  Polarization of $\Lambda$ hyperons and their antiparticles is calculated in a
3+1 dimensional viscous hydrodynamic model with initial state from UrQMD
hadron/string cascade. We find that, along with recent results from STAR, the
mean polarization at fixed centrality decreases as a function of collision
energy from 1.5% at $\sqrt{s_{\rm NN}}=7.7$ GeV to 0.2% at $\sqrt{s_{\rm
NN}}=200$ GeV. We explore the effects which lead to such collision energy
dependence, feed-down corrections and a difference between $\Lambda$ and
$\bar\Lambda$.
