Experimental characterisation of cessations and flow mode transitions in turbulent liquid-metal convection

Flow reorganisation and intermittent cessations of the large-scale convection (LSC) are investigated in turbulent liquid-metal Rayleigh–Bénard convection at a Prandtl number italic probability almost equals 0.03 Pr ≈ 0.03 Pr 0.03 . Flow measurements are performed at Rayleigh numbers of italic Ra equals 9.33 times 10 Superscript 6 Baseline comma 5.31 times 10 Superscript 7 Ra = 9.33 × 10 6 , 5.31 × 10 7 Ra = 9.33 × 10⁶, 5.31 × 10⁷ and 6.02 times 10 Superscript 8 6.02 × 10 8 6.02 × 10⁸ in a cylindrical sample of aspect ratio upper Gamma Γ = 0.5 filled with the ternary alloy GaInSn. The LSC structure is characterised using two complementary diagnostics: the classical multi-thermoprobe method based on sidewall temperature measurements and contactless inductive flow tomography (CIFT), which reconstructs the three-dimensional velocity field and enables a proper orthogonal decomposition of the dominant flow modes. While both diagnostics provide consistent descriptions of well-organised circulation states, they increasingly diverge during strongly three-dimensional flow reorganisation as they probe different physical quantities. The volumetric CIFT reconstructions reveal that cessations are associated with a temporary loss of global flow coherence accompanied by a redistribution of kinetic energy from the dominant single-roll and double-roll circulation modes towards higher-order structures. In contrast, localised flow reorganisations are identified as events representing transitions between competing circulation modes without complete loss of global coherence. The transition pathways are highly stochastic and show no unique modal sequence. The results provide the first structurally resolved experimental characterisation of the dynamics of flow cessations in turbulent liquid-metal convection and demonstrate the role of flow mode transitions in triggering large-scale intermittency at very low Prandtl numbers.