AI 中文总结
该研究通过直接数值模拟发现，管道流中Re≈2900以下的层流间隙是与湍流泡类似的独特动力学状态，由剪切自调节机制稳定，寿命呈指数分布，其消失对应时空均匀湍流的起始。
AI 中文摘要
在雷诺数处于中间区间的管道流中，层流与完全湍流状态之间，呈现出多种时空间歇性相，其中湍流与层流状态共存。在较低区间，即雷诺数Re∈(1750,2300)，湍流以被称为“湍流泡（puffs）”的局域行波结构形式出现，这些结构形成长寿命的混沌动力学状态，其随机衰减与分裂控制稳态间歇性。在较高区间，即Re∈(2300,3000)，湍流泡被扩展的湍流状态取代，其中间歇性形成并消失层流 pocket（囊区）。采用Re=2400、2450、2500、2550的管道流直接数值模拟，我们提供证据表明这些层流间隙形成与湍流泡类似的独特动力学状态：即湍流环境中的行波层流囊区，由依赖剪切的自调节机制稳定。我们分析这些间隙的平均空间剖面，表明其寿命呈指数分布，提示间隙闭合对应从混沌鞍点逃逸。最后，我们提出这些层流间隙在有限雷诺数Re≈2900时变得不稳定并消失，该点可解释为时空均匀湍流的 onset（起始）点。
英文摘要
Pipe flow at intermediate Reynolds numbers, between the laminar and fully turbulent regimes, takes the form of several spatially and temporally intermittent phases in which turbulent and laminar states coexist. In the lower range, $Re\in (1750,2300)$, turbulence appears in the form of localized traveling structures called "puffs", which form long-lived chaotic dynamical states, whose stochastic decays and splits control the steady state intermittency. At the other end, $Re\in (2300,3000)$, puffs are replaced by an extended turbulent state, with laminar pockets intermittently forming and disappearing within it. Using direct numerical simulations of pipe flow at $Re = 2400, 2450, 2500, 2550$, we provide evidence that these laminar gaps form a distinct dynamical state analogous to puffs: a traveling laminar pocket in a turbulent surrounding, stabilized by a shear-dependent self-tuning mechanism. We analyse the mean spatial profile of these gaps and show that their lifetimes are exponentially distributed, suggesting that gap closing corresponds to an escape from a chaotic saddle. Finally, we suggest these laminar gaps become unstable and disappear at a finite Reynolds number, $Re\sim 2900$, which can be interpreted as the onset point of spatially and temporally homogeneous turbulence.
Comments10 pages, 6 figures
