Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session T02: Minisymposia: The Fluid Mechanics of Microplastics Transport |
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Chair: Michelle DiBenedetto, University of Washington; Nicholas Ouellette, Stanford Univ Room: Sagamore 4567 |
Monday, November 21, 2022 4:10PM - 4:36PM |
T02.00001: The dynamics of buoyant particles at the ocean surface: implications for fate and transport Invited Speaker: Michelle H DiBenedetto Buoyant microplastics concentrate near the ocean surface, where their vertical distribution is controlled by particle buoyancy and vertical mixing. This distribution is relevant to multiple processes. For example, near-surface currents are strongly sheared, so horizontal transport couples to vertical position. In addition, solar irradiance decays with depth, thus microplastic vertical position controls photodegradation rates. In order to predict the vertical distribution of microplastics, we need to better understand the interactions between the particle properties and flows characteristic of the near-surface ocean boundary layer. |
Monday, November 21, 2022 4:36PM - 5:02PM |
T02.00002: Effects of Turbulence on the Transport of Poistively Buoyant Particles in the Ocean Mixed Layer Invited Speaker: Marcelo Chamecki Positively buoyant particles such as microplastic have large residence times in the ocean mixed layer (OML), and their large-scale horizontal transport is strongly influenced by the physical processes within this thin, turbulent layer. We introduce a general framework to predict the fate of positively buoyant materials in the OML under a broad spectrum of oceanic mixed layer regimes produced by various combinations of wind, waves, and surface cooling. The framework is based on the estimation of a turbulence velocity scale for the vertical mixing of buoyant materials under different combinations of wind shear, Stokes drift, and surface buoyancy flux. By combining this velocity scale with the particle's terminal rise velocity and an eddy diffusivity parameterization, an analytical prediction for the vertical profile of material concentration can be obtained. This prediction is shown to be a reasonably accurate and general representation for oceanic mixed layer regimes, and it allows simple estimates of important parameters that govern the large-scale transport, such as the depth of the center of mass and the horizontal speed and direction of material transport. In agreement with large-eddy simulations of particle transport in the OML, theoretical results highlight the strong sensitivity of large-scale transport to the balance between vertical turbulent mixing and particle buoyancy. |
Monday, November 21, 2022 5:02PM - 5:28PM |
T02.00003: Settling and transport of microplastics: shape, size, and mass distribution effects Invited Speaker: Margaret L Byron Microplastics are widely recognized as a contaminant of emerging concern by both the scientific community and the general public. They appear throughout the earth’s ecosystems, from the top of Mount Everest to the bottom of the Mariana Trench—and everywhere in between. However, the fluid dynamics of microplastic transport remains an active area of study. Microplastics occur in a wide variety of shapes, including not only spheres (and other shapes typically canonical to studies of particle-laden flow) but also fragments, films, foams, fibers, and more. They also span a range of sizes, from millimeters to nanometers. Commonly used plastics include both positively and negatively buoyant materials. These physical features—shape, size, and composition—are integral to our understanding of how microplastics are transported. A further complication is that these properties can be dynamic: plastic can be degraded or colonized by microorganisms, changing both the overall density as well as the mass distribution (and in some cases, size and shape as well). We will examine how shape and mass distribution affect settling velocity and trajectory for two cases: an idealized case, in which low-aspect-ratio cylinders are fabricated with specified mass distributions, and a realistic case, in which microplastics are exposed to bacterial growth. In the idealized case, we compare uniform-density cylinders with bipartite cylinders of the same overall size, shape, and mass density. In the realistic case, we culture a bacterial biofilm on either one side or all sides of positively buoyant polyethylene nurdles. We compare the still-water settling and rising velocities for each case, and find significant differences. We will discuss these findings in the context of overall microplastics transport, and what our results might imply for the future of microplastics research. |
Monday, November 21, 2022 5:28PM - 5:54PM |
T02.00004: Particle transport along the non-wavy free surface above turbulent water Invited Speaker: Filippo Coletti Because a large fraction of all plastics are less dense than water, the fate of microplastics depends on the dynamics of free-surface turbulence. Past studies in this area have mostly focused on the influence exerted by the surface on the flow underneath, while the characterization of the transport along the surface itself remains incomplete. I will summarize our recent experiments on tracer particles floating in turbulent water, using laboratory-scale and field-scale facilities: grid turbulence in a large open-channel flow, homogeneous turbulence in a zero-mean flow tank, and an outdoor stream. We focus on regimes in which surface waves are too small to affect the transport. Single-point and two-point statistics in both Eulerian and Lagrangian frames are explored. Although the tracers move in two dimensions, their motion is consistent with the hallmarks of Kolmogorov's theory for three-dimensional turbulence, as revealed by the velocity fluctuations, structure functions, energy cascade, and Lagrangian dispersion. Uniquely to the compressibility of the free surface, the tracers cluster strongly and do so over spatial and temporal scales comparable to the integral scales of the turbulence. Finally, the effect of particle size, shape and concentration will be discussed. |
Monday, November 21, 2022 5:54PM - 6:20PM |
T02.00005: Buoyant microplastic settling mediated by clay flocculation Invited Speaker: Bruce R Sutherland A significant fraction of plastic waste entering the ocean does so through muddy rivers and estuaries which carry suspended clay and silt. Individual particles of clay are micron-sized, and so take hours to settle just a few centimetres. However, when clay suspended in fresh water comes into contact with salt water, the particles flocculate to form effectively larger particles that settle rapidly. Through laboratory experiments, we examine the interaction of clay with buoyant plastic particles in salt water solutions. The results show that, in the presence of surfactants, clay can flocculate onto the plastic particles making them effectively more dense and causing a significant fraction of them to settle. In separate laboratory experiments in which solid particles precipitate out of a solution, we find that the descending particles can either form a settling front or act collectively to induce counter-rotating cells in the ambient fluid. Challenges for numerically modelling these experiments will be discussed. |
Monday, November 21, 2022 6:20PM - 6:46PM |
T02.00006: Modeling the fragmentation of brittle object in turbulence Invited Speaker: Gautier Verhille The mechanism and the rate of fragmentation of plastic litters in the oceans are of paramount importance to model the fate of microplastic in open sea. The more likely scenario is a fragmentation of large plastic objects during storms after a fragilisation by UV radiation, salinity corrosion, ... From a fundamental point of view, this raise the question of the process of fragmentation within turbulent flow. This problem has been investigated for the fragmentation of particle aggregates but not for brittle object. Here we perform experiments and compelementary numerical simulations of the fragmentation of a single deformable objects that behaves elastically up to breakage. We exhibit a comprehensive fragmentation scenario, further modeled by an evolution equation. Our results demonstrate that the fragmentation process is limited at small scales by a physical cutoff length originating from the fluid-structure interactions between the objects and turbulence, and therefore independent of the brittleness of the fibers. This scenario leads to the accumulation of fragments with a typical length slightly longer than the cutoff scale, as smaller fragments are too short to be deformed and broken by turbulence. |
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