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On the nature of the entrainment interface of a two-layer fluid subjected to zero-mean-shear turbulence

Published online by Cambridge University Press:  20 April 2006

Harindra J. S. Fernando  and
Robert R. Long
Harindra J. S. Fernando
Affiliation:
Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, MD 21218 Present address: W. M. Keck Laboratory of Hydraulics and Water Resources, California Institute of Technology, Pasadena, CA 91125.
Robert R. Long
Affiliation:
Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, MD 21218
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Abstract

An experimental study was performed to further understanding of turbulent mixing in a two-layer fluid subjected to shear-free turbulence. At low Richardson numbers Ri (= ΔbD3*/K2, where Δb is the buoyancy jump, D* is the depth of a mixed layer and K is ‘action’) the entrainment seems to occur through the eroding effect of large eddies, whereas at high Ri the large eddies flatten at the density interface and the quasi-isotropic eddies near the interface are responsible for the entrainment. The buoyancy transfer can be well described by a gradient-transport model when the eddy diffusivity is properly defined. At or just above the entrainment interface, the buoyancy flux is of the same order as the dissipation, and the diffusive-flux Richardson number tends to a constant.

The thickness h of the interfacial layer was measured in three different ways and was found to grow linearly with D* in agreement with preliminary findings of an earlier investigation of Fernando & Long (1983). The buoyancy gradient in the interfacial layer was found to be constant, and the resulting buoyancy conservation law was experimentally verified. The frequency of the interfacial-layer waves appears to vary as Ri½. The present results, together with the results of the earlier work of Fernando & Long, show a good agreement with a theory of Long (1978b) for behaviour at high values of Ri. The closure assumptions of that theory were also verified by our measurements.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 151 , February 1985 , pp. 21 - 53
Copyright
© 1985 Cambridge University Press

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