Spontaneous flow reversals in Rayleigh-Bénard convection of a liquid metal

Abstract

We report a finding of spontaneous flow reversals of roll-like patterns in liquid gallium Rayleigh-Bénard convection. The vessel has a square geometry with an aspect ratio of 5, and a horizontal magnetic field is applied to align the rolls. The flow patterns were visualized by ultrasonic velocity measurements, and the processes of the reversal were clearly observed. The basic flow pattern observed in the vessel is a four-roll structure with its axis parallel to the magnetic field. Emergence of a new circulation at a corner of the vessel causes flow reversal with reorganization of the whole pattern. The flow keeps relatively steady four-roll structure for most of the duration, while the reversal of it is over in a short time. The reversals of the flow occur randomly with the interval time between reversals being much longer than the circulation time.

Convection patterns in a liquid metal under an imposed horizontal magnetic field

“…The conditions and parameter range for the occurrence of flow reversals is not clear, and the relation to steady roll structures and their instabilities should be elucidated further. This paper extends the studies by [22] for a larger Ra range and various Chandrasekhar numbers Q using the same geometry of square vessel, with simultaneous pointwise measurements of temperature fluctuations in the layer. The aim of the present study is to draw up a regime diagram of flow behavior including the region of flow reversals in relation to Ra and Q and to give a consistent description of the transition of flow patterns.…”

“…This regime IV includes the flow reversals described in Ref. [22], where the parameters were Ra = 1.5 × 10 4 and Q = 1.0 × 10 3 . Regime V is a state with a steady two-dimensional (2D) roll structure: V a with five-rolls and V b with four-rolls.…”

“…Figure 1 shows schematic illustrations of the setting of the experiments. The vessel has a square geometry with aspect ratio 5, which is the same geometry in the previous study [22]. The inside measurements of the vessel are 200 mm 2 and 40 mm deep (L).…”

“…Flow patterns, free from sidewall constraints, can be expected to emerge in larger horizontal vessels. Yanagisawa et al [22] used a wider square vessel with a fixed intensity of imposed horizontal magnetic field and observed spontaneous flow reversals characterized by random reversals of the flow direction in the roll structure. Flow reversals were interpreted as a reorganization process of the roll-like patterns moderately constrained by a magnetic field, but the mechanism that induces reversals is not fully understood.…”

“…[19]. Applications of the UVP to the RB convection of liquid gallium have shown flow patterns and their fluctuations for cases both with and without imposed magnetic fields [20][21][22]. In general, the UVP can be used for flows with velocities much larger than the detection limit and for RB convection with layer thicknesses of the order of centimeters, which means that the Rayleigh number needs to be several times higher than the Ra c value, requiring flow velocities of ∼mm/s.…”