Transition phenomena in unstably stratified turbulent flows

Abstract

We study experimentally and theoretically the transition phenomena caused by external forcing from Rayleigh-Bénard convection with large-scale circulation (LSC) to the limiting regime of unstably stratified turbulent flow without LSC, where the temperature field behaves like a passive scalar. In the experiments we use the Rayleigh-Bénard apparatus with an additional source of turbulence produced by two oscillating grids located near the sidewalls of the chamber. When the frequency of the grid oscillations is larger than 2 Hz, the 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 [(ℓ{x}∇{x}T)²+(ℓ{y}∇{y}T)² + (ℓ{z}∇{z}T)²]/<θ²> varies slightly (even in the range of parameters where the behavior of the temperature field is different from that of the passive scalar). Here ℓ{i} are the integral scales of turbulence along the x,y,z directions, and T and θ are the mean and fluctuating parts of the fluid temperature. At all frequencies of the grid oscillations we have detected 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.

Turbulent thermal diffusion in strongly stratified turbulence: Theory and experiments

“…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 temperature fluctuations in forced stably stratified turbulent flows

“…The details of this experimental setup and measurements are given in Ref. 33 , in which the Rayleigh-Bénard apparatus with an additional source of turbulence produced by the two oscillating grids located nearby the side walls of the chamber was used. Additional forcing for turbulence allows to observe evolution of the mean temperature and velocity fields during the transition from turbulent convection with the large-scale circulations (LSC) for very small frequencies of the grid oscillations, to the limiting regime of unstably stratified flow without LSC for very high frequencies of the grid oscillations.…”

“…In the latter case of the unstably stratified flow without LSC the temperature field behaves like a passive scalar. 33 In our experiments with the stably stratified turbulence, we determined the dependence of the r.m.s. of the temperature fluctuations θ 2 versus the frequency f of the grid oscillations (see Fig.…”

Tracer Dispersion in the Turbulent Convective Layer

“…is of significant importance for a number of fields of science and technology covering many physical applications including: environmental pollutant dispersion, heat transfer, combustion, reacting flows, meteorology and oceanology. Due to the buoyancy effect and the associated anisotropy, convective turbulence produces much more complex phenomenology than the classical Kolmogorov model of isotropic turbulence and this imposes new challenges for its analytical treatment [1][2][3]. In spite of this complexity there has been remarkable progress in understanding this phenomena based on the application of modern methods of theoretical physics (see [4][5][6] for comprehensive reviews).…”

“…In spite of this complexity there has been remarkable progress in understanding this phenomena based on the application of modern methods of theoretical physics (see [4][5][6] for comprehensive reviews). Examples include the derivation of Kolmogorov and Bolgiano spectra in thermal convection, scaling of statistical moments of concentration, multi-fractal structure of tracer statistics and some others (see [2,[7][8][9][10] and references therein).…”