Three-dimensional flow structures in turbulent Rayleigh-Bénard convection at low Prandtl number Pr = 0.03

In this paper we report 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 at Rayleigh numbers of Ra = 9.33×10⁶ , 5.31×10₇ and 6.02×10₈ 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. This is complemented with the multi-thermal-probe method for capturing the azimuthal temperature variation induced by the LSC at the sidewall. We use proper orthogonal decomposition (POD) to identify the dominating modes of the turbulent convection. The analysis reveals that a single-roll structure (SRS) of the LSC alternates in short succession with double-roll structures (DRS) or a three-roll structure (TRS). 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.