Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session P06: Suspensions: General (3:10pm - 3:55pm CST)Interactive On Demand
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P06.00001: A granular model for the transient response of subglacial till Katarzyna Warburton, Duncan Hewitt, Jerome Neufeld Glaciers frequently rest on beds of subglacial till - a mixture of clay, grains, and water - over which the ice slides. The dynamics of glaciers are tied to the degree of deformation in the till. Many ice streams show velocity variations that have been linked to tidal fluctuations in subglacial water pressure. Experimental evidence characterises till as a plastic material with pressure dependent yield stress, contrary to current models which treat it as a viscous layer. To understand the tidal response requires extending the models of these steady-state experiments to include the time-dependent response of the till. We start from a one-dimensional, two-phase model of coupled fluid and solid flows, using Darcy flow for the fluid phase and a wet granular rheological model for the solid part. After verifying our model against experimental steady-state rheology, we force the model with a fluctuating confining pressure at its upper surface and infer the resulting relationship between porosity, applied shear stress, and deformation throughout the till. We find that shear dilation introduces internal pressure variations and hysteretic behaviour in low-permeability materials, which may help explain the large-scale transient response of ice sheets to changes in the hydrological system. [Preview Abstract] |
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P06.00002: Deviations from Jeffery’s theory in the dynamics of atomically-thin sheet-like molecules in shear flow Simon Gravelle, Catherine Kamal, Lorenzo Botto The rotational dynamics of anisotropic colloidal particles in shear flow is well known for objects satisfying the hydrodynamic no-slip boundary condition. Jeffery's theory predicts that, at high Péclet numbers, a no-slip particle rotates continuously about one of its axis. However, our recent results for particles constrained to move in the flow-gradient plane suggest that Jeffery's theory fails to predict the rotational dynamics in the case of large hydrodynamic slip. For a slip length larger than its thickness, a particle is predicted to find an equilibrium orientation at a small angle with the flow [Kamal et al., Nat. Commun., 11, 2020]. The dynamics in shear flow of atomically-thin molecules must therefore be reconsidered. Using molecular dynamics, we study the flow dynamics of aromatic molecules, whose features are similar to graphene nanosheets. We show that aromatic molecules align in the direction of the shear flow with a constant average orientation angle, and that this alignment is due to hydrodynamic slip, in keeping with the theory. In the case of a single aromatic molecule, the trend of average molecular orientation for varying Péclet numbers is captured by a simplified Fokker-Planck equation. Multiple particle simulations illustrate the importance of clustering. [Preview Abstract] |
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P06.00003: Aligning self-propelling particles in confinement Enkeleida Lushi, Nathaniel Netznik, Katherine Wall, Shang-Huan Chiu We present a model for self-propelling aligning particles and look at the collective motion for such swimmers in non-trivial confined domains. We discuss the complex behavior in circular convex domains and racetracks for a variety of densities, confinement sizes and alignment distances. Phase diagrams for different geometries summarize the behavior and give insight into the dynamics. Lastly, we compare the results to experiments in active matter systems such as motile colloids, swimming bacteria or larva fish, and note the qualitative similarities and differences. [Preview Abstract] |
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P06.00004: Interplay of gravity and diffusion in crystallization of on hard-sphere colloids. Boris Khusid, Lou Kondic, Michael Lam, William V. Meyer Vital for a variety of industries, from 3D printing to photonics, electronics, chemicals, and pharmaceuticals, colloids also serve as an excellent model system to reveal crystallization mechanisms in condensed matter at a particle level. Despite extensive studies, the nature of the glass transition in hard-sphere suspensions unexpectedly discovered in terrestrial experiments about 30 years ago still remains elusive and hotly debated. The presented theory and comparison of data on crystallization in microgravity and on Earth show that the observed glass transition is caused by the swirling of settling particles. Presented findings bring a novel insight into the interplay between gravity and diffusion in colloidal crystallization and open the door to development of novel materials of tailored structures. [Preview Abstract] |
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P06.00005: Fiber alignment in oscillating confined shearing flows Scott Strednak, Jason Butler, Laurence Bergougnoux, Elisabeth Guazzelli Rigid, non-colloidal fibers suspended at high concentration in a Newtonian fluid were aligned in the flow-gradient plane (vorticity direction) by an oscillatory shearing flow. Measurements of the alignment were performed over a range of strain amplitudes, fiber aspect ratio, concentrations, and confinement using a custom flow cell; simulations that account for the hydrodynamic drag and excluded volume of the fibers predict the alignment, largely in agreement with the measurements. The vorticity alignment of fibers was influenced by the confinement, and the orientation of fibers was spatially dependent. For some conditions, nearly perfect alignment of fibers with the vorticity direction occurs adjacent to the bounding walls, while fibers in the center of the gap are significantly more aligned in the flow direction. Overall, the alignment is a complicated function of the particle concentration, confinement, and fiber aspect ratio. [Preview Abstract] |
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P06.00006: An opensource tool for filtered two-fluid simulations of fluidized gas-particle flows Oliver Daisey, Federico Municchi, Jan Hendrik Cloete In the past decades, computational fluid dynamics (CFD) tools based on the two-fluid (Euler-Euler) approach have been developed on the basis of the kinetic theory of granular flows (hence, assuming a local homogeneously cooling state). More recently, filtered two-fluid models have emerged, incorporating the effects of mesoscale structures (clusters or bubbles) within their constitutive laws and enabling the use of coarser grids. These approaches bear numerous similarities with the Large Eddy Simulation (LES) methods employed in single-phase CFD. However, they are generally more complicated due to the multiphase nature of the problem. In this work, we present a numerical solver based on the opensource finite volume library OpenFOAM that solves the filtered two-fluid equations for fluidized gas-particle flows using a wide range of diverse models developed in recent literature. We illustrate the phase coupling algorithm and discuss the implementation of anisotropic drag and pressure forces, as well as detailing the practical implementation of different classes of models (dynamic, with or without the solution of additional transport equations). Finally, we compare the predictions from different constitutive models against experimental results. [Preview Abstract] |
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P06.00007: Scaling law of Brownian rotation in dense hard-rod suspensions Sheng Chen, Wen Yan, Tong Gao Self-diffusion in dense rod suspensions are subject to strong geometric constraints because of steric interactions. This topological effect is essentially anisotropic when rods are nematically-aligned with their neighbors, raising considerable challenges in understanding and analyzing their impacts on the bulk physical properties. Via a classical Doi-Onsager kinetic model with the Maier-Saupe potential, we characterize the long-time rotational Brownian diffusivity for dense suspensions of hard rods of finite aspect ratios, based on quadratic orientation auto-correlation functions. Furthermore, we show that the computed non-monotonic scalings of the diffusivity as a function of volume fraction can be accurately predicted by a new {\it tube} model in the nematic phase. [Preview Abstract] |
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P06.00008: Stable alignment of a flexible sheet-like particle in shear flow: effect of surface slip and edges. Catherine Kamal, Simon Gravelle, Lorenzo Botto Very thin sheet-like particles presenting hydrodynamic surface slip (e.g., graphene colloids and other 2D nanomaterials) can attain a constant orientation in a shear flow when the slip length exceeds a length scale comparable to the particle thickness. To study the effect of bending deformations on this phenomenon, we develop a 2D fluid-structure interaction model, based on coupling the Euler-Bernoulli beam equation with a Boundary Integral method, of a flexible plate rotating in a simple shear flow. We find that: i) a stable alignment is observed even for relatively flexible particles - non-dimensional bending rigidity$\sigma_{B} /(\mu \dot{{\gamma }}a^{3})<<1$, where $\sigma_{B} $is the bending rigidity, $a$is the major semi-axis, $\dot{{\gamma }}$is the shear rate, and$\mu $is the fluid viscosity; ii) the effect of the edges on the shape of the plate is important, for values of the aspect ratio $a/b$ at least as large as 100. In our parameter range, the mild effect of flexibility on orientation is primarily due to the markedly reduced axial compressive stresses that a flow-oriented sheet presenting slip experiences, compared to a no-slip sheet. Our results are particularly relevant in view of recent research on graphene suspensions. [Preview Abstract] |
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P06.00009: Simulating colloidal hydrodynamics near a solid surface with a modified approach: more physics, less mathematics Md Mahmudur Rahman, Stuart Williams We studied behavior of hard-sphere colloids located near a solid surface in a cylindrical confinement which was rotated horizontally at slow and uniform speed. The flow field caused by a single particle's mobility near a solid surface was determined using an image system. Pair-wise flow field was further modified by the reflection from an entrained particle due to the disturbance flow, caused by a moving particle, in the form of either torque, stresslet, or both depending on entrained particle's location with respect to the direction of the moving particle's motion. To ensure non-overlapping particle's mobility, we used hypothetical center-to-center colloid repulsion model which was balanced by the viscous drag. To ensure particles' motion were confined within the sidewall boundary, we used reduced force model which mostly affect to the nearby particles at the boundary. This modification estimated reduced drag and caused error near the boundary which is minimized by the sufficiently larger simulation space. We validated observed simulated structures formation through experimentation. We observed colloids, both from experimentation and simulation, formed similar dynamic structures and clusters while laterally migrating away from the surface. [Preview Abstract] |
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