Abstract

The interplay of gravity with mass density inhomogeneities introduces natural (thermal) convection and represents the essential mechanism by which heat is transported in natural flows. Simultaneously, natural flows are often far more extended in the horizontal direction than in the vertical one. Motivated by these two observations and the various geo- and astrophysical applications (e.g. the solar convection zone), 3-dimensional Rayleigh-Bénard convection as the paradigm of thermal convection has been studied. This talk will cover some recent results from studying the impact of thermal (and mechanical) boundary conditions on large-scale flow structures in Rayleigh-Bénard convection by means of direct numerical simulations. It will be shown that thermal boundary conditions are crucial to the formation of long-living large-scale (turbulent) flow structures. In particular, a slow transient aggregation process — that only stops once the horizontal extent of the domain is reached — can be found once the fluid layer is subjected to Neumann-type constant heat flux boundary conditions. As a result, the temperature field in the domain is separated into one extended hot and another extended cold region. We trace this mechanism of self-organisation of flow structures back to secondary instabilities as well as an inverse cascade in spectral space. The talk will finish with a brief overview of our work on the identification of those large-scale flow structures by the use of unsupervised machine learning based on Lagrangian particle data.