近日，奥地利科学技术研究所的Björn Hof及其研究小组与美国伊利诺伊大学香槟分校的Nigel Goldenfeld等人合作并取得一项新进展。经过不懈努力，他们揭示了向管道湍流转捩附近的定向渗透和气泡堵塞现象。相关研究成果已于2024年5月27日在国际知名学术期刊《自然—物理学》上发表。
据悉，自从第一次观察到转捩的空间异质性以来，管道流动中湍流的发生一直没有得到详细的理解。最近在更简单的剪切驱动流动中的理论研究和实验表明，湍流的开始是定向渗透非平衡相变，但这些发现是否具有普遍性，是否也适用于开放或压力驱动的流动尚不清楚。在管道流动中，转捩附近的极长时间尺度使得直接观察临界行为几乎是不可能的。
该研究团队成功找到了一种技术解决方案，突破了这一限制，并证明了转捩的普适类为定向渗透。在定向渗透的过程中，当达到临界点以上时，会出现气泡的堵塞相位。他们的研究方法是通过实验详细描绘了局部湍流气泡之间的所有成对相互作用，并将这些相互作用作为重整化群的输入。
随后，他们将最小模型的计算机模拟外推到更长的长度和时间尺度。堵塞机制的强相互作用使得研究人员能够精确地测量湍流分数，并验证了模型的预测。这项研究工作不仅表明定向渗透标度适用于简单的封闭剪切流，更强调了统计力学在湍流及其相位研究中能够提供的深刻、定量和预测性的见解。
附：英文原文
Title: Directed percolation and puff jamming near the transition to pipe turbulence
Author: Lemoult, Gregoire, Mukund, Vasudevan, Shih, Hong-Yan, Linga, Gaute, Mathiesen, Joachim, Goldenfeld, Nigel, Hof, Bjorn
Issue&Volume: 2024-05-27
Abstract: The onset of turbulence in pipe flow has defied detailed understanding ever since the first observations of the spatially heterogeneous nature of the transition. Recent theoretical studies and experiments in simpler, shear-driven flows suggest that the onset of turbulence is a directed-percolation non-equilibrium phase transition, but whether these findings are generic and also apply to open or pressure-driven flows is unknown. In pipe flow, the extremely long time scales near the transition make direct observations of critical behaviour virtually impossible. Here we find a technical solution to that limitation and show that the universality class of the transition is directed percolation, from which a jammed phase of puffs emerges above the critical point. Our method is to experimentally characterize all pairwise interactions between localized patches of turbulence puffs and use these interactions as input for renormalization group and computer simulations of minimal models that extrapolate to long length and time scales. The strong interactions in the jamming regime enable us to explicitly measure the turbulent fraction and confirm model predictions. Our work shows that directed-percolation scaling applies beyond simple closed shear flows and underscores how statistical mechanics can lead to profound, quantitative and predictive insights on turbulent flows and their phases.
DOI: 10.1038/s41567-024-02513-0
Source: https://www.nature.com/articles/s41567-024-02513-0