Experimental study of temperature fluctuations in forced stably stratified turbulent flows

TL;DR

This experimental study investigates temperature fluctuations in forced stably stratified turbulence, revealing the behavior of temperature as a passive or active scalar depending on oscillation frequency, and confirming theoretical predictions with experimental data.

Contribution

The paper provides the first detailed experimental analysis of temperature fluctuations in stably stratified turbulence, comparing passive and active scalar behavior and validating theoretical models.

Findings

The ratio involving temperature gradients and fluctuations is nearly constant and independent of oscillation frequency.

At high oscillation frequencies, temperature acts as a passive scalar.

At low frequencies, temperature behaves as an active scalar, influencing turbulence dynamics.

Abstract

We study experimentally temperature fluctuations in stably stratified forced turbulence in air flow. In the experiments with an imposed vertical temperature gradient, the turbulence is produced by two oscillating grids located nearby the side walls of the chamber. Particle Image Velocimetry is used to determine the turbulent and mean velocity fields, and a specially designed temperature probe with sensitive thermocouples is employed to measure the temperature field. We found that the ratio $\big[(\ell_x \nabla_x \bar{T})^2 + (\ell_y \nabla_y \bar{T})^2 + (\ell_z \nabla_z \bar{T})^2\big] / <\theta^2 >$ is nearly constant, is independent of the frequency of the grid oscillations and has the same magnitude for both, stably and unstably stratified turbulent flows, where $\ell_i$ are the integral scales of turbulence along $x, y, z$ directions, $\bar{T}$ and $\theta$ are the mean and turbulent fluctuations components of the fluid temperature. We demonstrated that for large frequencies of the grid oscillations, the temperature field can be considered as a passive scalar, while for smaller frequencies the temperature field behaves as an active field. The theoretical predictions based on the budget equations for turbulent kinetic energy, turbulent potential energy ($\propto <\theta^2 >)$ and turbulent heat flux, are in a good agreement with the experimental results. Detailed comparison with experimental results obtained previously in unstably stratified forced turbulence is performed.