[M-IS24] Geophysical fluid dynamics-Transfield approach to geoscience

[MIS24-02] Quasi-periodic oscillations of convection patterns and heat transport in horizontally finite geometries

*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.Department of Earth and Planetary Science, University of Tokyo)

Keywords:convection pattern, quasi-periodic oscillation, heat transport

Pattern of Rayleigh-Bernad convection at low Prandtl numbers has been a hot topic in recent years. In laboratory experiments, patterns existing in turbulent convection of liquid metals are observed and quantified by ultrasonic velocity measurements. In numerical simulations, calculations of convection in horizontally wide geometries are realized and so-called superstructures are identified in turbulence. We reported transitions from roll-like to cell-like structure in turbulent convection of a liquid metal experiment with the increase of the Rayleigh number (Ra) for a vessel having aspect ratio five (Akashi et al., 2019, Phys. Rev. Fluids). Both roll- and cell-like structures were marked by quasi-periodic oscillations whose periods are comparable to the turn-over time of the flow.
Here we studied the dependency of the features on aspect ratios of the square geometry confined by no-slip side walls by numerical simulations. The aspect ratio of the geometry (=A) is defined by the horizontal length of square vessel to its layer thickness, and we examined A from 0.5 to 20. The value of Prandtl number was fixed at 0.025 and Ra numbers were set around 10^5, where features of turbulence are clearly recognized. There exist organized flow structures in turbulence. When A < 4, the patterns are roll-like showing strong directionality. When 4 < A < 6, the patterns are cell-like with similar flow velocities for two horizontal directions. In both cases the patters are consist of a single or a pair of circulation and show quasi-periodic oscillations. For geometries with larger A, the patterns are composed of multiple cells and rolls; quasi-periodic oscillations are not observed any more. The degree of heat transport and its fluctuation in time are closely related to the convection pattern. We quantified them by using time averaged value of the Nusselt numbers (Nu) and their standard deviations. The dependence of Nu on A is not monotonic but complicated reflecting the transition of patterns. We found that Nu shows maximum where pattern changes from roll to cell, and that the fluctuation of Nu is very small at that point.