Unravelling the large-scale circulation modes in turbulent Rayleigh-Bénard convection^(a)

¹ Centre for Fluid and Complex Systems, Coventry University - Coventry CV1 5FB, UK
² Department of Mathematics, Imperial College London - South Kensington Campus, London SW7 2AZ, UK
³ Physical Science and Engineering Division, King Abdullah University of Science and Technology Thuwal 23955, Saudi Arabia
⁴ Department of Earth, Planetary, and Space Sciences, University of California - Los Angeles, CA 90095, USA

^(a) susanne.horn@coventry.ac.uk (corresponding author)

Received: 1 August 2021
Accepted: 26 November 2021

Abstract

The large-scale circulation (LSC) is the most fundamental turbulent coherent flow structure in Rayleigh-Bénard convection. Further, LSCs provide the foundation upon which superstructures, the largest observable features in convective systems, are formed. In confined cylindrical geometries with diameter-to-height aspect ratios of , LSC dynamics are known to be governed by a quasi-two-dimensional, coupled horizontal sloshing and torsional (ST) oscillatory mode. In contrast, in cylinders, a three-dimensional jump rope vortex (JRV) motion dominates the LSC dynamics. Here, we use dynamic mode decomposition (DMD) on direct numerical simulation data of liquid metal to show that both types of modes co-exist in and cylinders but with opposite dynamical importance. Furthermore, with this analysis, we demonstrate that ST oscillations originate from a tilted elliptical mean flow superposed with a symmetric higher-order mode, which is connected to the four rolls in the plane perpendicular to the LSC in tanks.