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SUMMARY:Basic Properties of MHD turbulence - Andrey Beresnyak (Ruhr-Univer
 sitat Bochum)
DTSTART:20120508T120000Z
DTEND:20120508T130000Z
UID:TALK37627@talks.cam.ac.uk
CONTACT:Jérôme Guilet
DESCRIPTION:Astrophysical fluids are conductive\, magnetized and turbulent
 . This entails a variety of phenomena\, two most basic of which is the dyn
 amo and the energy cascade. Very well known empirically in hydrodynamics\,
  so called "zeroth law of turbulence"\, states that even if viscosity goes
  to zero\, energy dissipation does not\, but goes to a constant. It turns 
 out that in MHD not only this still holds true\, but another basic law\, w
 hich I call "zeroth law of dynamo"\, is valid\, namely that if Reynolds nu
 mbers are sufficiently high and magnetic energy is low\, the latter will g
 row at a constant rate\, which is a fraction of the total dissipation rate
 . Another point of interest for an astrophysicist is the properties of MHD
  cascade in the inertial range. I will argue that both theory and numerics
  favor Kolmogorov -5/3 slope and not -3/2 slope that was reported earlier.
  The most challenging problem is so-called imbalanced\, or cross-helical c
 ase which appear whenever there is a localized source of perturbations\, s
 uch as the Sun for the solar wind turbulence or the central engine in AGN 
 jets. The standard Goldreich-Sridhar model does not apply in this case and
  it eluded theoretical description for a long time. The keys to understand
  energy cascades in the imbalanced case are the anisotropies of the Elsass
 er fields which turn out to be different. I will show the results of one o
 f the highest resolution simulations ever performed\, which were very help
 ful in discriminating between various viable models of MHD turbulence.
LOCATION:MR14\,  Centre for Mathematical Sciences\, Wilberforce Road\, Cam
 bridge
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