  We present a direct numerical simulation on the dynamics of large-scale
circulation (LSC) of turbulent Rayleigh-B\'enard 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.
