The role of boundaries in the MagnetoRotational Instability

TL;DR

This study uses 3D simulations to explore how boundaries influence the MagnetoRotational Instability in cylindrical flows, revealing boundary effects on bifurcation, mode structure, and the emergence of non-axisymmetric modes related to shear layer instabilities.

Contribution

It demonstrates the significant impact of boundaries on MRI behavior and identifies the formation of Kelvin-Helmholtz modes due to boundary-induced shear layers in laboratory setups.

Findings

Boundaries alter the bifurcation and saturation of MRI.

Large non-axisymmetric modes emerge at strong magnetic fields.

Kelvin-Helmholtz instability explains non-axisymmetric mode formation.

Abstract

In this paper, we investigate numerically the flow of an electrically conducting fluid in a cylindrical Taylor-Couette flow when an axial magnetic field is applied. To minimize Ekman recirculation due to vertical no-slip boundaries, two independently rotating rings are used at the vertical endcaps. This configuration reproduces setup used in laboratory experiments aiming to observe the MagnetoRotational Instability (MRI). Our 3D global simulations show that the nature of the bifurcation, the non-linear saturation, and the structure of axisymmetric MRI modes are significantly affected by the presence of boundaries. In addition, large scale non-axisymmetric modes are obtained when the applied field is sufficiently strong. We show that these modes are related to Kelvin-Helmoltz destabilization of a free Shercliff shear layer created by the combined action of the applied field and the rotating rings at the endcaps. Finally, we compare our numerical simulations to recent experimental results obtained in the Princeton MRI experiment.