Abstract

In this talk we will first report on a series of numerical MHD experiments on the turbulent relaxation of braided magnetic fields in plasmas of high magnetic Reynolds numbers (Wilmot-Smith et al. 2009, 2010). These experiments have produced relaxed states which in some cases differ drastically from the predictions of the Taylor hypothesis, that is the assumption that the final state of a turbulent relaxation is a linear force-free field with the same total helicity as the initial state.

We present a method to determine the topological degree of the field line mapping which shows that there are further constraints on the relaxation process beyond the conservation of the total helicity (A. Yeates et al., Phys. Rev. Lett. 105, 2010). These constraints can prevent the system from relaxing to a Taylor state and hence limit the energy which can be released.

We will then report on a new series of experiments where we test whether similar constraints hold in the hydrodynamic case, that is we investigate the relaxation of incompressible flows with braided vorticity field lines.