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Defining and testing a granular continuum element

Description: Continuum mechanics relies on the fundamental notion of amesoscopic volume "element" in which properties averaged over discreteparticles obey deterministic relationships. Recent work on granularmaterials suggests a continuum law may be inapplicable, revealinginhomogeneities at the particle level, such as force chains and slow cagebreaking. Here, we analyze large-scale Discrete-Element Method (DEM)simulations of different granular flows and show that a "granularelement" can indeed be defined at the scale of dynamical correlations,roughly three to five particle diameters. Its rheology is rather subtle,combining liquid-like dependence on deformation rate and solid-likedependence on strain. Our results confirm some aspects of classicalplasticity theory (e.g., coaxiality of stress and deformation rate),while contradicting others (i.e., incipient yield), and can guide thedevelopment of more realistic continuum models.
Date: December 3, 2007
Creator: Rycroft, Chris H.; Kamrin, Ken & Bazant, Martin Z.
Partner: UNT Libraries Government Documents Department

‘Granular Elasticity’ and the loss of elastic stability in granular materials

Description: A recently proposed hyperelastic model for granular materials, called "granular elasticity", identifies a yield angle as a result of thermodynamic instability. GE gives yield angles that are smaller than those found in real materials; a generalization of the theory is considered here that includes dependence on the third strain invariant. This generalization proves unsuccessful, as it gives smaller, not larger, yield angles. Fully convex hyperelastic models are identified as a point for future investigation.
Date: July 1, 2009
Creator: Humrickhouse, P. W.
Partner: UNT Libraries Government Documents Department

Assessing continuum postulates in simulations of granular flow

Description: Continuum mechanics relies on the fundamental notion of a mesoscopic volume"element" in which properties averaged over discrete particles obey deterministic relationships. Recent work on granular materials suggests a continuum law may be inapplicable, revealing inhomogeneities at the particle level, such as force chains and slow cage breaking. Here, we analyze large-scale three-dimensional Discrete-Element Method (DEM) simulations of different granular flows and show that an approximate"granular element" defined at the scale of observed dynamical correlations (roughly three to five particle diameters) has a reasonable continuum interpretation. By viewing all the simulations as an ensemble of granular elements which deform and move with the flow, we can track material evolution at a local level. Our results confirm some of the hypotheses of classical plasticity theory while contradicting others and suggest a subtle physical picture of granular failure, combining liquid-like dependence on deformation rate and solid-like dependence on strain. Our computational methods and results can be used to guide the development of more realistic continuum models, based on observed local relationships betweenaverage variables.
Date: August 26, 2008
Creator: Rycroft, Chris; Kamrin, Ken & Bazant, Martin
Partner: UNT Libraries Government Documents Department

Theoretical model of granular compaction

Description: Experimental studies show that the density of a vibrated granular material evolves from a low density initial state into a higher density final steady state. The relaxation towards the final density follows an inverse logarithmic law. As the system approaches its final state, a growing number of beads have to be rearranged to enable a local density increase. A free volume argument shows that this number grows as N = {rho}/(1 {minus} {rho}). The time scale associated with such events increases exponentially e{sup {minus}N}, and as a result a logarithmically slow approach to the final state is found {rho} {infinity} {minus}{rho}(t) {approx_equal} 1/lnt.
Date: November 1, 1997
Creator: Ben-Naim, E.; Knight, J.B.; Nowak, E.R.; Jaeger, H.M. & Nagel, S.R.
Partner: UNT Libraries Government Documents Department

Lightweight alumina refractory aggregate. Phase 2, Pilot scale development

Description: Kilogram quantities of refractory aggregate were prepared from both a paste and a pelletized form of extruder feed material in both bench and pilot-scale equipment. The 99{sup +} % alumina aggregate exhibited a bulk density approaching 2.5 g/cm{sup 3} and a fired strength slightly lower than fused alumina. Based on initial evaluation by two refractory manufacturers in brick or castable applications, the new aggregate offered adequate strength with thermal conductivity reductions up to 34%, depending on the temperature and application of the new aggregate in these initial trials. The new aggregate was simply substituted for Tabular{trademark} in the refractory formulation. Thus, there is room for improvement through formulation optimization with the lightweight aggregate. The new aggregate offers a unique combination of density, strength, and thermal properties not available in current aggregate. To this point in time, technical development has led to a pelletized formulation with borderline physical form leaving the Eirich mixer. The formulation requires further development to provide more latitude for the production of pelletized material without forming paste, while still reducing the bulk density slightly to reach the 2.5 g/cm{sup 3} target. The preferred, pelletized process flowsheet was outlined and a preliminary economic feasibility study performed based on a process retrofit into Alcoa`s Arkansas tabular production facilities. Based on an assumed market demand of 20,000 mt/year and an assumed selling price of $0.65/lb (25% more than the current selling price of Tabular{trademark}, on a volume basis), economics were favorable. Decision on whether to proceed into Phase 3 (full- scale demonstration) will be based on a formal market survey in 1994 October.
Date: November 1, 1994
Creator: Swansiger, T.G. & Pearson, A.
Partner: UNT Libraries Government Documents Department

[Development of a ballistic furnace for shell production]. Annual report 1998

Description: During the fourth contract year, the authors continued to develop Ballistic technology of shell formation. A new upgraded version of Ballistic Furnace with longer hot zone (1.56m) and cooling one (1.2m) had been finally assembled, and a lot of shell formation experiments had been carried out. The change of the Ballistic Furnace configuration has led to significant changing in operational conditions suitable for shells production. They had found optimal operational conditions for some grades of initial granules giving them high yield of good shells. Serious attention was paid on initial granules preparation. In the experiments some unexpected results were obtained--first of all it was a strong influence of temperature profile, an initial granule velocity and a trajectory angle on good quality shells yield. Those observations made them consider some additional physical phenomena (initial granule defragmentation and gas convection inside hot zone) to explain good shell formation. Appropriate estimations of the velocity of possible convectional gas currents in the hot zone, strength of formed shells, thermal stress in an initial granule caused by its fast heating in the ballistic furnace etc. were made. Good quality shells up to 2mm in diameters with high yield were produced. Although a production of good quality shells in diameter range > 1.8 mm stays an easy job, their experience led them to declare that Ballistic technology hasn't reach its boundaries, and future development will allow them to obtain perfect results.
Date: December 31, 1998
Creator: Cook, R. & Isakov, A. I.
Partner: UNT Libraries Government Documents Department

Spontaneous spirals in vibrated granular chains

Description: We present experimental measurements on the spontaneous formation of compact spiral structures in vertically-vibrated granular chains. Under weak vibration when the chain is quasi two-dimensional and self-avoiding, spiral structures emerge from random initial configurations. We compare the spiral geometry with that of an ideal tight spiral. Globally, the spiral undergoes a slow rotation such that to keep itself wound, while internally, fast vibrational modes are excited along the backbone with transverse oscillations dominating over longitudinal ones.
Date: January 1, 2002
Creator: Ecke, R. E. (Robert E.); Daya, Z. A. (Zahir A.); Rivera, M. K. (Michael K.) & Ben-Naim, E. (Eli)
Partner: UNT Libraries Government Documents Department

High-pressure shock behavior of WC and Ta2O5 powders.

Description: Planar shock experiments were conducted on granular tungsten carbide (WC) and tantalum oxide (Ta{sub 2}O{sub 5}) using the Z machine and a 2-stage gas gun. Additional shock experiments were also conducted on a nearly fully dense form of Ta{sub 2}O{sub 5}. The experiments on WC yield some of the highest pressure results for granular materials obtained to date. Because of the high distention of Ta{sub 2}O{sub 5}, the pressures obtained were significantly lower, but the very high temperatures generated led to large contributions of thermal energy to the material response. These experiments demonstrate that the Z machine can be used to obtain accurate shock data on granular materials. The data on Ta{sub 2}O{sub 5} were utilized in making improvements to the P-{lambda} model for high pressures; the model is found to capture the results not only of the Z and gas gun experiments but also those from laser experiments on low density aerogels. The results are also used to illustrate an approach for generating an equation of state using only the limited data coming from nanoindentation. Although the EOS generated in this manner is rather simplistic, for this material it gives reasonably good results.
Date: October 1, 2011
Creator: Knudson, Marcus D. (Sandia National Laboratories, Albuquerque, NM); Reinhart, William Dodd (Sandia National Laboratories, Albuquerque, NM); Vogler, Tracy John & Root, Seth (Sandia National Laboratories, Albuquerque, NM)
Partner: UNT Libraries Government Documents Department

Fast spot-based multiscale simulations of granular drainage

Description: We develop a multiscale simulation method for dense granular drainage, based on the recently proposed spot model, where the particle packing flows by local collective displacements in response to diffusing"spots'" of interstitial free volume. By comparing with discrete-element method (DEM) simulations of 55,000 spheres in a rectangular silo, we show that the spot simulation is able to approximately capture many features of drainage, such as packing statistics, particle mixing, and flow profiles. The spot simulation runs two to three orders of magnitude faster than DEM, making it an appropriate method for real-time control or optimization. We demonstrateextensions for modeling particle heaping and avalanching at the free surface, and for simulating the boundary layers of slower flow near walls. We show that the spot simulations are robust and flexible, by demonstrating that they can be used in both event-driven and fixed timestep approaches, and showing that the elastic relaxation step used in the model can be applied much less frequently and still create good results.
Date: May 22, 2009
Creator: Rycroft, Chris H.; Wong, Yee Lok & Bazant, Martin Z.
Partner: UNT Libraries Government Documents Department

A generalization of Reiner’s mathematical model for wet sand

Description: In this paper we modify the constitutive relation derived by Reiner (1945), to describe dilatancy in wet sand, by suggesting that the shear viscosity would depend on the shear rate and the volume fraction. We then look at the flow of a saturated densely packed bed of particles (with liquid in the pores) between two horizontal flat plates. We obtain exact solutions for a very special case.
Date: January 1, 2011
Creator: Massoudi, Mehrdad
Partner: UNT Libraries Government Documents Department

Granular filtration in a fluidized bed

Description: Successful development of advanced coal-fired power conversion system often requires reliable and efficient cleanup devices that can remove particulate and gaseous pollutants from high-temperature, high- pressure gas streams. A novel filtration concept for particulate cleanup has been developed at the U.S. Department of Energy`s Morgantown Energy Technology Center (METC). The filtration system consists of a fine metal screen filter immersed in a fluidized bed of granular material. As the gas stream passes through the fluidized bed, a layer of the bed granular material is entrained and deposited at the screen surface. This material provides a natural granular filter to separate fine particles from the gas stream passing through the bed. Since the filtering media is the granular material supplied by the fluidized bed, the filter is not subjected to blinding like candle filters. Because only the in-flowing gas, not fine particle cohesive forces, maintains the granular layer at the screen surface, once the thickness and permeability of the granular layer are stabilized, it remains unchanged as long as the in-flowing gas flow rate remains constant. The weight of the particles and the turbulent nature of the fluidized bed limits the thickness of the granular layer on the filter leading to a self-cleaning attribute of the filter. The granular filtration testing system consisted of a filter, a two-dimensional fluidized bed, a continuous powder feeder, a laser-based, in-line particle counting, sizing, and velocimeter (PCSV), and a continuous solid feeding/bed material withdrawal system. The two-dimensional, transparent fluidized bed allowed clear observation of the general fluidized state of the granular material and the conditions under which fines are captured by the granular layer.
Date: December 31, 1996
Creator: Mei, J.S. & Yue, P.C.
Partner: UNT Libraries Government Documents Department

Shock wave structure in heterogeneous reactive media

Description: Continuum mixture theory and mesoscale modeling are applied to describe the behavior of shock-loaded heterogeneous media. One-dimensional simulations of gas-gun experiments demonstrate that the wave features are well described by mixture theory, including reflected wave behavior and conditions where significant reaction is initiated. Detailed wave fields are resolved in numerical simulations of impact on a lattice of discrete explosive {open_quotes}crystals{close_quotes}. It is shown that rapid distortion first occurs at material contact points; the nature of the dispersive fields includes large amplitude fluctuations of stress over several particle pathlengths. Localization of energy causes {open_quotes}hot-spots{close_quotes} due to shock focusing and plastic work as material flows into interstitial regions.
Date: June 1, 1997
Creator: Baer, M.R.
Partner: UNT Libraries Government Documents Department

Studies of complexity in fluid systems

Description: This is the final report of Grant DE-FG02-92ER25119, ''Studies of Complexity in Fluids'', we have investigated turbulence, flow in granular materials, singularities in evolution of fluid surfaces and selective withdrawal fluid flows. We have studied numerical methods for dealing with complex phenomena, and done simulations on the formation of river networks. We have also studied contact-line deposition that occurs in a drying drop.
Date: June 12, 2000
Creator: Nagel, Sidney R.
Partner: UNT Libraries Government Documents Department

Ceramic powder compaction

Description: With the objective of developing a predictive model for ceramic powder compaction we have investigated methods for characterizing density gradients in ceramic powder compacts, reviewed and compared existing compaction models, conducted compaction experiments on a spray dried alumina powder, and conducted mechanical tests and compaction experiments on model granular materials. Die filling and particle packing, and the behavior of individual granules play an important role in determining compaction behavior and should be incorporated into realistic compaction models. These results support the use of discrete element modeling techniques and statistical mechanics principals to develop a comprehensive model for compaction, something that should be achievable with computers with parallel processing capabilities.
Date: December 31, 1995
Creator: Glass, S.J.; Ewsuk, K.G. & Mahoney, F.M.
Partner: UNT Libraries Government Documents Department

Studies of granular flow down an inclined chute. Final technical report

Description: An inclined chute facility and its associated diagnostics has been developed and utilized to study the flow of granular materials. A variety of flow regimes and flow phenomena were observed. Fully developed flows were observed over a bumpy base for a range of slopes. Under some conditions, these flows were dominated by friction and under other conditions, collisions played a dominant role. A variety of unsteady flows were also observed. These include decelerating flows, accelerating flows, and wavy (periodic) flows. The characteristics of the base strongly influenced the flow regime and flow dynamics.
Date: June 1, 1996
Creator: Hanes, D.M.
Partner: UNT Libraries Government Documents Department

Project progress report

Description: Works of two variety have been fulfilled: first, research of polystyrene shells formation conditions in drop tower furnace and ballistic furnace; second, creation of computer codes for simulation of shells formation processes, including numerous nucleation. Besides that polystyrene shells with diameter up to 2 mm transmitted to LLNL in parcel.
Date: December 31, 1997
Creator: Isakov, A. I.
Partner: UNT Libraries Government Documents Department

Final technical report

Description: Granular flows of nearly elastic, identical spheres down bumpy inclines are described. A numerical technique is developed to overcome the difficulties associated with the ill-defined `tops` of these flows.
Date: August 1, 1996
Creator: Richman, M.
Partner: UNT Libraries Government Documents Department

A theoretical and numerical study of the flow of granular materials. Quarterly report, April 1, 1992--June 30, 1992

Description: Linearized stability equations for flow of granular materials down an inclined plane are derived for a continuum model [cf. Goodman and Cowin (1971), Rajagopal and Massoudi (1990)]. The basic flow exhibits multiplicity of solutions, one in which the volume fraction increases monotonically from the inclined plane to the free surface, and the other in which the volume fraction increases monotonically. Solutions to the basic equations were presented in the previous report. Next, we have to solve the stability equations, numerically.
Date: December 31, 1992
Creator: Rajagopal, K.R.
Partner: UNT Libraries Government Documents Department

Force models for particle-dynamics simulations of granular materials

Description: Engineering-mechanics contact models are utilized to describe the inelastic, frictional interparticle forces acting in dry granular systems. Simple analyses based on one-dimensional chains are utilized to illustrate wave propagation phenomena in dense and dilute discrete particulates. The variation of restitution coefficient with impact velocity is illustrated for a variety of viscous and hysteretic normal force models. The effects of interparticle friction on material strength in discrete-particle simulations are much closer to measured values than are theories that do not allow article rotations.
Date: December 1, 1994
Creator: Walton, O.R.
Partner: UNT Libraries Government Documents Department

Wet solids flow enhancement

Description: The objective was to visualize the flow of granular materials in the silo using Nuclear Magnetic Resonance. This was done by introducing traces. Mustard seeds and poppy seeds were used as trace particles. The region sampled was a cylinder 25 mm in diameter and 40 mm in length. Eight slices containing 128 by 128 to 256 by 256 pixels were generated for each image.
Date: July 1, 1997
Creator: Caram, H.S.; Agrawal, D.K. & Foster, N.
Partner: UNT Libraries Government Documents Department