Large-scale thermal motions of turbulent Rayleigh–Bénard convection in a wide aspect-ratio cylindrical domain

The large-scale structures that occur in turbulent Rayleigh-Bénard convection in a wide-aspect-ratio cylindrical domain are studied by means of direct numerical simulation. The simulation is performed in a 6.3 aspect-ratio cylindrical cell with a Rayleigh number of 9.6 × 10⁷ and Prandtl number equal to 6.7. Single-point and double-point statistics compare well against experimental results under nearly identical conditions. Large-scale thermal motions with coherence times exceeding 20 eddy-turnovers (∼600 free-fall time units) are seen in the instantaneous fields. Temporally filtering them by integrating over approximately one eddy-turnover time scale reveals a clear pattern consisting of seven discrete thermal structures: three warm, rising sectors, three cool, falling sectors and a single plume of warm, rising fluid that wanders around the center of the cylindrical cell. Smoothing over still longer times (10 and 20 eddy turn-over time scales) yields a clear hub-and-spoke pattern of warm and cool sectors in a dominantly 120° periodic pattern separated by concentrations of radial vortex lines (the spokes) plus a nearly circular plume at the center of the test section (the hub). The similarity of the patterns in the instantaneous fields and the long-time smoothed fields demonstrates long persistence of these structures, a defining characteristic of coherent structures in turbulence. The warm and cool sectors are intimately linked with conical roll-cells rotating about the spokes, and these circulations are likely the analogs of the ‘wind of turbulence’ found in low-aspect-ratio RBC experiments.