Nonaxisymmetric simulations of the Princeton magnetorotational instability experiment with insulating and conducting axial boundaries.

Abstract

Stability and nonlinear evolution of rotating magnetohydrodynamic flows in the Princeton magnetorotational instability (MRI) experiment are examined using three-dimensional non-axisymmetric simulations. In particular, the effect of axial boundary conductivity on a free Stewartson-Shercliff layer (SSL) is numerically investigated using the spectral finite-element Maxwell and Navier Stokes (SFEMaNS) code. The free SSL is established by a sufficiently strong magnetic field imposed axially across the differentially rotating fluid with two rotating rings enforcing the boundary conditions. Numerical simulations show that the response of the bulk fluid flow is vastly different in the two different cases of insulating and conducting end caps. We find that, for the insulating end caps, there is a transition from stability to instability of a Kelvin-Helmholtz-like mode that saturates at an azimuthal mode number m=1, whereas for the conducting end caps, the reinforced coupling between the magnetic field and the bulk fluid generates a strong radially localized shear in the azimuthal velocity resulting in axisymmetric Rayleigh-like modes even at reduced thresholds for the axial magnetic field. For reference, three-dimensional nonaxisymmetric simulations have also been performed in the MRI unstable regime to compare the modal structures.