Effects of strong fringing magnetic fields on turbulent thermal convection

TL;DR

This study investigates how fringing magnetic fields influence turbulent thermal convection, revealing that magnetic field strength and fringe-width significantly affect flow structures, heat transport, and wall-localized modes.

Contribution

It provides new insights into the effects of fringing magnetic fields on turbulent convection, including the dependence of heat transport and flow structures on magnetic field strength and fringe-width.

Findings

Large-scale structures become thinner and align perpendicular to sidewalls with increasing magnetic field.

Global heat transport decreases with fringe-width for strong magnetic fields.

Wall modes exhibit non-monotonic amplitude dependence on fringe-width.

Abstract

We study the influence of fringing magnetic fields on turbulent thermal convection in a horizontally extended rectangular domain. The magnetic field is created in the gap between two semi-infinite planar magnetic poles, with the convection layer located near the edge of the gap. We employ direct numerical simulations in this setup for fixed Rayleigh and small Prandtl numbers, but vary the fringe-width by controlling the gap between the magnetic poles and the convection cell. The magnetic field generated by the magnets is strong enough to cease the flow in high magnetic flux region of the convection cell. We observe that as the local vertical magnetic field strength increases, the large scale structures become thinner and align themselves perpendicular to the longitudinal sidewalls. We determine the local Nusselt and Reynolds numbers as functions of the local Hartmann number (based on the vertical component of the magnetic field) and estimate the global heat and momentum transport. We show that the global heat transport decreases with increasing fringe-width for strong magnetic fields but increases with increasing fringe-width for weak magnetic fields. In the regions of large vertical magnetic fields, the convective motion becomes confined to the vicinity of the sidewalls. The amplitudes of these wall modes show a non-monotonic dependence on the fringe-width.