Philipp P. Vieweg, Janet D. Scheel, and Jörg Schumacher
Turbulent flows are highly chaotic and characterized by a cascade of irregular vortices, however our daily experience shows that such flows are often organized into prominent large-scale and long-living patterns. One prominent example is turbulent convection in the outer shell of the Sun which carries heat and momentum to the surface. Solar convection manifests at the surface in the form of granules – the optically visible convection cells – which cover the approximately 30 times larger long-living supergranules which can be detected seismically, but have not been reproduced consistently in numerical models.
Our present three-dimensional numerical investigation demonstrates that such a supergranule aggregation can be observed in a much simpler turbulent convection flow than solar surface convection – the Rayleigh-Bénard convection case – once a constant heat flux is maintained at the top and bottom of the fluid layer instead of the more commonly-used constant temperature boundary conditions. We show that this requires very long-term simulations and reveal the basic instability mechanisms that drive the fully developed turbulent system from an aggregation of granules to a supergranule. In this instability, a flow structure at a given scale is found to give rise to successively larger scales until the horizontal extension has been reached.
Our studies suggest that investigations of simplified configurations, in which the multi-physical processes of solar (and stellar) convection are disentangled, lead to a deeper understanding of structure formation. They can then serve as a starting point for a stepwise increase to physically more complex models that are able to reproduce the observational results.