Transition from convection rolls to large-scale cellular structures in turbulent Rayleigh-Bénard convection in a liquid metal layer

Megumi Akashi, Takatoshi Yanagisawa, Yuji Tasaka, Tobias Vogt, Yuichi Murai, and Sven Eckert
Phys. Rev. Fluids 4, 033501 – Published 8 March 2019

Abstract

Turbulent Rayleigh-Bénard convection was investigated within a liquid metal layer, Prandtl number Pr=0.03, in a square vessel having a moderate aspect ratio, Γ=5. Laboratory experiments were performed at moderate Rayleigh numbers, 7.9×103<Ra<3.5×105. Ultrasonic velocity profiling was used to visualize the spatiotemporal flow structure in two horizontal planes, while temperature fluctuations were monitored simultaneously in the fluid layer. By using multiple ultrasonic sensors, a grid of orthogonal measurement lines was created. This configuration enabled the identification of coherent flow structures showing periodic oscillations. In particular, oscillatory roll-like structures were observed for Ra6×104, while the transition to a new-found, fully three-dimensional cellular structure occurs around Ra=7×104. The Fourier analysis of the temperature fluctuations indicates that the convection reaches the developed state of thermal turbulence at this Ra number. This cellular structure of the flow field is recognized as a representation of the large-scale circulation in thermal turbulence for the specific situation of confined convection in the rectangular vessel. The transition from laminar convection to thermal turbulence manifests itself in the occurrence of unstable intermediate regimes accompanied by a stepwise increment in the horizontal scale. We suggest scaling laws for the characteristic velocity and the dominating oscillation frequency and based on that for the horizontal length scale as a function of the Ra number. The comparison to corresponding values of characteristic length scales published for thermal convection in air in larger aspect ratios [Pr=0.7, T. Hartlep et al., Phys. Rev. Lett. 91, 064501 (2003), A. Pandey et al., Nat. Commun. 9, 2118 (2018), and D. E. Fitzjarrald, J. Fluid Mech. 73, 693 (1976)] reveals a different Ra number dependence of the horizontal wave number.

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  • Received 3 August 2018

DOI:https://doi.org/10.1103/PhysRevFluids.4.033501

©2019 American Physical Society

Physics Subject Headings (PhySH)

Fluid Dynamics

Authors & Affiliations

Megumi Akashi1,2, Takatoshi Yanagisawa3,1, Yuji Tasaka1,*, Tobias Vogt2, Yuichi Murai1, and Sven Eckert2

  • 1Laboratory for Flow Control, Hokkaido University, Sapporo 060-8638, Japan
  • 2Helmholtz Zentrum Dresden-Rossendorf (HZDR), Dresden 01328, Germany
  • 3Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan

  • *tasaka@eng.hokudai.ac.jp

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Vol. 4, Iss. 3 — March 2019

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