The spatial structure of advective transport in two-dimensional homogeneous Rayleigh–Bénard (HRB) convection is investigated by means of direct numerical simulations. The convective driving leads to the emergence of thermal plumes. These create dynamically changing pathways characterised by localised mean flows. The resulting large-scale anisotropy of the system diminishes with increasing nominal Rayleigh number ( Ra ). The key components of advective transport are extracted via a network-based analysis of Lagrangian trajectories. This reveals a coherent structure based on plume-related pathways that governs the transport of heat and matter. A reduced description of the structure is given by the zero isoline of scale-filtered vorticity. While its essential large-scale characteristics display only a weak dependency on Ra , geometric analysis shows that the decrease of large-scale anisotropy with increasing Ra is due to a reduction of the length of vertical transport paths. Mean profiles with respect to the transport paths suggest that this reduction is caused by an enhanced turbulent transfer of temperature fluctuations into adjacent shear layers and vortices. This process leads to a spatial decorrelation of temperature and velocity and, consequently, to a reduced structural impact of the thermal driving on the flow. Spatially resolved nonlinear fluxes indicate that shear layers and vortices next to the transport paths are associated with a spectrally inverse flux of enstrophy. The observed structure of advective transport in HRB convection also displays asymptotically scale-invariant characteristics, contrasting the structural properties of wall-bounded classical Rayleigh–Bénard convection.
Moczarski et al. (Mon,) studied this question.