  Rotating clusters or vortices are formations of agents that rotate around a
common center. These patterns may be found in very different contexts: from
swirling fish to surveillance drones. Here, we propose a minimal model for
self-propelled chiral particles with inertia, which shows different types of
vortices. We consider an attractive interaction for short distances on top of
the repulsive interaction that accounts for volume exclusion. We study cluster
formation and we find that the cluster size and clustering coefficient increase
with the packing of particles. Finally, we classify three new types of
vortices: encapsulated, periodic and chaotic. These clusters may coexist and
their proportion depends on the density of the ensemble. The results may be
interesting to understand some patterns found in nature and to design agents
that automatically arrange themselves in a desired formation while exchanging
only relative information.
