Large-scale flow and spiral core instability in Rayleigh-Bénard convection

The spiral core instability, observed in large aspect-ratio Rayleigh-Bénard convection, is studied numerically in the framework of the Swift-Hohenberg equation coupled to a large-scale flow. It is shown that the instability leads to nontrivial core dynamics and is driven by the self-generated vorticity. Moreover, the recently reported transition from spirals to hexagons near the core is shown to occur only in the presence of a nonvariational nonlinearity, and is linked to the spiral core instability. Qualitative agreement between the simulations and the experiments is demonstrated.