$v_{2}[2]$ with $|\Delta\eta|>$ 0 (circles), $|\Delta\eta| >$ 0.4 (diamonds) and $|\Delta\eta| >$ 0.8 (squares) for various centrality classes in Pb--Pb collisions at $\sqrt{s_{_{\rm NN}}} = 2.76$ TeV. Hydrodynamic calculations with MC-Glauber initial conditions  and $\eta/s =$ 0.08, with MC-KLN initial conditions and $\eta/s =$ 0.20 , with Trento initial conditions and temperature dependent $\eta/s$  and AMPT initial conditions and $\eta/s =$ 0.08  are shown in green dot-dashed and orange dashed curves, and magenta and grey shaded areas, respectively.