Abstract
In this study we consider the effect of a horizontal magnetic field on the Rayleigh-Bénard convection in a finite liquid metal layer contained in a cuboid vessel of aspect ratio . Laboratory experiments are performed for measuring temperature and flow field in the low melting point alloy GaInSn at Prandtl number and in a Rayleigh number range . The field direction is aligned parallel to one pair of the two side walls. The field strength is varied up to a maximum value of 320 mT (, , definitions of all nondimensional numbers are given in the text). The magnetic field forces the flow to form two-dimensional (2D) rolls whose axes are parallel to the direction of the field lines. The experiments confirm the predictions made by Busse and Clever [Busse and Clever, Stability of convection rolls in the presence of a horizontal magnetic field, J. de Méc. Théor. et Appl. 2, 495 (1983)] who showed that the application of the horizontal magnetic field extends the range in which steady 2D roll structures exist (Busse balloon) towards higher Ra numbers. A transition from the steady to a time-dependent oscillatory flow occurs when Ra exceeds a critical value for a given Chandrasekhar number Q, which is also equivalent to a reduction of the ratio . Our measurements reveal that the first developing oscillations are clearly of 2D nature, in particular a mutual increase and decrease in the size of adjacent convection rolls is observed without the formation of any detectable gradients in the velocity field along the magnetic field direction. At a ratio of , the first 3D structures appear, which initially manifest themselves in a slight inclination of the rolls with respect to the magnetic field direction. Immediately in the course of this, there arise also disturbances in the spaces between adjacent convection rolls, which are advected along the rolls due to the secondary flow driven by Ekman pumping. The transition to fully developed 3D structures and then to a turbulent regime takes place with further lowering .
7 More- Received 31 July 2020
- Accepted 13 January 2021
DOI:https://doi.org/10.1103/PhysRevFluids.6.023502
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