Localized turbulence propagating through laminar flow
Parameters
Re = 2200 regime: transitional
Puff speed: — U_bulk
Active puffs: 0
Turbulent puffs appear in pipe flow near Re≈2000-3000. The puff is a localized turbulent slug that propagates slower than the bulk flow. At Re>2500 puffs can split (puff splitting / slug formation). Below Re≈1700 they always decay. This subcritical transition is studied via directed percolation and extreme value statistics.