Large-scale instabilities in a nonrotating turbulent convection

Formation of large-scale coherent structures in a turbulent convection viaexcitation of large-scale instability is studied. The redistribution of theturbulent heat flux due to non-uniform large-scale motions plays a crucial rolein the formation of the coherent large-scale structures in the turbulentconvection. The modification of the turbulent heat flux results in strongreduction of the critical Rayleigh number (based on the eddy viscosity andturbulent temperature diffusivity) required for the excitation of thelarge-scale instability. The mean-field equations which describe thelarge-scale instability, are solved numerically. We determine the keyparameters that affect formation of the large-scale coherent structures in theturbulent convection. In particular, the degree of thermal anisotropy and thelateral background heat flux strongly modify the growth rates of thelarge-scale instability, the frequencies of the generated convective-shearwaves and change the thresholds required for the excitation of the large-scaleinstability. This study elucidates the origins of the large-scale circulationsand rolls in the atmospheric convective boundary layers and the meso-granularstructures in the solar convection.Comment: 13 pages, 13 figures, Physics of Fluids, in pres