Prandtl number dependence of flow topology in quasi-two-dimensional turbulent Rayleigh-Benard convection

To date, a comprehensive understanding of the influence of the Prandtl number (Pr) on flow topology in turbulent Rayleigh-Benard convection (RBC) remains elusive. In this study, we present an experimental investigation into the evolution of flow topology in quasi-two-dimensional turbulent RBC with 7.0 <= Pr <= 244.2 and 2.03 x 10(8) <= Ra <= 2.81 x 10(9). Particle image velocimetry (PIV) measurements reveal the flow transitions from multiple-roll state to single-roll state with increasing Ra, and the transition is hindered with increasing Pr, i.e. the transitional Rayleigh number Rat increases with Pr. We mapped out a phase diagram on the flow topology change on Ra and Pr, and identified the scaling of Rat on Pr: Rat similar to Pr-0.93 in the low Pr range, and Rat similar to Pr-3.3 in the high Pr range. The scaling in the low Pr range is consistent with the model of balance of energy dissipation time and plume travel time that we proposed in our previous study, while the scaling in the high Pr range implies a new governing mechanism. For the first time, the scaling of Re on Ra and Pr is acquired through full-field PIV velocity measurement, Re similar to Ra-0.63 Pr (-0.87). We also propose that increasing horizontal velocity promotes the formation of the large-scale circulation (LSC), especially for the high Pr case. Our proposal was verified by achieving LSC through introducing horizontal driving force RaH by tilting the convection cell with a small angle.