In this numerical and theoretical work, we study the turbulent channel flow of Newtonian and elastoviscoplastic fluids. The coherent structures in these flows are identified by means of higher order dynamic mode decomposition (HODMD), applied to a set of data non-equidistant in time, to reveal the role of the near-wall streaks and their breakdown, and the interplay between turbulent dynamics and non-Newtonian effects. HODMD identifies six different high-amplitude modes, which either describe the yielded flow or the yielded–unyielded flow interaction. The structure of the low- and high-frequency modes suggests that the interaction between high- and low-speed streamwise velocity structures is one of the mechanisms triggering the streak breakdown, dominant in Newtonian turbulence where we observe shorter near-wall streaks and a more chaotic dynamics. As the influence of elasticity and plasticity increases, the flow becomes more correlated in the streamwise direction, with long streaks disrupted for short times by localised perturbations, reflected in reduced drag. Finally, we present streamwise-periodic dynamic mode decomposition modes as a viable tool to describe the highly complex turbulent flows, and identify simple well-organised groups of travelling waves.

中文翻译：

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弹粘塑性流体湍流通道流动中的相干结构

在这项数值和理论工作中，我们研究了牛顿流体和弹粘塑性流体的湍流通道流动。这些流动中的相干结构通过高阶动态模式分解 (HODMD) 识别，应用于一组时间不等距的数据，以揭示近壁条纹的作用及其击穿，以及之间的相互作用湍流动力学和非牛顿效应。HODMD 确定了六种不同的高振幅模式，它们要么描述屈服流，要么描述屈服-非屈服流相互作用。低频和高频模式的结构表明，高速和低速流向速度结构之间的相互作用是触发条纹破裂的机制之一，在牛顿湍流中占主导地位，我们观察到较短的近壁条纹和更多混沌动力学。随着弹性和塑性的影响增加，流动在流向方向上变得更加相关，长条纹被局部扰动在短时间内中断，反映在阻力减少上。最后，我们将流向周期性动态模式分解模式作为一种可行的工具来描述高度复杂的湍流，并识别简单的组织良好的行波组。