Experimental study of the global flow state transformation in a rectangular Rayleigh-Bénard sample

Abstract

Large-scale flow structures were measured in a rectangular Rayleigh-Bénard sample of equal length and height of 10 cm and a depth of 2.5 cm. The working fluid was water at a Prandtl number of Pr = 6.9. Aiming to capture flow structures to their full extent and throughout the entire sample, the three-dimensional Lagrangian measurement technique 3D Particle Tracking Velocimetry (3DPTV) was employed. The study provides direct confirmation that the global mean flow field changes significantly in dependence of the Rayleigh number Ra within the investigated range of 2.1 x 10^6 < Ra < 4.5 x 10^8. Several distinguishable flow states were observed in the laminarturbulent transition regime complementing the well-known mean wind. A 2D mode decomposition reveals a break down of the mean wind during the transition phase and its new formation in the turbulent regime. Further, the global distribution of magnitudes of the instantaneous velocity fields was used to extract characteristic velocities in the flow field. It is showed, that the Reynoldsnumber-dependent scaling law Re ~ Ra^gamma can be unambiguously determined with directly measured Lagrangian velocities, since the same scaling behavior holds for all chosen reference velocities. In this context, using the product of a polynomial with an exponential function, the global velocity distribution is described analytically in dependence of Ra and four independent fit parameters in the sub-range 2.8 x 10^7 < Ra < 4.5 x 10^8.