Elastically driven Kelvin-Helmholtz-like instability in straight channel flow
Elastically driven Kelvin-Helmholtz-like instability in straight channel flow
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DOI：
10.1073/pnas.2105211118
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发表时间：
2021-08-24
影响因子：
11.1
通讯作者：
Steinberg, Victor
中科院分区：
文献类型：
--
作者：
Jha, Narsing K.;Steinberg, Victor
最初，开尔文-亥姆霍兹不稳定性（KHI）描述了在分离不同密度的牛顿流体的反向传播流与底部的较重流体的界面处的扰动的增长。广义KHI也被用来描述速度和密度连续变化的自由剪切层的不稳定性。KHI是研究最多的剪切流不稳定性之一。它在自然界中广泛存在于层流和湍流中，并作用于不同的空间尺度，从银河系到土星带，海洋和气象流动，以及实验室和工业规模。在这里，我们报告的观察弹性驱动KH类不稳定性直粘弹性通道流，观察到的弹性湍流（ET）。目前的研究结果与界面扰动在惯性可忽略时是稳定的这一既定观点相矛盾。流动揭示了速度脉动的弱不稳定相干结构（CSs），即条纹自组织成CSs的自持循环过程，并与伴随的弹性波同步。在ET的每个周期中，反向传播条纹被弹性KH类不稳定性破坏。它的动力学非常类似于牛顿的KHI，但尽管相似，不稳定机制却明显不同。扰动条纹界面上的速度差使流动不稳定，界面扰动处的曲率产生稳定的环向应力。后者是克服速度差失稳的主要稳定因素。失稳机制是弹性波与壁面法向涡量的相互作用导致界面扰动放大。弹性波能量从主流中提取并泵入壁面法向涡量增长，从而破坏条纹。
Originally, Kelvin-Helmholtz instability (KHI) describes the growth of perturbations at the interface separating counterpropagating streams of Newtonian fluids of different densities with heavier fluid at the bottom. Generalized KHI is also used to describe insta-bility of free shear layers with continuous variations of velocity and density. KHI is one of the most studied shear flow instabilities. It is widespread in nature in laminar as well as turbulent flows and acts on different spatial scales from galactic down to Saturn's bands, oceanographic and meteorological flows, and down to lab-oratory and industrial scales. Here, we report the observation of elastically driven KH-like instability in straight viscoelastic channel flow, observed in elastic turbulence (ET). The present findings con-tradict the established opinion that interface perturbations are stable at negligible inertia. The flow reveals weakly unstable coher-ent structures (CSs) of velocity fluctuations, namely, streaks self-organized into a self-sustained cycling process of CSs, which is syn-chronized by accompanied elastic waves. During each cycle in ET, counter propagating streaks are destroyed by the elastic KH-like instability. Its dynamics remarkably recall Newtonian KHI, but de-spite the similarity, the instability mechanism is distinctly different. Velocity difference across the perturbed streak interface destabilizes the flow, and curvature at interface perturbation generates stabiliz-ing hoop stress. The latter is the main stabilizing factor overcoming the destabilization by velocity difference. The suggested destabiliz-ing mechanism is the interaction of elastic waves with wall-normal vorticity leading to interface perturbation amplification. Elastic wave energy is drawn from the main flow and pumped into wall -normal vorticity growth, which destroys the streaks.