Dynamics of reorientations and reversals of large scale flow in Rayleigh-Benard convection

TL;DR

This study numerically investigates the mechanisms and characteristics of large-scale flow reversals and reorientations in Rayleigh-Bénard convection, highlighting the roles of azimuthal reorientation, Fourier mode dynamics, and flow cessation.

Contribution

It provides detailed numerical analysis of flow reorientations in Rayleigh-Bénard convection, including the distinction between complete and partial reversals and the identification of rotation-led and cessation-led reorientations.

Findings

Reversals are caused by azimuthal reorientation of the large-scale circulation.

Complete reversals involve a 180° phase change in the Fourier mode.

Cessation-led reorientations show increased second Fourier mode amplitude.

Abstract

We present a numerical study of the reversals and reorientations of the large scale circulation (LSC) of convective fluid in a cylindrical container of aspect ratio one. We take Prandtl number to be 0.7 and Rayleigh numbers in the range from $6\times10^5$ to $3\times10^7$. It is observed that the reversals of the LSC are induced by its reorientation along the azimuthal direction, which are quantified using the phases of the first Fourier mode of the vertical velocity measured near the lateral surface in the mid plane. During a ``complete reversal'', the above phase changes by around $180^0$ leading to reversals of the vertical velocity at all the probes. On the contrary, the vertical velocity reverses only at some of the probes during a ``partial reversal'' with phase change other than $180^0$. Numerically we observe rotation-led and cessation-led reorientations, in agreement with earlier experimental results. The ratio of the amplitude of the second Fourier mode and the first Fourier mode rises sharply during the cessation-led reorientations. This observation is consistent with the quadrupolar dominant temperature profile observed during the cessations. We also observe reorientations involving double cessation.