Multi-scale analysis of turbulent Rayleigh-Bénard convection

We report the results from a direct numerical simulation of turbulent Rayleigh-Bénard convection for Rayleigh number of 105 and Prandtl number of 0.7. The flow topology is characterized by the presence of coherent structures, the so-called thermal plumes, consisting of localized portions of fluid having a temperature contrast with the background. Two distinct events are identified close to the walls by using the wall-parallel divergence divπ of the velocity field: the impingement (divπ > 0) and the ejection of thermal plumes (divπ < 0). The impingement leads to the formation of larger velocity and temperature structures in the wall-parallel planes. Contrary to the classical picture of turbulence consisting of a direct transfer of energy from large toward smaller turbulent fluctuations, the impingement is conjectured to be probably responsible for a reverse transfer from small towards large scales in the near-wall region.