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Entrainment and mixed layer dynamics of a surface-stress-driven stratified fluid

Published online by Cambridge University Press:  28 January 2015

G. E. Manucharyan  and
C. P. Caulfield
G. E. Manucharyan*
Affiliation:
Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
C. P. Caulfield
Affiliation:
BP Institute, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK
*
†Email address for correspondence: gmanucharyan@whoi.edu
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Abstract

We consider experimentally an initially quiescent and linearly stratified fluid with buoyancy frequency $N_{Q}$ in a cylinder subject to surface-stress forcing from a disc of radius $R$ spinning at a constant angular velocity ${\rm\Omega}$. We observe the growth of the disc-adjacent turbulent mixed layer bounded by a sharp primary interface with a constant characteristic thickness $l_{I}$. To a good approximation the depth of the forced mixed layer scales as $h_{F}/R\sim (N_{Q}/{\rm\Omega})^{-2/3}({\rm\Omega}t)^{2/9}$. Generalising the previous arguments and observations of Shravat et al. (J. Fluid Mech., vol. 691, 2012, pp. 498–517), we show that such a deepening rate is consistent with three central assumptions that allow us to develop a phenomenological energy balance model for the entrainment dynamics. First, the total kinetic energy of the deepening mixed layer $\mathscr{E}_{KF}\propto h_{F}u_{F}^{2}$, where $u_{F}$ is a characteristic velocity scale of the turbulent motions within the forced layer, is essentially independent of time and the buoyancy frequency $N_{Q}$. Second, the scaled entrainment parameter $E={\dot{h}}_{F}/u_{F}$ depends only on the local interfacial Richardson number $Ri_{I}=(N_{Q}^{2}h_{F}l_{I})/(2u_{F}^{2})$. Third, the potential energy increase (due to entrainment, mixing and homogenisation throughout the deepening mixed layer) is driven by the local energy input at the interface, and hence is proportional to the third power of the characteristic velocity $u_{F}$. We establish that internal consistency between these assumptions implies that the rate of increase of the potential energy (and hence the local mass flux across the primary interface) decreases with $Ri_{I}$. This observation suggests, as originally argued by Phillips (Deep-Sea Res., vol. 19, 1972, pp. 79–81), that the mixing in the vicinity of the primary interface leads to the spontaneous appearance of secondary partially mixed layers, and we observe experimentally such secondary layers below the primary interface.

JFM classification

Turbulent Flows: Shear layer turbulence Turbulent Flows: Stratified turbulence Turbulent Flows: Turbulent Flows
Type
Papers
Information
Journal of Fluid Mechanics , Volume 765 , 25 February 2015 , pp. 653 - 667
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Copyright
© 2015 Cambridge University Press 

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