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We present a detailed analysis of the flow in a 180 sharp bend of squarecross-section. Besides numerous applications where this generic configurationis found, its main fundamental interest resides in the co-existence of arecirculation bubble in the outlet and a pair of Dean vortices driven fromwithin the turning part of the bend, and how their interaction may drive theflow dynamics. A critical point analysis first shows that the complex flowtopology that results from this particular configuration can be reduced tothree pairs of critical points in the symmetry plane of the bend (with a focusand a half-saddle each). These pairs respectively generate the firstrecirculation bubble, the pair of Dean vortex tubes and a third pair of vortextubes located in the upper corner of the bend, akin to the Dean vortices but ofmuch lower intensity and impact on the rest of the flow. The Dean flow bycontrast drives a strong vertical jet that splits the recirculation bubble intotwo symmetric lobes. Unsteadiness sets in at $Re\lesssim800$ through asupercritical bifurcation, as these lobes start oscillating antisymmetrically.These initially periodic oscillations grow in amplitude until the lobes breakaway from the main recirculation. The system then settles into a secondperiodic state where streamwise vortices driven by the Dean flow arealternatively formed and shed on the left and right part of the outlet. Thisnovel mechanism of periodic vortex shedding results from the subtle interactionbetween the recirculation bubble in the outlet and the pair of Dean vorticesgenerated in the turning part, and in this sense, they are expected to be aunique feature of the 180$^o$ bend with sufficiently close side walls.

Dean flow and vortex shedding in a three-dimensional 180$^\circ$ sharp bend