From localized to expanding turbulence
In the transitional regime turbulence in pipe, channel and Couette flow appears in localized patches, sometimes called spots or puffs. These have a fixed length and a propagation speed which depends on the Reynolds number. It has been demonstrated in recent studies of pipe flow that such turbulent puffs have a finite lifetime and that their decay is a memoryless process [1, 2]. Generally this behaviour is consistent with the assumption that the turbulent state forms a chaotic saddle in phase space.