The Ekman-Hartmann layer in MHD Taylor-Couette flow

TL;DR

This paper investigates magnetic effects, including Ekman and Hartmann currents, in a finite aspect ratio MHD Taylor-Couette flow, revealing their influence on flow stability and implications for experimental design.

Contribution

It demonstrates the magnetic effects induced by end-plates in finite aspect ratio MHD Taylor-Couette flow and analyzes their impact on flow stability.

Findings

End-plates induce magnetic effects similar to infinite plates.

Hartmann currents penetrate the fluid and influence flow stability.

Magnetic boundary conditions critically affect the flow profile.

Abstract

We study magnetic effects induced by rigidly rotating plates enclosing a cylindrical MHD Taylor-Couette flow at the finite aspect ratio $H/D=10$. The fluid confined between the cylinders is assumed to be liquid metal characterized by small magnetic Prandtl number, the cylinders are perfectly conducting, an axial magnetic field is imposed $\Ha \approx 10$, the rotation rates correspond to $\Rey$ of order $10^2-10^3$. We show that the end-plates introduce, besides the well known Ekman circulation, similar magnetic effects which arise for infinite, rotating plates, horizontally unbounded by any walls. In particular there exists the Hartmann current which penetrates the fluid, turns into the radial direction and together with the applied magnetic field gives rise to a force. Consequently the flow can be compared with a Taylor-Dean flow driven by an azimuthal pressure gradient. We analyze stability of such flows and show that the currents induced by the plates can give rise to instability for the considered parameters. When designing an MHD Taylor-Couette experiment, a special care must be taken concerning the vertical magnetic boundaries so they do not significantly alter the rotational profile.