Transport of Non-Spherical Particles in Wavy Flows

塑料垃圾是世界海洋日益严重的环境问题；每年有数百万公吨的塑料进入海洋。大部分塑料碎片以5毫米或更短的不规则小颗粒的形式存在，被称为“微塑料”，对海洋生物、环境和人类健康构成威胁。该研究项目涉及将有关微粒流的最新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，并正在开发预测海洋中微塑料运动和颗粒在海岸附近停留时间的工具。研究小组正在与地球科学家、生物学家和政策制定者合作，将所得知识整合到保护工作中。调查人员还密切参与指导研究生，向本科生讲授海洋微塑料的运输，并为高中生和中学生开展推广活动。本研究项目旨在识别和分类波浪环境中控制各向异性粒子输运和翻滚的机制，并根据这些粒子在波浪中的大小和形状来量化这些粒子的分散速率和分选。虽然在一定范围内对颗粒流进行了很好的研究，但表面重力波中涉及大的各向异性颗粒的颗粒流的复杂性由于涉及的长度和时间尺度的广泛范围而变得复杂。特别是，直接的数值模拟在计算上变得昂贵，因此，实验是一个至关重要的工具。研究人员拥有先进的设备来产生和量化波浪流，并正在进行实验和经验模型来表征这些流中的粒子传输。这些实验将使研究人员能够解决各向异性颗粒（如相对于球形颗粒具有额外旋转自由度的微塑料）如何在与海浪、湍流和海洋中发现的分层相结合时导致复杂的动力学。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。控制非量纲参数（密度比、波弗劳德数和斯托克斯数）在与海洋相关的范围内变化。用激光多普勒风速仪测量点向流动特性和湍流；波浪是通过波浪计来表征的；通过粒子跟踪测量斯托克斯漂移引起的平均输运。各向异性粒子，包括棒状、圆盘状和椭球体，都是通过3D打印创建的，这些粒子的分散是通过释放粒子云来测量的，然后测量云随时间和空间的相位平均形状。研究结果有望增强我们对波浪中非球形颗粒传输的基本物理学的理解，并为海洋中微塑料的经验建模提供信息。

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.