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SUMMARY:The stratified inclined duct: what we have learned about stratifie
 d turbulence in a maintained shear flow - Paul Linden (DAMTP)
DTSTART:20230120T124500Z
DTEND:20230120T134500Z
UID:TALK195136@talks.cam.ac.uk
CONTACT:Paras Vadher
DESCRIPTION:The stratified inclined duct (SID) is a relatively new experim
 ental paradigm that produces a sustained shear flow between two counterflo
 wing layers of fluid supplied by reservoirs at each end of the duct contai
 ning fluids of different density. The duct can be tilted at a small angle 
 θ to the horizontal and\, for a given fluid\, the flow is determined by t
 wo nondimensional parameters θ and the\nReynolds number. We have observed
  four different flow regimes in SID: Laminar when the interface between th
 e layers remains undisturbed\, Holmboe characterised by sharp cusped waves
  on the interface\, Intermittent when the flow has bursts of turbulence fo
 llowed by relatively calm periods and Turbulent when the turbulence occurs
  throughout the duct and is sustained in time. The laminar regime occurs a
 t low Re and θ\, and transitions to the other regimes occur successively 
 as Re and θ increase\, so SID allows a systematic study of the different 
 regimes. One of the most important questions in stratified turbulence is t
 he efficiency with which the fluid\nis mixed. When the stratification is s
 table\, with density decreasing with height\, work needs to be done agains
 t gravity to move light fluid downwards and dense fluid upwards so that ir
 reversible mixing can occur. The ‘tax’ that this irreversible mixing i
 mposes on the kinetic energy of the flow\, the so-called ‘mixing efficie
 ncy’ is important to parameterise mixing in ocean and climate models. In
  this talk I will discuss the philosophy behind SID and explain why the ex
 periment is relevant to this issue\, particularly in the context of the en
 ergetics of the flow. We focus first on the self-organisation properties o
 f the flows\, wherein more strongly turbulent flows tend to an asymptotic 
 state characterised by a uniform gradient Richardson number of order 0.1-0
 .2 across the shear layer. We then summarise our results on turbulent ener
 getics and mixing statistics. We derive the kinetic and scalar energy budg
 ets and explain the specificity and scalings of SID turbulence. We assess 
 the relevance of standard mixing parameterisations models\, and we compare
  representative values with the literature. The dependence of these measur
 es of mixing on controllable flow parameters provides asymptotic estimates
  that may be extrapolated to more strongly turbulent flows.  Complementing
  the experiments we introduce the first accurate 3D DNS for SID. Implement
 ing a suitable forcing method and boundary conditions allow us to maintain
  steady exchange flow for an arbitrarily long time at a minimal computatio
 nal cost. With the newly developed numerical model\, we explore the divers
 e transitions in SID from a numerical perspective. 
LOCATION:LR5
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