  We study two models of overdamped self-propelled disks in two dimensions,
with and without aligning interactions. Active mesoscale flows leading to
chaotic advection emerge in both models in the homogeneous dense fluid away
from dynamical arrest, forming streams and vortices reminiscent of multiscale
flow patterns in turbulence. We show that the characteristics of these flows do
not depend on the specific details of the active fluids, and result from the
competition between crowding effects and persistent propulsions. Our results
suggest that dense active fluids present a type of `active turbulence' distinct
from collective flows reported in other types of active systems.
