Numerical simulations of current generation and dynamo excitation in a mechanically-forced, turbulent flow

The role of turbulence in current generation and self-excitation of a magnetic field has been studied using a three dimensional MHD computation. A simple impeller model drives a flow that can generate a growing magnetic field, depending upon the magnetic Reynolds number Rm = mu 0sigmaVa and the fluid Reynolds number Re = Va/nu of the flow. For Re < 420 the flow is laminar and the dynamo transition is governed by a simple threshold in Rm > 100, above which a growing magnetic eigenmode is observed that is primarily of a dipole field tranverse to axis of symmetry of the flow. In saturation, the Lorentz force slows the flow such that the magnetic eigenmode becomes marginally stable. For Re > 420 and Rm ∼ 100 the flow becomes turbulent and the dynamo eigenmode is suppressed. The mechanism of suppression is due to a combination of a time varying large-scale flow and the presence of fluctuation-driven currents which effectively enhance the magnetic diffusivity. For higher Rm a dynamo reappears, however the structure of the magnetic field is often different from the laminar dynamo; it is dominated by a dipolar magnetic field which is aligned with the axis of symmetry of the mean-flow, apparently generated by fluctuation-driven currents.; The magnitude and structure of the fluctuation-driven currents has been studied by applying a weak, axisymmetric seed magnetic field to laminar and turbulent subcritical flows. An Ohm's law analysis of the axisymmetric currents allows the fluctuation-driven currents to be identified which show features consistent with predictions from mean field theory. In laminar flow, shear couples with a uniform magnetic field to cause a poloidal current which give rise to the toroidal magnetic field (the omega-effect) and poloidal flow compresses magnetic flux near the symmetry axis. In turbulent flow, the mean flow is qualitatively similar to the steady state flow in the laminar simulation and features of the laminar fluid limit remain. However, magnetic fields generated by the fluctuations are significant: a dipole moment aligned with the symmetry axis of the mean-flow is generated similar to those observed in the experiment, and both toroidal and poloidal flux expulsion are observed.