title: Breakdown of the large-scale circulation in Γ=1/2 rotating Rayleigh-Bénard flow.
abstract: Experiments and simulations of rotating Rayleigh-Bénard convection in cylindrical samples have revealed an increase in heat transport with increasing rotation rate. This heat transport enhancement is intimately related to a transition in the turbulent flow structure from a regime dominated by a large-scale circulation (LSC), consisting of a single convection roll, at no or weak rotation to a regime dominated by vertically aligned vortices at strong rotation. For a sample with an aspect ratio Γ=D/L=1 (D is the sample diameter and L is its height) the transition between the two regimes is indicated by a strong decrease in the LSC strength. In contrast, for Γ=1/2, Weiss and Ahlers [J. Fluid Mech. 688, 461 (2011)] revealed the presence of a LSC-like sidewall temperature signature beyond the critical rotation rate. They suggested that this might be due to the formation of a two-vortex state, in which one vortex extends vertically from the bottom into the sample interior and brings up warm fluid while another vortex brings down cold fluid from the top; this flow field would yield a sidewall temperature signature similar to that of the LSC. Here we show by direct numerical simulations for Γ=1/2 and parameters that allow direct comparison with experiment that the spatial organization of the vertically aligned vortical structures in the convection cell do indeed yield (for the time average) a sinusoidal variation of the temperature near the sidewall, as found in the experiment. This is also the essential and nontrivial difference with the Γ=1 sample, where the vertically aligned vortices are distributed randomly.
authors.affiliation: Department of Science and Technology and J.M. Burgers Center for Fluid Dynamics, University of Twente, Post Office Box 217, 7500 AE Enschede, The Netherlands.
authors.forename: Richard J A M
authors.initials: RJ
authors.lastname: Stevens
authors.forename: Herman J H
authors.initials: HJ
authors.lastname: Clercx
authors.forename: Detlef
authors.initials: D
authors.lastname: Lohse
mesh_terms: D000054:Acceleration; D003198:Computer Simulation; D008962:Models, Theoretical; D012212:Rheology / Q000379:methods*; D012399:Rotation; D012996:Solutions / Q000737:chemistry*; D013696:Temperature
chemical_list: D012996:Solutions