Plume structure in high Rayleigh number convection

Puthenveettil, Baburaj A and Arakeri, Jaywant H (2005) Plume structure in high Rayleigh number convection. [Preprint]

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Abstract

Near wall structures in turbulent natural convection at Rayleigh numbers of $10^{10}$ to $10^{11}$ at Schmidt number of 602 are visualised by a new method of driving the convection across a one membrane using concentration differences of NaCl. The visualisations show the near wall flow to consist of sheet plumes. A wide variety of large scale row cells, scaling with the cross section dimension, are observed. Multiple large scale row cells are seen at AR =0.65, while only a single circulation cell is detected at AR =0.435. The cells (or the mean wind) are driven by plumes coming together to form columns of rising lighter fluid. The wind in turn aligns the sheet plumes along the direction of shear. The mean wind direction is seen to change with time. The near wall dynamics show plumes initiated at points, which elongate to form sheets and then merge. Increase in Rayleigh number results in larger number of closely and regularly spaced plumes. The plume spacings show a common log-normal probability distribution function, independent of the Rayleigh number and the aspect ratio. We propose that the near wall structure is made of laminar natural convection boundary layers, which become unstable to give rise to sheet plumes, and show that the predictions of a model constructed on this hypothesis match the experiments. Based on these findings, we conclude that in the presence of a mean wind, the local near wall boundary layers associated with each sheet plume in high Rayleigh number turbulent natural convection are likely to be laminar mixed convection type.

Item Type:	Preprint
Keywords:	Plumes;Rayleigh number;Convection;Large scale circulation;Near wall dynamics;Natural convection
Department/Centre:	Division of Mechanical Sciences > Mechanical Engineering
Date Deposited:	03 Aug 2005
Last Modified:	19 Sep 2010 04:19
URI:	http://eprints.iisc.ac.in/id/eprint/3478

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