Collective Flow in Heavy-Ion Collisions from AGS to SPS

Collective transverse flow in heavy-ion collisions at incident energies E_{lab} = (1 -160)A GeV is analyzed within the model of 3-fluid dynamics (3FD). Simulations are performed with purely hadronic equation of state (EoS). At the AGS energies the flow turns out to be sensitive to the stopping power of nuclear matter rather than only to the stiffness of the EoS. When the stopping power is fixed to reproduce other observables, the flow data favor more and more soft EoS with the incident energy rise, which can be associated with ``a transition from hadronic to string matter'' reported in the Hadron-String-Dynamics (HSD) model. Problems, which are met in simultaneous reproduction of directed and elliptic flows within the 3FD, suggest that the transverse flow is very sensitive to the character of the transverse-momentum nonequilibrium at the initial stage of collision. Arguments in favor of "early-stage" nature of the flow observable are put forward. This suggests that the flow (especially the directed one) is determined by early-stage evolution of the collision rather than freeze-out stage.