标题： 管道流中湍流泡的不同寿命尺度定律 显示英文标题

标题： Distinct Lifetime Scaling Laws of Turbulent Puff in Duct Flow

摘要： 局部湍流脉冲的时空动力学$-$以及方形管道流动中特征过渡结构$-$的研究是通过直接数值模拟和理论分析进行的。 结果表明，湍流脉冲是瞬态结构，在临界雷诺数$Re_c\simeq1450$处表现出不同的层流化区域。 亚临界区域（$Re<Re_c$）中脉冲的平均寿命随着$Re$的增加遵循平方根标度律，在超临界区域（$Re > Re_c$）则转变为超指数标度。 通过实施模式保持近似，雷诺-奥尔动能方程被简化为一个带有噪声的鞍点分岔方程，该方程从亚临界区域的临界缓慢下降所主导的确定性衰减，以及由随机波动激活的突然衰减的角度解释了观察到的标度律。 尽管几何约束引发了独特的二次流动，例如角落定位的流向涡对、角落对齐的高速条纹和分叉的低速条纹，但脉冲寿命统计与管道流动中的相似，这表明在过渡壁包围湍流的衰减机制中存在几何不变性。 显示英文摘要

摘要： The spatio-temporal dynamics of localized turbulent puffs $-$ the characteristic transitional structures in square duct flows $-$ are investigated through direct numerical simulations and theoretical analyses. It is revealed that the turbulent puffs are transient structures, exhibiting distinct relaminarization regimes bifurcated at a critical Reynolds number $Re_c\simeq1450$. Puff's mean lifetimes at the subcritical regime ($Re<Re_c$) follow a square-root scaling law with increasing $Re$, transitioning to a super-exponential scaling in the supercritical regime ($Re > Re_c$). By implementing pattern preservation approximation, the Reynolds-Orr kinetic energy equation is reduced to a noisy saddle-node bifurcation equation, which explains the observed scaling laws in terms of the deterministic decay governed by the critical slowing down at the subcritical regime, and the abrupt decay activated by the stochastic fluctuations. Despite geometric confinement inducing unique secondary flows, e.g., corner-localized streamwise vortex pairs, corner-aligned high-speed streaks, and forked low-speed streaks, the puff lifetime statistics remain analogous to those in pipe flows, suggesting geometric invariance in decay mechanisms for transitional wall-surrounded turbulence.