Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session H14: Granular Flows: General (5:45pm  6:30pm CST)Interactive On Demand

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H14.00001: Blockages Associated with Defluidization and Refluidization of Narrow Beds Fernando C\'u\~nez, Erick Franklin Fluidized beds are widely found in industrial applications due to their high rates of heat and mass transfers. In the case of narrow beds, different behaviors are induced by high confinement effects, such as the formation of alternating regions of high and low compactness known as plugs and bubbles, respectively. When the fluid velocity is reduced (defluidization), unusual behaviors can occur at velocities still above that for minimum fluidization, $U_{mf}$. Here, we present experiments in narrow fluidized beds, for which we performed defluidizations where we kept fluid velocities above $U_{mf}$, and we varied the grain types and deceleration rates. Once the defluidization process reaches velocities slightly higher than $U_{mf}$, for which fluidization should occur, the macroscopic motion stops: grains are arranged in lattice structures occupying all the tube crosssection, and presenting small fluctuations. Afterward, if the fluid velocity is slightly increased, a jamming state is reached, where grains are completely trapped and their fluctuations disappear. Our results raise the questions of fluidization conditions and minimum fluidization in narrow beds. [Preview Abstract] 

H14.00002: Granular Rheology with Sliding, Rolling and Twisting Friction Andrew Santos, Ishan Srivastava, Leonardo Silbert, Jeremy Lechman, Gary Grest Intuition tells us that a rolling or spinning sphere on a table will eventually stop due to the presence of friction. Nonetheless, rolling and twisting friction are often neglected in particlebased flow simulations. Rolling and twisting friction are important in granular rheology, where the flow field induces rotations. Granular rheology is simulated using particlebased, discrete element simulations with sliding, rolling and twisting friction in bulklike stresscontrolled shear flows. Normal stress difference, stress ratios and inertial numbers are used to characterize the rheology for different friction states. The increase in critical shear stress to flow, for bulklike rheology, due to rolling and twisting friction is measured. Fabric tensors are used to characterize the structure of the flowing states. Beyond the contact, normal and tangential fabric tensors, the two rotational fabric tensors, from rolling and twisting friction, are also calculated. [Preview Abstract] 

H14.00003: Machine learning characterization for strain localization of granular particles floating at an airoil interface under tensile deformation Hongyi Xiao, Robert Ivancic, Ge Zhang, Robert Riggleman, Andrea Liu, Douglas Durian Understanding the interplay between plastic deformation and local structural change is important for disordered systems. In this study, quasistatic tensile experiments were performed using a monolayer of polydisperse granular spheres floating at an airoil interface that induces capillary attractions between particles. Under tensile deformation, the strain in the monolayer localizes into an inclined shear band, upon which failure occurs, and the ductility of the monolayer can be tuned by controlling the capillary interactions via the particle size. Local plastic rearrangements and the corresponding structural changes were extracted from early stage deformation. Using machine learning methods, we developed a scalar field, softness, which indicates the likelihood of a certain structure to rearrange. During a rearrangement, the local softness tends to revert back to the mean system softness, while the near field softness tends to increase, possibly leading to more nearby rearrangements. The yield strain and the strain field near the rearrangements were also examined, and these structuredynamic relations can be potentially used to inform an elastoplastic model that incorporates the influence of the local disordered structure. [Preview Abstract] 

H14.00004: Coefficient of Restitution in a Vertically Driven Experiment with Two Particles Kai Yang, Alex Sabey, Jeffrey Olafsen The velocity dependence of the coefficient of restitution (COR) has been investigated both theoretically and experimentally for as few as one bouncing ball to granular gases with thousands of particles. Numerous experiments and simulations have been performed to describe the dependence of the COR on the collision velocity. Here, we present an experimental study of the COR with two identical particles driven vertically in a nearly onedimensional channel. Rather than prior experiments that seek to constrain the experiment to an ideal onedimensional collision, the aim here is to examine more freely colliding events that can be filtered for different types of dynamics. Both spherical particles in this experiment are Delrin with a diameter, d, of 5 mm. Multiple sets of data are obtained for frequencies, f, from 26 to 32 Hz and acceleration magnitudes, $\Gamma $, from 1.79 to 3.52 g. High speed digital imaging is used to extract the positions and velocities of the two particles from the experiment for analysis. Most of our data suggests that the COR has a linear dependence on the precollision velocity, but different behaviors are observed in the low and high collision velocity regimes, respectively. [Preview Abstract] 

H14.00005: Normal and Tangential Coefficients of Restitution of a single particle at a boundary. Jeffrey Olafsen, Martin Martinez The interesting dynamics in driven granular systems can be attributed, in part, to the complicated question of the mechanism of dissipation between two particles in collision with each other, or a particle in collision with a boundary. The exact description of the coefficient of restitution (COR) even in a low dimensional system, has been investigated over decades. It remains an important detail in understanding the dynamics of both flow and jamming in collections of larger numbers of macroscopic particles studied in a variety of geometries and for different methods of energy injection. Here, we present the results from two experimental investigations for a horizontally driven, vertical boundary of wedge, parabolic and hyperbolic shapes. A single particle free to move within the cell collides with the boundary twice per shaking cycle. High speed photography allows for the velocity of the particle both before and after the collision to be extracted from the experiment and analyzed. A value for the COR can be determined in two ways: from either the velocity or the total energy before and after the collision at the boundary. The results demonstrate regimes of both surprisingly simple and more complex behavior dependent upon the shape of the boundary. [Preview Abstract] 

H14.00006: Impact of loading geometry on steadystate flow of frictional granular packings Joel Clemmer, Ishan Srivastava, Jeremy Lechman, Gary Grest Studies of granular rheology often focus on a single loading geometry or stress state such as simple shear. However, different loading geometries or stress states such as triaxial extension or compression have different yield conditions and result in distinct flows. We explored this dependence using discrete element model simulations. Systems are deformed to large strains to reach steadystate flow using generalized KraynikReinelt boundary conditions. Rheology is characterized for different constant bulk pressures, values of interparticle friction, strain rate, and loading geometries. Results are compared to common yield criteria such as the MohrCoulomb and Drucker Prager models. Additionally, we will discuss different methods of maintaining constant pressure in flow and compare results to constant volume simulations. Supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy's National Nuclear Security Administration under contract DENA0003525. [Preview Abstract] 

H14.00007: Timedependent granular flow down an inclined plane Sumit Kumar, Satyabrata Patro, Anurag Tripathi The viscoplastic rheological model based on the inertial number captures the steady state rheology of granular materials in various configurations very well. In comparison, relatively little attention has been given to study the timedependent granular flow. In this study, we numerically solve the unsteady state momentum balance equations using the constitutive model given by the $\muI$ rheology and predict the timedependent flow profiles of the shear stress, pressure and velocity. We also compute these flow properties by performing DEM simulations of frictional, inelastic, spherical particles. The particles flow under the influence of gravity starting from a settled state after the inclination of the plane is suddenly increased to a desired angle. The predictions of the momentum balance equations obtained from the numerical results are compared with the DEM simulations. The validity of the numerical method for low as well as moderate inertial numbers will be discussed in this work. [Preview Abstract] 

H14.00008: Nonmonotonic $\mu(I)$rheology of granular flows at high inertial numbers Satyabrata Patro, Anurag Tripathi The inertial number based rheology for the dense flow regime has been used to describe flows for inertial number $I<0.6$. We perform 3D DEM simulations of dry, inelastic, frictional spherical particles flowing over a bumpy inclined plane under the influence of gravity starting from a settled state spanning a large range of restitution coefficients and inclination angles. In contrast to the widely reported monotonically increasing variation of the effective friction coefficient that seems to saturate at high values of the inertial numbers, we find that the effective friction becomes maximum at moderate values of the inertial number and decreases with further increase in inertial number. The results suggest that both the first and the second normal stress differences become significant and cannot be ignored for $I\sim1$. Steady state flow properties are obtained by averaging the flow properties for sufficiently long time duration in the DEM simulations. We also compute the steady state flow properties by solving the momentum balance equations analytically and accounting for both the normal stress differences. Analytical results obtained from the theory will be compared with the results obtained from DEM simulations to explore whether the $\muI$ rheology can be used for $I>1$. [Preview Abstract] 

H14.00009: Predicting a clog: characterizing 2D granular hopper flows using machine learning methods Jesse Hanlan, Douglas Durian In contrast to other fluids, grains flowing from a hopper discharge at a constant rate and form stable arches, clogging the system. Thomas and Durian (PRL 2015) showed that measuring the fraction of flow microstates that cause a clog supports the assertion that the average mass of grains discharged between clogs is an exponential function of outlet diameter. The accompanying physical intuition is that hopper flows are a Poisson process, wherein the flow randomly samples states until it finds one that forms a stable arch. To better understand this, Koivisto and Durian (PRE 2017) found similar scaling of the fraction of flow states versus outlet diameter for both dry and submerged hoppers, suggesting the momentum states are not as dominant as the configuration states for the clogging behavior. We wish to fully characterize the properties of microstates that ‘cause’ a clog compared to those that continue flowing. We use particle tracking to extract the positions throughout a hopper flow, and construct a method to differentiate between configurations. We apply new machine learning techniques in order to best utilize the data in a highly asymmetric classification problem: to differentiate between the single clogging microstate and the many flow states that preceded it. [Preview Abstract] 

H14.00010: Erosion of a cohesive granular material by an impinging turbulent jet Mingze Gong, Adrien Gans, Philippe Gondret, Alban Sauret The erosion of a cohesive soil by an impinging turbulent jet is observed, for instance, during the landing of a spacecraft and is often an undesirable effect. But this situation is also used in the socalled ``jet erosion test'' to obtain valuable information on the mechanical properties of the soil using empirical laws. To provide a quantitative understanding, we perform experiments using a cohesion controlled granular material that allows us to finely tune the cohesion between particles while keeping the other properties constant. We then investigate the response of this cohesive granular bed when subjects to an impinging normal turbulent jet. We characterize experimentally the effects of the cohesion on the erosion threshold and the development of the crater. We demonstrate that the results can be rationalized by introducing a cohesive Shields number that accounts for the interparticles cohesion force. The results of our experiments highlight the crucial role of cohesion in erosion processes. [Preview Abstract] 

H14.00011: Shock wave interactions in granular flows. Aqib Khan, Shivam Verma, Yash Jaiswal, Yazur Gupta, Rakesh Kumar, Sanjay Kumar Due to frictional and collisional dissipation, granular flows become supersonic at velocities of the order of 1 m/s and exhibit shock waves in daytoday applications and in landslides and avalanches. In the present work the phenomenon of shockshock interactions is investigated by performing experiments in gravity driven rapid granular streams produced in a glass channel of width 300 mm and height 5 mm. An array of triangular wedges are placed inside the channel in close proximity. The shock waves formed on the wedges interact and results in the formation of structures which were never observed before. Due to complex dynamics involved in granular collisions a central streak of concentrated grains is formed which becomes unstable and starts oscillating to and fro around the wedges under specific experimental conditions. The instability is observed for the first time and is unique to granular shocks as no such flow feature exist in gas dynamic shock waves. A detailed investigation is being carried out to explore the underlying mechanism of these instabilities for parameters that cover a wide range of applications. [Preview Abstract] 

H14.00012: Dynamics of Granular Intrusion Under Localized Point Source Fluidization Andras Karsai, Daniel Goldman Resistive forces during simple intrusion into dry granular media increase rapidly when no additional mechanism anchors the intruder. The increasing lithostatic pressure gradient can prevent unanchored intruders like robots from burrowing without pushing themselves out via reaction force. We investigate in both experiment and CFDDEM simulations how rapid downward airflow from an intruder's tip can create transient cavities in the media and help reduce resistive force for cylindrical intruders of 3 cm diameter in both 425850 micron granular sand and 3 mm glass beads. When the fluidizing intruder tip is above the granular surface, the airflowinduced cavities vary from tight downward fountains of flowing grains to wide static craters as the airflow disperses and slows with travel distance. Blow the surface, the resistive force per unit depth on the intruder decreases as a function of airflow rate until a characteristic intrusion depth, where the force increases rapidly and approaches the resistive force for the same intruder with no airflow. Our results show how intrusions' net resistive force depends on the relation between local airflow velocity and external lithostatic pressure due to the twoway coupling of these phases of dense granular solids and localized rapid airflow. [Preview Abstract] 

H14.00013: Rheology of Dense Granular Flows near the FlowArrest Transition Ishan Srivastava, Leonardo Silbert, Gary Grest, Jeremy Lechman Granular materials exhibit a dynamical transition between arrested and steady flowing states at a critical ratio of shear stress and hydrostatic pressure. Although their simpleshear steadystate rheology is now wellcharacterized, the transition itself is accompanied by interesting dynamical phenomena such as transient dilatancy and shear jamming, which are not well understood. This transition is highly stochastic, which makes its continuum modeling quite challenging. Additional complications are introduced by a dependence on deformation paths, with important differences between the rheology of shearinduced and compression/extensioninduced flows. We demonstrate such complex rheological scenarios that emerge at the flowarrest transition of granular materials using stresscontrolled discrete element simulations. We introduce and calibrate a continuum rheological model, derived from a dissipative rheological theory, which is applicable to viscometric and extensional flow regimes, and incorporates important rheological features such as normal stress effects and flowarrest transitions. [Preview Abstract] 

H14.00014: Stickslip dynamics in pointloaded granular media comprised of disks or polygonal grains Ryan Kozlowski Dry granular packings, granular suspensions, and colloids can exhibit significant changes in rheological properties when the rotational symmetry of idealized spherical or circular particles is broken. Here, we report on experiments in which a quasitwodimensional layer of either disks or pentagons is penetrated by a grainscale intruder, with the packing fraction adjusted to yield stickslip dynamics. We track particle positions and orientations and use photoelastic measurements to characterize propagation of forces throughout the medium. We observe that the propagated stress on the channel boundaries does not extend as far in front of the intruder for pentagons as for disks, and we connect this difference to the grain and mesoscale structures in the force networks and the velocity and rotation fields during slip events. We observe that disks tend to flow collectively around the intruder as it moves, whereas pentagons do not, and that force chains tend to curve more strongly for disk packings, leading to an increased probability of forces propagating behind the intruder. Our results indicate that grain rotation constraints, a key feature of real granular media found in nature, significantly modify mesoscale force transmission and correspondingly enhance resistance to flow. [Preview Abstract] 

H14.00015: Drag anisotropy of a cylinder in fluidsaturated granular beds Ankush Pal, Arshad Kudrolli We study the drag of a solid cylindrical intruder moving perpendicular and parallel to its long axis in fluid sedimented granular bed of polystyrene particles under steady state conditions. The drag in parallel and perpendicular directions are both found to increase nonlinearly with speed from a nonzero value in the zerospeed limit consistent with a Herschel–Bulkley fluid rheology. The drag anisotropy (ratio of perpendicular to parallel drag) varies with the intruder aspect ratio of the intruder. The ratio of perpendicular to parallel drag is observed to be greater than in case of a Newtonian fluid and is further observed to increase as the aspect ratio of the intruder increases. The contribution of drag arising from circular frontal lobe of the cylinder in parallel direction is much higher compared to the drag arises from the sideways circular lobe in the perpendicular direction. The drag anisotropy is found to further increase after removing the contribution from both circular lobes of the cylinder in parallel and perpendicular direction. [Preview Abstract] 

H14.00016: Simple realization of nonlocal granular fluidity rheology based on finite volume method solver Dorian Faroux, Kimiaki Washino, Takuya Tsuji, Toshitsugu Tanaka Despite the ubiquity of granular materials in industries and natural phenomena, the modeling of dense granular flows whose behavior is at the frontier between solids and liquids and has yet to reach a consensus. Local theories like the popular $\mu(I)$ rheology has been successfully applied to a wide range of applications but fail to capture the influence of grain interaction on a mesoscopic level such as smooth transition between a jamming and a flowing state or various grain sizedependent effects. One of the nonlocal models proposed to address those shortcomings is the nonlocal granular fluidity (NGF) introduced by Kamrin & Koval (2012). While the model has been validated against several reference cases and has been given a finite element method formulation, it has yet to be used with other type of computational fluid dynamics (CFD) solvers. In order to ease the use of nonlocal simulations and deepen our knowledge regarding the abilities and limitations of the NGF model, we propose a simple realization of the NGF rheology within the finite volume method (FVM) framework using a standard incompressible NavierStokes solver. We then compare our results and find good agreement with both experiments and FEM data for several geometries including planar and annular shear flows. [Preview Abstract] 

H14.00017: Building magnetic sand castles Ganga Prasath S, Gaurav Chaudhary, Fabio Giardina, Mahadevan Lakshminarayanan 4D printed structures have recently shown much promise as a strategy for designing complex shapes that can morph from planar prints to 3d structures. However, these approaches require a sophisticated implementation infrastructure, are limited in the number of possible equilibrium configurations, and cannot be easily designed on demand. One way to circumvent these difficulties is to use externally actuated granular systems that are infinitely morphable, and can be easily manipulated using external fields. Using dry magnetic particles made out of Iron filings subject to an externally applied magnetic field, we create structures that can be morphed from one shape to another on demand. A theoretical framework for our observations takes the form of a lubrication equation for magnetic Bingham plastic fluid that explains our results and allows us to pose and solve the inverse problem of calculating the optimal magnetic field required to reach a target shape. We demonstrate this by showing how to a create a prototypic magnetic sand castle! [Preview Abstract] 

H14.00018: Velocity Fluctuations and Granular Fluidity: An Experimental Exploration Rebecca N. Poon, Amalia L. Thomas, Nathalie M. Vriend To construct nonlocal constitutive equations for granular flow, Kamrin and Koval (Phys. Rev. Lett. 108 (2012)) defined a granular fluidity, $g=\dot\gamma/\mu$, relating two macroscopic quantities: the shear rate $\dot\gamma$, and the stress ratio $\mu$. Zhang and Kamrin (ZK: Phys. Rev. Lett. 118 (2017)) subsequently proposed that microscopically, $g=\frac{\delta v}{d}F(\Phi)$, where $\delta v$ is the average magnitude of singleparticle velocity fluctuations around the spatial mean, $F(\Phi)$ is some function of the local volume fraction $\Phi$ (which may depend on e.g. flow geometry) and $d$ is the particle diameter. ZK validated their proposal by numerical simulations in three different 3D geometries. Here we present the first experimental validation of the relation between these microscopic and macroscopic fluidities, in a fourth geometry: an avalanche down a 2D chute. We access $\delta v$ and $\Phi$ by particle tracking, and use photoelasticity to obtain $\mu$. We observe that, indeed, $g=\frac{\delta v}{d}F(\Phi)$ in our system, indicating the robustness of the ZK definition to changes of dimension and geometry. We discuss our form of $F(\Phi)$, which is similar to that found by ZK, and interpret its shape in terms of the jamming transition in dense granular flows. [Preview Abstract] 
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