Laminar gaps mirror turbulent puffs in pipe flow

Pipe flow at intermediate Reynolds numbers, between the laminar and fully turbulent regimes, exhibits several spatially and temporally intermittent phases where turbulent and laminar states coexist. This study aims to analyze the behavior of turbulent puffs and laminar gaps within the transitional range of Reynolds numbers, specifically Re ∈ (1750, 3000). Through direct numerical simulations conducted at Reynolds numbers of 2400, 2450, 2500, and 2550, we provide evidence that laminar gaps, analogous to turbulent puffs, correspond to distinct dynamical states characterized by localized traveling structures stabilized by shear-dependent self-tuning mechanisms. We investigate the mean spatial profile of these gaps and demonstrate that their lifetimes follow an exponential distribution, which indicates that the closure of gaps corresponds to an escape from a chaotic saddle. Moreover, we suggest that these laminar gaps become unstable, leading to their disappearance at a critical threshold around Re ∼ 2900, which marks the onset of homogeneous turbulence. Our findings contribute to the understanding of the intermittent nature of flow transitions and the inherent complexities within the context of fluid mechanics.