Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Oral

M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS15] Geophysical fluid dynamics-Transfield approach to geoscience

Sun. May 20, 2018 1:45 PM - 3:15 PM 106 (1F International Conference Hall, Makuhari Messe)

convener:Keita Iga(Atmosphere and Ocean Research Institute, The University of Tokyo), Shigeo Yoshida(Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University), Takatoshi Yanagisawa(海洋研究開発機構 地球深部ダイナミクス研究分野, 共同), Hidenori AIKI(Nagoya University), Chairperson:Iga Keita(Atmosphere and Ocean Research Institute, The University of Tokyo)

2:45 PM - 3:00 PM

[MIS15-05] Horizontal scale of patterns in liquid metal convection

*Takatoshi Yanagisawa1, Megumi Akashi2, Yuji Tasaka2, Yuichi Murai2, Tobias Vogt3, Sven Eckert3 (1.Department of Deep Earth Structure and Dynamics Research, Japan Agency for Marine-Earth Science and Technology, 2.Faculty of Engineering, Hokkaido University, 3.Helmholtz Zentrum Dresden-Rossendorf)

Keywords:low Prandtl number fluid, convection pattern, thermal turbulence

Convection occurring in a liquid metal is an essential process for understanding dynamics of the Earth and planetary cores. The characteristics of flow pattern in liquid metal convection are, however, less understood as flows in ordinary fluids because of the opaqueness of it. Recently, ultrasound techniques are widely applicable for the study of flow in opaque fluids. We performed laboratory experiments on Rayleigh-Benard convection with a liquid metal in a square geometry having an aspect ratio five. Horizontal velocity profiles of convective flow were measured at several lines by using ultrasonic velocity profiling. By combining the information from profiles, we can reconstruct organized flow structures with turbulent fluctuations. Systematic variation of the structure was detected with increasing the Rayleigh number (Ra) up to 10^5, in which a quasi-two-dimensional roll changes to a cell having a relatively larger horizontal scale. In addition, we found that the organized large-scale structure, whether it is roll or cell, show quasi-periodic oscillation whose representative period is approximately same as the circulation time of the large-scale flow. We also performed numerical simulations of convection with the same geometry as our experiments by setting a small Prandtl number (Pr = 0.025) like a liquid metal. Quantitative comparison on the velocity profiles between experiments and simulations provided quite satisfactory agreement, and we analyzed the whole structure of the flow and the style of oscillation in detail based on the result of simulation. By integrating results from experiments and simulations, we propose a scaling low on the Ra dependence of horizontal size of large-scale flow structure, and estimate an enlarged value of effective momentum diffusivity by turbulence in a liquid metal convection.