Reorientation of large-scale circulation of turbulent Rayleigh-Bénard convection in a cubic cell

We present a direct numerical simulation on the dynamics of large-scale circulation (LSC) of turbulent Rayleigh-Bénard convection of air (Pr = 0.7) contained in a cubic cell for Rayleigh number range 2X10^6 < Ra < 10^9 . The strength and orientation of LSC are quantified by the amplitude and phase of the first Fourier mode of the vertical velocity. The plane containing LSC is generally aligned along one of the diagonals of the box accompanied by a four-roll structure in the other. However, an abnormal single-roll state with substructures is noted at the face planes for low Ra. In addition to the primary roll, two secondary corner-roll structures are also observed in the LSC plane which grow in size and destabilize the LSC resulting in partial and complete reversals. In addition to previously reported rotation-led reorientations, we also observe cessation events which are rare in cubic cells. The distribution of turbulent kinetic energy shows that the energy is primarily extracted from the non-LSC plane and fed to the LSC plane by the transport mechanism. We observe that the corner-rolls reduce in size and the substructures diminish at higher Ra, which leads to the reduction in the occurrence of reorientations of LSC.