Continuation and stability of rotating waves in the magnetized spherical Couette system: Secondary transitions and multistability

TL;DR

This study investigates the bifurcation, stability, and multistability of rotating waves in a magnetized spherical Couette system at moderate Reynolds number, revealing multiple flow regimes, transitions, and the influence of magnetic field strength.

Contribution

It provides the first detailed numerical analysis of rotating wave bifurcations and stability in the magnetized spherical Couette system under conditions relevant to the HEDGEHOG experiment.

Findings

Multiple multistable wave regimes identified for different azimuthal wave numbers.

Transitions to quasiperiodic flows and modulated rotating waves observed.

Maximum nonaxisymmetric flow component occurs at a critical magnetic field strength.

Abstract

Rotating waves (RW) bifurcating from the axisymmetric basic magnetized spherical Couette (MSC) flow are computed by means of Newton-Krylov continuation techniques for periodic orbits. In addition, their stability is analysed in the framework of Floquet theory. The inner sphere rotates whilst the outer is kept at rest and the fluid is subjected to an axial magnetic field. For a moderate Reynolds number ${\rm Re}=10^3$ (measuring inner rotation) the effect of increasing the magnetic field strength (measured by the Hartmann number ${\rm Ha}$) is addressed in the range ${\rm Ha}\in(0,80)$ corresponding to the working conditions of the HEDGEHOG experiment at Helmholtz-Zentrum Dresden-Rossendorf. The study reveals several regions of multistability of waves with azimuthal wave number $m=2,3,4$, and several transitions to quasiperiodic flows, i.e modulated rotating waves (MRW). These nonlinear flows can be classified as the three different instabilities of the radial jet, the return flow and the shear-layer, as found in previous studies. These two flows are continuously linked, and part of the same branch, as the magnetic forcing is increased. Midway between the two instabilities, at a certain critical ${\rm Ha}$, the nonaxisymmetric component of the flow is maximum.