Bulk Turbulence in Polymer Solutions: Beyond Friction Drag Reduction

PI：Ouellette，Nicholas Proposal number：1436423拟议研究的目标是探索在大部分湍流中添加聚合物的影响。当少量的添加剂，包括长链聚合物、表面活性剂或微泡被加入到湍流中时，流动的结构和动力学可能会发生巨大的变化。添加剂最著名的减阻效果是表面摩擦减阻：在湍流的壁面边界流动中，微量的聚合物或表面活性剂可以将阻力降低高达80%。经过多年的研究，科学界开始就这种减阻的基本物理问题达成共识。然而，表面摩擦力只是沉浸在流体中的船舶或其他物体所经历的总流体阻力的一部分。边界层的分离和湍流尾迹的动力学导致了能量损失，而能量损失往往主导着物体上的摩擦损失。要了解添加剂如何影响这些其他类型的阻力，需要研究远离任何壁面的体积湍流在添加剂存在的情况下的修改情况。虽然我们知道体积湍流确实会随着添加剂的变化而改变，但目前的模型不能解释实验观察。这里提出的研究将通过对聚合物溶液中整体湍流的详细实验测量来解决这一差距。在教育和推广方面，将开发一个基于非牛顿湍流的本科生实验室课程的新模块。此外，PI将制作关于研究成果的电影，将在校园内的咖啡馆和YouTube上展示，并将通过当地的科学博览会计划参与K-12推广活动。该项目的主要目标是了解聚合物对湍流动力学的影响的哪些方面取决于墙的存在，以及哪些方面更普遍。为了解决这个问题，将测量能量注入速率、尺度间传递速率和耗散速率作为聚合物浓度的函数，并研究对湍流衰减的修正。此外，还将对流动的空间结构进行测量，以验证聚合物与小尺度涡度相互作用并抑制小尺度涡度的常见假设。实验测量将在拉格朗日框架内进行，使用拉格朗日粒子跟踪，其中使用多摄像机立体成像装置来跟踪流动平流时三维和时间上的小示踪粒子的运动。

PI: Ouellette, NicholasProposal Number: 1436423The goal of the proposed research is to explore the effects of adding polymers in the bulk of turbulent flows. When small amounts of additives, including long-chain polymers, surfactants, or microbubbles, are added to a turbulent flow, the structure and dynamics of the flow can change dramatically. The most celebrated effect of additives is skin-friction drag reduction: tiny amounts of polymers or surfactants can reduce the drag in a turbulent wall-bounded flow by up to 80%. After years of research, the scientific community is beginning to reach consensus on the fundamental physics of this drag reduction. Skin friction, however, is only a part of the total fluid drag experienced by ships or other objects immersed in a fluid. Separation of the boundary layer and the dynamics of turbulent wakes lead to energy losses that frequently dominate the frictional losses over the object. Understanding how additives affect these other types of drag requires the study of the modification of bulk turbulence, far from any walls, in the presence of additives. Although it is known that bulk turbulence does change with additives, current models cannot explain the experimental observations. The research proposed here will address this gap via detailed experimental measurements of bulk turbulence in polymer solutions. In terms of education and outreach, a new module for an undergraduate laboratory class based on non-Newtonian turbulence, will be developed. In addition, the PI will develop movies about the research results that will be displayed in an on-campus coffee shop and on YouTube, and will participate in K-12 outreach via a local science-fair program.The primary goal of the project is to understand what aspects of the modification of the turbulent dynamics by polymers is dependent on the presence of a wall, and what aspects are more universal. To address this question, the rates of energy injection, scale-to-scale transfer, and dissipation will be measured as a function of polymer concentration, and modification to the decay of turbulence will be studied. In addition, measurements will be made of the spatial structure of the flow, in order to test the common hypothesis that polymers interact with and suppress small-scale vorticity. The experimental measurements will be made in the Lagrangian framework, using Lagrangian particle tracking, where a multicamera stereoimaging setup is used to follow the motion of small tracer particles in three dimensions and in time as they are advected by the flow.