Analysis for Polymeric Flows based on Molecular Dynamics

这项工作为尝试分析基于分子动力学的聚合物流动提供了第一步。本文将高展弦比纤维的稀悬液在牛顿流体中表示为聚合物液体的简单模型，然后研究了纤维在简单流动中的取向和稀纤维悬浮液在复杂进入流中的流动规律。并将纤维悬浮液流动与聚合物入口流动进行了比较。纤维的取向几乎是随机的，在纤维悬浮液的初始简单剪切流动中，纤维的取向迅速转变为流线。纤维取向的快速演变可能导致瞬态流动特性，如应力超调，这在牛顿流动中没有表现出来。然而，由于实验中使用的是刚性直纤维悬浮液，因此在纤维悬浮液中无法观察到聚合物液体启动流中出现的剧烈应力超调现象。从分子动力学的角度来看，为了使聚合物分子在流动开始后立即接近平衡状态，大的剪切应力似乎是必要的。然而，目前还没有聚合物链解开的证据，因此需要进一步的研究工作。纤维的存在极大地改变了入口流场：显著的角涡随着纤维的体积分数和/或长径比的增加而增加。然而，在刚性纤维悬浮流动中，即使拐角涡的增长很大，旋涡边界也变得相当直。这种涡旋边界形状与柔性分子系统中更为凸出的涡旋边界形成鲜明对比。此外，突出角涡长度几乎与流量无关，在纤维悬浮流动中随着雷诺数的增加而减小。另一方面，在柔性聚合物体系中，凸角涡的大小随着流速的增加而迅速增大。因此，我们的实验计划的下一步将是对柔性宏观纤维系统的研究，并介绍纤维结构在流动过程中变化的影响，这对于开发更好和简化的聚合物液体模型流体是必要的。少

This work provides a first step in attempting to analyze polymeric flows based on the molecular dynamics. In this analysis dilute suspensions of high-aspect ratio fibers in Newtonian fluids are represented as a simpler model of polymer liquids,then both the fiber orientation in simple flows and the flow patterns in complex entry flows of dilute fiber suspensions are studied. The fiber suspension flows are also compared with polymeric entry flows. Fibers, which are almost randomly oriented, change rapidly their orientations to the streamlines in a start-up simple shear flow of fiber suspensions. The rapid evolution of fiber orientation may cause the transient flow properties, such as stress overshoot, which are not exhibited in Newtonian flows. However, the suspensions of rigid straight fibers are used in the experiments, thus the drastic stress overshoot exhibited in start-up flows of polymer liquids can not be observed in fiber suspension flows. From the viewpoint of molecular dynamic … More s,large shear stress seems to be necessary in order to unravel considerably the polymer molecules close to equilibrium immediately after the start of flow.However,evidence of polymer chain unravelling is not available, thus the further research work should be required. The presence of fibers drastically changes the entry flow field: the salient corner vortex grows as the volume fraction and/or aspect ratio of fibers increase. However, even in large growth of the corner vortex, the vortex boundary becomes rather straight in rigid fiber suspension flows. This vortex boundary shape presents striking contrast to the vortex boundary being more convex in flexible molecule systems. Furthermore, the length of the salient corner vortex is almost independent of flow rate and it decreases as the Reynolds number is further increased in fiber suspension flows. On the other hand, the salient corner vortex increases in size rapidly with an increase in flow rate for flexible polymer systems. Thus, a next step in our experimental program will be the examination of flexible macroscopic fiber systems and an introduction of the effects of the change of fiber configuration during flow is necessary to develop better and simplified model fluids of polymer liquids. Less