Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

MHD Rayleigh-Benard convection was studied experimentally using a liquid metal inside a box with square horizontal cross section and aspect ratio 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 2x10^3 < Q < 10^4 and 5x10^3 < Ra < 3x10^5, thus extending the regime diagram provided by Yanagisawa et al. (Physical Review E, 2013). Three new regimes were identified, whereas 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. (2013). We performed the POD analysis on the spatio-temporal velocity distribution and detected that the regular flow reversals can be interpreted as a periodic emergence of a 4-rolls state in a dominant 5-rolls state. The POD analysis