Laminar to Turbulent Transition in Pipe Flow: Effect of Dispersed Phase

PI: Longmire， EllenProposal Number: 1605719本次拟研究的目的是通过实验和理论探讨管道中两相流从层流到湍流的转变。虽然之前已经有很多关于单相流体相变的工作，但这一提议的主题在很大程度上是未知的。当两个相中的一个分散在另一个相中时，所提出的工作将对理解过渡到湍流的基本概念产生影响。这项工作的技术应用非常重要，尤其是在3D打印和增材制造过程中。最近对管道流动的实验表明，当液体流动中包含不混相液滴或刚性颗粒的第二相时，过渡的临界雷诺数可能会显著降低或延迟到更高的值。位移的方向取决于离散相的体积分数以及管道与颗粒直径的比值。这些转变背后的物理机制尚不清楚。本研究的目的是通过实验和分析来揭示和描述这些机制。据推测，至少存在两种机制：较大的液滴或颗粒可以促进由颗粒和周围流体速度之间的局部差异引起的不稳定性，较小的液滴或颗粒可以抑制生长或以其他方式破坏更大规模的不稳定性和连贯结构。实验计划在透明管道设施中进行，在那里将确定液体/液体和液体/固体分散的临界雷诺数。将对结果进行分析，以确定随时间变化的流动特征、总体平均统计、与过渡流动结构相关的动态显著模式，以及最终影响过渡的具体机制。提议的教育活动包括本科生和研究生参与研究，与小学教师合作制定科学课程，以及积极指导高中和大学女学生，与国会议员举行PI会议以促进科学和工程等外展活动。

PI: Longmire, EllenProposal Number: 1605719The goal of the proposed research is to experimentally and theoretically explore the transition from laminar to turbulent flow in the case of two phase flow in a pipe. While a lot of prior work has been done on transition for single phase fluids, the theme of this proposal is largely unexplored. The proposed work will have an impact on understanding the fundamental notions in transition to turbulence when one of the two phases is dispersed in the other. The technological applications of this work are important, most notably in the process of 3D printing and in additive manufacturing.Recent experiments on pipe flows suggest that the critical Reynolds number for transition can either be shifted to significantly lower values or delayed to much higher values when a liquid flow includes a second phase of immiscible liquid drops or rigid particles. The direction of the shift depends on the volume fraction of the discrete phase as well as the ratio of pipe-to-particle diameter. The physical mechanisms behind these transition changes are unknown. The objective of the proposed research is to uncover and characterize these mechanisms through experimentation and analysis. It is hypothesized that at least two mechanisms are present: larger drops or particles can promote instabilities caused by local differences between the particle and surrounding fluid velocity, and smaller drops or particles can suppress growth of or otherwise disrupt larger scale instabilities and coherent structures. Experiments are planned in a transparent pipe facility where critical Reynolds number will be determined for liquid/liquid and liquid/solid dispersions. Results will be analyzed to determine time evolving flow features, ensemble averaged statistics, and dynamically significant modes associated with transitional flow structures, and ultimately the specific mechanisms affecting transition. The educational activities proposed include undergraduate and graduate student involvement in research, engagement with elementary school teachers to develop science curriculum, and outreach activities ranging from actively mentoring female high school and college students to meetings of the PI with members of Congress in order to promote science and engineering.