title: 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)PLEEE81539-375510.1103/PhysRevE.88.063020]. 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.
authors.affiliation: Laboratory for Flow Control, Hokkaido University, Sapporo, Japan.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Tasaka
authors.affiliation: Laboratory for Flow Control, Hokkaido University, Sapporo, Japan.
authors.forename: Kazuto
authors.initials: K
authors.lastname: Igaki
authors.affiliation: Department of Deep Earth Structure and Dynamics Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan.
authors.forename: Takatoshi
authors.initials: T
authors.lastname: Yanagisawa
authors.affiliation: Institute of Fluids Dynamics, Helmholtz Zentrum Dresden-Rossendorf (HZDR), Dresden, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Vogt
authors.affiliation: Institute of Fluids Dynamics, Helmholtz Zentrum Dresden-Rossendorf (HZDR), Dresden, Germany.
authors.forename: Till
authors.initials: T
authors.lastname: Zuerner
authors.affiliation: Institute of Fluids Dynamics, Helmholtz Zentrum Dresden-Rossendorf (HZDR), Dresden, Germany.
authors.forename: Sven
authors.initials: S
authors.lastname: Eckert