  In our work we have studied a two-dimensional suspension of finite-size
Vicsek hard-disks, whose time evolution follows an event-driven dynamics
between subsequent time steps. Having compared its collective behaviour with
the one expected for a system of scalar Vicsek point-like particles, we have
analysed the effect of considering two possible bouncing rules between the
disks: a Vicsek-like rule and a pseudo-elastic one, focusing on the
order-disorder transition. Next, we have added to the two-dimensional
suspension of hard-disk Vicsek particles disk-like passive obstacles of two
types: either fixed in space or moving according to the same event-driven
dynamics. We have performed a detailed analysis of the particles' collective
behaviour observed for both fixed and moving obstacles. In the fixed obstacles
case, we have observed formation of clusters at low noise, in agreement with
previous studies. When using moving passive obstacles, we found that that order
of active particles is better destroyed as the drag of obstacles increases. In
the no drag limit an interesting result was found: introduction of low drag
passive particles can lead in some cases to a more ordered state of active
flocking particles than what they show in bulk.
