Hagen-Poiseuille pipe flow belongs to the class of shear flows that do not
show a linear instability of the laminar profile. Nevertheless,
experiments indicate a transition to turbulence for Reynolds numbers
between 1800 and 2500. The transition is accompagnied by strong temporal
and spatial intermittency. Numerical investigations show a sensitive
dependence on initial conditions in the transition region and rare
longlived turbulent events down to Reynolds numbers near 1700. They also
reveal novel coherent structures in the form of 3-d travelling waves down
to Reynolds numbers of about 1230. These findings suggest that the
transition is connected with the formation of a transient chaotic saddle.
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