Abstract
An experimental study of the three-dimensional spatial structure of low-frequency temperature oscillations in cylindrical Rayleigh–Bénard convection of a fluid with a Prandtl number Pr = 12.3, aspect ratio Γ ≡ D/L = 1.00 (D is the diameter, and L is the height) and Rayleigh-number 5 × 1010 < Ra < 3 × 1011 is reported. The flow structure was measured using 3 sets of 8 thermal probes, each distributed uniformly around the periphery at heights L/4, L/2, and 3L/4 from the bottom. At the top/bottom layer, the large-scale circulation (LSC) consisted of two well-identified cold/hot flows. These cold/hot flows traveled to mid-height, where only the fluctuation in the temperature reveals the existence of two cold/hot flows. The oscillatory frequency corresponding to the turnover frequency of the LSC was only found at the location where the cold/hot flows were present. There is a discrepancy between the Reynolds number based on the turnover frequency of the LSC in the present work and GL prediction. This discrepancy is consistent with the study by Brown, Funfschilling, and Ahlers (J. Stat. Mech. 2007, P10005-1–P10005-22), indicating that there is a new state in Ra > Ra* where the LSC is no longer a coherent single-roll structure. Ra* for Pr = 12.3 is 1 × 1010.
Highlights
Turbulent Rayleigh–Bénard (RB) convection, the flow confined between a heated bottom plate and a cooled top plate, is an ideal model to study the turbulence involving heat transport.1–3 The coherent structure of turbulent RB convection, large-scale circulation (LSC), has attracted the attention of researchers and has been studied extensively.4–20 The visualization study by Xi, Lam, and Xia21 has demonstrated that the hot/cold plumes emitted from the bottom/top boundary layer organize themselves into a quasi-2D single roll, which is referred to as LSC, which spans almost the whole cell
As reported by previous work, with regard to the apparatus used in the present work, the results obtained with two different materials, copper and aluminum plates, agreed with each other, which means there is no need to correct the influence of infinite plate conductivity
A structure consisting of two hot flows and cold flows was used in the present work
Summary
Brown, Funfschilling, and Ahlers found a discrepancy between the Reynolds numbers based on twisting and sloshing motions when Ra is larger than Ra∗ = 1 × 109. We investigate the 3D structure of LSC and its oscillation frequency in the convection with a Prandtl number of 12.3. Since the LSC is a coherent structure exhibiting sloshing motion at the mid-height plane and twisting motion over top and bottom plates, the characteristic velocity of the LSC could be used to calculate the Reynolds number Re = UL/ν. II, we started with the experimental setup and analysis method that were used IV, the general conclusions of this paper will be presented
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