  Microswimmers exhibit an intriguing, highly-dynamic collective motion with
large-scale swirling and streaming patterns, denoted as active turbulence --
reminiscent of classical high-Reynolds-number hydrodynamic turbulence. Various
experimental, numerical, and theoretical approaches have been applied to
elucidate similarities and differences to inertial hydrodynamic and active
turbulence. These studies reveal a wide spectrum of possible structural and
dynamical behaviors of active mesoscale systems, not necessarily consistent
with the predictions of the Kolmogorov-Kraichnan theory of turbulence. We use
squirmers embedded in a mesoscale fluid, modeled by the multiparticle collision
dynamics (MPC) approach, to explore the collective behavior of bacteria-type
microswimmers. Our model includes the active hydrodynamic stress generated by
propulsion, and a rotlet dipole characteristic for flagellated bacteria. We
find emergent clusters, activity-induced phase separation, and swarming,
depending on density, active stress, and the rotlet dipole strength. The
analysis of the squirmer dynamics in the swarming phase yields
Kolomogorov-Kraichnan-type hydrodynamic turbulence and energy spectra for
sufficiently high concentrations and strong rotlet dipoles. This emphasizes the
paramount importance of the hydrodynamic flow field for swarming and bacterial
turbulence.
