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Localised turbulent structures such as puffs or oblique stripes are buildingblocks of the intermittency regimes in subcritical wall-bounded shear flows.These turbulent structures are investigated in incompressible pressure-drivenannular pipe flow using direct numerical simulations in long domains. For lowenough radius ratio $\eta$, these coherent structures have a dynamicscomparable to that of puffs in cylindrical pipe flow. For $\eta$ larger than0.5, they take the shape of helical stripes inclined with respect to the axialdirection. The transition from puffs to stripes is analysed statistically byfocusing on the axisymmetry properties of the associated large-scale flows. Itis shown that the transition is gradual : as the azimuthal confinement relaxes,allowing for an azimuthal large-scale component, oblique stripes emerge aspredicted in the planar limit. The generality of this transition mechanism isdiscussed in the context of subcritical shear flows.

Turbulent bifurcations in intermittent shear flows: from puffs to oblique stripes