Computational Mechanics Group, Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, West Midnapore, Bengal 721302, India.
Phys Rev E. 2019 Aug;100(2-1):023109. doi: 10.1103/PhysRevE.100.023109.
A high level of mixing by passive means is a desirable feature in microchannels for various applications, and use of flexible obstacles (or plates) is one of the prime choices in that regard. To gain further insight, we carry out two-dimensional numerical simulations for flow past one or two flexible plates anchored to a channel's opposite walls using a fluid-structure interaction framework. For the inlet flow Reynolds number vs the Strouhal number plane, we observe a sudden flow change from a laminar to a time-periodic vortex shedding state when flexible plates are present in the channel. We found the critical Reynolds number to be Re_{cr}≈370 when a single plate is anchored on the channel wall and Re_{cr}≈290 or even lower when two plates are anchored. With an increase in the inlet flow Reynolds number (up to 3200), we found that vortices detach regularly at the plates' tips, which causes the flow to meander in the channel. In a two-plate anchored configuration, primary vortices generated at the first plate are constrained by the second plate and result in an energetic secondary vortex generation in the downstream side. The overall flow features and the energy dissipation in the channel are mainly controlled by the separation gap between the plates. At high-inlet-flow Reynolds numbers (≥1600), the probability density function (F) of the kinetic energy dissipation in a flexible plate configuration shows a stretched exponential shape in the form F(Z)∼1/sqrt[Z]e^{-pZ^{q}}, where Z is the normalized kinetic energy dissipation and the constants p=0.89 and q=0.86. The observed increase in energy dissipation comes at the cost of an increase in pressure loss in the channel, and we found that the loss is inversely related to the plates' separation gap. From our simulations, we found that if high mixing levels are desired, then two flexible plates anchored to the channel walls is a better choice than a channel flow without obstacles or flow past a single plate. The two-plate configuration with zero separation gap between the plates is best suited to achieve a high mixing level.
在微通道中，通过被动手段实现高度混合是各种应用的理想特征，而使用柔性障碍物（或板）是这方面的首要选择之一。为了深入了解这一点，我们使用流固耦合框架对流过固定在通道相对壁上的一个或两个柔性板的流动进行二维数值模拟。在入口雷诺数与斯特劳哈尔数平面上，当通道中存在柔性板时，我们观察到从层流向周期性涡旋脱落状态的突然流动变化。我们发现，当单个板固定在通道壁上时，临界雷诺数为 Re_{cr}≈370，而当两个板固定时，Re_{cr}≈290 甚至更低。随着入口雷诺数的增加（高达 3200），我们发现涡旋在板的尖端定期脱落，这导致流动在通道中蜿蜒。在两板固定配置中，在第一块板上产生的主涡旋被第二块板约束，从而在下游产生高能的二次涡旋生成。通道中的整体流动特征和能量耗散主要由板之间的分离间隙控制。在高入口流动雷诺数（≥1600）下，柔性板配置中动能耗散的概率密度函数（F）呈伸展指数形式，形式为 F(Z)∼1/sqrt[Z]e^{-pZ^{q}}，其中 Z 是归一化动能耗散，常数 p=0.89 和 q=0.86。观察到的能量耗散增加是以通道中压力损失增加为代价的，我们发现损失与板之间的分离间隙成反比。从我们的模拟中，我们发现如果需要高混合水平，则将两块柔性板固定在通道壁上比没有障碍物的通道流或流过单个板的流更好。两块板之间没有间隙的配置最适合实现高混合水平。
