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Abstract:
Alternating laminar and turbulent helical bands appearing in shear flows between counterrotating cylinders
are accurately computed and the near-wall instability phenomena responsible for their generation identified.
The computations show that this intermittent regime can only exist within large domains and that its spiral
coherence is not dictated by endwall boundary conditions. A supercritical transition route, consisting of a
progressive helical alignment of localized turbulent spots, is carefully studied. Subcritical routes disconnected
from secondary laminar flows have also been identified.
