Effect of large-scale coherent structures on turbulent convection

TL;DR

This study investigates how large-scale coherent structures influence the global properties of turbulent convection, revealing shear-driven turbulence and confirming theoretical scalings in laboratory experiments with air flow.

Contribution

It provides new insights into the role of large-scale structures and shear in turbulent convection, supported by experimental data and theoretical agreement.

Findings

Turbulence is mainly shear-driven rather than buoyancy-driven.

Global parameters scale with temperature difference as predicted.

Inhomogeneity of turbulence with structures is small.

Abstract

We study an effect of large-scale coherent structures on global properties of turbulent convection in laboratory experiments in air flow in a rectangular chamber with aspect ratios $A \approx 2$ and $A\approx 4$ (with the Rayleigh numbers varying in the range from $5 \times 10^6$ to $10^8$). The large-scale coherent structures comprise the one-cell and two-cell flow patterns. We found that a main contribution to the turbulence kinetic energy production in turbulent convection with large-scale coherent structures is due to the non-uniform large-scale motions. Turbulence in large Rayleigh number convection with coherent structures is produced by shear, rather than by buoyancy. We determined the scalings of global parameters (e.g., the production and dissipation of turbulent kinetic energy, the turbulent velocity and integral turbulent scale, the large-scale shear, etc.) of turbulent convection versus the temperature difference between the bottom and the top walls of the chamber. These scalings are in an agreement with our theoretical predictions. We demonstrated that the degree of inhomogeneity of the turbulent convection with large-scale coherent structures is small.