Transport of Non-Spherical Particles in Wavy Flows

塑料碎片是世界海洋日益令人关注的环境问题；每年有数百万吨塑料进入海洋。许多塑料碎片以长度在5毫米或更小的不规则小颗粒形式存在，被称为“微塑料”，对海洋生物、环境和人类健康构成威胁。这项研究项目涉及将最先进的关于颗粒流动的研究扩展到与微塑料问题相关的情况。人们正在研究波浪流中的不规则颗粒，以了解如何控制这些微粒在海洋和近岸环境中的运输和分类。正在进行实验室实验和建模，并正在开发预测微塑料在海洋中的运动和颗粒在海岸附近停留的时间长度的工具。研究小组正在与地球科学家、生物学家和政策制定者合作，将由此产生的知识整合到保护工作中。调查人员还密切参与指导研究生，教授本科生有关海洋微塑料运输的知识，并为高中生和中学生创建外联活动。本研究项目是对控制各向异性颗粒在波浪环境中传输和翻滚的机制进行识别和分类，并根据波浪中颗粒的大小和形状来量化这些颗粒的分散速度和分类。虽然在一定的范围内对颗粒流进行了很好的研究，但由于涉及的长度和时间范围很广，表面重力波中包含大的各向异性颗粒的颗粒流的复杂性是复杂的。特别是，直接的数值模拟在计算上变得昂贵，因此，实验是一个重要的工具。研究人员拥有产生和量化波状流的先进设备，并正在进行实验和经验建模，以表征这些流中的颗粒传输。这些实验将使研究人员能够解决各向异性粒子，如微塑料，它们相对于球形粒子具有额外的旋转运动自由度，在与波浪、湍流和海洋中发现的层化相结合时，可能会导致复杂的动力学。更好地理解这些相互作用将为简化模型中的假设提供指导。控制无量纲参数(密度比、波弗劳德数和斯托克斯数)在与海洋中的参数相关的范围内变化。点状流动特性和湍流是用激光多普勒风速仪测量的；波浪是通过波长计测量的；斯托克斯漂移引起的平均输运是通过颗粒跟踪测量的。包括棒状、圆盘和椭球体在内的各向异性粒子是通过3D打印创建的，这些粒子的分散度是通过释放粒子云，然后测量云在时间和空间上的相平均形状来测量的。这些结果有望加深我们对非球形粒子在波浪中传输的基本物理的理解，并为海洋中微塑料的经验建模提供参考。

Plastic debris is a growing environmental concern for the world's oceans; several million metric tons of plastic enter the oceans each year. Much of the plastic debris is present as small, irregular particles five millimeters or less in length, called "microplastics," that pose a threat to marine organisms, the environment, and human health. This research project involves extending state-of-the-art research about particle-laden flows to situations relevant to the microplastics problem. Irregular particles are being studied in wavy flows to understand how to control the transport and sorting of these microparticles in the ocean and in near-shore environments. Both laboratory experiments and modeling are being performed and tools for predicting the motion of microplastics in the ocean and the length of time particles spend near the shore are being developed. The research team is working with earth scientists, biologists, and policy makers to integrate the resulting knowledge into conservation efforts. The investigators also are closely engaged in mentoring graduate students, teaching undergraduates about the transport of marine microplastics, and creating outreach activities for high school and middle school students. This research project is identifying and classifying the mechanisms that control transport and tumbling of anisotropic particles in wavy environments and quantifying the dispersion rate and sorting of these particles based on size and shape in waves. While particle-laden flows are well studied in certain limits, the complexities of particle-laden flows involving large, anisotropic particles in surface gravity waves is complicated by the wide range of length and time scales involved. In particular, direct numerical simulations become computationally expensive, and as a result, experiments are a crucial tool. The investigators have advanced facilities for generating and quantifying wavy flows and are performing experiments and empirical modeling to characterize particle transport in these flows. The experiments will allow the researchers to address how anisotropic particles, like microplastics with their additional degrees of freedom for rotational motion relative to spherical particles, may lead to complex dynamics when coupled with waves, turbulence, and the stratification found in the ocean. A better understanding of these interactions will provide guidance regarding assumptions going into simplified models. The governing nondimensional parameters (the density ratio, wave Froude number, and Stokes number) are being varied over ranges relevant to those in the ocean. Pointwise flow characteristics and turbulence are being measured using laser Doppler anemometry; waves are being characterized via wave gauges; and the mean transport due to Stokes drift is being measured via particle tracking. Anisotropic particles, including rods, disks, and ellipsoids, are being created via 3D printing, and the dispersion of these particles is being measured by releasing a cloud of particles and then measuring the phase-averaged shape of the cloud over time and space. The results are expected to enhance our understanding of the fundamental physics governing the transport of non-spherical particles in waves and to inform the empirical modeling of microplastics in the ocean.