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Flow regimes of Rayleigh–Bénard convection in a vertical magnetic field

Published online by Cambridge University Press:  11 May 2020

Till Zürner Open the ORCID record for Till Zürner [Opens in a new window] ,
Felix Schindler Open the ORCID record for Felix Schindler [Opens in a new window] ,
Tobias Vogt Open the ORCID record for Tobias Vogt [Opens in a new window] ,
Sven Eckert Open the ORCID record for Sven Eckert [Opens in a new window]  and
Jörg Schumacher Open the ORCID record for Jörg Schumacher [Opens in a new window]
Till Zürner*
Affiliation:
Institute of Thermodynamics and Fluid Mechanics, Technische Universität Ilmenau, Postfach 100565, D-98684Ilmenau, Germany
Felix Schindler
Affiliation:
Department of Magnetohydrodynamics, Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden – Rossendorf, Bautzner Landstraße 400, D-01328Dresden, Germany
Tobias Vogt
Affiliation:
Department of Magnetohydrodynamics, Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden – Rossendorf, Bautzner Landstraße 400, D-01328Dresden, Germany
Sven Eckert
Affiliation:
Department of Magnetohydrodynamics, Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden – Rossendorf, Bautzner Landstraße 400, D-01328Dresden, Germany
Jörg Schumacher
Affiliation:
Institute of Thermodynamics and Fluid Mechanics, Technische Universität Ilmenau, Postfach 100565, D-98684Ilmenau, Germany
*
†Email address for correspondence: [email protected]
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Abstract

The effects of a vertical static magnetic field on the flow structure and global transport properties of momentum and heat in liquid metal Rayleigh–Bénard convection are investigated. Experiments are conducted in a cylindrical convection cell of unity aspect ratio, filled with the alloy GaInSn at a low Prandtl number of $Pr=0.029$. Changes of the large-scale velocity structure with increasing magnetic field strength are probed systematically using multiple ultrasound Doppler velocimetry sensors and thermocouples for a parameter range that is spanned by Rayleigh numbers of $10^{6}\leqslant Ra\leqslant 6\times 10^{7}$ and Hartmann numbers of $Ha\leqslant 1000$. Our simultaneous multi-probe temperature and velocity measurements demonstrate how the large-scale circulation is affected by an increasing magnetic field strength (or Hartmann number). Lorentz forces induced in the liquid metal first suppress the oscillations of the large-scale circulation at low $Ha$, then transform the one-roll structure into a cellular large-scale pattern consisting of multiple up- and downwellings for intermediate $Ha$, before finally expelling any fluid motion out of the bulk at the highest accessible $Ha$ leaving only a near-wall convective flow that persists even below Chandrasekhar’s linear instability threshold. Our study thus proves experimentally the existence of wall modes in confined magnetoconvection. The magnitude of the transferred heat remains nearly unaffected by the steady decrease of the fluid momentum over a large range of Hartmann numbers. We extend the experimental global transport analysis to momentum transfer and include the dependence of the Reynolds number on the Hartmann number.

JFM classification

MHD and Electrohydrodynamics: Magneto convection
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 894 , 10 July 2020 , A21
Copyright
© The Author(s), 2020. Published by Cambridge University Press

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