Turbulent transition in Rayleigh-Benard convection with fluorocarbon(a)

We present measurements of the global heat transfer and the velocity field in two Rayleigh-Benard cells (aspect ratios 1 and 2). We use Fluorinert FC770 as the working fluid, up to a Rayleigh number 2x10(12). 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 yields 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 Re-5/2 scaling to a turbulent Re-3 scaling. We propose that the turbulent transition in Rayleigh-Benard convection is controlled by a threshold Peclet number rather than a threshold Rayleigh number, which may explain the apparent discrepancy in the literature regarding the "ultimate" regime of convection. focus article Copyright (C) 2022 EPLA