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SUMMARY:Diapycnal mixing in stratified plane Couette flows - Dr. Qi Zhou\,
  DAMTP\, Cambridge University
DTSTART:20161021T120000Z
DTEND:20161021T130000Z
UID:TALK68679@talks.cam.ac.uk
CONTACT:Akshath Sharma
DESCRIPTION:Stratified shear flows are common in natural and engineering s
 ystems. It is\nimportant to understand the ‘mixing efficiency’ at whic
 h kinetic energy is\nconverted to potential energy via irreversible diapyc
 nal mixing occurring in\nthese systems. A specific task is to determine ho
 w the flux Richardson number\nR f varies with characteristic flow paramete
 rs\, such as the gradient Richardson number Rig and the buoyancy Reynolds 
 (Gibson) number Reb . In this\ntalk\, I will review experimental and numer
 ical results in the literature and\npresent new data obtained by direct nu
 merical simulations of stratified plane\nCouette flows. Particular attenti
 on in the latter is paid to the effects of varying Prandtl number Pr from 
 0.7\, 7 and 70 motivated by the wide range of Pr\nthat is encountered in g
 eophysical flows. The fluid between the oppositely\nmoving walls is either
  continuous stratified or initialized with a two-layer\ndensity profile\, 
 which enables us to investigate various mixing scenarios. In\nthe continuo
 usly stratified set-up\, we characterise the mixing properties in the\nfra
 mework of Monin-Obukhov similarity theory and bridge the gap between\nRig 
 - and Reb -based parameterizations of R f . In the layered set-up\, we inv
 estigate possible self-maintaining mechanism of sharp density interfaces m
 otivated by Phillips’ proposal (Deep-Sea Res.\, vol. 19\, 1972) on how l
 ayers and\ninterfaces can spontaneously form due to vertical variation of 
 diapycnal flux.\nI will present a ‘self-sharpening’ mechanism which is
  due to the combined\neffects of the ‘scouring’ induced by the turbule
 nce external to the interface\nand comparatively weak molecular diffusion 
 across the core region of the interface. The effective diapycnal diffusivi
 ty and irreversible buoyancy flux are\nquantified in the tracer-based refe
 rence coordinate proposed by Winters and\nD’Asaro (J. Fluid Mech.\, vol.
  317\, 1996) and Nakamura (J. Atmos. Sci.\,\nvol. 53\, 1996)\, which enabl
 es a detailed investigation of the self-sharpening\nprocess by analysing t
 he local budget of buoyancy gradient in the reference\ncoordinate. Some th
 eoretical development in refining Phillips’ work will also\nbe discussed
 .\n
LOCATION:JDB Seminar Room\, CUED
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