Breakdown of the large-scale wind in aspect ratio Γ=1/2 rotating Rayleigh-Bénard flow

Details Breakdown of the large-scale wind in aspect ratio Γ=1/2 rotating Rayleigh-Bénard flow Breakdown of the large-scale wind in aspect ratio Γ=1/2 rotating Rayleigh-Bénard flow

2011-12-02

arxiv.org/abs/1112.0411v2

Physics

Fluid Dynamics

18 pages, 9 figures

Scientific paper

Experiments and simulations of rotating Rayleigh-Benard convection in cylindrical samples have revealed an increase in heat transport with increasing rotation rate. This heat transport enhancement is intimately related with 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 \Gamma = D/L = 1 (D is the sample diameter and L its height) the transition between the two regimes is indicated by a strong decrease in the LSC strength. In contrast, for \Gamma = 1/2 Weiss and Ahlers [J. Fluid Mech. 2011 (DOI:10.1017/jfm.2011.392] showed with sidewall temperature measurements that the relative LSC strength does not decrease when the heat-transport enhancement sets in. 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 the same sidewall temperature-signature as the LSC. Here we show by direct numerical simulations for \Gamma= 1/2 and parameters that allow direct comparison with experiment that vortex structures, which are somewhat more complex than just two separate vortices, does indeed yield (for the time average) a sinusoidal variation of the temperature near the sidewall, as found in the experiment. Furthermore, we confirm the experimental results, as a function of 1/Ro, of the vertical temperature gradient in the bulk. In addition, we present direct numerical simulation results for the kinetic boundary layer thickness and for certain Reynolds-number averages which shed light on the effect of rotation on the transition in the turbulent flow structure.

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