Transition phenomena in unstably stratified turbulent flows

TL;DR

This study investigates how external forcing affects the transition from Rayleigh-Benard convection with large-scale circulation to a regime of unstably stratified turbulence without LSC, revealing consistent temperature behavior and long-term oscillations.

Contribution

It provides experimental and theoretical insights into the transition phenomena and temperature behavior in unstably stratified turbulent flows under external forcing.

Findings

Destruction of LSC occurs at grid oscillation frequencies above 2 Hz.

The ratio of temperature gradient contributions remains nearly constant across regimes.

Long-term nonlinear temperature oscillations are observed regardless of forcing frequency.

Abstract

We study experimentally and theoretically transition phenomena caused by the external forcing from Rayleigh-Benard convection with the large-scale circulation (LSC) to the limiting regime of unstably stratified turbulent flow without LSC whereby the temperature field behaves like a passive scalar. In the experiments we use the Rayleigh-B\'enard apparatus with an additional source of turbulence produced by two oscillating grids located nearby the side walls of the chamber. When the frequency of the grid oscillations is larger than 2 Hz, the large-scale circulation (LSC) in turbulent convection is destroyed, and the destruction of the LSC is accompanied by a strong change of the mean temperature distribution. However, in all regimes of the unstably stratified turbulent flow the ratio $\big[(\ell_x \nabla_x T)^2 + (\ell_y \nabla_y T)^2 + (\ell_z \nabla_z T)^2\big] / < \theta^2 >$ varies slightly (even in the range of parameters whereby the behaviour of the temperature field is different from that of the passive scalar). Here $\ell_i$ are the integral scales of turbulence along x, y, z directions, T and \theta are the mean and fluctuating parts of the fluid temperature. At all frequencies of the grid oscillations we have detected the long-term nonlinear oscillations of the mean temperature. The theoretical predictions based on the budget equations for turbulent kinetic energy, turbulent temperature fluctuations and turbulent heat flux, are in agreement with the experimental results.