Collaborative Research: Swimming and Settling in Stratified Fluids

1066545/1066566Ardenkani/Stoker许多水生系统的特点是水密度随深度变化，通常是由于温度或盐度梯度。这些跃层与强烈的生物活动有关，并可以通过减缓颗粒的下降来影响碳通量。低到中等的雷诺数特别重要，因为绝大多数生物和颗粒都很小(微米-厘米)，它们的运动主要是粘性的。尽管如此，层状流体中沉降和游泳的基本流体动力学在很大程度上仍未被探索。这在一定程度上是因为人们普遍认为，层化的相关长度范围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到小至O(100？m)的颗粒和生物的流场。这打开了一系列关于分层流体中粘性运动的新问题的大门，这是流体力学的一个新领域。这项拟议的研究将通过实验、理论和计算研究相结合的方式，确定层化对游泳生物和细长颗粒沉降的影响，并使之合理化，从而向这一新领域迈出第一步。将开发新的工具来求解分层流体中游泳生物的流场，并广泛、深入地研究浮力、粘度、惯性和扩散对基本水动力参数的影响，包括游泳速度、速度衰减率和能量消耗。这项拟议的研究将侧重于伸长对沉降的作用，从而解决自然颗粒和生物体几何复杂性的重要组成部分。在这一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：浮力诱导的扭矩会改变细长颗粒的方向，并显著影响它们的下沉。在过去的几十年里，已经发现流体层结区域与广泛的环境过程之间的重要关联，包括藻类水华、海洋雪粒的积累和水生生物的垂直迁移。尽管这通常是水生科学家和海洋学家的领域，但在这个新的、未被探索的区域，缺乏对流体力学的基本了解，在这种区域，层化和粘性效应都很重要。这项研究将在颗粒沉降和生物体运动的广泛背景下，首次对这一区域的流体动力学产生物理见解。这些新的见解，加上将要开发的最先进的实验和数值技术，将：(1)为流体力学与水生科学之间的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；(2)使水生生态系统中的各种过程具有生态和社会价值，例如，有助于改进管理做法，以防止富营养化(例如藻类大量繁殖)，为生物地球化学海洋模型提供更好的颗粒通量估计，加深对海洋环境中油羽散落的油滴去向的理解。这笔助学金将为三名研究生提供培训。将通过圣母大学的妇女工程方案和在一所女子学院(圣玛丽学院)的演讲，大力鼓励妇女和任职人数不足群体的成员参加。PIS将通过圣母大学和麻省理工学院的本科生研究机会计划，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.