Abstract
We conduct magnetized spherical Couette (MSC) flow experiments in the return flow instability regime with GaInSn as the working fluid, the ratio of the inner to the outer sphere radii ri/ro = 0.5, the Reynolds number Re = 1000, and the Hartmann number Ha ∈ [27.5, 40]. Rotating waves with different azimuthal wavenumbers m ∈ {2, 3, 4} manifest in certain ranges of Ha in the experiments, depending on whether the values of Ha were fixed or varied from different initial values. These observations demonstrate the multistability of rotating waves, which we attribute to the dynamical system representing the state of the MSC flow tending to move along the same solution branch of the bifurcation diagram when Ha is varied. In experiments with both fixed and varying Ha, the rotation frequencies of the rotating waves are consistent with the results of nonlinear stability analysis. A brief numerical investigation shows that differences in the azimuthal wavenumbers of the rotating waves that develop in the flow also depend on the azimuthal modes that are initially excited.
Highlights
The interaction of magnetic fields with the flow of electrically conducting fluids and plasmas is a common astrophysical phenomenon
Despite great efforts and promising initial results obtained in spherical Couette7 and Taylor–Couette8 experiments, as well as in an interesting spring-mass analog,9 the unequivocal proof of the standard magnetorotational instability (MRI), with a purely axial geometry of the applied magnetic field, has not yet been found
To ensure that we are in the return flow instability regime, we selected the lower bound of Ha for the experiments at Ha = 27.5 to be slightly above the critical value
Summary
The interaction of magnetic fields with the flow of electrically conducting fluids and plasmas is a common astrophysical phenomenon. Dynamo theory postulates that planetary, stellar, and galactic magnetic fields are self-excited in flows of electrically conducting fluids and plasmas. Another important effect is the magnetorotational instability (MRI), a process by which differentially rotating conducting fluids are destabilized in the presence of magnetic fields. Experimental evidence of the MRI has been found at Helmholtz-Zentrum Dresden-Rossendorf for both helical and azimuthal geometries of the applied magnetic field. Despite great efforts and promising initial results obtained in spherical Couette and Taylor–Couette experiments, as well as in an interesting spring-mass analog, the unequivocal proof of the standard MRI, with a purely axial geometry of the applied magnetic field, has not yet been found
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