[M-IS21] Geophysical fluid dynamics-Transfield approach to geoscience

[MIS21-P03] Emergence of quasi-periodic behavior of thermal convection in square vessels

*Takatoshi Yanagisawa¹, Megumi Akashi², Yuji Tasaka³, Ataru Sakuraba⁴ (1.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology, 2.Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, 3.Faculty of Engineering, Hokkaido University, 4.School of Science, The University of Tokyo)

Keywords:convection pattern, oscillation, Prandtl number, effect of walls

Organized flow patterns in turbulence are widely observed in nature. As one of the most fundamental configurations, turbulent flow in Rayleigh-Benard convection has been provided many keys to understand mechanisms for generating and sustaining them. In laboratory experiments on turbulent thermal convection, flow patterns are often organized as a single large-scale circulation, whose horizontal scale extends to that of vessels. When vessels are sufficiently wide, plural circulations are observed. There should exist a transition point from single to plural circulations in vessels having medium horizontal scale. We studied patterns of thermal convection in a liquid metal whose Prandtl number (Pr) is 0.03 in a square vessel with aspect ratio (horizontal scale/layer thickness) five, and found the transition point in increasing the Rayleigh number (Ra) (Akashi et al., 2019, Phys. Rev. Fluids). The style of circulation at higher Ra is marked by quasi-periodic oscillations of single cellular structure. We succeeded in simulating the behavior by high resolution numerical simulations (Akashi et al., 2022, J. Fluid Mech.). On the other hand, we do not observe such oscillations in the same geometry for water convection whose Pr~7. One reason of the difference is the role of Pr, that is, the flow can be turbulent for smaller Pr for the same Ra. In the present study, we use the Reynolds number (Re) calculated by the typical flow velocity of convection as an index of turbulence, and compare the style of large-scale circulations in various Pr (0.001-100) in vessels having medium horizontal scales. Most of results are obtained by numerical simulations because available materials with these Pr are limited. We found that the range of emergence for quasi-periodic behavior of cells (what we observed in a liquid metal), are limited for Pr < 0.1. At higher Pr, continuously changing irregular patterns are observed. This suggests that the oscillation at lower Pr is closely related to the oscillatory instability of the Busse balloon.