Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D18: Flow of Dense Granular Materials and SuspensionInvited
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Sponsoring Units: GSNP Chair: Jeremy Lechman, Sandia National Laboratories Room: 205 |
Monday, March 2, 2020 2:30PM - 3:06PM |
D18.00001: Shear Jamming in Dense Suspensions Invited Speaker: Heinrich M. 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 most models from capturing transient 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. |
Monday, March 2, 2020 3:06PM - 3:42PM |
D18.00002: Investigating viscous to inertial transitions in granular suspensions with internal imaging Invited Speaker: Arshad Kudrolli We discuss experiments that probe the rheology of fluid-saturated granular beds coupled with the micromechanics of their flow using refractive index matching (RIM) techniques. These studies are fundamental to problems of erosion, sedimentation, and biolocomotion in such mediums. We will first discuss a sedimented bed which is sheared in a conical rheometer geometry which allows us to examine the system over prolonged periods under steady driving conditions. The shear experienced as a function of prescribed shear rate can be measured in this system along with the motion of the granular phase and the fluid phase. This allows us to determine the granular and fluid component of shear above and below the bed. We demonstrate that the appropriate control variable for the onset of bed motion with increasing shear is in fact the granular packing of the bed rather than the strength or the duration of the applied shear. Then, we show that the velocity profile of the suspended phase which develops between the clear fluid phase on top and a dense creeping granular bed below can be obtained from the applied strain rate and the Krieger-Dougherty model for the effective viscosity. We shall also discuss the drag and the unsteady flow of the medium resulting from the motion of a large solid intruder through the medium. The differences in the flow and drag encountered versus those observed in a viscous fluid and in dry granular media will be discussed going from the frictional to the inertial regime. We will highlight the difficulties encountered in interpreting the encountered rheology simply in terms of inertial and viscous numbers even in cases where the grains are weakly sedimenting. |
Monday, March 2, 2020 3:42PM - 4:18PM |
D18.00003: 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 simulations of dense, bidisperse particles and propose a continuum model for coupled size-segregation and flow in dense, bidisperse granular systems. In our discrete simulations, we consider four flow configurations: (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, flow configuration size, fraction of large/small grains, and grain-size ratio. Selected discrete 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 - across all considered flow configurations, driving conditions, and mixture properties. |
Monday, March 2, 2020 4:18PM - 4:54PM |
D18.00004: 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. |
Monday, March 2, 2020 4:54PM - 5:30PM |
D18.00005: 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. |
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