  The polarizations of $\Lambda$ and $\bar{\Lambda}$ hyperons are important
quantities in extracting the fluid vorticity of the strongly coupled quark
gluon plasma and the magnitude of the magnetic field created in off-central
heavy-ion collisions, through the spin-vorticity and spin-magnetic coupling. We
computed the energy dependence of the global $\Lambda$ polarization in
off-central Au+Au collisions in the energy range $\sqrt{s_{NN}}=7.7-200$ GeV
using a multiphase transport model. The observed polarizations with two
different impact parameters agree quantitatively with recent STAR measurements.
The energy dependence of the global $\Lambda$ polarization is decomposed as
energy dependence of the $\Lambda$ distribution at hadronization and the
space-time distribution of the fluid-vorticity field. The visualization of both
the $\Lambda$ distribution and the fluid-vorticity field show a smaller tilt at
higher collisional energies, which indicates that the smaller global
polarization at higher collisional energies is caused by a smaller angular
momentum deposition at midrapidity.
