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SUMMARY:Beyond Coherent Structures - Javier Jimenez (Universidad Politécn
 ica de Madrid)
DTSTART:20220107T130000Z
DTEND:20220107T140000Z
UID:TALK166441@talks.cam.ac.uk
DESCRIPTION:Much of what we know about interscale transfer in turbulence i
 s due to Kolmogorov (for energy)\, or to Townsend (for momentum in shear f
 lows). We also have a reasonably good understanding of the viscous vortice
 s at the small-scale end of the energy cascade\, and a (less definite) one
  that the energy input at the large-scale end of the momentum cascade is a
  linear process\, in the sense that the fluctuations extract their energy 
 from the mean shear.While we know little about the geometry of the flow in
  the inertial range\, and it is doubtful that the concept of shape applies
  to it\, the dissipative vortices and the production mechanism have defini
 te shapes and dimensions\, leading to the concept of coherent structures. 
 These are good representations of shear flows. They evolve under their own
  dynamics to a first approximation\, and account for most of the dissipati
 on and of the energy production. Some of these dynamics are well understoo
 d: shear instabilities\, lift-up\, Orr bursts\, the self-sustaining cycle\
 , etc. They will be briefly reviewed\, with emphasis on what is universal 
 and what is particular about them.But we will also argue that a representa
 tion in terms of structures is incomplete. In the first place\, their desc
 ription as self-contained dynamical systems begs the question of how are t
 hey initiated? Secondly\, turbulence is clearly not a fully linear process
 \, and the effect of nonlinearity is poorly understood: what limits linear
  growth? How do the different scales interact? We will argue that a furthe
 r level of description is required in which the causal connection of the d
 ifferent coherent processes is explicitly addressed\, and that current com
 puters are beginning to make such an analysis possible.
LOCATION:Seminar Room 1\, Newton Institute
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