Research into hydrodynamic (HD) turbulence, without magnetic fields, has offered substantial results over the past years. On the other hand, progress in understanding magnetohydrodynamic (MHD) turbulence has been limited, mainly because of the lack of experimental data. To overcome the difficulty in experimenting with systems where MHD applies, scientists carried out numerical simulations of unprecedented resolution. The MHDTURB (Nonuniversal statistics in MHD turbulence) project focused on the unsolved problem of universality in turbulence of electrically conducting fluids. While there are similarities with HD, to simulate turbulent flow states in MHD, highly complicated models were needed. The complexity is a result of the dependence on initial conditions and translates to increased resolution of the numerical simulations that, in turn, requires substantial computational power. The MHDTURB team took numerical simulations of MHD turbulence to a higher level using the computational resources of the Partnership for Advanced Computing in Europe (PRACE). The results show different behaviours that can be observed depending on the initial conditions. The differences identified were attributed to structures that are formed. Nevertheless, as the achieved 'strength' of turbulence is increased, transition to a potentially universal behaviour can be observed, similarly to HD turbulent flows where the initial conditions tend to be forgotten. Furthermore, the simulation results have revealed that in the absence of any perturbation, symmetries were preserved at the examined Reynolds numbers. In the presence of even the smallest perturbation, certain symmetries were broken, provided the Reynolds number was high enough. Currents sheets formed in regions of magnetic discontinuities were found to be responsible for the power law that the energy distribution among scales follows in decaying MHD turbulent flows. An additional discovery was that transient spectra appear under random initial conditions in the case of a close correlation between velocity and current. Overall, MHDTURB has offered insights into MHD turbulence that are set to support experimental studies of this multi-scale phenomenon, abundant in astrophysics and plasma physics.
Universality, plasma, turbulence, astrophysical, magnetohydrodynamic, MHDTURB