Collaborative Research: Swimming and Settling in Stratified Fluids

许多水生系统的特征是水密度随深度变化的区域，通常是由于温度或盐度梯度。这些吡咯啉与强烈的生物活性有关，可以通过减缓颗粒的下降来影响碳通量。低到中等雷诺数是特别重要的，因为绝大多数生物和粒子都很小（µm-cm），它们的运动主要是粘性的。尽管如此，在分层流体中沉降和游动的基本流体动力学在很大程度上仍未被探索。这部分是由于人们普遍认为，分层的相关长度尺度比生物体大几个数量级。pi最近表明这不是真的，典型的水生分层实际上可以影响小到0 （100 ìm）的颗粒和生物体的流场。这打开了一扇门，一套广泛的新的问题，粘滞运动在分层流体流体力学的一个新领域。拟议的研究将通过实验、理论和计算研究的结合，确定和合理化分层对游泳生物和细长颗粒沉降的影响，从而迈出进入这个新领域的第一步。将开发新的工具来求解分层流体中游泳生物的流场，并对浮力、粘度、惯性和扩散对基本流体动力学参数（包括游泳速度、速度衰减率和能量消耗）的影响进行广泛而深入的研究。拟议的研究将解决自然粒子和生物体的几何复杂性的重要组成部分，通过集中在沉降伸长率的作用。在这个提议中提出了一个新的假设，并将在理论和实验上进行验证：浮力引起的扭矩使拉长的粒子重新定向，并在很大程度上影响它们的下降。在过去的几十年里，已经发现了流体分层区域与广泛的环境过程之间的重要相关性，包括藻华、海洋雪颗粒的积累和水生生物的垂直迁移。虽然这通常是水生科学家和海洋学家的领域，但在这种新的、未探索的环境中，缺乏对流体力学的基本理解，在这种环境中，分层和粘性效应都很重要。这项研究将在粒子沉降和有机体运动的广泛背景下产生关于该制度的流体动力学的第一个物理见解。这些新的见解，以及即将开发的最先进的实验和数值技术，将(i)在流体力学和水生科学之间的界面上为广泛的其他研究人员（数学家、工程师、海洋学家、湖泊学家、生态学家）提供肥沃的土壤；（二）为水生生态系统中具有生态和社会价值的广泛过程提供信息，例如有助于改进管理做法，防止富营养化（例如藻华），为海洋生物地球化学模型提供更好的颗粒通量估计，并进一步了解从油羽状流中分散的油滴在海洋环境中的命运。这笔补助金将为三名研究生提供培训。通过圣母大学的女性工程项目和一所女子学院（圣玛丽学院）的演讲，将大力鼓励女性和代表性不足的群体成员的参与。通过圣母大学和麻省理工学院的本科生研究机会计划，pi将确保本科生的参与，特别是在项目的实验方面。

1066545/1066566Ardenkani/StockerMany aquatic systems are characterized by regions where water density varies over depth, often due to temperature or salinity gradients. These pycnoclines are associated with intense biological activity and can affect carbon fluxes by slowing the descent of particles. The low to moderate Reynolds number regime is particularly important, because the vast majority of organisms and particles are small (µm-cm) and their motion predominantly viscous. Despite this, the fundamental fluid dynamics of settling and swimming in a stratified fluid have remained largely unexplored. This is partly due to the widespread belief that the relevant length scale of stratification is orders of magnitude larger than organisms. The PIs have recently showed this not to be true, and that typical aquatic stratifications can in fact affect the flow field of particles and organisms as small as O(100 ìm). This opens the door to a broad new set of questions on viscous motion in stratified fluids a novel area of fluid mechanics. The proposed research will take first strides into this new area by determining and rationalizing the effects of stratification on swimming organisms and settling of elongated particles through a combination of experimental, theoretical, and computational research. New tools will be developed to solve for the flow field of swimming organisms in stratified fluids and conduct a broad, in-depth investigation on the effects of buoyancy, viscosity, inertia and diffusion on fundamental hydrodynamic parameters, including swimming speed, velocity decay rates and energy expenditure. The proposed research will address the important component of the geometrical complexity of natural particles and organisms, by focusing on the role of elongation on settling. A novel hypothesis is developed in this proposal and will be tested both theoretically and experimentally: that a buoyancy-induced torque reorients elongated particles and considerably affects their descent.During the last few decades, important correlations have been discovered between regions of fluid stratification and a wide range of environmental processes, including algal blooms, accumulation of marine snow particles, and vertical migration of aquatic organisms. Although this is often the realm of aquatic scientists and oceanographers, what is missing is a fundamental understanding of the fluid mechanics in this new, unexplored regime where both stratification and viscous effects are important. This study will yield the first physical insights on the hydrodynamics of this regime within the broad context of particle settling and organism motility. These new insights, along with the state-of-the-art experimental and numerical techniques to be developed, will (i) provide fertile ground for a broad range of other researchers (mathematicians, engineers, oceanographers, limnologists, ecologists) at the interface between fluid mechanics and the aquatic sciences; and (ii) inform a broad range of processes in aquatic ecosystems, of ecological and societal value, for example by contributing to improved management practices to prevent eutrophication (e.g. algal blooms), providing better estimates of particle fluxes for biogeochemical ocean models and furthering the understanding of the fate of oil droplets dispersed from oil plumes in the marine environment. This grant will provide training for three graduate students. The participation of women and members of underrepresented groups will be strongly encouraged through the Women's Engineering Program at Notre Dame and presentations at an all-women's college (Saint Mary's college). The PIs will ensure the participation of undergraduates, particularly in the experimental aspects of the project, through the Undergraduate Research Opportunities Program at both Notre Dame and MIT.