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Dynamics of fluctuations in a fluid below the onset of Rayleigh-Benard convection



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Oh, Jaechul and Ortiz de Zárate Leira, José María and Sengers, Jan V. and Ahlers, Guenter (2004) Dynamics of fluctuations in a fluid below the onset of Rayleigh-Benard convection. Physical Review E, 69 (2). ISSN 1539-3755

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Official URL: http://dx.doi.org/10.1103/PhysRevE.69.021106


We present experimental data and their theoretical interpretation for the decay rates of temperature fluctuations in a thin layer of a fluid heated from below and confined between parallel horizontal plates. The measurements were made with the mean temperature of the layer corresponding to the critical isochore of sulfur hexafluoride above but near the critical point where fluctuations are exceptionally strong. They cover a wide range of temperature gradients below the onset of Rayleigh-Benard convection, and span wave numbers on both sides of the critical value for this onset. The decay rates were determined from experimental shadowgraph images of the fluctuations at several camera exposure times. We present a theoretical expression for an exposure-time-dependent structure factor which is needed for the data analysis. As the onset of convection is approached, the data reveal the critical slowing down associated with the bifurcation. Theoretical predictions for the decay rates as a function of the wave number and temperature gradient are presented and compared with the experimental data. Quantitative agreement is obtained if allowance is made for some uncertainty in the small spacing between the plates, and when an empirical estimate is employed for the influence of symmetric deviations from the Oberbeck-Boussinesq approximation which are to be expected in a fluid with its density at the mean temperature located on the critical isochore.

Item Type:Article
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©2004 The American Physical Society. The research of J. Oh and G. Ahlers was supported by U.S. National Science Foundation Grant No. DMR02-43336. G. Ahlers and J. M. Ortiz de Zárate acknowledge support through a grant under the Del Amo Joint Program of the University of California and the Universidad Complutense de Madrid. The research at the University of Maryland was supported by the Chemical Sciences, Geosciences and Biosciences Division of the Office of Basic Energy of the U.S. Department of Energy under Grant No. DE-FG-02-95ER14509.

Uncontrolled Keywords:Thermal-equilibrium; Nonequilibrium fluctuations; Sulfur-hexafluoride; Light-scattering; Critical-point; Instability; Liquid; Viscosity; Stability; Gravity
Subjects:Sciences > Physics > Thermodynamics
ID Code:27356
Deposited On:17 Nov 2014 14:01
Last Modified:26 Feb 2015 15:49

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