CMG Collaborative Research: Interactions of Phytoplankton with Dissipative Vortices

学术价值：该项目的目的是开发协调的实验室实验和计算模型，以解决海洋学中的一个基本问题，即在单个生物体的空间尺度上，湍流对浮游植物和其他颗粒的影响的大小和机制。湍流形式的外部能量在决定不同种类浮游植物相对成功程度方面的重要性可以追溯到Munk和Riley(1952)和Margalef(1978)的开创性分析。Margalef的“曼陀罗”断言，高营养浓度和湍流强度有利于硅藻的优势，而较低的值有利于非赤潮的甲藻。随后的工作揭示了甲藻物种之间广泛的湍流效应，包括生长刺激。然而，支配这些效应的物理化学机制在很大程度上仍有待确定。通过创新的数值模型和实验之间的迭代，研究人员将缩小教科书上对湍流的理解与对悬浮生物和颗粒的后果的理解之间日益扩大的差距。模型和实验已经使用一维剪切来评估单胞和链水平上的湍流效应。流体过滤对浓度场和细胞旋转的影响已经被预测，并对细胞生长和运动的影响被记录在案。然而，目前对湍流的理解更多地强调涡度、涡度梯度和单个浮游植物细胞经历的耗散尺度的涡旋。我们建议开发一个框架，用于数值和模拟评估细胞在粘性尺度涡旋中和附近经历的影响，捕捉涡度和流体变形、浓度场的演变和流体-结构相互作用的影响。涡度和涡度梯度在决定单体运动从而形成浓度场方面的作用被低估，部分原因是到目前为止一直被用作理论模型和模拟装置的主要基础的二维流动中不可能存在湍流的一个特征，即涡旋拉伸。数值方法将使用两个小尺度涡旋结构和演化的简化模型，即Burgers涡和Lundgren伸展螺旋涡旋，特别关注涡量在两者内和远离两者的扩散。我们将探索衰变旋涡和平衡旋涡。细胞和细胞链的模型将基于实际细胞和链的形状和弯曲硬度。每一个都将被连续放置在涡旋内部和附近的一系列位置，并将完全机械地耦合到流体中。感兴趣的行为是细胞和链的平移、旋转和变形，以及它们对捕食者可以用来定位细胞的局部速度场和涡量场的反馈。标量(以细胞为汇的营养物质或以细胞为源的代谢物)的扩散也将被模拟，从而能够计算用于养分获取的扩散通量和预测食草动物使用的化学场。研究人员将进一步利用他们现有的鞭毛周围流动模型，在建模和测量方案中包括移动的甲藻。模拟实验将利用这样一个事实，即柯尔莫戈罗夫尺度附近的流动是由粘度主导的，就像在早期的Couette实验中一样，但将纳入真实的3D时间变化。借鉴微流体学中新兴的各种几何形状，研究人员将建造各种小型设备，利用散落的涡街、温和的射流和空腔流动来匹配变形速率、涡量和其中的梯度，在他们的涡旋数值模型中对浮游植物产生有趣的影响。这些类比将被用来检验模型的预测，并对模型提出新的问题。更广泛的影响：浮游植物的结果很容易扩展到其他重要现象，如在动荡的环境中(例如，鲍鱼和其他底栖无脊椎动物)产卵的引诱剂的扩散和相应的精子游动能力。它们对其他重要的遭遇过程也有影响，如颗粒凝聚和沉淀、水溶胶过滤和捕食者与猎物的相互作用。这一新方法既提供了从更大尺度的直接湍流数值模拟(DNS)模型到这些个别尺度的湍流效应的天然桥梁，也提供了在更大尺度的流体动力学模型中对这些效应进行参数化的逻辑路径。湍流强度是最可能受到气候变化影响的参数之一，研究人员将与海洋科学教育卓越海洋系统中心(COSEE-OS)密切合作，该中心选择了气候变化下的海洋作为主要研究重点。他们还将在为研究生、本科生和高中教师提供低雷诺数生物力学教学和推广材料的历史基础上再接再厉。他们将用专业制作的重要现象的视觉动画来补充这两方面的工作，他们确定这些现象将纳入COSEE-OS网站。

Intellectual merit: The aim of this project is to develop coordinated laboratory experiments and computational models to address a fundamental question in oceanography concerning magnitudes and mechanisms of turbulence effects on phytoplankton and other particles at the spatial scale of individual organisms. The importance of external energy in the form of turbulence in determining relative success of different kinds of phytoplankton dates to the seminal analysis of Munk and Riley (1952) and Margalef (1978). Margalef's "mandala" asserts that high nutrient concentrations and turbulence intensities favor dominance by diatoms, whereas low values favor non-red-tide dinoflagellates. Subsequent work has revealed a wide spectrum of turbulence effects among species of dinoflagellates, including growth stimulation. The physicochemical mechanisms that govern these effects largely remain to be determined, however.Through iteration between innovative numerical models and experiments, the investigators will close a growing gap between textbook understanding of turbulent flows and understanding of consequences for suspended organisms and particles. Models and experiments have used one-dimensional shear to assess turbulence effects at the level of single cells and chains. Effects of fluid straining on concentration fields and cell rotation have been predicted, and effects on cell growth and motion, documented. Current understanding of turbulence, however, places greater emphasis on vorticity, gradients in vorticity and vortices at dissipation scales experienced by individual phytoplankton cells. We propose to develop a framework for both numerical and analog evaluation of effects that cells experience from being in and near viscous-scale vortices, that capture effects of vorticity as well as fluid deformation, evolution of concentration fields, and fluid-structure interactions. Roles of vorticity and gradients in vorticity in determining cell motions and thereby shaping concentration fields have been underappreciated, partly because a signature feature of turbulence, i.e., vortex stretching, is impossible in the two-dimensional flows that so far have been used as theoretical models and the primary basis of analog devices.Numerical approaches will use two simplified models of small-scale vortex structure and evolution, the Burgers vortex and the Lundgren stretched-spiral vortex, giving particular attention to diffusion of vorticity within and away from both. Both decaying and equilibrium vortices will be explored. Models of cells and chains of cells will be based on shapes and flexural stiffnesses of actual cells and chains. Each will be placed successively at a range of positions within and near a vortex and will be fully coupled mechanically to the fluid. Behaviors of interest are cell and chain translation, rotation and deformation and their feedbacks to local velocity and vorticity fields that could be used by grazers to locate a cell. Also to be modeled is the diffusion of scalars (nutrients with cell as sink or metabolites with cell as source), allowing calculation of diffusive fluxes for nutrient acquisition and prediction of chemical fields used by grazers. The investigators will further take advantage of their existing models of flow around flagella to include motile dinoflagellates in the modeling and measurement scheme.Analog experiments will exploit the fact that flows near Kolmogorov scale are dominated by viscosity, just as in earlier Couette experiments, but will incorporate realistic, 3D time variation. Borrowing from a burgeoning variety of geometries used in microfluidics, the investigators will construct a variety of small devices that utilize shed vortex streets, mild jets and cavity flows to match deformation rates, vorticities and gradients in them that produce interesting effects on phytoplankton in their numerical models of vortices. These analogs will be used to test the model predictions and to pose new questions of the models.Broader impacts: Results for phytoplankton extend easily to other important phenomena such as diffusion of attractants from eggs spawned in a turbulent environment (e.g., by abalone and other benthic invertebrates) and corresponding sperm swimming capabilities. They have implications for other important encounter processes such as particle coagulation and sedimentation, hydrosol filtration, and predator-prey interactions. This new approach provides both a natural bridge from larger-scale, direct numerical simulation (DNS) models of turbulence to these individual-scale effects of turbulence and a logical path to parameterizing these effects in larger-scale fluid dynamic models.Turbulence intensity is one of the parameters most likely to be influenced by climate change, and the investigators will work closely with the Center for Ocean Sciences Education Excellence Ocean Systems (COSEE-OS) that has chosen oceans under climate change as its major focus. They will also build on their history of providing teaching and outreach materials in biomechanics at low Reynolds numbers for graduate students, undergraduates and high-school teachers. They will complement both of these efforts with professionally produced, evocative visual animations of the important phenomena that they identify for incorporation into the COSEE-OS website.