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


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

Zürner, T.; Schindler, F.; Vogt, T.; Eckert, S.; Schumacher, J.

The effects of a vertical constant 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 field structure with increasing magnetic field strength are probed using multiple ultrasound Doppler velocimetry sensors and thermocouples for a Rayleigh number range of 10^6 < Ra < 6x10^7 and Hartmann numbers Ha < 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, then transform the one-roll structure into a cellular large-scale pattern consisting of multiple up- and downwellings, before finally expelling any fluid motion out of the bulk leaving only a near-wall convective flow that persists even below Chandrasekhar's linear instability threshold. Our study thus proofs experimentally the existence of wall modes in confined magnetoconvection. The magnitude of the transferred heat remains nearly unaffected by the steady decrease of the velocity momentum over a large range of Hartmann numbers. Previous experiments and direct numerical simulations are consistent with our results.

Keywords: magnetoconvection Rayleig-Bénard convection; liquid metal

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Publ.-Id: 30445