Experimental study of temperature fluctuations in forced stably stratified turbulent flows

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 (ℓx∇xT ) 2 + (ℓy∇yT ) 2 + (ℓz∇zT ) 2 / θ 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 ℓi are the integral scales of turbulence along x, y, z directions, T and θ 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 (∝ θ 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.

Turbulent thermal diffusion in strongly stratified turbulence: Theory and experiments

“…To validate the theory of turbulent thermal diffusion in strongly stratified turbulent flows we perform laboratory experiments in different set-ups: in the oscillating grid turbulence (see, e.g., [27][28][29][39][40][41]) and in the multifan produced turbulence (see, e.g., [30]). We also validate the theory of turbulent thermal diffusion against data of meteorological observations (see, e.g., [32]).…”

“…In this section we describe very briefly the experimental set-up and measurement facilities in the oscillating grid turbulence. The details of the experimental set-up and measurements in the oscillating grid turbulence can be found in [29,[39][40][41]. The experiments in stratified turbulence have been conducted in rectangular chamber with dimensions 26 × 58 × 26 cm 3 in air flow.…”

Experimental study of turbulent transport of nanoparticles in convective turbulence

“…Similar measurement techniques and data processing procedure have been used by us previously in the experimental study of different aspects of turbulent convection and stably stratified turbulence [52][53][54][55], investigations of turbulent thermal diffusion [22][23][24][25] of micron-size particles in the oscillating grid turbulence [28,[30][31][32] and in turbulence produced by the multi-fan generator [33], and investigations of small-scale particle clustering [56][57][58] in the oscillating grid turbulence [59].…”

Experimental study of turbulent thermal diffusion of particles in inhomogeneous and anisotropic turbulence

“…The temperature effect on these characteristics is negligibly small. Similar experimental set-up and data processing procedure have been previously used by us in the experimental study of different aspects of turbulent convection [37,57,58], stably stratified turbulence [59], phenomenon of turbulent thermal diffusion in homogeneous turbulence [33][34][35][36][37] and small-scale particle clustering [19].…”