Self-organization and generation of large scale flows in quasi 2D turbulence
Date/Time: 09:00 08-Aug-2014
The self-consistent generation of large scale flows - zonal flows - by the rectification of small scale turbulent fluctuations is a generic feature of quasi-2D turbulent flows. It is of great importance both in geophysical flows and in magnetically confined plasmas. These flows, which are generally sheared, will regulate the turbulence, suppressing the small scale structures and set up effective transport barriers.
The purpose of this presentation is to discuss the morphology of zonal flows, basic mechanisms for their generation and their influence on turbulence and the associated transport in magnetized plasmas and rotating fluids.
Zonal flow generation will be illustrated in a simple fluid experiment performed in a rotating tank with radial symmetric bottom topography . The results are discussed by the concept of the potential vorticity, PV, which is a Lagrangian conserved quantity. An effective mixing that homogenises PV will lead to replacing the high PV near the centre of the tank with low PV from the outside, and this will appear as an anti-cyclonic vortex over the centre, hence a large scale flow.
In magnetically confined hot plasma turbulent transport is the dominating channel for radial density, energy and momentum transport across the confining magnetic field. However, spontaneously generated sheared poloidal flows strongly reduce the radial turbulent transport and are instrumental in the rapid transition to an enhanced confinement state (the H-mode), with suppressed turbulent transport. The transition from low (L-mode) to the high (H-mode) confinement is still not understood from first principles. Recent investigations of zonal flow generation in plasma turbulence and the role of these in the L-H transition will be discussed, and illustrated by low-dimensional models of the predator-prey type .
 Rasmussen, J. Juul, Garcia, O.E., Naulin, V., Nielsen, A.H., Stenum, B., van Bokhoven, L.J.A., and Delaux, S., On the generation of zonal flows in rotating fluids and magnetized plasmas, Physica Scripta (2006) T 122, 44-51.
 Dam, M., Br?ns, M., Rasmussen, J. Juul, Naulin, V., and Xu, G.S., Bifurcation analysis and dimension reduction of a predator-prey model for the L-H transition. Physics of Plasmas (2013) 20, 102302 (11-pages).
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