Session R46: Particle-Laden Flow: Non-Spherical Particles I

Positively buoyant particle dynamics in wave run-up

Plastic pollution remains a significant threat to the health of ocean ecosystems. Given that the majority of plastic pollution originates from land-based sources, understanding the transport of plastic debris in nearshore regions is crucial. And yet, while coastal sediment transport is a longstanding area of research, there are minimal research efforts for positively buoyant microplastic transport at the coastline. Specifically, we consider the transport of microplastics in the swash zone, which encompasses the region where broken waves travel up and down the slope, (known as uprush and backwash). In this talk, we present results from an experiment designed to study the transport of positively buoyant particles in an idealized swash flow. We track particles starting from rest on a sloped, impermeable boundary during one wave event. Our results indicate the importance of initial condition on particle transport, as well as the size of the particle relative to the incoming wave.   

Non-spherical particle orientation dynamics under a wind-driven free surface boundary layer

Buoyant particles, such as microplastics, debris, and ice crystals, are ubiquitous at the surface of the ocean. However, it is not known how particle size, shape, and inertia interact with wind-driven turbulence and waves at the free surface to affect their transport and dispersion in the water column. Experiments are performed to measure particle depth and orientation in a laboratory wind-wave tank over a range of wind speeds relevant to the ocean surface. Buoyant rod-, disk-, and sphere-shaped plastic particles are seeded in the flow and are tracked using a large-scale shadow imaging technique. The results provide insight into the physical processes governing particle transport in the ocean surface layer. Particle concentration and orientation are investigated as a function of depth, and are used to explore the effects of particle size and shape on the amount of solar irradiation nonspherical particles would receive in the environment.   

Settling of magnetic rods in quiescent fluid

The precise description of the motion of anisotropic particles in a flow rests on the understanding of the forces and torques acting on them. Here, we investigate experimentally magnetic rods settling at low Reynolds number when a homogeneous vertical magnetic field is applied. We show that the competition between the magnetic torque and the inertial torque from the fluid acting on the rod results in a bifurcation where the rod settle vertically for large magnetic fields while it settles with a steady oblique angle for moderate and small magnetic fields, until eventually reaching the classical horizontal settling regime for the non-magnetic case. Besides, we show that using magnetic rods offers a fantastic strategy to accurately measure the angular dependency of the inertial torque, which is a key ingredient for the understanding and modelling of small anisotropic particles transported by turbulence, with important applications to many natural and industrial multi-phase flows.   

Orientation dynamics of chiral particles in turbulence

Particles that break their own mirror symmetry generally have coupled rotational-translation dynamics. Here we will discuss the dynamics of such chiral particles in turbulent Taylor-Couette flows with Reynolds numbers up to 10⁵. We experimentally track the location and orientation of the inertial chiral particles (Stokes number ~ 10³) and obtain statistics on the coupling between the particle rotation and the particle velocity. Based on their rotation-translation coupling dynamics we can reveal the handedness (left or right) of the particles.   

Chiral coupling of particles to fluid flows

Rigorous quantification of chiral geometry and chiral coupling in fluids has proved challenging. As the fluid dynamics community is developing the ability to build increasingly complex objects and measure their motion and the flow around them, it is essential that we develop efficient tools for quantifying chiral coupling. In the Stokes flow regime we have a rigorous formulation using resistance and mobility tensors, but computing or measuring these tensors for specific geometries remains challenging. This talk will discuss several simple chiral objects including isotropic helicoids, helical fibers, and helical ribbons and the state of the art in measuring and computing the dynamics of these particles.   

Sedimentation Dynamics of Helical Ribbons

We consider the dynamics of helical ribbons sedimenting at low Reynolds number. Experimentally, we use images from three cameras to reconstruct 3D dynamics with high spatiotemporal resolution. Theoretically, we build ribbons out of hydrodynamically interacting spheres and compute resistance tensor components using the moment expansion of Durlovsky et al. We observe that the typical trajectories of these particles are periodic orbits in orientation space. A simplified picture is that these oscillations occur because the center of projected area shifts relative to the center of mass as the particle spins about its symmetry axis. Mathematically, the translation-rotation mobility tensor has three eigenvalues and when all three are distinct, there are stable and unstable orientations with associated periodic oscillations. We find there are special chiral particles with two degenerate eigenvalues where the stable and unstable points disappear and sedimentation is steady. For our chiral ribbon shape, there is a sequence of these axisymmetric helicoids with steady motion at discrete values of ribbon length.   

A Computational Modeling Approach For Magnetic Resonance Navigation In Targeted Embolization: Impact of Aggregate Shape and Hemodynamics Forces

Magnetic resonance navigation (MRN) is gaining popularity in the treatment of liver cancer. The method involves magnetic particles that form aggregates in response to the MRI magnetic field. These aggregates are injected into the bloodstream, where they are directed towards the targeted branch by the combination of the magnetic gradient force and gravity. However, this procedure's success depends highly on the aggregate shape. Prior studies have been limited to a few forces investigation, drag approximation or bead-chain models of aggregates unsuitable for MRN. In addition, the wall effect on aggregates is not well known. To resolve it, we devise a computational model: particle trajectories are simulated with the point-particle approach, solving the modified Maxey-Riley equation. Additionally, we employ the immersed boundary method (IBM) to construct an effective drag library, enabling accurate modeling of hemodynamic forces acting on the aggregates based on their specific sizes and shapes. Preliminary findings exhibit promising outcomes for various aggregate sizes, validating IBM's effectiveness in determining drag coefficients. The results align well with both the bead drag model and experimental data. However, it is essential to conduct further research to generalize the impact of hemodynamic forces on aggregates with diverse shapes.   

Motion of elliptical objects in idealized vortical flow

In many environmental flows, dynamics of interest often lay below the surface and can only be indirectly measured. Typically if we want to measure a flow field we use small spherical tracers, however in the environment we are often limited to what "imperfect" tracers are already floating on the surface (e.g. logs, debris, ice floes). These imperfect tracers can be irregularly shaped, can have length scales larger than the scales of the flow, and can have inertia which will lead to complex responses to the flow. Here we aim to understand these complex interactions by simulating the motion of inertialess elliptical objects in ideal 2D flows such as the Taylor-Green Vortex. We run simulations where we vary the scale, the aspect ratio, and starting position of the objects. We then analyze the kinematics of the objects–including mean transport, dispersion, and orientation–so that we can relate it to the flow field kinematics. We find that the aspect ratio and the size of the object relative to the length scales of flow are important parameters in determining the kinematics of these objects.