Robust wall modes and their interplay with bulk turbulence in confined rotating Rayleigh-B\'enard convection

TL;DR

This study investigates the dynamics of wall modes and their interaction with bulk turbulence in confined rotating Rayleigh-Bénard convection through direct numerical simulations, revealing their structure, evolution, and influence on flow patterns.

Contribution

It establishes a relation between wall mode drift frequency and onset frequency, and uncovers how wall-bulk interactions generate large-scale vortices and influence turbulence.

Findings

Wall modes precess anti-cyclonically along side walls.

A relation links wall mode drift frequency with onset frequency.

Radial jets from wall modes induce large-scale vortices.

Abstract

In confined rotating convection, a strong zonal flow can develop close to the side wall with a modal structure that precesses anti-cyclonically (counter to the applied rotation) along the side wall. It is surmised that this is a robust non-linear evolution of the wall modes observed before the onset of bulk convection. Here, we perform direct numerical simulations of cylindrically confined rotating convection at high rotation rates and strong turbulent forcing. Through comparison with earlier work, we find a fit-parameter-free relation that links the angular drift frequency of the robust wall mode observed far into the turbulent regime with the critical wall mode frequency at onset, firmly substantiating the connection between the observed boundary zonal flow and the wall modes. Deviations from this relation at stronger turbulent forcing suggest early signs of the bulk turbulence starting to hamper the development of the wall mode. Furthermore, by studying the interactive flow between the robust wall mode and the bulk turbulence, we identify radial jets penetrating from the wall mode into the bulk. These jets induce a large scale multipolar vortex structure in the bulk turbulence, dependent on the wavenumber of the wall mode. In a narrow cylinder the entire bulk flow is dominated by a quadrupolar vortex driven by the radial jets, while in a wider cylinder the jets are found to have a finite penetration length and the vortices do not cover the entire bulk. We also identify the role of Reynolds stresses in the generation of zonal flows in the region near the sidewall.