  Recent LHC results on the appearance of sub-leading flow modes in PbPb
collisions at 2.76~TeV, related to initial-state fluctuations, are analyzed and
interpreted within the HYDJET++ model. Using the newly introduced Principal
Component Analysis (PCA) method applied to two-particle azimuthal correlations
extracted from the model calculations, the leading and the sub-leading flow
modes are studied as a function of the transverse momentum ($p_{T}$) over a
wide centrality range. The leading modes of the elliptic ($v^{(1)}_{2}$) and
triangular ($v^{(1)}_{3}$) flow calculated within the HYDJET++ model reproduce
rather well the $v_{2}\{2\}$ and $v_{3}\{2\}$ coefficients experimentally
measured using the two-particle correlations. Within the $p_{T} \le $~3~GeV/c
range where hydrodynamics dominates, the sub-leading flow effects are greatest
at the highest $p_{T}$ of around 3~GeV/c. The sub-leading elliptic flow mode
($v^{(2)}_{2}$), which corresponds to $n = 2$ harmonic, has a small non-zero
value and slowly increases from central to peripheral collisions, while the
sub-leading triangular flow mode ($v^{(2)}_{3}$), which corresponds to $n = 3$
harmonic, is even smaller and does not depend on centrality. For $n = $~2, the
relative magnitude of the effect measured with respect to the leading flow mode
shows a shallow minimum for semi-central collisions and increases for very
central and for peripheral collisions. For $n = $~3 case, there is no
centrality dependence. The sub-leading flow mode results obtained from the
HYDJET++ model are in a rather good agreement with the experimental
measurements of the CMS Collaboration.
