Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H04: Granular Flows I |
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Chair: Alban Sauret, University of California Santa Barbara Room: 203 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H04.00001: Localized rotational effects on granular temperature Jonathan Higham, Avinash Vaidheeswaran, William Fullmer, Jonas Saffon The granular temperature of a granular flow is seen to be analogous to a the Reynolds stress inside of a fluid flow. In both cases they represent how energy / momentum is passed around the medium. The main difference between a fluid flow and a granular flow is a fluid flows physics is governed by its viscosity, whilst a granular flow is governed by collisions and surface roughness. In a recent paper by Higham et al. (2019) it was shown a combination of individual surface roughnesses and collisions can cause rotational moments to be passed between the individual grains. These three dimensional rotations are not typically taken into consideration in simulations or in experiments, but can have quite an effect on the individual energy / momentum fluxes. In this presentation we present an experimental investigation of a two-dimension driven vortex in a granular flow. We use particle tracking to determine the spatial and rotational translations. From these data we determine what effect the localized rotations have of the granular temperature. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H04.00002: A Depth-Averaged Method for Estimating Velocity Profile Evolution in a Granular Flow Benjamin Young, Stuart Dalziel, Nathalie Vriend We present a method for estimating the depth-dependent velocity profiles of free surface granular flows from evolving surface velocity and flow-depth data. We first derive a quasi-two-dimensional three-variable depth-averaged model for free surface flows that accommodates the inclusion of a varying basal boundary condition. This model is then used in conjunction with an ensemble Kalman Filter (enKF) and free surface data to examine the inverse problem: “What is the internal velocity field of the flow, given our model and free surface data?”. We demonstrate the capabilities of this method by applying our algorithm to the Blasius boundary-layer problem described in Tsang et al. (2018). Synthetic, evolving free surface data is generated from discrete particle model (DPM) simulations and then input into the enKF/depth-averaged algorithm in order to estimate the internal velocity. These estimates are then compared to the coarse-grained DPM velocity field and are shown to provide a good fit to the synthetic data. We believe our method has clear potential, not just in modelling free surface flows, but also as a powerful data-analysis tool for experimentally validating modelling work done within the granular community. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H04.00003: Using directional-specific shear rates to correlate mass flow rate and velocity profiles in a granular conveyor Nicholas Pohlman, Hannah Higgins, Michael Roeing-Donna, Jifu Tan Velocity profiles and total mass flow rate of an industrial-style conveyor system with a storage hopper is explored using both experiments and simulation. Despite expectations of quasi-two-dimensional behavior, the velocity profiles observed at the side walls are not consistent throughout the stored material above the flighted conveyor belt. Integrating velocity profiles from high speed imaging of the experiment proved to underestimate the total mass flow rate of the system when different opening sizes and belt speeds were used. Simulations using the LIGGGHTS platform with boundary conditions similar to the experimental parameters confirm that velocity adjacent to the flights may be constant but non-uniform due to the jam-prevention gaps between flights and the walls. Results indicate that the shear rate decays differently in gravitational versus transverse directions. Fitting parameters for two unique shear rates were applied to allow better correlation of the velocity profiles and mass flow rates that were measured. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H04.00004: Slow granular flows in a Split-Bottom Couette device Peter Dsouza, Prabhu Nott We present DEM simulations and experiments of slow shear of granular materials in a split-bottom Couette device. This device is a cylindrical cup with a split base, wherein a central disc that is flush with the bottom rotates, while all other walls of the device are stationary[1]. Shear originates at the split between this disc and the rest of the base. This device has been widely used to validate rheological models for granular flows since it exhibits wide shear bands. We show that by changing the fill height of the device, we change the location of shear in the material. We then show the presence of system spanning vortices in addition to the primary azimuthal flow. These vortices are like those seen in the cylindrical Couette device[2]. We show that the form of the vortices can be explained by accounting for shear-induced dilation in the system, validating the arguments made for the vortices seen in the Cylindrical Couette device[2]. No current rheological models for slow granular flow allow for shear-induced dilation and thus can't capture these vortices. We thus make the case that dilation needs to be included in any rheological model for slow granular flows. 1.Dijksman, {\&} van Hecke, 2010, Soft Matter 2.Krishnaraj, {\&} Nott, 2016, Nature Comm [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H04.00005: Experimental study of the collapse of cohesion-controlled granular materials Alban Sauret, Adrien Gans, Mingze Gong, Olivier Pouliquen, Maxime Nicolas Cohesive granular media are encountered in many geophysical and industrial applications, examples being cement, pharmaceutical powders, or flours. Many progress has been made in the description of dry granular flow, but the flow behavior of powders remains elusive. In particular, one difficulty lies in the cohesion force between the particles. Using a recently developed method to create a Cohesion Controlled Granular Material (CCGM) and relate the interparticle cohesion to the macroscopic behavior, we consider experimentally the collapse of a column made of cohesive grains. This configuration has been extensively studied in the case of dry granular material: when the grains are released, the granular mass spreads and stops at a finite distance. The morphology of the deposit is mainly controlled by the initial aspect ratio and is independent of the material properties. Yet, for powders the inter-particle forces strongly affect the collapse. Here, we characterize the effects of cohesion on the collapse dynamics, the run-out length, and the final morphology of the deposit. These experiments illustrate that cohesive forces between particles introduced an additional complexity in this system. [Preview Abstract] |
(Author Not Attending)
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H04.00006: Energy evolution in the subaqueous granular column collapse process. Yi An, Wentao Zhang, Qingquan Liu We studied the energy evolution in the subaqueous granular column collapse process experimentally in this work to understand the unusual phenomena of long-runout reported by V. Topin et al. (PRL, 2012). We set up a well-controlled experimental facility with only one granular width to study the one layer subaqueous granular column collapse problem. We obtained the velocity field for both granular and liquid simultaneously by employing the refractive index matching and planar laser-induced fluorescence. We use Hough transform to identify and track the spherical granular and the 2D Particle Image Velocimetry to obtain the velocity field outside the granular body. We find the total kinetic energy E$_{\mathrm{k}}$ of the granular materials drop dramatically when granular particles turn horizontal with a sharp angle, which implies the granular collision is the key factor determining the runout. The viscosity of the ambient liquid may play multiple roles, on one hand, it reduces the total local collision events, on the other hand, it not only dissipates the kinematic energy but also extends the duration of the entire event. Thus a non-dimensional number describing the role of the viscosity is suggested. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H04.00007: A friction model for packings of arbitrary shaped, non-convex bodies. Damien Huet, Anthony Wachs Granular flows are present in nature and countless major industrial applications, and in most cases the shape of the particles involved is non-spherical or even non-convex. The non-sphericity and non-convexity of the particles play a tremendous role in the dynamics of the system. Therefore, developing and using reliable simulation tools is critical to accurately capture the actual behaviour of granular flows. In this work, we apply the smooth Discrete Elements Method (DEM) to study packings of arbitrary-shaped particles. In smooth DEM, the collision forces are computed explicitly with a contact model. The classical models are only valid until the system reaches a pseudo steady-state, in which particles are not able to reach a zero velocity. This numerical artifact eventually overpacks the system. Our contact model follows the approach of Costa et. al. (PRL E, 2015) and uses three spring-dashpot models: two in the translational direction and another one in the angular direction. We apply this friction model to non spherical and/or non-convex particles. We show that non-convex bodies are able to reach a pure static state. We also show that we are able to capture macroscopic properties, such as packing porosity, that agree well with experimental data. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H04.00008: Bed-load characteristics over evolving and developed subaqueous barchan dunes Erick M. Franklin, Carlos A. Alvarez In the morphodynamics of crescent-shaped dunes, known as barchans, many complex aspects are involved. One of them concerns the trajectories of individual grains over the dune, and how they affect its shape. In this study, we investigate experimentally the formation and evolution 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. We measured the trajectories of grains migrating to horns of both evolving and developed dunes and showed that most of the grains came from peripheral regions upstream of the dune centroid, with significant transverse displacements. These results diverge from the generally accepted description that the barchan horns form from the advance of the lateral dune flanks. Hence, our results reveal a new mechanism for barchan formation that might be complementary to that accepted so far. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H04.00009: Unsteady Shearing of a Granular Material in an Annular Couette Cell Han-Hsin Lin, Melany Hunt We study the transition from unsteady to steady state shearing of spherical glass beads and irregular sands in a Couette cell. By initially fluidizing the bed or compressing with a constant force, we ensure the initial state is controlled and repeatable. By comparing to simulations, we are able to capture the structure change inside the bulk. When controlling the torque, the system cannot reach a steady state when it is below a critical stress. When controlling the speed of the boundary, the shear stress at the wall increases slowly over a period of time that depends on the initial state of the bed, wall friction, shear rate, and flow along the free surface. At steady state, the stress decreases at the highest rotation speeds. Simulations with LAMMPS using Hooke's contact model show a recirculation cell driven by gravity and the free surface, which results in the increasing stress observed in the measurements. The relations of wall friction angle to normal stress for different samples have different trends. The effective friction of the inner wall matters. When using the smooth cylinder, the system needs more time to reach a steady state than using the rough cylinder. At steady state, the wall stress decreases more significantly at the highest rotation speeds compared to the rough cylinder. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H04.00010: Wiggling arthropods induce flow in granular materials Karen Daniels, Melia Kendall, Shih-Yuan Chen, Emily Brown, Bjorn Sumner, Michael Mann Just as heating a viscous fluid causes its viscosity to drop, we observe that the introduction of active particles into a passive granular material can increase its flowability. This effect can be observed, for instance, in the historical practice of aging Milbenk\"{a}se cheese in mixtures of flour and mites. In our experiments, we examine this effect by introducing flour beetle larvae (\textit{Tribolium confusum}) into agricultural grains of various sizes. We measure the timescale for bulk flow via the relaxation of a sloping pile, and the timescale for particle-scale rearrangements via diffusing wave spectroscopy. We find that the macroscopic and microscopic timescales are approximately proportional; both timescales decrease as the fraction of larvae increases, but only for samples in which the grains are smaller than the larvae. For samples in which the larvae and grains are of similar size, these two timescales decouple. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H04.00011: Experimental measurements of the torque and normal force on a helix rotating in a granular material Rogelio Valdez, Melany Hunt, Roberto Zenit The torque and the normal force produced by a helix rotating in granular matter were measured experimentally. The experiments were conducted using the rheometer, with a powder cell, for a wide range of rotational speeds. Two granular media were considered: mustard seeds and glass beads with diameter 0.203 mm. The experiments considered changes in the geometry of the helix. For a first set of tests, seven helices with the same total length but with different helix angle and wavelength were considered. For the second group, ten helices with the same geometric shape but with different numbers of turns, from 1 to 9, were used. The results show that torque and normal force are strongly dependent on the helical geometry. A maximum normal force is reached when the helix angle is around 55 degrees while the peak for the torque occurs when the helix angle is close to 40 degrees. In both cases, the measurements are nearly independent on the rotational speed of the helix. Both force and torque increase linearly with the number of coil turns for small number of coils; however, in contrast to what may be expected for a viscous fluid, the increase is not linear when the number of coils is larger than 3. Comparisons with calculations from granular resistive force theory will be presented. [Preview Abstract] |
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