Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J18: Flow of Dense Granular Materials and SuspensionsInvited Live
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Sponsoring Units: GSNP Chair: Ishan Srivastava |
Tuesday, March 16, 2021 3:00PM - 3:36PM Live |
J18.00001: Shear Jamming in Dense Suspensions* Invited Speaker: Heinrich Jaeger Dense suspensions of hard particles in a simple liquid have become a model system in the soft condensed matter, granular materials, and rheology communities for the investigation of strongly non-Newtonian behaviors. A key aspect underlying the recent surge of activity has been the realization that, in addition to hydrodynamic interactions, frictional contact between particles can occur. In fact, friction forces were found to be essential in order to explain some of the most striking phenomena observed, such as an abrupt, essentially discontinuous onset of shear thickening, whereby the viscosity can jump up by over an order of magnitude as a critical shear rate is exceeded. So far, however, friction has typically been modelled as a parameter without considering its origin. Furthermore, a focus on the steady-state response has prevented these approaches from capturing dynamic phenomena, most notably the propagating jamming fronts associated with the transition from a shear-thickened to a solid-like, shear jammed state. Thus, there remain fundamental questions both at the nano-scale about the nature of the frictional interactions, and at the macro-scale about the relation between steady-state and transient dynamic phenomena. |
Tuesday, March 16, 2021 3:36PM - 4:12PM Live |
J18.00002: Continuum modeling of flow and size-segregation in dense granular materials Invited Speaker: David Henann Dense granular systems that consist of particles of disparate size segregate based on size during flow, resulting in complex, coupled segregation and flow fields. In this talk, we study size-segregation phenomenology using discrete-element method (DEM) simulations of dense, bidisperse particles and propose a continuum model for coupled size-segregation and flow in dense, bidisperse granular systems. In our DEM simulations, we consider four flow geometries: (1) gravity-driven flow down a long vertical chute, (2) annular shear flow, (3) gravity-driven flow down a rough, inclined surface, and (4) planar shear flow in the presence of gravity - all while varying system parameters, such as the flow rate, system size, fraction of large/small grains, and grain-size ratio. Selected DEM simulation data inform continuum constitutive equations for the relative flux of large and small particles. The segregation model accounts for two driving forces - shear-strain-rate-gradients and pressure-gradients. When coupled with the nonlocal granular fluidity model - a nonlocal continuum model for dense granular flow - we show that both flow fields and segregation dynamics may be simultaneously captured using the coupled, continuum model. |
Tuesday, March 16, 2021 4:12PM - 4:48PM Live |
J18.00003: Bedform dynamics: interaction, attraction and repulsion of dunes Invited Speaker: Nathalie Vriend Bedforms are fascinating and captivating self-organising patterns; from wind-blown dunes on Earth and other planets to regular ripple patterns on riverbeds or coastal beaches. Loose sediment reorganizes when the aeolian (wind-driven) or aqueous (fluid-driven) forcing exceeds a critical value for mobilization of grains. The resulting sediment ripples can coarsen into larger-scale dunes. The migration speed of individual dunes depends inversely on their dimensions: the larger the dune, the slower it migrates. |
Tuesday, March 16, 2021 4:48PM - 5:24PM Live |
J18.00004: Nonlocal rheology of granular materials Invited Speaker: Karen Daniels Granular materials are inherently heterogeneous, and it is therefore difficult to construct a continuum model that successfully spans all the way from creeping to well-developed flows. This deficiency has serious consequences for making predictions in geological and industrial flows, but also presents a very interesting challenge for physicists. Local rheologies, such as the μ(I) rheology, relate the local shear rate to the local stresses. However, they fail to describe creeping flows, non-trivial particle size scaling, and the influence of small vibrations on the flow. Recently, the development of nonlocal rheologies has made inroads into solving this problem by allowing the fluidity at any position in the flow to depend on a spatially-extended region. In my talk, I will describe several experiments on two-dimensional granular materials which bridge particle-scale, meso-scale, and continuum-scale approaches. We test the efficacy of local and nonlocal models for describing flows across various particle shapes, particle stiffness, packing fractions, shear rates, and geometries. Through a combination of photoelastic force measurements, boundary stress measurements, and particle-tracking, it is possible to both fully-characterize the flows and test the assumptions of nonlocal models. We find that a single set of material parameters is able to capture the rheology of a particular granular material under a variety of flow conditions. Our measurements confirm the prediction that there is a growing lengthscale at a finite yield stress ratio associated with a frictional yield criterion. Finally, we observe rearrangements of the force network extending into quasi-static regions of the flow where shear rates vanish, and propose connections between their dynamics and the mechanisms responsible for nonlocal behaviors. |
Tuesday, March 16, 2021 5:24PM - 6:00PM Live |
J18.00005: Active intruder dynamics in granular suspensions Invited Speaker: Arshad Kudrolli We discuss the dynamics of intruders in the form of solid rods, anguilliform robots, and limbless worms moving through sedimented granular medium based on experiments and rheology models coupled with resistive force theory. Index-matching techniques are used to directly visualize the motion of the intruder and the flow of the medium to diagonize the rheology of the medium. Friction, inertia and viscous forces are demonstrated to play important roles in determining the interaction of the medium depending on the intruder speed and depth, and the physical properties of the granular medium. We show that inertial and viscous numbers, first introduced in the context of steady linearly sheared medium, can be extended to unsteady flows as is the case in intruder dynamics by considering the shear rate given by the ratio of the intruder speed and size to describe the effective friction encountered by the intruder [1,2]. The effect of greater drag anisotropy observed in a granular medium is found to lead to greater propulsive force in active intruders and greater locomotion speeds compared to Newtonian mediums. The unique rheology of granular suspensions is then demonstrated to enable novel dual stroke biolocomotion [3]. |
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