Characterization of LSC flow regimes in turbulent liquid metal Rayleigh-Benard convection

This paper reports on an experimental study focusing on the manifestation and dynamics of the large-scale circulation (LSC) in turbulent liquid metal convection. The experiments are performed inside a cylinder of aspect ratio Γ = 0.5 filled with the ternary alloy GaInSn, which has a Prandtl number of Pr = 0.03. The large-scale flow structures are classified and characterized by means of the contactless inductive flow tomography (CIFT) which enables the full reconstruction of the three-dimensional flow structures in the entire convection cell. The analysis reveals that a single-roll structure of the LSC alternates in short succession with double-roll structures or a three-roll structure. This is accompanied by dramatic fluctuations of the Reynolds number, whose instantaneous values can deviate by more than 50% from the time-average value. No coherent oscillations are observed, whereas a correlation analysis indicates a residual contribution of the torsion and sloshing modes. Results of the POD analysis suggest a stabilisation of the single-roll LSC with increasing Ra at the expense of flow structures with multiple rolls. Moreover, the relative lifetime of all identified flow states, measured in units of free-fall times, increases with rising Ra.