The role of magnetic boundaries in kinematic and self-consistent magnetohydrodynamic simulations of precession-driven dynamo action in a closed cylinder

We numerically examine dynamo action generated by a flow of an electrically conducting fluid in a precessing cylindrical cavity. We compare a kinematic approach with the results from a self-consistent three-dimensional simulation of the complete set of magnetohydrodynamic (MHD) equations. In all cases, we observe a minimum for the onset of dynamo action in a transitional regime, within which the hydrodynamic flow undergoes a change from a large-scale to a more small-scale, turbulent behavior. Significant differences in the critical magnetic Reynolds number occur depending on the physical properties of an external layer surrounding the flow active domain. The strong influence of the electromagnetic properties of this outer layer can be related to two different branches with dynamo action. In contrast to the kinematic models, the nonlinear MHD simulations reveal a small-scale dynamo with the magnetic energy remaining significantly smaller than the kinetic energy. In irregular intervals, we observe dynamo bursts with a local concentration of the magnetic field, resulting in a global increase in the magnetic energy by a factor of 3–5. However, diffusion of the local patches caused by strong local shear is too rapid, causing these features to exist for only a short period so that their dynamical impact on the dynamo remains small. A connection with the kinematic models can be derived by looking at the time-averaged field of the MHD dynamo solution. This is comparable to the eigenmode of the inefficient branch of the kinematic models, which explains their large critical magnetic Reynolds number.