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SUMMARY:The subtle intermittency of elastic turbulence - Dhrubaditya Mitra
  (NORDITA)
DTSTART:20240611T092000Z
DTEND:20240611T101000Z
UID:TALK214672@talks.cam.ac.uk
DESCRIPTION:Turbulence is a state of irregular\, chaotic and unpredictable
  fluid motion at very high Reynolds numbers (Re)\, which is the ratio of t
 ypical inertial forces over typical viscous forces in a fluid. &nbsp\;Conc
 eptually\, the fundamental problem of turbulence shows up in the simplest 
 setting of statistically stationary\, homogeneous and isotropic turbulent 
 (HIT) flows: What are the statistical properties of velocity fluctuations?
  &nbsp\;In other words\, what is the probability distribution function of 
 velocity differences (PDF) across a length scale? Experiments and numerica
 l simulations over the last seventy years have shown that this PDF &nbsp\;
 is non-Gaussian\, not only because it has non-zero odd moments but also be
 cause &nbsp\;moments of all orders are important in determining the nature
  of the PDF. This is a phenomena called intermittency. A systematic theory
  of intermittency starting from the Navier-Stokes equation is the goal of 
 turbulence research.\nTurbulent flows\, both in nature and industry\, are 
 often multiphase\, i.e. they are laden with particles\, may comprise of fl
 uid mixtures\, or contain additives such as polymers. Of these\, polymeric
  flows are probably the most curious and intriguing: the addition of high 
 molecular weight (about 107) polymers in 10&ndash\;100 parts per million (
 ppm) concentration to a turbulent pipe flow reduces the friction factor (o
 r the drag) up to 5&ndash\;6 times (depending on concentration). A straigh
 tforward parameterization of the importance of elastic effects is the Debo
 rah number\, De \, which is the ratio of the characteristic scale of the p
 olymer over some typical time scale of the flow.&nbsp\;\nResearch in polym
 eric flows turned into a novel direction when it was realized that even ot
 herwise laminar flows may become unstable due to the instabilities driven 
 by the elasticity of polymers. Even more dramatic is the phenomena of elas
 tic turbulence (ET)\, where polymeric flows at low Reynolds but high Debor
 ah numbers are chaotic and mixing. &nbsp\;We\, for the first time\, show t
 hat such flows are also intermittent but unlike the usual turbulence\, the
 y are intermittent in a subtle way. The velocity field is smooth\, i.e.\, 
 the velocity difference across a length scale r\, is proportional to r but
 \, crucially\, with a non-trivial sub-leading contribution r^(3/2) which w
 e extract by using the second difference of velocity. The structure functi
 ons of second difference of velocity &nbsp\;show clear evidence of intermi
 ttency.&nbsp\;
LOCATION:Seminar Room 1\, Newton Institute
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