  Measurements of longitudinal flow decorrelations for charged particles are
presented in the pseudorapidity range $|\eta| < 1$ using a reference detector
at 2.1 $< |\eta_{\mathrm{ref}}| <$ 5.1 in Au+Au collisions at $\sqrt{s_{NN}}$ =
27 GeV by STAR. The flow decorrelation for $v_2$ shows a strong centrality
dependence, while a weak centrality dependence for $v_3$. Results are compared
with the results in Au+Au collisions at 200 GeV as a function of $\eta$ scaled
by beam-rapidity, i.e. $\eta/y_{beam}$. No energy dependence is observed for
$v_2$ decorrelation, but clear energy dependence for $v_3$ decorrelation. These
results provide new insights into the longitudinal structure of the
initial-state geometry in heavy-ion collisions.
