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SUMMARY:Negative effective viscosity in strongly stratified turbulence - Q
 i Zhou (University of Calgary)
DTSTART:20240116T154500Z
DTEND:20240116T163000Z
UID:TALK208480@talks.cam.ac.uk
DESCRIPTION:In this talk\, we examine the possible self-organization of st
 rongly stratified turbulence around a local critical state\, by analyzing 
 a dataset of a numerically simulated stratified turbulent wake. To facilit
 ate the analysis\, the turbulent flow field is decomposed into a 'large-sc
 ale' flow of horizontal scales greater than the Ozmidov scale\, lO\, and a
  'small-scale' flow of scales below lO. A local gradient Richardson number
 \, Ri\, characterizing the large-scale flow\, is calculated and then utili
 zed to produce conditional sampling of various turbulence statistics relev
 ant to the local dynamics. While the bulk turbulence is observed to decay 
 by approximately one order of magnitude in terms of the dissipation rate\,
  the median Ri has remained nearly constant due to the self-organization o
 f flow structures under strong stratification\; the subsampled Ri distribu
 tion peaks around 1/4 for regions in the upper quartile of local dissipati
 on. Regions of small Ri are found to be associated with large dissipation 
 and large net transfer of energy to the small scales. Regions of 'back-sca
 tter' of kinetic energy to large scales\, where the local eddy viscosity t
 akes a negative value\, are also observed. Occurrence of large magnitude o
 f both positive and negative eddy viscosity appears to be most frequent ar
 ound the critical value of Ri~1/4\, indicating an intense two-way exchange
  of kinetic energy between the large and small scales around the local cri
 tical state. The threshold behaviour of Ri underscores the dynamical signi
 ficance of the critical Ri of 1/4 for locally self-sustained turbulence in
  a strongly stratified configuration and bears some resemblance to the cel
 ebrated self-organized criticality dynamics [Bak et al.\, Phys. Rev. Lett.
 \, 59\, 381 (1987)].\nSee the full paper:&nbsp\;\nZhou\, Q. (2022). Thresh
 old behavior of local gradient Richardson number in strongly stratified no
 nequilibrium turbulence. Physical Review Fluids\, 7(10)\, 104802.
LOCATION:Seminar Room 1\, Newton Institute
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