The three-dimensionalities of wall bounded MHD turbulence

Flows of electrically conducting fluids in an external magnetic field tend to become 2D. We show experimentally that in channels flows normal to the field and electrically forced through one of the channel walls, the intensity of turbulent fluctuations scales as Re when the flow is quasi-2D, Re²/³ when the flow is 3D but strong near both walls, and Re¹/² if it is mostly concentrated along one wall only. We also partly confirm the theoretical prediction of [5] that structures of transversal wavelength k_⊥ become quasi-2D when they exceed a critical size k_⊥ ∼ N_t¹/², where the true interaction parameter Nt represents the ratio of momentum diffusion by the Lorentz force to inertia for the large scales.