Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session R32: Granular Flows V |
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Chair: Troy Shinbrot, Rutgers University Room: 33C |
Tuesday, November 20, 2012 1:00PM - 1:13PM |
R32.00001: A numerical study of unsteady shear flows of fluid-saturated granular materials in the presence of gravity Christos Varsakelis, Miltiadis Papalexandris In this talk we present results from a numerical study of unsteady, shear flows of fluid-saturated granular materials in the presence of gravity. In our study, we employ a two-pressure, two-velocity continuum model for the mixtures of interest. The governing equations are integrated via a predictor-corrector algorithm that combines a projection method for the pressure of each phase and an interface-tracking scheme. Initially, a high particle concentration ball is placed between two parallel plates while the rest of the domain is filled with a carrier fluid. The mixture is set in motion by the horizontal movement of the upper plate with constant speed. Because of the developing shear stresses and the onset of the Rayleigh-Taylor instability, the ball deforms to a wavy finger-like shape whose length increases with time. Further, fluid entrainment produces a mushroom pattern in its frontal part. At the same time, this granular finger descends due to gravity and once it reaches the bottom plate it forms an asymmetric granular pile. This talk concludes with results from a parametric study with respect to the shear rate and the diameter of the particles. [Preview Abstract] |
Tuesday, November 20, 2012 1:13PM - 1:26PM |
R32.00002: ABSTRACT MOVED TO H32.00004 |
Tuesday, November 20, 2012 1:26PM - 1:39PM |
R32.00003: Erosion dynamics of a wet granular medium Gautier Lefebvre, Pierre Jop Liquid may give strong cohesion properties to the granular medium, and confers a solid-like behavior. We study the evolution of a fixed aggregate of wet granular matter subjected to a flow of dry grains. In the confined geometry of a thin cell, the aggregate is hold by the walls, and the dry matter flowing around will pull grains out of the aggregate. Thus, by granular erosion, the interface is modified. Image treatment allows us to follow the shape of the aggregate, and to quantify the erosion speed. We have set-up two configurations of erosion. At the center of a half-filled rotating drum, we introduce the wet material with a determined liquid content. During the rotation, the dry grains flow around the fixed obstacle and grains are pulled-out of the aggregate, reducing its size. This provides an erosion rate, related to the liquid and grains physical properties. We analyze the influence of liquid properties (surface tension, viscosity) and quantity in this geometry. A model based on the fluctuations of the flow explains the dependencies observed in the experiment. In an open cell between vertical plates, we can form a heap-shaped aggregate. Then, with a funnel of constant outlet, we inject dry grains, flowing on top of the cohesive heap. We observe destabilization of an initial flat profile in certain conditions. The coupling between the flow stress and the shape of the heap creates periodical structures, which propagate to the top through the erosion process. [Preview Abstract] |
Tuesday, November 20, 2012 1:39PM - 1:52PM |
R32.00004: An introduction to the Hele-Shaw beach experiments Anthony Thornton, Bram Van der Horn, Devaraj Van der Meer, Wout Zweers, Onno Bokhove The sea, as well as being a destructive force can also be constructive and can move great quantities of sand often forming a beach. Waves can move material both up and down the beach, leading to the construction of sloping beaches. Wave-sand dynamics are studied via experiments. The tank is narrow, just over one-particle diameter wide, creating a quasi-2D set-up also geared towards mathematical modelling. There is strong two-way feedback between the free-surface waves and the beach morphology. The waves transport the particles, changing the basal topography, causing the waves to transform from rolling to breaking. ``All'' classical breaker types (plunging, collapsing, spilling and surging) are observed on a time-scale of about a second. Finally, on longer time-scales many steady beach morphologies are observed, including dry and wet beaches, dry berms/dunes, and bars. The highlight being dry dunes which have dynamic waves crashing on the seaward-side and quiescent water on the far side. [Preview Abstract] |
Tuesday, November 20, 2012 1:52PM - 2:05PM |
R32.00005: Multiphase flow description of material pulverization Duan Zhang, Xia Ma, Balaji Jayaraman Material failure and crack growth are traditionally studied in solid mechanics. However, rapid material failure often results in growth of numerous cracks and pulverization of a material. Before pulverization, the motion of the material is described by the set of equations for solid. After pulverization, if the size of the debris piece is sufficiently small, the effect of surrounding media, such as air and water, is important, and the motion of the material is often modeled as a disperse multiphase flow. Numerical simulation of the process encounters two significant challenges. The first challenge is quite practical. That is how to interface a solid code with a fluid code. The second challenge is more subtle and difficult. That is how to describe the transition from a continuum to a granular state, and provide a proper initial condition for the multiphase calculation. In this talk, we first introduce a framework of equations capable of describing the entire pulverization process based on multiphase flow formulation, and then a numerical method capable of unifying solid and fluid calculations. Despite the need to study suitable closure models for the equations, in this talk we present a few numerical examples that have be obtained by using simple closure relations. [Preview Abstract] |
Tuesday, November 20, 2012 2:05PM - 2:18PM |
R32.00006: Fast X-ray Imaging Applications for Granular Physics Yujie Wang, Yixin Cao, Chengjie Xia, Binquan Kou, Haohua Sun, Xianghui Xiao, Kamel Fezzaa Studying granular systems with x-ray imaging technique, including x-ray computed tomography (CT) and ultrafast x-ray projection imaging, has great superiority. Due to the penetrating properties of x-ray, internal structures of a granular system could be obtained. Using x-ray CT technology, we studied packing problems with various granular systems, such as mono-dispersed hard spheres, wet spheres, rods, poly-dispersed foams, etc. At the same time, ultrafast x-ray phase contrast imaging technique provides a projective realization of evolving systems, which is one of the few experimental methods that can probe dynamic properties of granular systems. These experimental works will contribute to revealing some important properties of granular systems. [Preview Abstract] |
Tuesday, November 20, 2012 2:18PM - 2:31PM |
R32.00007: Two-level hierarchical structure in nano-powder agglomerates in gas media Lilian de Martin, Wim G. Bouwman, J. Ruud van Ommen Nanoparticles in high concentration in a gas form agglomerates due to the interparticle van der Waals forces. The size and the internal structure of these nanoparticles agglomerates strongly influence their dynamics and their interaction with other objects. This information is crucial, for example, when studying inhalation of nanoparticles. It is common to model the structure of these agglomerates using a fractal approach and to compare their dimension with the dimension obtained from aggregation models, such diffusion limited aggregation (DLA). In this work we have analyzed the structure of nanoparticles agglomerates in situ by means of Spin-Echo Small-Angle Neutron Scattering (SESANS), while they were fluidized in a gas stream. The advantage of SESANS over conventional SANS is that SESANS can measure scales up to 20 microns, while SANS does not exceed a few hundred of nanometers. We have observed that when agglomerates interact, their structure cannot be characterized by using only one scaling parameter, the fractal dimension. We have found that there are at least two structure levels in the agglomerates and hence, we need at least two parameters to describe the autocorrelation function in each level. [Preview Abstract] |
Tuesday, November 20, 2012 2:31PM - 2:44PM |
R32.00008: Shock Wave Instability in Dissipative Granular Gases Nick Sirmas, Matei Radulescu The current study addresses the stability of shock waves propagating through dissipative granular gases. We perform molecular dynamics simulations of colliding hard disks accelerated by a piston. The collisions between the particles are modeled with a constant coefficient of restitution for activated collisions. An activated state is first established through shock compression, followed by a relaxation period until equilibrium is reached. Due to the existence of the activation threshold, the compacted region retains some thermal motion. Our numerical experiments reveal that the structure of these shock waves is unstable. Distinctive high density non-uniformities are formed, which take the form of convective rolls. We find that the characteristic spacing between the bumps is correlated with the relaxation length scale, which is dependent on the coefficient of restitution and shock strength. The results are also investigated in the framework of shock wave theory. Using analytical and numerical results for the shock Hugoniot, we show that both D'yakov-Kontorovich instability, and Bethe-Zel'dovich-Thompson instability can be ruled out. Instead, the results suggest that the clustering instability of Goldhirsch and Zanetti is the dominant mechanism controlling the shock instability. [Preview Abstract] |
Tuesday, November 20, 2012 2:44PM - 2:57PM |
R32.00009: Clustering Instabilities in Homogeneously Cooling Particulate Flows Peter Mitrano, Steven Dahl, John Zenk, Christopher Ewasko, Christine Hrenya Particulate flow instabilities, such as particle clustering, are commonly observed in industrial applications (e.g., gasifiers and fluidized beds). The particle dynamics associated with such instabilities have been studied through experiment, theory, and discrete-particle simulation. However, most previous theoretical analysis has been limited to linear stability analyses, and no quantitative predictions about instabilities have been obtained via numerical simulations of hydrodynamic models or via direct simulation Monte Carlo. In this work, we use a combination of numerical hydrodynamic simulations, linear stability analyses, and discrete-particle simulations to quantitatively assess the ability of hydrodynamics to describe instabilities in particulate flows. We find excellent agreement between discrete-particle simulations and hydrodynamic simulations for the onset of particle clustering. Such agreement demonstrates the aptitude of the Enskog equation in describing particulate flows and (since velocity gradients exist) the versatility of the small-Knudsen-number expansion. A systematic under prediction of clustering onset by linear analyses exemplifies the importance of nonlinear mechanisms (e.g., viscous heating) in cluster formation. [Preview Abstract] |
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