Titre/Title :
High Rayleigh number convection driven by concentration differences
Contact :
Emil Hopfinger
Résumé/Abstract :
The talk deals with the role of near wall coherent structures in the flux scaling in high Rayleigh
Number (Ra ∼ 10^11 ) – high Schmidt number (S c ∼ 601) turbulent natural convection. We visualise
the near wall structures in this regime for the first time by driving the convection across a fine
membrane using concentration differences of NaCl ; a layer of brine lies over a layer of water,
separated by a horizontal membrane. The basic coherent structure in the near wall flow is identified
as a line plume fed by boundary layers on either side. The near wall dynamics show plumes initiated
at points, which elongate and then merge. The large scale flow (or mean wind) cells are driven by
plume columns ; the wind in turn aligns the line plumes along the direction of shear. We propose that
the near wall structure is made of local laminar natural convection boundary layers, which become
unstable to give rise to line plumes ; the predictions of a model constructed on this hypothesis
match the experiments. We observe that the probability distribution function of plume spacings
follow the standard log-normal distribution and show the same underlying multifractal spectrum
of singularities ; we infer a common underlying generating mechanism for the plume structures.
The last part of the talk discusses the phenomena observed when a coarser membrane allows an
advective flow across it.