Pseudo--magnetorotational instability in a Taylor-Dean flow between electrically connected cylinders

TL;DR

This paper introduces a pseudo-magnetorotational instability in a Taylor-Dean flow with electrically connected cylinders under a helical magnetic field, highlighting its centrifugal nature and complex interactions with hydrodynamic instabilities.

Contribution

It reveals a new pseudo-MRI mechanism driven by radial currents in electrically connected cylinders, distinct from true MRI, and discusses its implications for experiments and simulations.

Findings

Pseudo-MRI destabilizes flow via centrifugal-like mechanism.

Radial currents induce azimuthal electromagnetic forces leading to instability.

Hydrodynamic instability can occur at any strong differential rotation, complicating MRI detection.

Abstract

We consider a Taylor-Dean-type flow of an electrically conducting liquid in an annulus between two infinitely long perfectly conducting cylinders subject to a generally helical magnetic field. The cylinders are electrically connected through a remote, perfectly conducting endcap, which allows a radial electric current to pass through the liquid. The radial current interacting with the axial component of magnetic field gives rise to the azimuthal electromagnetic force, which destabilizes the base flow by making its angular momentum decrease radially outwards. This instability, which we refer to as the pseudo--magnetorotational instability (MRI), looks like an MRI although its mechanism is basically centrifugal. In a helical magnetic field, the radial current interacting with the azimuthal component of the field gives rise to an axial electromagnetic force, which drives a longitudinal circulation. First, this circulation advects the Taylor vortices generated by the centrifugal instability, which results in a traveling wave as in the helical MRI (HMRI). However, the direction of travel of this wave is opposite to that of the true HMRI. Second, at sufficiently strong differential rotation, the longitudinal flow becomes hydrodynamically unstable itself. For electrically connected cylinders in a helical magnetic field, hydrodynamic instability is possible at any sufficiently strong differential rotation. In this case, there is no hydrodynamic stability limit defined in the terms of the critical ratio of rotation rates of inner and outer cylinders that would allow one to distinguish a hydrodynamic instability from the HMRI. These effects can critically interfere with experimental as well as numerical determination of MRI.