Coherent structures in turbulent convection: observations, theory and experiments

Coherent semi-organized structures are observed in the atmospheric convective boundary layers (CBLs) and in laboratory experiments and they do not exhibit real similarity with turbulence. Their lifetimes are much larger than the largest time-scales of turbulence. In the atmospheric shear-free convection these structures represent large, three-dimensional, long-lived Benard-type cells (cloud cells) composed of narrow uprising plumes and wide downdraughts. They usually embrace the entire CBL (of the order of 1-3 km in height) and include pronounced convergence flow patterns close to the surface. In the sheared convective flows, the structures represent CBL-scale rolls stretched along the mean wind (cloud streets). A new mean-field theory of turbulent convection is developed based on the idea that only the small-scale part of spectra is considered as turbulence, whereas its large-scale part, including both regular and semi-organized motions, is treated as a kind of a mean flow. In the shear-free regime, this theory predicts the convective wind instability, which causes formation of large-scale semi-organized motions in the form of cells (similar to cloud cells in the atmosphere). In the presence of wind shear, the theory predicts another type of instability, which causes generation of convective-shear waves with nonzero hydrodynamic helicity and formation of large-scale semi-organized structures in the form of rolls (similar to cloud streets). The spatial characteristics of the structures, such as the minimum size of the growing perturbations and the size of perturbations with the maximum growth rate, are determined. Predictions of this theory are in good qualitative agreement with modern knowledge about large eddies in atmospheric convective boundary layers. We also studied experimentally formation of coherent structures in turbulent convection in air flow. We investigated turbulent convection in a box equipped with two heat exchangers mounted at the top and bottom walls of the chamber. We used Particle Image Velocimetry to determine the turbulent and mean velocity fields, and a specially designed temperature probe with twelve sensitive thermocouples to measure the temperature field and the local convective heat flux. The hysteresis phenomenon in turbulent convection was found by varying the temperature difference between the bottom and the top walls of the chamber. The hysteresis loop comprises the one-cell and two-cells flow patterns while the aspect ratio is kept constant. The developed theory of coherent structures in turbulent convection is in agreement with the experimental observations. The observed coherent structures are superimposed on a small-scale turbulent convection. The redistribution of the turbulent heat flux plays a crucial role in the formation of coherent large-scale circulations in turbulent convection.