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Observations and other characteristics of thermals

Published online by Cambridge University Press:  29 March 2006

E. M. Sparrow ,
R. B. Husar  and
R. J. Goldstein
E. M. Sparrow
Affiliation:
University of Minnesota, Minneapolis
R. B. Husar
Affiliation:
University of Minnesota, Minneapolis
R. J. Goldstein
Affiliation:
University of Minnesota, Minneapolis
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Abstract

Experiments have been performed to explore the qualitative and quantitative characteristics of thermals which ascend through the fluid environment above a heated horizontal surface. With water as the participating fluid, an electrochemical technique was employed which made the flow field visible and facilitated the direct observation of thermals. Measurements were also made of the fluid temperature above an active site of thermal generation.

As seen in flow field photographs, a thermal has a mushroom-like appearance, with a blunted nearly hemispherical cap. At a given heating rate, thermals are generated at fixed sites which are spaced more or less regularly along the span of the heated surface. At these sites, the generation of thermals is periodic in time, thereby validating a prediction of Howard. Both the spatial frequency of the sites and the rate of thermal production increase with increases in heating rate. The break-up Rayleigh number of the conduction layer is shown to be a constant (within the uncertainties of the experiment), which is in accord with Howard's phenomenological model.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 41 , Issue 4 , 15 May 1970 , pp. 793 - 800
Copyright
© 1970 Cambridge University Press

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References

Baker, D. J. 1966 A technique for the precise measurement of small fluid velocities. J. Fluid Mech. 26, 573.Google Scholar
Howard, L. N. 1966 Convection at high Rayleigh number. Proc. 11th Int. Cong. Appl. Mech. (Ed. H. Görtler.) Berlin: Springer.
Townsend, A. A. 1959 Temperature fluctuations over a heated horizontal surface. J. Fluid Mech. 5, 209.Google Scholar