MHD TURBULENCE AT THE LABORATORY-SCALE - ESTABLISHED IDEAS AND NEW CHALLENGES

The properties of MHD turbulence in the electrically conducting fluids available in the laboratory (where the magnetic Reynolds number is si gnificantly smaller than unity) may be summarised as follows: (1) The Alfven waves, even under their degenerated form at this scale, are res ponsible for a tendency to two-dimensionality. Eddies tend to become a ligned with the applied magnetic field and inertia tends to restore is otropy. The competition between these mechanisms results in a spectral law t<SUP>-2</SUP>k<SUP>-3</SUP>.<SUP> </SUP>(2) When insulating wall s, perpendicular to the magnetic field, are present and close enough t o each other, two-dimensionality can be established with a good approx imation within the large scales, and the predominant mechanism is the inverse energy cascade. (3) These columnar eddies are nevertheless sub mitted to a dissipation within the Hartmann boundary layers present at their ends, whose time scale is independent of the wave number. When this damping effect is negligible, ordinary 2D turbulence is observed with k(-5/3) spectra. On the contrary when this (ohmic and viscous) da mping is significant this 2D turbulence exhibits k(-3) spectra. Beside s these homogeneous (except within the Hartmann layers) conditions, fo r instance in shear flows such as mixing layers, almost nothing is kno wn except that two-dimensionality may be well established. The first r esults of a recent experimental investigation (still in development) a re presented. Some challenging questions are raised, such as the inter pretation of a surprising difference between the transport of momentum and the transport of a scalar quantity (heat) across that layer. A vi deo was shown during the oral presentation of this paper, illustrating the energy transfer toward the large scales and the weakness of the d issipation suffered by this 2D velocity field.