  Heavy-ion collisions at center-of-mass nucleon collision energies 4.5--11.5
GeV are analyzed within the PHSD transport model. Spectator nucleons are
separated, and the transfer of the initial angular momentum of colliding nuclei
to the fireball formed by participants is studied. The maximal angular momentum
is carried by the fireball in gold-gold collisions with the impact parameter
about 5 fm corresponding to centrality class 10--20\%. The obtained participant
distributions were fluidized and the energy and baryon number densities,
temperature, and velocity fields are obtained in the Landau frame. It is shown
that the velocity field has dominantly Hubble-like transversal and longitudinal
expansion with the vortical motion being only a small correction on top of it.
The vorticity field is calculated and illustrated in detail. The formation of
two oppositely-rotating vortex rings moving in opposite directions along the
$z$ axis is demonstrated. Other characteristics of the vortical motion such as
the Lamb vector field and the kinematic vorticity number are considered. The
magnitude of the latter one is found to be smaller than that for the Poiseuille
flow and close to the pure shear deformation corresponding to just a flattening
of fluid cells. The field of hydrodynamic helicity, which is responsible for
the axial vortex effect, is calculated. The separation of positive and negative
helicities localized upper and lower semi-planes with respect to the reaction
plane is shown. It is proved that the areas with various helicity signs can be
probed by the selection of $\Lambda$ hyperons with positive and negative
projections of their momenta orthogonal to the reaction plane.
