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 F04: Granular Solids, Fluids, and Gases |
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Chair: Nathan Keim, California State Polytechnic University Room: Georgia World Congress Center B206 |
Monday, November 19, 2018 8:00AM - 8:13AM |
F04.00001: Continuum Dynamics of Dense Granular Materials Michael Shearer, Nico Gray Recent developments in the continuum modeling of granular materials address the long-standing problem of ill-posedness in the evolution equations. The μ(I)-rheology, in which the friction coefficient μ depends on shear rate through the dimensionless inertial number I, introduced more than a decade ago by Pouliquen and co-authors, is a crucial step in establishing regimes in which equations for incompressible 2-D granular flow are well-posed, leaving specific thresholds for the onset of ill-posedness. The introduction of compressibility, a physically relevant property captured by a variable packing fraction φ, is introduced first by prescribing φ as a decreasing function Φ(I) of inertial number. However, the resulting dynamic equations are shown to be ill-posed. A more sophisticated approach, via constitutive laws derived from Critical State Soil Mechanics, results in a set of conditions under which granular flow is well posed. This significant breakthrough in the continuum modeling of granular materials is being further explored through detailed simulation of prototypical flows. |
Monday, November 19, 2018 8:13AM - 8:26AM |
F04.00002: Dilatancy-driven secondary flows in sheared granular systems Peter Varun Dsouza, Prabhu R Nott Granular materials are an important part of many industrial processes, yet a suitable continuum model that adequately describes their flow is lacking. Krishnaraj and Nott [1] showed that shearing granular materials in a Cylindrical Couette device gives rise to a secondary flow in the form of a system-spanning toroidal vortex. Unlike the centrifugally-driven Taylor-Couette vortices, this vortex is driven by the dilatancy of the material. The vortex leaves an experimentally measured large rheological signature [2]. In this work, we show that the dilatancy-driven vortices are a generic feature in flows where the gravity and shear directions are not co-linear. Using simulations and experiments, we show that such vortices arise in a split-bottom Couette device [3]. By varying the fill height, we can change the sense of rotation and number of vorticies. We show how the form of the primary flow can explain the nature of the secondary flow. Dilation at constant pressure is not included in any available continuum model – our results show the necessity of incorporating this feature. References: 1. Krishnaraj, K. P. & Nott, P. R., Nat Commun, 2016. 2. Gutam, K. J., Mehandia, V. & Nott, P. R. Physics of Fluids, 2013 3. A. Dijksman, J. & Hecke, M. van. Soft Matter, 2010 |
Monday, November 19, 2018 8:26AM - 8:39AM |
F04.00003: How do large-force networks, or force chains, arise in granular materials? K. P. Krishnaraj, Prabhu R Nott Stress transmission in static and slowly deforming granular materials is characterized by the emergence of an inhomogeneous distribution of quasi-linear large force-carrying network of particles, called force chains. Despite many studies, there is little understanding of how such a correlated network of force-bearing chains arises from an essentially disordered assembly of grains, and what the relevance of these force chains to the macroscopic mechanics of the granular assembly is. Here we show from a classification of a subset of the contact network based on a simple definition of its linearity, there emerges a sharp transition from an essentially disordered network to an ordered, quasi-linear network of grains. This transition is remarkably similar to that of a continuous phase transition in thermodynamic systems. The sub-network of particles at the critical value of linearity are the force chains, and explain many of the observed statistical features of granular force networks. More significantly, it explains several interesting mechanical features observed in granular materials confined in various geometries. |
Monday, November 19, 2018 8:39AM - 8:52AM |
F04.00004: The Return-Point Memory of a Soft Jammed Solid Nathan C. Keim, Jacob Hass, Dani Medina, Benjamin Kauffman, Brian Kroger A soft 2D jammed material can form memories of past deformations. Our experiments cyclically shear a material made of repulsive particles at an oil-water interface, observing the motion of many particles. After many cycles, the system reaches an approximately steady state in which few new rearrangements are created. We show that in this state the system has a memory of past strain amplitudes that is similar to return-point memory in ferromagnets and many other condensed-matter systems. While the system may appear to be changed upon decreasing the strain amplitude, returning to the previous value restores the system's state, allowing readout of multiple encoded memories. We discuss similarities and differences with memory behaviors in non-jammed suspensions. |
Monday, November 19, 2018 8:52AM - 9:05AM |
F04.00005: Low-frequency oscillations in a granular column Loreto Oyarte Galvez, Nicolás Rivas, Devaraj van der Meer A vertically vibrated granular bed exhibits many phenomena, such as convection and Faraday-like surface waves. However, when the lateral dimensions of the bed are confined such that a quasi-one-dimensional geometry is formed, the only phenomena that remain are the bouncing bed and granular Leidenfrost states, where in the latter a dense cluster is floating on top of a dilute, gaseous layer of particles. This setup thus permits the observation of the granular Leidenfrost state for a wide range of energy injection parameters and more specifically allows for a thorough characterization of the low-frequency oscillation (LFO) that is present in this state. We experimentally and numerically determine the LFO frequency from the power spectral density of the center-of-mass signal of the grains, varying many parameters of the system. We find that the LFO frequency (i) is inversely proportional to the fast inertial timescale and (ii) decorrelates with a typical decay time proportional to the slow dissipative timescale in the system. Using a Langevin approach, we show that the latter is consistent with the view that the LFO is driven by the inherent noise that is present in the granular Leidenfrost state with a low number of particles. |
Monday, November 19, 2018 9:05AM - 9:18AM |
F04.00006: Thermal convection in a granular gas of hard disks system limited by dissipative lateral walls, under zero gravity. Álvaro Rodríguez-Rivas, Miguel Ángel López-Castaño, Francisco Vega Reyes We have studied the properties of the thermal convection in a granular system with no gravity (i.e., g = 0); this 2D gas is enclosed by a rectangular region provided with a pair of walls acting as kinetic energy source and other two walls (“lateral” walls) characterized by inelastic wall-particle collisions (and with a coefficient of normal restitution for wall-particle collisions αw). |
Monday, November 19, 2018 9:18AM - 9:31AM |
F04.00007: The microscopic structure of a granular gas fluidized by turbulent air wakes. Miguel Ángel López-Castaño, Álvaro Rodríguez-Rivas, Francisco Vega Reyes We study the properties of the velocity distribution function of granular gas of identical spheres; this gas is produced in our laboratory by placing ping-pong balls on a metallic mesh. We then dispose a fan whose air current intensity is controlled at will. The air stream causes turbulent wakes near the balls surface, thus producing Brownian dynamics on the gas. In particular, we analyze the case of not large average packing fractions; i.e., the system can be regarded as a granular gas since particles contacts are short and only between two spheres (thermality in the system is driven only through binary collisions). Using a high speed camera (Phantom VEO410L) we are able to track particles positions at short time invervals. In our presentation, we explain in detail the particle tracking method we use and the information conveyed on the structure of the single particle distribution function as a function of applied air flow. We also provide an analysis in order to deduce the values of particle collision frequency and mean free path in our experiment. |
Monday, November 19, 2018 9:31AM - 9:44AM |
F04.00008: Phase-coexisting patterns, horizontal segregation, and controlled convection in vertically vibrated binary granular mixtures Mohammed Istafaul Haque Ansari, Meheboob Alam We report patterns consisting of coexistence of synchronous and asynchronous states [for example, a granular gas co-existing with (i) bouncing bed, (ii) undulatory subharmonic waves, and (iii) Leidenfrost-like states] in experiments on vertically vibrated binary granular mixtures in a Hele-Shaw cell. Most experiments have been carried out with equimolar binary mixtures of glass and steel balls of same diameter by varying the total layer height (F) for a range of shaking acceleration (\Gamma). The segregation of heavier and lighter particles along the horizontal direction is shown to be the progenitor of such phase-coexisting patterns as confirmed in experiments. At strong shaking we uncover a partial convection state in which a pair of convection rolls is found to coexist with a Leidenfrost-like state. The crucial role of the relative number density of two species on controlling the buoyancy-driven granular convection is demonstrated. |
Monday, November 19, 2018 9:44AM - 9:57AM |
F04.00009: Confined Packing of Granular Rods: Bulk Experiments Julian Oneal Freeman, Cong Cao, Sean Peterson, Eric R Weeks, Scott V Franklin, Yujie Wang We conduct experiments to observe the effects surfaces have on the internal packing structure of randomly packed rods. We use cylindrical containers of different diameters, and rods of aspect ratios ranging from 4 to 32. We find that the rods packed into smaller cylindrical containers yielded lower volume fractions than in larger containers. The results are extrapolated to an infinite container size, leading to a volume fraction which decreases for increasing aspect ratio, in agreement with previous simulations. The results also suggest that the surface effect on internal packing decreases with aspect ratio as well. By comparing containers with different heights and diameters, our results suggest that the influence of the top and bottom boundaries is less significant than the influence of the side boundaries. The implication is that gravity plays a role in these experimental packings. |
Monday, November 19, 2018 9:57AM - 10:10AM |
F04.00010: Confined Packing of Granular Rods: X-ray Tomography Experiments Cong Cao, Julian Oneal Freeman, Sean Peterson, Scott V Franklin, Yujie Wang, Eric R Weeks We study 3D rod packings in cylindrical containers with two sizes by a computed tomography scanner. The aspect ratio of our rods is 1:8. We observe rods pack to a higher volume fraction phi in the larger container, suggesting a strong influence of the walls on the packing. In both containers, we observe that particles pack isotropically in the bulk, but pack differently near the boundaries. Near the bottom boundary, the rods lie flat against the container boundary but nearly maintain a same volume fraction as bulk area. In contrast, rods are much loosely packed near the side boundary and top boundary. For all three boundary layers, the layer thickness is shorter than a rod length. The differences between the bottom and top boundaries show that gravity plays an important role in the packing of rods. |
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