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Large scale magnetic fields in MHD turbulence

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High Reynolds number magneto-hydro-dynamic turbulence in the presence of zero-flux large scale magnetic fields is investigated as a function of the magnetic field strength. For a variety of flow configurations the energy dissipation rate ε follows the scaling ε ∝ Urms^3/l
even when the large scale magnetic field energy is twenty times larger than the kinetic. Further increase of the magnetic energy showed a transition to the ε ∝ Urms^2 Brms/l scaling implying that magnetic shear becomes more efficient at this point at cascading the energy
than the velocity fluctuations. Strongly helical configurations form non-turbulent helicity condensates and do not result in viscosity independent scaling. Weak turbulence scaling was absent from the investigation. Finally, the magnetic energy spectra showed support for the Kolmogorov spectrum k−5/3 while kinetic energy spectra are closer to the Iroshnikov-Kraichnan spectrum k^−3/2.

Author(s):

Alexandros Alexakis    
Laboratoire de Physique Statistique, École Normale Supérieure
France