Re-orientations of the large scale flow in turbulent convection with cubic confinement

Large-eddy simulations (LES) of turbulent Rayleigh-B\'{e}nard convection were conducted with a fluid of Prandtl number $Pr=0.7$ in a fully three dimensional cubic confinement of characteristic width-to-height aspect ratio unity for $Ra$ = $10^6$ and $10^8$. The model solves the unsteady Navier-Stokes equations under the Boussinesq approximation, using a dynamic Smagorinsky model with a Lagrangian averaging technique for the subgrid scale terms. Under fully developed conditions the flow topology is characterized by a large scale circulation (LSC) or mean wind developing in a plane containing one of the diagonals of the cell, while two counter-rotating vortices develop in the other diagonal plane, resulting in inflow at the midplane. This flow structure is not stable in time, undergoing non-periodic re-orientation or switching between the two diagonal planes. The time-interval over which the flow maintains a particular orientation is not constant. We contrast the three-dimensional time-averaged flow structures with single point measurements (time-series) to shed light on the dynamics of the re-orientations. We observe that as $Ra$ increases the LSC becomes more robust and attains a more squarish-like shape.