  The transient processes of a turbulent large-scale convective circulation
(LSC) in a cubic cell are investigated using large-eddy simulations for
Rayleigh number $\Ray=10^8$ and Prandtl number $\Pran=0.7$. For the first time,
we have explicitly shown that LSC is accompanied by large-scale azimuthal flows
with non-zero total angular momentum. It is also shown that solid-body rotation
of the entire fluid is not realized. It is found that correlation between
rotation of LSC plane and the mean azimuthal motion is high during
quasiperiodic oscillations of LSC near the diagonal plane and relatively weak
during LSC reorientations. We propose a new plausible scenario for the
reorientations of the LSC in a cube that does not involve a mean azimuthal
flow. Instead of a single-roll, we introduce the superposition of a pair of
large-scale orthogonal quasi-two-dimensional (Q2D) rolls and the reorientation
of the LSC occurs as a result of the cessation of one of the Q2D rolls. This
scenario is consistent with all known experimental and numerical data.
