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Stratified shear instability in a field of pre-existing turbulence

Published online by Cambridge University Press:  11 January 2019

A. K. Kaminski Open the ORCID record for A. K. Kaminski [Opens in a new window]  and
W. D. Smyth Open the ORCID record for W. D. Smyth [Opens in a new window]
A. K. Kaminski*
Affiliation:
College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA
W. D. Smyth
Affiliation:
College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA
*
†Present address: Applied Physics Laboratory, University of Washington, Seattle, WA 98105, USA. Email address for correspondence: [email protected]
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Abstract

Turbulent mixing of heat and momentum in the stably-stratified ocean interior occurs in discrete events driven by vertical variations of the horizontal velocity. Typically, these events have been modelled assuming an initially laminar stratified shear flow which develops wavelike instabilities, becomes fully turbulent, and then relaminarizes into a stable state. However, in the real ocean there is always some level of turbulence left over from previous events. Using direct numerical simulations, we show that the evolution of a stably-stratified shear layer may be significantly modified by pre-existing turbulence. The classical billow structure associated with Kelvin–Helmholtz instability is suppressed and eventually eliminated as the strength of the initial turbulence is increased. A corresponding energetics analysis shows that potential energy changes and dissipation of kinetic energy depend non-monotonically on initial turbulence strength, with the largest effects when initial turbulence is present but insufficient to prevent billow formation. The mixing efficiency decreases with increasing initial turbulence amplitude as the development of the Kelvin–Helmholtz billow, with its large pre-turbulent mixing efficiency, is arrested.

JFM classification

Geophysical and Geological Flows: Stratified flows Instability: Transition to turbulence Mixing: Turbulent mixing
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 862 , 10 March 2019 , pp. 639 - 658
Copyright
© 2019 Cambridge University Press 

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