, J.-P. Hulin.

The buoyant mixing of two fluids of different densities is a widespread phenomenon in natural systems (oceanography, hydrology, atmospheric sciences) with large potential consequences on the environment. It is also frequently encountered in chemical or petroleum engineering.

We study the buoyant mixing of two fluids of different densities initially separated in an unstable configuration (the heavier fluid is above the lighter fluid) in a confined geometry of a tilted tube (the angle is fixed for each experiment).

Fig. 1 - Experimental setup.

The flow results from a competition between:

  • Fluids interpenetration due to the longitudinal gravity component.
  • Transverse mixing due to Kelvin-Helmholtz instabilities at the interface.
  • Segregation of the fluids due to the transverse gravity component.

The flow displays a variety of regime depending on the two control parameters, the tube angle and density difference. For a tube close to vertical and for a large density contrast, the flow is turbulent and a strong transverse mixing is observed. In contrast, for a nearly horizontal tube and a small density contrast, there is a laminar counterflow without mixing. Between these two extreme cases, the flow is laminar with intermittent turbulent bursts.

Fig. 2 - Normalized concentration during the mixing of miscible fluids in a tube inclined at 45°.

This project is in collaboration with the Institute of fluid mechanics of Toulouse (Y. Hallez and J. Magnaudet) and the Laboratory of Physics of ENS Lyon (J.-C. Tisserand, F. Chilla and B. Castaing) and in the framework of an ANR program.


  • Lock-exchange flows in inclined pipes: the relevance of the Prandtl mixing length model
    Y. Tanino, F. Moisy and J.-P. Hulin, J. Turbulence 16 (5), 484-502 (2015)
  • Laminar-turbulent cycles in inclined lock-exchange flows
    Y. Tanino, F. Moisy, J.-P. Hulin, Phys. Rev. E 85, 066308 (2012).
    [Abstract | PDF]
  • Flow structure and momentum transport for buoyancy driven mixing flows in long tubes at different tilt angles
    J. Znaien, F. Moisy, J.-P. Hulin, Phys. Fluids 23, 035105 (2011).
    [Abstract | PDF]
  • Experimental and numerical investigation of flow structure and momentum transport in a turbulent buoyancy-driven flow inside a tilted tube
    J. Znaien, Y. Hallez, F. Moisy, J. Magnaudet, J.-P. Hulin, D. Salin, E. J. Hinch, Phys. Fluids 21, 115102 (2009).
    [Abstract | PDF]
  • Front dynamics and macroscopic diffusion in buoyant mixing in a tilted tube
    T. Séon, J. Znaien, E. J. Hinch, B. Perrin, D. Salin, and J. P. Hulin, Phys. Fluids, 19,125105 (2007)
  • Transient buoyancy-driven front dynamics in nearly horizontal tubes
    T. Séon, J. Znaien, E. J. Hinch, B. Perrin, D. Salin, and J. P. Hulin, Phys. Fluids , 19,123603 (2007)