Supergranule aggregation for constant heat flux-driven turbulent convection

Abstract

Supergranule aggregation, i.e., the gradual aggregation of convection cells to horizontally extended networks of flow structures, is a unique feature of constant heat flux-driven turbulent convection. In the present study, we address the question if this mechanism of self-organisation of the flow is present for any fluid. Therefore, we analyse three-dimensional Rayleigh-Bénard convection at a fixed Rayleigh number Ra ≈ 2.0 × 10 5 across 4 orders of Prandtl numbers Pr ∈ 10 −2 , 10 2 by means of direct numerical simulations in horizontally extended periodic domains with aspect ratio Γ = 60. Our study confirms the omnipresence of the mechanism of supergranule aggregation for the entire range of investigated fluids. Moreover, we analyse the effect of Pr on the global heat and momentum transport, and clarify the role of a potential stable stratification in the bulk of the fluid layer. The ubiquity of the investigated mechanism of flow self-organisation underlines its relevance for pattern formation in geophysical and astrophysical convection flows, the latter of which are often driven by prescribed heat fluxes.

Rotation suppresses giant-scale solar convection

“…mechanisms (6,37,(40)(41)(42). To date, no model self-consistently demonstrates how the supergranular scale might arise.…”

“…Against expectations, however, these motions appear weak compared with the smaller-scale supergranular and granular flows. A great deal of theory and simulation work has attempted to solve the supergranulation problem, including direct formation mechanisms ( 6 , 37 , 40 – 42 ). To date, no model self-consistently demonstrates how the supergranular scale might arise.…”

“… Why and exactly how do observations seem to contradict numerical models? These questions all essentially ask the same thing: “Where is the large-scale convective power?” Some authors have recently studied possible direct formation mechanisms for supergranulation ( 6 , 37 , 40 – 42 ). In particular, recent work by Schumacher and coworkers ( 6 , 37 , 42 ) finds a continual amalgamation of convection on the largest scales admitted by the computational domain.…”

“…These questions all essentially ask the same thing: “Where is the large-scale convective power?” Some authors have recently studied possible direct formation mechanisms for supergranulation ( 6 , 37 , 40 – 42 ). In particular, recent work by Schumacher and coworkers ( 6 , 37 , 42 ) finds a continual amalgamation of convection on the largest scales admitted by the computational domain. Featherstone and Hindman ( 5 ) pointed out that rotational effects provide a natural explanation for the last two of these questions.…”

Spatially distributed chaos and turbulence in clouds

“…(1)(2)(3), reported in Ref. [5], power spectra were computed for the vertical component of the velocity field.…”