Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session A20: Granular Flows: ApplicationsGranular
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Chair: Nicholas Pohlman, Northern Illinois University Room: 704 |
Sunday, November 19, 2017 8:00AM - 8:13AM |
A20.00001: The growth and equilibrium of barchan dunes Erick Franklin, Carlos Alvarez Barchan dunes are commonly found in oil pipelines, on river beds, on Earth deserts, and on the surface of other planets. The main feature of a barchan dune is its horns pointing downstream; however, their time evolution has yet to be fully understood. Here, we investigate experimentally the formation of subaqueous barchan dunes in a closed conduit. In our experiments, granular heaps of conical shape were placed on the bottom wall of a rectangular channel and they were entrained by turbulent water flows. For each heap, horns appear and grow until an equilibrium length is reached. The experimental results show the existence of two time scales, one for the growth and the other for the equilibrium of horns, equal to $0.5t_c$ and $2t_c$, respectively, where $t_c$ is a characteristic time scaling with the grains diameter, gravity acceleration, densities of the fluid and grains, and shear and threshold velocities. [Preview Abstract] |
Sunday, November 19, 2017 8:13AM - 8:26AM |
A20.00002: Granular flow in silos with moving exit Kiwing To We conducted granular flow experiments of mono-disperse plastic beads falling out of a cylindrical silos through a circular orifice at the bottom. When the diameter of the orifice is about twice that of the beads, no finite flow rate can be sustained because of clogging at the orifice. We constructed a silo with a bottom that can rotate with respect to the wall of the silo. Then one can rotate the bottom of the silo so that the orifice can rotate (or move in a circle if the orifice is off centered) with respect to the beads. In such a silo with rotating bottom, a finite flow rate can be sustained. While the flow rate $Q$ depends on the angular frequency $\omega$ of the rotating bottom as well as the distance $R$ of the orifice from the axis of the silo, $Q$ at different $\omega$ and $R$ can be collapsed to a single curve when $Q$ when plotted against the product of $\omega$ and $R$. [Preview Abstract] |
Sunday, November 19, 2017 8:26AM - 8:39AM |
A20.00003: The self-propulsion of a helix in granular matter Rogelio Valdes, Veronica Angeles, Elsa de la Calleja, Roberto Zenit The effect of the shape of helicoidal on the displacement of magnetic robots in granular media is studied experimentally. We quantify the influences of three main parameters of the shape of the helicoidal swimmers: body diameter, step, and the angle. We compare the experimental measurements with an empirically modified resistive force theory prediction that accounts for the static friction coefficient of the particles of the granular material, leading to good agreement. Comparisons are also made with the granular resistive force theory proposed by Goldman and collaborators. We found an optimal helix angle to produce movement and determined a relationship between the swimmer size and speed. [Preview Abstract] |
Sunday, November 19, 2017 8:39AM - 8:52AM |
A20.00004: Dynamic similarity in granular locomotion Ken Kamrin, James Slonaker, Qiong Zhang To model the flow of granular media with high accuracy, a number of subtleties arise and complex constitutive relations are needed to address them. However, making certain rheological simplifications produces a framework that is simple enough to obtain global rule-sets that can be used to aid in design without having to solve any partial differential equations or perform discrete element simulations. This talk will show how reduced-order rule-sets such as the Resistive Force Theory can be obtained from a basic frictional plasticity model, and how plasticity can further be used to produce a family of scaling laws in granular locomotion reminiscent of `wind tunnel' scaling laws in fluid dynamics. These are verified with experiments and numerical simulations. [Preview Abstract] |
Sunday, November 19, 2017 8:52AM - 9:05AM |
A20.00005: A square-force cohesion model and its extraction from bulk measurements Peiyuan Liu, Casey LaMarche, Kevin Kellogg, Christine Hrenya Cohesive particles remain poorly understood, with order of magnitude differences exhibited for prior, physical predictions of agglomerate size. A major obstacle lies in the absence of robust models of particle-particle cohesion, thereby precluding accurate prediction of the behavior of cohesive particles. Rigorous cohesion models commonly contain parameters related to surface roughness, to which cohesion shows extreme sensitivity. However, both roughness measurement and its distillation into these model parameters are challenging. Accordingly, we propose a ''square-force'' model, where cohesive force remains constant until a cut-off separation. Via DEM simulations, we demonstrate validity of the square-force model as surrogate of more rigorous models, when its two parameters are selected to match the two key quantities governing dense and dilute granular flows, namely maximum cohesive force and critical cohesive energy, respectively. Perhaps more importantly, we establish a method to extract the parameters in the square-force model via defluidization, due to its ability to isolate the effects of the two parameters. Thus, instead of relying on complicated scans of individual grains, determination of particle-particle cohesion from simple bulk measurements becomes feasible. [Preview Abstract] |
Sunday, November 19, 2017 9:05AM - 9:18AM |
A20.00006: Experiments in a flighted conveyor comparing shear rates in compressed versus free surface flows Nicholas Pohlman, Hannah Higgins, Kamila Krupiarz, Ryan O'Connor Uniformity of granular flow rate is critical in industry. Experiments in a flighted conveyor system aim to fill a gap in knowledge of achieving steady mass flow rate by correlating velocity profile data with mass flow rate measurements. High speed images were collected for uniformly-shaped particles in a bottom-driven flow conveyor belt system from which the velocity profiles can be generated. The correlation of mass flow rates from the velocity profiles to the time-dependent mass measurements will determine energy dissipation rates as a function of operating conditions. The velocity profiles as a function of the size of the particles, speed of the belt, and outlet size, will be compared to shear rate relationships found in past experiments that focused on gravity-driven systems. The dimension of the linear shear and type of decaying transition to the stationary bed may appear different due to the compression versus dilation space in open flows. The application of this research can serve to validate simulations in discrete element modeling and physically demonstrate a process that can be further developed and customized for industry applications, such as feeding a biomass conversion reactor. [Preview Abstract] |
Sunday, November 19, 2017 9:18AM - 9:31AM |
A20.00007: Simulation of a Magneto-Rheological Fluid Based, Jamming, Soft Gripper Using the Soft Sphere DEM in LIGGGHTS Thomas Leps, Christine Hartzell, Norman Wereley, Young Choi Jamming soft grippers are excellent universal grippers due to their low dependence on the shape of objects to be grabbed, and low stiffness, mitigating the need for object shape data and expensive force control of a stiff system. These grippers now rely on jamming transitions of dry grains under atmospheric pressure to hold objects. In order to expand their use to space environments, a gripper using magnetic actuation of a magneto-rheological fluid (MR Gripper) is being developed. The MR fluid is a suspension of $\mu$m scale iron grains in a silicone oil. When un-magnetized the fluid behaves as a dense suspension with low Bagnold number. When magnetized, it behaves like a jammed granular material, with magnetic forces between the grains dominating. We are simulating the gripper using LIGGGHTS, an open-source soft sphere DEM code. We have modeled both the deformable gripper membrane and the MR fluid itself using the LIGGGHTS framework. To our knowledge, this is the first time that the induced magnetic dipoles required to accurately simulate the jamming behavior of MR fluids have been modeled in LIGGGHTS. This simulation allows the rapid optimization of the hardware and magnetic field geometries, as well as the fluid behavior, without time consuming, and costly prototype revisions. [Preview Abstract] |
Sunday, November 19, 2017 9:31AM - 9:44AM |
A20.00008: Transient response in granular bounded heap flows Hongyi Xiao, Julio M. Ottino, Richard M. Lueptow, Paul B. Umbanhowar Heap formation, a canonical granular flow, is common in industry and is also found in nature. Here, we study the transition between steady flow states in quasi-2D bounded heaps by suddenly changing the feed rate from one fixed value to another. During the transition, in both experiments and discrete element method simulations, an additional wedge of flowing particles propagates over the rising free surface. The downstream edge of the wedge - the wedge front - moves downstream with velocity inversely proportional to the square root of time. An additional longer duration transient process continues after the wedge front reaches the downstream wall. The transient flux profile during the entire transition is well modeled by a diffusion-like equation derived from local mass balance and a local linear relation between the flux and the surface slope. Scalings for the transient kinematics during the flow transitions are developed based on the flux profiles. [Preview Abstract] |
(Author Not Attending)
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A20.00009: Density-wave fronts on the brink of wet granular condensation Kai Huang, Andreas Zippelius From sand dunes to Faraday heaping, driven granular matter, i.e., large agglomeration of macroscopic particles, is rich pattern forming system. When a granular material is partially wet (e.g., wet sand on the beach), a different pattern forming scenario arises due to the cohesive particle-particle interactions. Here, we focus on the formation of density-wave fronts in an oscillated wet granular layer undergoing a gas-liquid-like transition \footnote{A. Zippelius and K. Huang, \textbf{Sci. Rep.} 7, 3613}. The threshold of the instability is governed by the amplitude of the vertical vibrations. Fronts, which are curved into a spiral shape, propagate coherently along the circular rim of the container with leading edges. They are stable beyond a critical distance from the container center. Based on the measurement of the critical distance and the rotation frequency, we propose a model for the pattern formation by considering the competition between the time scale for the collapse of cohesive particles and that of the energy injection resisting this process. [Preview Abstract] |
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