  Active turbulence, or chaotic self-organized collective motion, is often
observed in concentrated suspensions of motile bacteria and other systems of
self-propelled interacting agents. To date, there is no fundamental
understanding of how geometrical confinement orchestrates active turbulence and
alters its physical properties. Here, by combining large-scale experiments,
computer modeling, and analytical theory, we have discovered a generic sequence
of transitions occurring in bacterial suspensions confined in cylindrical wells
of varying radii. With increasing the well's radius, we observed that
persistent vortex motion gives way to periodic vortex reversals, four-vortex
pulsations, and then well-developed active turbulence. Using computational
modeling and analytical theory, we have shown that vortex reversal results from
the nonlinear interaction of the first three azimuthal modes that become
unstable with the radius increase. The analytical results account for our key
experimental findings. To further validate our approach, we reconstructed
equations of motion from experimental data. Our findings shed light on the
universal properties of confined bacterial active matter and can be applied to
various biological and synthetic active systems.
