Other
Scientific paper
Feb 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000nyasa.898..122k&link_type=abstract
Annals of the New York Academy of Sciences: Astrophysical Turbulence and Convection, vol. 898, p. 122
Other
1
Low Frequencies, Oscillations, Turbulence, Rayleigh-Benard Convection, Rayleigh Number, Reynolds Stress, Prandtl Number, Aspect Ratio, Plumes, Power Spectra, Time Series Analysis
Scientific paper
Large fluid systems such as the atmosphere of the Earth or the convection zone of the Sun are no doubt turbulent, but contain considerable organization on many scales. Not all of these scales are directly forced; some arise as a result of internal arrangements of energy and momentum, giving rise, in the Earth's atmosphere, to such phenomena as the atmospheric jet streams. This paper is about an investigation of large-scale organization that arises in turbulent Rayleigh-Benard convection. We report on observations of low frequency oscillations in the Rayleigh number (R) range 107 - 108, with Prandtl number (Pr) equal to seven. It has been known that for convecting layers with large aspect ratio A (where A is the ratio of fluid layer width to depth), a steady large-scale now sets in at R = 2 x 106. Tilted transient plumes embedded in this now, and maintaining it through Reynolds stresses, drift in one direction along the bottom of the layer, and in the opposite direction along the top. At a fixed point near the bottom or top boundary, there is a variability associated with the passage of these plumes. We call this the high-frequency variability. A new kind of organization is observed for 107 < R < 108; clusters of transient tilted plumes travel in a horizontal direction as coherent units. These clusters are separated from each other by quiescent zones with almost no plumes. Now at a fixed point near the bottom boundary, there is a low-frequency variability associated with the passage of clusters, as well as the high frequency variability from the passage of plumes within the cluster. Quantitative information on this low-frequency oscillation derived from space-time portraits and from temperature-time series is described. A mathematical model of the low frequency oscillation is presented.
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