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

PI：Longmire，EllenProposal number：1605719本研究的目的是从实验和理论上探索管内两相流从层流到湍流的转变。虽然之前已经做了大量关于单相流体转变的工作，但这项提议的主题在很大程度上还没有被探索。这项拟议的工作将对理解当两个相中的一个分散在另一个相中时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打印和添加剂制造过程中。最近的管道流动实验表明，当液体流动包括不相容液滴或刚性颗粒的第二相时，转变的临界雷诺数可以移动到明显较低的值，也可以推迟到很高的值。移动的方向取决于离散相的体积分数以及管径与颗粒直径的比。这些转变变化背后的物理机制尚不清楚。拟议研究的目的是通过实验和分析来揭示和描述这些机制。假设至少存在两种机制：较大的液滴或颗粒可以促进由颗粒和周围流体速度的局部差异引起的不稳定性，较小的液滴或颗粒可以抑制或以其他方式破坏较大尺度的不稳定性和共格结构的增长。实验计划在透明管道装置中进行，在那里将确定液体/液体和液体/固体分散体的临界雷诺数。将对结果进行分析，以确定时间演变的流动特征、集合平均统计数据和与过渡流动结构相关的动态显著模式，并最终确定影响过渡的具体机制。拟议的教育活动包括本科生和研究生参与研究，与小学教师接触以制定科学课程，以及从积极指导女高中生和大学生到为促进科学和工程学而与国会议员举行的会议等外联活动。

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.