  We present a detailed analysis of the flow in a 180 sharp bend of square
cross-section. Besides numerous applications where this generic configuration
is found, its main fundamental interest resides in the co-existence of a
recirculation bubble in the outlet and a pair of Dean vortices driven from
within the turning part of the bend, and how their interaction may drive the
flow dynamics. A critical point analysis first shows that the complex flow
topology that results from this particular configuration can be reduced to
three pairs of critical points in the symmetry plane of the bend (with a focus
and a half-saddle each). These pairs respectively generate the first
recirculation bubble, the pair of Dean vortex tubes and a third pair of vortex
tubes located in the upper corner of the bend, akin to the Dean vortices but of
much lower intensity and impact on the rest of the flow. The Dean flow by
contrast drives a strong vertical jet that splits the recirculation bubble into
two symmetric lobes. Unsteadiness sets in at $Re\lesssim800$ through a
supercritical bifurcation, as these lobes start oscillating antisymmetrically.
These initially periodic oscillations grow in amplitude until the lobes break
away from the main recirculation. The system then settles into a second
periodic state where streamwise vortices driven by the Dean flow are
alternatively formed and shed on the left and right part of the outlet. This
novel mechanism of periodic vortex shedding results from the subtle interaction
between the recirculation bubble in the outlet and the pair of Dean vortices
generated in the turning part, and in this sense, they are expected to be a
unique feature of the 180$^o$ bend with sufficiently close side walls.
