Observation of a Free-Shercliff-Layer Instability in Cylindrical Geometry

TL;DR

This paper reports the experimental observation of a free-Shercliff-layer instability in a liquid metal Taylor-Couette setup, demonstrating a Kelvin-Helmholtz-type instability influenced by magnetic fields across a wide range of Reynolds numbers.

Contribution

First experimental observation of a free-Shercliff-layer instability in cylindrical geometry with detailed analysis and simulation validation.

Findings

Instability occurs at Elsasser number above unity.

Azimuthal mode number increases with Elsasser number.

Experimental results agree with linear and nonlinear simulations.

Abstract

We report on observations of a free-Shercliff-layer instability in a Taylor-Couette experiment using a liquid metal over a wide range of Reynolds numbers, $Re\sim 10^3-10^6$. The free Shercliff layer is formed by imposing a sufficiently strong axial magnetic field across a pair of differentially rotating axial endcap rings. This layer is destabilized by a hydrodynamic Kelvin-Helmholtz-type instability, characterized by velocity fluctuations in the $r-\theta$ plane. The instability appears with an Elsasser number above unity, and saturates with an azimuthal mode number $m$ which increases with the Elsasser number. Measurements of the structure agree well with 2D global linear mode analyses and 3D global nonlinear simulations. These observations have implications for a range of rotating MHD systems in which similar shear layers may be produced.