Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session L06: Forces, Flows, and Locomotion in Granular Media |
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Chair: Daniel Goldman, Georgia Institute of Technology Room: Georgia World Congress Center B208 |
(Author Not Attending)
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L06.00001: Avalanches and force chains in a granular stick-slip experiment Aghil Abed Zadeh, Jonathan Barés, Robert P Behringer We perform a stick-slip experiment to characterize avalanches for granular materials. In our experiment, a constant speed stage pulls a slider which rests on a vertical bed of circular photoelastic particles in a 2D system. The stage is connected to the slider by a spring. We measure the force on the spring by a force sensor attached to the spring. We study the avalanche size statistics, and other seismicity laws of slip avalanches. We analyze the effect of macroscopic system's parameters such as driving rate, the spring stiffness, pressure and system size. We identify a crackling to periodic transition as we change driving rate and system stiffness. We provide a phase diagram of different regimes and the scaling laws of the transition. |
Monday, November 19, 2018 4:18PM - 4:31PM |
L06.00002: Development of strong intermittency in densely packed soft particles under shearing JC Tsai, JR Huang We study experimentally the dynamics of centimeter-sized fully-immersed PDMS particles that are densely packed and are subject to smooth constant-speed shearing. In addition to previously reported internal imaging, we further characterize the development of intermittency by measuring the fluctuations of boundary force simultaneously. Fluctuation measurements allow us to identify the development of intermittency (with driving rates) and the role of interparticle frictions, which can otherwise be missed in the conventional time-averaged rheology. By varying the compostion of particles, we also vary their elasticity and find a non-trivial dependence of intermittency, suggesting a rich dynamics behind these soft contacts. |
Monday, November 19, 2018 4:31PM - 4:44PM |
L06.00003: Rapid Wheeled Locomotion in Dry Granular Media Andras Karsai, Shashank Agarwal, Kenneth N Kamrin, Daniel I Goldman Wheeled locomotion can be an energy efficient means of traversing ground, but when applied to soft granular terrain, excessive wheel slippage and sinkage due to overly fast wheel actuations can entrap vehicles. We examine the scenarios of a rapidly turning wheel in dry granular media using our lab’s “robophysics” principles by developing an automated experimental apparatus based on terramechanics testbeds. This setup allows us to compare wheeled locomotion experiments to rheological models of intruder-substrate interaction, such as Resistive Force Theory and granular plasticity, in high-speed regimes where rapid shearing can cause changes in the rheological behavior. For a rigid wheel with circumferential protrusions (grousers), we observe that increasing wheel rotational speed leads to sudden increased slippage and sinkage into granular media at a critical rotation speed, after which horizontal speed cannot increase. This phenomenon appears to enforce a kind of speed limit in locomotion through granular media, mediated by the rheological properties of the medium itself. |
Monday, November 19, 2018 4:44PM - 4:57PM |
L06.00004: Mechanics of undulatory swimming on the surface of granular matter. Perrin E Schiebel, Jennifer M Rieser, Christian Hubicki, Alex M Hubbard, Henry C Astley, Kelimar Diaz Cruz, Daniel I Goldman Elongate, limbless animals from the microscopic C. elegans to eels and snakes use flexural waves of the body to move. The swimming of such organisms immersed in homogeneous fluids is well-studied, but little is known about movement on deformable terrestrial materials. We used as a model system the sand-specialist snake C. occipitalis (~40 cm and 20 g) slithering quickly (body segment speeds of 30-100 cm/s) on the surface of homogeneous granular matter (GM). Surface drag measurements revealed that the ratio of thrust to drag forces, a critical component determining animal performance, was largely independent of drag distance, speed, or depth over an order of magnitude. As a result, resistive force theory (RFT) accurately predicted snake performance without accounting for the observed interface complexities like hysteresis. RFT revealed that the observed stereotyped waveform of the snake conferred maximum speed given a limit on peak muscle power. Our study suggests that surface sand slithering is analogous to low-Re swimming in a frictional fluid even at the highest observed speeds. Therefore, terrestrial “swimmers” may not need to contend with changing material dynamics as they increase speed. |
Monday, November 19, 2018 4:57PM - 5:10PM |
L06.00005: Photoelastic study of dense granular free-surface flows Amalia L Thomas, Nathalie M Vriend We use a novel experimental method to reveal the distribution of dynamic forces in the bulk of granular free-surface flows. We release photoelastic discs from an incline to create steady 2D monodisperse and bidisperse avalanches. These gravity-driven dry granular flows are in the slow to intermediate regime (I=0.1-1), dense and thin (h≈10d). We measure constant density and quasi-linear velocity profiles through particle tracking at several points down the chute, for two different basal topographies. The photoelastic technique allows the visualisation and quantification of instantaneous forces transmitted between particles during collisions. We use this information to obtain force measurements and coarse-grained profiles of the stress tensor components at various points along the chute. We first analyse how particle size distribution affects the force network. Furthermore, we specify the ways in which granular flow behaviour is analogous to that of a continuous fluid flow. We observe a hydrostatic increase of the mean pressure with depth, and show that the dynamic average force diagram is the same for discrete and continuous flows. We also show that the highly fluctuating force chains within the flow form preferentially in the directions of the forces acting on the bulk. |
Monday, November 19, 2018 5:10PM - 5:23PM |
L06.00006: Abstract Withdrawn
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Monday, November 19, 2018 5:23PM - 5:36PM |
L06.00007: Granular flow in silo with moving exit Kiwing To, Yun Yen, Chien-Hung Yi In experiments of mono-disperse plastic beads falling out of a cylindrical silo through a circular exit at the bottom, no continuous flow can be sustained when the diameter D of the exit is about 5 times that of the beads. If the exit of the silo moves horizontally (e.g. circular motion or linear oscillation) with respect to the wall of the silo, finite flow rate can be sustained even at small D. When the exit rotates at off-center distance R with angular frequency ω, the flow rate W increases with Rω. Similarly, when the exit oscillates with amplitude A and oscillation frequency f, W increases with Af. On the other hand, if D is large, W varies non-monotonically with either the angular frequency ω or oscillation frequency f. |
Monday, November 19, 2018 5:36PM - 5:49PM |
L06.00008: Non-local effects in intermediate granular avalanching flows Nathalie Vriend, Amalia L Thomas, Zhu Tang, Karen Daniels In this work we showed that photoelastic analysis allows the quantification of rheologies in an intermediate chute flow. We characterize stresses and velocities in a flow layer with regions below and above the yield ratio mu_s. We compare our experimental results with a nonlocal rheology. Preliminary results show that the timescale over which the force chains decorrelate is strongly associated with the inverse of the fluidity, a viscosity-like quantity. This provides a physical interpretation of its particle-scale origins. |
Monday, November 19, 2018 5:49PM - 6:02PM |
L06.00009: Steady chute flow of inelastic discs beyond the maximum angle from μ-I rheology Anurag Tripathi, Mahesh Prasad, Puneet Kumar, Ayushi Tripathi Studies investigating granular rheology in chute flow configuration suggest that steady flows occur only up to an angle tanθmax=μmax where μmax is the maximum value of the effective friction coefficient in the μ-I rheology description. To the best of our knowledge, studies beyond this maximum value of the inclination angle are not reported and it has been assumed that the flow becomes unsteady and keeps accelerating beyond this angle. We show that this assumption is incorrect and steady, fully developed chute flows well beyond the angle corresponding to maximum effective friction are indeed possible. The flows at such high inclinations show a significant slip at the base along with huge dilation of the layer. Angles as large as 44o corresponding to the inertial number I~1.7 are observed in our DEM simulations of inelastic, frictional disks. The dip in the effective friction at higher inertial numbers, observed previously in plane-shear flows, is found to be present in chute flow as well. A modified μ-I rheology predicts the properties in the bulk of the layer reasonably well even at these high values of θ, after which transition of the flow to a different regime is observed. |
Monday, November 19, 2018 6:02PM - 6:15PM |
L06.00010: Effects of external factors on segregation of a granular chute flow Saleema Panda Granular mixtures can be seen in various industries like food processing, pharmaceuticals, paints, chemicals, cosmetics, etc. During different handling procedures, the end products in these industries may vary due to the non-uniformity, i.e., segregation occurs, which is sometimes a problem as it affects the product quality. Many researchers have investigated the differences in particles size, density, and shape that leads to segregation. But, it is always not possible to change the internal factors. Here in this paper, we have presented the results by studying a dry granular chute flow. We investigated on the effect of the external factors like chute inclination, fill volume and wall roughness which can minimize the segregation. We carried out small experiments for an understanding of the flow of two-sized particle mixture. Then we conducted the numerical simulation using discrete element method by an open source code – LIGGGHTS to study the velocity and volume fraction of the species. |
Monday, November 19, 2018 6:15PM - 6:28PM |
L06.00011: Soft Particle Clogging in 2D Hoppers Mia Morrell, Eric R Weeks We study the outflow of soft, low-friction hydrogel beads from a quasi-2D hopper, examining the probability of clog formation as a function of hopper exit width. By tilting the hopper chamber, we vary the force of gravity driving the flowing bead system. We find that clogging of soft beads requires the hopper aperture to be only slightly larger than the bead diameter, and increasing the force driving the beads towards the exit results in a decreased probability of clogging, holding exit width constant. We then investigate the effects of vibration of the entire hopper system, finding that the addition of vibrations results in decreased clogging probability. We additionally conduct simulations, which suggest that the decrease in the clogging is controlled by the amplitude of the vibrations rather than the frequency. |
Monday, November 19, 2018 6:28PM - 6:41PM |
L06.00012: Sand particle retention in slurry flow using CFD-DEM approach Amira Shaffee, Wongngamlert Petchfa, Aditya Karnik, Omar K Matar Excessive sand production is a challenge in the oil-and-gas industry. Installation of a mechanical sand screen is a conventional method to reduce sand production at source. Our work investigates the efficiency of sand retention process of a wire-wrap screen via coupled Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) approach. The sedimentation of a dilute concentration of poly-dispersed particles are simulated. We find good agreement between the predicted sand productions through a sand screen with experimental data from literature. We further investigate the effect of screen inclination on sand screen efficiency. It is observed that for inclination less than 30°, sand production through the re-wrap screen is relatively unaffected. When particle adhesion is introduced, a reduction in sand production as a solid pack of particle agglomerate is formed over the sand screen. The formation of a stable sand pack allows the sand production to cease. However, when the inclination exceeds 30°, the sand pack is unstable, and continuous sand production is observed. These findings demonstrate the use of CFD-DEM for sand screen selection and design as an alternative to physical experiments. |
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