Turbulent transition in Rayleigh-Bénard convection with Fluorocarbon

We present measurements of the heat-transfer and the velocity field in two Rayleigh-Bénard cells (aspect ratios 1 and 2). We use Fluorinert FC770 as the working fluid, up to a Rayleigh number 2 × 10¹². The velocity field is inferred from sequences of Shadowgraph pattern using a Correlation Image Velocimetry (CIV) algorithm. Indeed the large number of plumes, and their small characteristic scale, make it possible to use the Shadowgraph pattern produced by the thermal plumes in the same manner as particles in Particle Image Velocimetry (PIV). The method is validated in water against PIV, and yield identical wind velocity estimates. The joint heat-transfer and velocity measurements allow to compute the scaling of the kinetic dissipation rate which features a transition from a laminar Re⁵/² scaling to a turbulent Re³ scaling. We propose that the turbulent transition in Rayleigh-Bénard convection is controlled by a threshold Péclet number rather than Rayleigh number, which may explain the apparent discrepancy in the literature regarding the “ultimate” regime of convection.

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