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Measurements of diapycnal diffusivities in stratified fluids

Published online by Cambridge University Press:  24 August 2001

MICHAEL E. BARRY
Affiliation:
Centre for Water Research, The University of Western Australia, Crawley 6009, Australia
GREGORY N. IVEY
Affiliation:
Centre for Water Research, The University of Western Australia, Crawley 6009, Australia
KRAIG B. WINTERS
Affiliation:
Applied Physics Laboratory, University of Washington, Seattle WA 98105-6698, USA
JÖRG IMBERGER
Affiliation:
Centre for Water Research, The University of Western Australia, Crawley 6009, Australia

Abstract

Linearly stratified salt solutions of different Prandtl number were subjected to turbulent stirring by a horizontally oscillating vertical grid in a closed laboratory system. The experimental set-up allowed the independent direct measurement of a root mean square turbulent lengthscale Lt, turbulent diffusivity for mass Kρ, rate of dissipation of turbulent kinetic energy ε, buoyancy frequency N and viscosity v, as time and volume averaged quantities. The behaviour of both Lt and Kρ was characterized over a wide range of the turbulence intensity measure, ε/vN2, and two regimes were identified.

In the more energetic of these regimes (Regime E, where 300 < ε/vN2 < 105), Lt was found to be a function of v, κ and N, whilst Kρ was a function of v, κ and (ε/vN2)1/3. From these expressions for Lt and Kρ, a scaling relation for the root mean square turbulent velocity scale Ut was derived, and this relationship showed good agreement with direct measurements from other data sets.

In the weaker turbulence regime (Regime W, where 10 < ε/vN2 < 300) Kρ was a function of v, κ and ε/vN2.

For 10 < ε/vN2 < 1000, our directly measured diffusivities, Kρ, are approximately a factor of 2 different to the diffusivity predicted by the model of Osborn (1980). For ε/vN2 > 1000, our measured diffusivities diverge from the model prediction. For example, at ε/vN2 ≈ 104 there is at least an order of magnitude difference between the measured and predicted diffusivities.

Type
Research Article
Copyright
© 2001 Cambridge University Press

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