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Bounds on the attractor dimension for low-R m wall-bound magnetohydrodynamic turbulence

Steady Low $R_m$ MHD turbulence is investigated here through estimates of upper bounds for attractor dimension. A flow between two parallel walls with an imposed perpendicular magnetic field is considered. The flow is defined by its maximum velocity and the intensity of the magnetic field. Given the corresponding Reynolds and Hartmann numbers, we derive an upper bound for the dimension of the attractor and find out which modes must be chosen to achieve this bound. Thier, are compared to heuristics estimates for the size of the smallest scales and anisotropy of the turbulence. We identify boundaries separating different sets of modes in the space of non-dimensional parameters, which are reminiscent of three important previously identified transitions observed in the real flow. The first boundary separates classical hydrodynamic sets of modes from MHD sets where anisotropy takes the form of a ``Joule cone''. The second boundary separates 3D MHD sets from quasi-2D MHD sets. The third separates sets where all the modes exhibit the same boundary layer thickness or so, and sets where many different ``boundary layer modes'' co-exists in the set. The non-dimensional relations defining these boundaries are then compared to the heuristics known for the transition between isotropic and anisotropic MHD turbulence, 3D and quasi-2D MHD turbulence and that between a turbulent and a laminar Hartmann layer. In addition to this 3D approach, we also determine upper bounds for the dimension of forced turbulent flows modelled using a 2D MHD equation, relevant in the quasi-2D MHD regime. The advantage of this 2D approach is that, while upper bounds are quite loose in three dimensions, optimal upper bounds exist for the 2D nonlinear term. This allows us to derive realistic attractor dimensions for quasi-2D MHD flows.