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Turbulent mixing at a shear-free density interface

Hannoun, Imad A. and List, E. John (1988) Turbulent mixing at a shear-free density interface. Journal of Fluid Mechanics, 189 . pp. 211-234. ISSN 0022-1120. doi:10.1017/S0022112088000977.

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The interaction of a sharp density interface with oscillating-grid-induced shear-free turbulence was experimentally investigated. A linear photodiode array was used in conjunction with laser-induced fluorescence to measure the concentration of dye that was initially only in the less dense layer. A laser-Doppler velocimeter was used to measure the vertical velocity in and above the density interface at a point where the dye concentration was also measured. Potential refractive-index-fluctuation problems were avoided using solutes that provided a homogeneous optical environment across the density interface. Internal wave spectra, amplitudes and velocities, as well as the vertical mass flux were measured. The results indicate that mixing occurs in intermittent bursts and that the gradient (local) Richardson number remains constant for a certain range of the overall Richardson number R_j, defined in terms of an integral lengthscale, buoyancy jump and turbulence intensity. The spectra of the internal waves decay as f^(−3) at frequencies below the maximum Brunt-Väisälä frequency. These findings give support to a model for oceanic mixing proposed by Phillips (1977) in which the internal waves are limited in their spectral density by sporadic local instabilities and breakdown to turbulence. The results also indicate that, for a certain R_j range, the thickness of the interfacial layer (normalized by the integral lengthscale of the turbulence) is a decreasing function of R_j. At sufficiently high R_j the interfacial thickness becomes limited by diffusive effects. Finally, we discuss a simple model for entrainment at a density interface in the presence of shear-free turbulence.

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Additional Information:© 1988 Cambridge University Press. (Received 31 October 1986 and in revised form 13 August 1987) The authors would like to thank Dr D. Papantoniou for setting up the data acquisition system associated with the Reticon camera. We would also like to thank Drs J. Skjelbreia and M. Hoochesfahani for their help, Professor J. Imberger for his valuable ideas while spending a sabbatical year at Caltech, and Dr H. J. S. Fernando for his support and friendship. The help of Dr P. Papanicolaou and the entire staff of the W. M. Heck Hydraulics Laboratory is deeply appreciated. The work was supported by US National Science Foundation Grant No. MSM84-12641, A01.
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NSFMSM84-12641, A01
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Official Citation:Imad A. Hannoun and E. John List (1988). Turbulent mixing at a shear-free density interface. Journal of Fluid Mechanics, 189, pp 211-234 doi:10.1017/S0022112088000977
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:66250
Deposited By: George Porter
Deposited On:18 Apr 2016 21:03
Last Modified:10 Nov 2021 23:55

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