Transient flows and reorientations of large-scale convection in a cubic cell

TL;DR

This study uses large-eddy simulations to analyze the transient behavior and reorientations of large-scale convection in a cubic cell, revealing the role of azimuthal flows and proposing a new reorientation mechanism.

Contribution

It introduces a novel scenario for LSC reorientations involving the cessation of orthogonal Q2D rolls, without requiring mean azimuthal flow.

Findings

LSC is accompanied by large-scale azimuthal flows with non-zero angular momentum.

Solid-body rotation of the entire fluid is not observed.

Reorientations occur via cessation of one of the orthogonal Q2D rolls.

Abstract

The transient processes of a turbulent large-scale convective circulation (LSC) in a cubic cell are investigated using large-eddy simulations for Rayleigh number $\Ray=10^8$ and Prandtl number $\Pran=0.7$. For the first time, we have explicitly shown that LSC is accompanied by large-scale azimuthal flows with non-zero total angular momentum. It is also shown that solid-body rotation of the entire fluid is not realized. It is found that correlation between rotation of LSC plane and the mean azimuthal motion is high during quasiperiodic oscillations of LSC near the diagonal plane and relatively weak during LSC reorientations. We propose a new plausible scenario for the reorientations of the LSC in a cube that does not involve a mean azimuthal flow. Instead of a single-roll, we introduce the superposition of a pair of large-scale orthogonal quasi-two-dimensional (Q2D) rolls and the reorientation of the LSC occurs as a result of the cessation of one of the Q2D rolls. This scenario is consistent with all known experimental and numerical data.