Regular flow reversals in Rayleigh-Bénard convection in a horizontal magnetic field

Abstract

Magnetohydrodynamic Rayleigh-Bénard convection was studied experimentally using a liquid metal inside a box with a square horizontal cross section and aspect ratio of five. Systematic flow measurements were performed by means of ultrasonic velocity profiling that can capture time variations of instantaneous velocity profiles. Applying a horizontal magnetic field organizes the convective motion into a flow pattern of quasi-two-dimensional rolls arranged parallel to the magnetic field. The number of rolls has the tendency to decrease with increasing Rayleigh number Ra and to increase with increasing Chandrasekhar number Q. We explored convection regimes in a parameter range, at 2 × 10 3 < Q < 10 4 and 5 × 10 3 < Ra < 3 × 10 5 , thus extending the regime diagram provided by Yanagisawa et al. [T. Yanagisawa et al., Phys. Rev. E 88, 063020 (2013)]. Three regimes were identified, of which the regime of regular flow reversals in which five rolls periodically change the direction of their circulation with gradual skew of the roll axes can be considered as the most remarkable one. The regime appears around a range of Ra/Q = 10, where irregular flow reversals were observed in Yanagisawa et al. We performed the proper orthogonal decomposition (POD) analysis on the spatiotemporal velocity distribution and detected that the regular flow reversals can be interpreted as a periodic emergence of a four-roll state in a dominant five-roll state. The POD analysis also provides the definition of the effective number of rolls as a more objective approach.

Competing forces in liquid metal electrodes and batteries

“…3a and 4a); the measured flow magnitude does not change. Previous work dealing with liquid metal convection in the presence of magnetic fields suggest that convection rolls will tend to align with magnetic field lines, producing a more ordered flow structure [114,115]. However, there are two other ways to explain the increase in order much better: electro-vortex flow and internally heated con- vection (IHC).…”

Quasistatic magnetoconvection: heat transport enhancement and boundary layer crossing

“…Various studies have been carried out in the past on RB convection in the presence of a magnetic field (Burr & Müller 2002). A particular interest is RB convection with an applied external horizontal magnetic field (Yanagisawa et al 2013;Tasaka et al 2016). It is observed that under strong magnetic field the convective rolls exhibit quasi-two dimensional flow patterns, which can be understood in terms of linear stability analysis (Tasaka et al 2016).…”

“…A particular interest is RB convection with an applied external horizontal magnetic field (Yanagisawa et al 2013;Tasaka et al 2016). It is observed that under strong magnetic field the convective rolls exhibit quasi-two dimensional flow patterns, which can be understood in terms of linear stability analysis (Tasaka et al 2016). For weak magnetic field, the magnetic damping effect is weak, which causes the convection pattern less coherent and hence losing its quasi-two-dimensional character.…”

Wall modes in magnetoconvection at high Hartmann numbers

“…Therefore, the quasistatic limit of small R m is applicable with simplifications of the full set of magnetohydrodynamic equations (Knaepen & Moreau 2008;Davidson 2016). It is actually also the operating point of most laboratory experiments on Rayleigh-Bénard convection (RBC) with external vertical (Cioni et al 2000;Burr & Müller 2001;Aurnou & Olson 2001) or horizontal magnetic fields (Fauve et al 1981;Tasaka et al 2016;Vogt et al 2018). Nakagawa (1955) and Chandrasekhar (1961) showed that a sufficiently strong vertical external magnetic field B 0 can suppress the onset of Rayleigh-Bénard convection.…”