Directed percolation (DP), a universality class of continuous phase transitions, has recently been established as a possible route to turbulence in subcritical wall-bounded flows. In canonical straight pipe or planar flows, the transition occurs via discrete large-scale turbulent structures, known as puffs in pipe flow or bands in planar flows, which either self-replicate or laminarize. However, these processes might not be universal to all subcritical shear flows. Here, we design a numerical experiment that eliminates discrete structures in plane Couette flow and show that it follows a different, simpler transition scenario: turbulence proliferates via expanding fronts and decays via spontaneous creation of laminar zones. We map this phase transition onto a stochastic one-variable system. The level of turbulent fluctuations dictates whether moving-front transition is discontinuous, or continuous and within the DP universality class, with profound implications for other hydrodynamic systems.

中文翻译：

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通过移动锋面向湍流的相变


定向渗流 (DP) 是一种连续相变的普遍性类别，最近已被确定为亚临界壁面流动中湍流的可能途径。在典型的直管流或平面流中，过渡是通过离散的大规模湍流结构发生的，称为管流中的泡流或平面流中的带，它们可以自我复制或层流化。然而，这些过程可能并不适用于所有亚临界剪切流。在这里，我们设计了一个数值实验，消除了平面库埃特流中的离散结构，并表明它遵循一种不同的、更简单的过渡场景：湍流通过扩展前沿而扩散，并通过层流区域的自发产生而衰减。我们将这种相变映射到随机单变量系统上。湍流波动的水平决定了移动前沿过渡是不连续的还是连续的并且在 DP 通用性类别内，这对其他流体动力系统具有深远的影响。