Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A6: Granular Flows: Force Transmission |
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Chair: Joshua Dijksman, Wageningen University Room: 105 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A6.00001: Quantitative Comparison of Experiments and Numerics in Granular Materials Joshua Dijksman, Jie Ren, Robert Behringer, Lenka Kovalcinova, Lou Kondic, Miro Kramar, Konstantin Mischaikow It is challenging to experimentally probe the microstructure of sheared granular media. Comparisons of numerical results to experiments are thus rare. We present a direct match of experimental and discrete element method results on a sheared two dimensional granular system. We compare the micro and mesostructural properties of the packing using several different metrics, among which measures that quantify the role of the force network topology. We find a quantitative match in the experimental and numerical approaches, and our results surprisingly indicate that the number of rattlers in the packing is a robust indicator of the mechanical and topological composition of the packing. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A6.00002: Local to global avalanches in sheared granular materials Dengming weng, Dong Wang, Thibault Bertrand, Jonathan Bares, Bob Berhinger Commonly, granular materials yield or flow if sufficiently large shear stress is applied, leading to avalanche-like behavior. Rearrangement phenomenon can produce dramatic events like snow avalanches, land-slides or earthquakes. For experimentally sheared media, we seek to understand the dynamics of the grain rearrangements from the local to the global scale. In this work, force networks and displacement fields are measured on two-dimensional sheared material for cyclically sheared photoelastic circular particles. Avalanches, their size, location and duration are extracted at the global scale from the rapid variation of the macroscopic energy stored in the system whereas at the local scale they are measured from the energy drop, displacement and rotation of each particle. Statistics of those different quantities are computed and correlated to test their intrinsic entanglement and analyze their universal dynamics. These results are quantitatively different from what has been observed for different analytic coarse-grained approaches and permit a clear measurement of the effect of the packing fraction and inter-particle friction coefficient on the statistical behavior. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A6.00003: Avalanches and local force evolution in a granular stick-slip experiment Aghil Abed Zadeh, Jonathan Bares, Robert Behringer We carry out experiments to characterize stick-slip 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. The distributions of energy release and time duration of avalanches during slip obey power laws. We analyze the power spectrum of the force signal to understand the effect of the loading speed and of the spring stiffness on the statistical behavior of the system. From a more local point of view and by using a high speed camera and the photoelastic properties of our particles, we characterize the internal granular structure during avalanches. By image processing and analyzing the skeleton of force network inside the media, we try to understand the flow of particles and evolution of force chains inside the media and during avalanches. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A6.00004: Force network in a three-dimensional sheared material Nicolas Brodu, Jonathan Bares, Joshua Dijksman, Bob Behringer Force chains in 2D granular material have been widely studied over the past decade. However the force network evolution when a 3D granular medium is sheared remains poorly understood due to the complexity of carrying out experimental observations. We present experiments using a novel apparatus to measure particle motion and inter-particle forces in the case of the quasi-static deformation of a 3D sphere packing subject to shear and compression. We perform these experiments on slightly polydisperse and very low-friction soft hydrogel spheres. We resolve the microscopic force network in a three dimensional packing by imaging the entire packing at each loading steps using a laser scanning technique. By resolving particle deformations via custom image analysis software, we extract all particle contacts and contact forces with a very good accuracy. We observe an increase of pressure, P, the Reynolds pressure during shear. This rise in pressure is associated with the evolution of the microscopic force network. The flow of particles is also investigated and correlated to the evolution of the network. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A6.00005: Forces and flows during high speed impacts on a non-Newtonian suspension Melody Lim, Jonathan Bares, Robert Behringer Above a certain mass fraction of particles, suspensions of dense granular particles in water exhibit non-Newtonian behavior, including impact-activated solidification. Although it has been suggested that the solidification of the suspension depends on interactions with the suspension boundary, quantitative experiments on the forces experienced by the boundaries of the suspension have not been reported. In the present experiments, we determine the magnitude and timings of impactor-driven events in both the boundaries and body of the suspension using high-speed video, tracer particles, and photoelastic container boundaries. We observe a shock-like propagation in the cornstarch suspension during impact. The dynamics of this shockfront are strongly correlated to the dynamics of the intruder. Additionally, we observe a second extremely fast shockfront, associated with the propagation of forces to the boundaries of the suspension. The dynamics of this shockfront do not depend on the intruder dynamics, but are correlated to the volume fraction of cornstarch particles in the suspension. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A6.00006: Fluid mechanical scaling of impact craters in unconsolidated granular materials Colin S. Miranda, David R. Dowling A single scaling law is proposed for the diameter of simple low- and high-speed impact craters in unconsolidated granular materials where spall is not apparent. The scaling law is based on the assumption that gravity- and shock-wave effects set crater size, and is formulated in terms of a dimensionless crater diameter, and an empirical combination of Froude and Mach numbers. The scaling law involves the kinetic energy and speed of the impactor, the acceleration of gravity, and the density and speed of sound in the target material. The size of the impactor enters the formulation but divides out of the final empirical result. The scaling law achieves a 98\% correlation with available measurements from drop tests, ballistic tests, missile impacts, and centrifugally-enhanced gravity impacts for a variety of target materials (sand, alluvium, granulated sugar, and expanded perlite). The available measurements cover more than 10 orders of magnitude in impact energy. For subsonic and supersonic impacts, the crater diameter is found to scale with the 1/4- and 1/6-power, respectively, of the impactor kinetic energy with the exponent crossover occurring near a Mach number of unity. The final empirical formula provides insight into how impact energy partitioning depends on Mach number. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A6.00007: Subsurface Explosions in Granular Media Shuyue Lai, Ryan Houim, Elaine Oran Numerical simulations of coupled gas-granular flows are used to study properties of shock formation and propagation in media, such as sand or regolith on the moon, asteroids, or comets. The simulations were performed with a multidimensional fully compressible model, GRAF, which solves two sets of coupled Navier-Stokes equations, one for the gas and one for the granular medium. The specific case discussed here is for a subsurface explosion in a granular medium initiated by an equivalent of 200g of TNT in depths ranging from 0.1m to 3m. The background conditions of 100K, 10 Pa and loose initial particle volume fraction of 25{\%} are consistent with an event on a comet. The initial blast creates a cavity as a granular shock expands outwards. Since the gas-phase shock propagates faster than the granular shock in loose, granular material, some gas and particles are ejected before the granular shock arrives. When the granular shock reaches the surface, a cap-like structure forms. This cap breaks and may fall back on the surface and in this process, relatively dense particle clusters form. At lower temperatures, the explosion timescales are increased and entrained particles are more densely packed. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A6.00008: A theoretical study of burrowing in dry soil using razor clam-inspired kinematics Amos Winter, Monica Isava This work investigates whether the digging kinematics of \textit{Ensis directus}, the Atlantic razor clam, could be utilized in dry soil. In wet soil, \textit{E. directus} uses contractions of its valves to relieve stress on the surrounding soil, and then draw water towards its body to create a pocket of fluidized substrate. This locally fluidized zone requires much less force to move through than static soil, resulting in burrowing energy that scales linearly with depth, rather than depth squared. In dry soil, if the valves of a clam-like device are contracted fast enough, the horizontal stress in the soil could be brought to a zero-stress state. This would correspondingly reduce the local vertical stresses to zero, which could drastically lower the forces required to burrow compared to moving through static dry soil. Using analytical models of soil failure mechanics, we investigated the critical timescales for inducing a zero-stress state in soil surrounding an \textit{E. directus}-like device with contracting valves. This device was modeled as a similar size to a real razor clam (15 mm wide). It was found that for most dry soils, the device would have to contract its valves in 0.02 seconds, a speed within the realm of possibility for a mechanical system. These results suggest that the burrowing method used by \textit{E. directus} could feasibly be adapted for digging in dry soil. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A6.00009: Resolving Two Dimensional Angular Velocity within a Rotary Tumbler Nathaniel Helminiak, David Helminiak, Vikram Cariapa, John Borg In this study, a horizontally oriented cylindrical tumbler, filled at variable depth with cylindrical media, was rotated at various constant speeds. A monoplane layer of media was photographed with a high-speed camera and images were post processed with Particle Tracking Velocimetry (PTV) algorithms in order to resolve both the translational and rotational flow fields. Although the translational velocity fields have been well characterized, contemporary resources enabled the ability to expand upon and refine data regarding rotational characteristics of particles within a rotary tumbler. The results indicate that particles rotate according to intermittent no-slip interactions between the particles and solid body rotation. Particles within the bed, not confined to solid body rotation, exhibited behavior indicative of gearing between particles; each reacting to the tangential component of contact forming rotation chains. Furthermore, it was observed that solid body interactions corresponded to areas of confined motion, as areas of high interaction dissuaded no-slip rotation, while areas of developing flow tended towards no-slip rotation. [Preview Abstract] |
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