  Using the newly developed hybrid model VISHNU which connects viscous
hydrodynamics with a hadron cascade model, we study the differential and
integrated elliptic flow v_2 at different centrality bins for 200 A GeV Au+Au
collisions and 2.76 A TeV Pb+Pb collisions. We find that the average Quark
Gluon Plasma (QGP) specific shear viscosity eta/s slightly increases from
Relativistic Heavy Ion Collider (RHIC) to Large Hadron Collider (LHC) energies.
However, a further study assuming different temperature dependencies for
(eta/s)_QGP shows that one cannot uniquely constrain the form of (eta/s)_QGP(T)
by fitting the spectra and v_2 alone. Based on our current understanding, the
question on whether the QGP fluid is more viscous or more perfect in the
temperature regime reached by LHC energies is still open.
