Large-scale transitional dynamics in pipe flow

Direct numerical simulation of transitional pipe flow is carried out in a long computational domain in order to characterize the dynamics within the saddle region of phase space that separates laminar flow from turbulent intermittency. A shoot & bisection method is used to compute critical trajectories. The chaotic saddle or edge state approached by these trajectories is studied in detail. For Re <= 2000 the edge state and the corresponding intermittent puff are shown to share similar averaged global properties. For Re >= 2200, the puff length grows unboundedly whereas the edge state varies but mildly with Re. In this regime, transition is shown to proceed in two steps: first the energy grows to produce a localized turbulent patch, which then, during the second stage, spreads out to fill the whole pipe.