Flow reversals in thermally driven turbulence

TL;DR

This study investigates the mechanisms and conditions leading to flow reversals in thermally driven turbulence, specifically Rayleigh-Bénard convection, using experiments and simulations to identify key flow structures and parameter dependencies.

Contribution

It reveals the role of corner flow rolls in flow reversals and maps the parameter space where reversals occur, providing new insights into the reversal mechanism.

Findings

Corner flow rolls grow and take over the main flow during reversals.

Reversal occurrence depends sensitively on Rayleigh and Prandtl numbers.

A typical time scale for reversals is identified.

Abstract

We analyze the reversals of the large scale flow in Rayleigh-B\'enard convection both through particle image velocimetry flow visualization and direct numerical simulations (DNS) of the underlying Boussinesq equations in a (quasi) two-dimensional, rectangular geometry of aspect ratio 1. For medium Prandtl number there is a diagonal large scale convection roll and two smaller secondary rolls in the two remaining corners diagonally opposing each other. These corner flow rolls play a crucial role for the large scale wind reversal: They grow in kinetic energy and thus also in size thanks to plume detachments from the boundary layers up to the time that they take over the main, large scale diagonal flow, thus leading to reversal. Based on this mechanism we identify a typical time scale for the reversals. We map out the Rayleigh number vs Prandtl number phase space and find that the occurrence of reversals very sensitively depends on these parameters.