John F. Peters

Pore-scale simulation of dispersion

Tracer dispersion has been simulated in three-dimensional models of regular and random sphere packings for a range of Peclet numbers. A random-walk particle-tracking (PT) method was used to simulate tracer movement within pore-scale flow fields computed with the lattice-Boltzmann (LB) method. The simulation results illustrate the time evolution of dispersion, and they corroborate a number of theoretical and empirical results for the scaling of asymptotic longitudinal and transverse dispersion with Peclet number. Comparisons with nuclear magnetic resonance (NMR) spectroscopy experiments show agreement on transient, as well as asymptotic, dispersion rates. These results support both NMR findings that longitudinal dispersion rates are significantly lower than reported in earlier experimental literature, as well as the fact that asymptotic rates are observed in relatively short times by techniques that employ a uniform initial distribution of tracers, like NMR.

Hydrodynamic dispersion in confined packed beds

Pore-scale simulations of monodisperse sphere packing and fluid flow in cylinders have reproduced heterogeneities in packing density and velocity previously observed in experiment. Simulations of tracer dispersion demonstrate that these heterogeneities enhance hydrodynamic dispersion, and that the degree of enhancement is related to the cylinder radius, R. The time scale for asymptotic dispersion in a packed cylinder is proportional to R2/DT, where DT represents an average rate of spreading transverse to the direction of flow. A generalization of the Taylor–Aris model of dispersion in a tube provides qualitative predictions of the long-time dispersion behavior in packed cylinders.

Shear Band Formation in Triaxial and Plane Strain Tests

This paper presents the results of a combined theoretical and laboratory study of the formation of shear bands in triaxial compression, triaxial extension, and plane strain compression. The principal finding is that shearbands are initiated more easily under plane strain than under axially symmetric conditions of the triaxial test. The triaxial compression test is most resistant to shear banding, although the theory tends to overestimate the stability of this test. The lack of agreement between the theory and experiment requires a reassessment of the suitability of isotropic hardening laws even for monotonic loading.

A poly‐ellipsoid particle for non‐spherical discrete element method

Purpose – The purpose of this paper is to present a simple non‐symmetric shape, the poly‐ellipsoid, to describe particles in discrete element simulations that incur a computational cost similar to ellipsoidal particles.Design/methodology/approach – Particle shapes are derived from joining octants of eight ellipsoids, each having different aspect ratios, across their respective principal planes to produce a compound surface that is continuous in both surface coordinate and normal direction. Because each octant of the poly‐ellipsoid is described as an ellipsoid, the mathematical representation of the particle shape can be in the form of either an implicit function or as parametric equations.Findings – The particle surface is defined by six parameters (vs the 24 parameters required to define the eight component ellipsoids) owing to dependencies among parameters that must be imposed to create continuous intersections. Despite the complexity of the particle shapes, the particle mass, centroid and moment of ine...

Video tracking for experimental validation of discrete element simulations of large discontinuous deformations

Abstract The discrete element method (DEM) simulates large discontinuous deformations as a natural outcome of discrete particle interactions. The method is well suited for problems such as plowing, penetration and hopper flows. However, verification of DEM simulations has been largely limited to comparisons with laboratory stress-strain diagrams of two dimensional simulations of at most a few thousand ideally-shaped particles. This paper presents an automated video tracking and digital image analysis system that has been developed to obtain soil particle displacement fields and velocities from small-scale laboratory experiments. A three-dimensional simulation of a laboratory plowing experiment is performed in which a one-to-one correspondence is achieved between the number of particles and their size distribution in simulation and physical experiment. The data obtained from the video tracking system are used to evaluate the qualitative and quantitative ability of the simulation to model the experiment kinematics.

Patterned Nonaffine Motion in Granular Media

AbstractVortex-like flow patterns are often observed in experiments on granular media for which uniform strain is expected based on the loading boundary conditions. These deformations become apparent when the motion associated with uniform strain is subtracted from the total particle motion. Besides presenting an interesting phenomenon that begs explanation, these vortex patterns suggest multiscale structure for nonaffine motion as suggested by modern continuum theories. Further, the authors note that the rotational velocity field added to a uniform strain field produces a planar slip field. Thus, these structures are associated with the slip-band fields that eventually form, which are generally associated with bifurcations in the solution path of the governing partial differential equations. The authors present a procedure to extract these motion fields from discrete-element simulations along with conjugate forces associated with these motions. A key finding from the simulations is that the motions that ...

A hierarchical search algorithm for discrete element method of greatly differing particle sizes

Purpose – A critical step toward an efficient contact detection algorithm is to localize the contact search to the immediate neighborhood of each particle. In particular, cell‐based algorithms are simple and require O(N) computations but become inefficient when the particles are not roughly the same diameter. The purpose of this paper is to describe a hierarchical search method with the simplicity and efficiency of the neighbor search algorithm but which is insensitive to size gradation.Design/methodology/approach – In this method, particles are allocated to cells based on their location and size within a nested hierarchical cell space. Contact searches are limited to neighboring particles of equal size within their own hierarchy and occasionally with particles of larger size when no contacts are found within their own hierarchy.Findings – The method is shown to be effective for the most severe case of highly segregated particle distributions in which a large particle is surrounded by particles of much sm...

Soil Moisture and Thermal Behavior in the Vicinity of Buried Objects Affecting Remote Sensing Detection: Experimental and Modeling Investigation

Improvements in buried mine detection using remote sensing technology rest on understanding the effects on sensor response of spatial and temporal variability created by soil and environmental conditions. However, research efforts on mine detection have generally emphasized sensor development, while less effort has been made to evaluate the effects of the environmental conditions in which the mines are placed. If the processes governing moisture and temperature distribution near the ground surface can be captured, sensor development and deployment can be more realistically tailored to particular operational scenarios and technologies. The objective of this study is to investigate the effects of the soil environment on landmine detection by studying the influence of the thermal boundary conditions at the land-atmosphere interface and the buried objects themselves on the spatial and temporal distribution of soil moisture around shallow-buried objects. Two separate large tank experiments were performed with buried objects with different thermal properties. Experimental results were compared to results from a fully coupled heat and mass transfer numerical model. Comparison of experimental and numerical results suggests that the vapor enhancement factor used to adjust the vapor diffusive flux described based on Ficks law is not necessary under dry soil conditions. Data and simulations from this study show that the thermal signature of a buried object depends on the complex interaction among a soils water content and its thermal and hydraulic properties. Simulated thermal and saturation contrasts were generally very different for a buried landmine than for other buried objects.

Some fundamental aspects of the continuumization problem in granular Media

The central problem of devising mathematical models of granular materials is how to define a granular medium as a continuum. This paper outlines the elements of a theory that could be incorporated in discrete models such as the Discrete-Element Method, without recourse to a continuum description. It is shown that familiar concepts from continuum mechanics such as stress and strain can be defined for interacting discrete quantities. Established concepts for constitutive equations can likewise be applied to discrete quantities. The key problem is how to define the constitutive response in terms of truncated strain measures that are a practical necessity for analysis of large granular systems.

Modeling Nanoindentation of Calcium Silicate Hydrate

The discrete element method (DEM) was used to model nanoindentation of calcium silicate hydrate (C-S-H). The interparticle forces consisted of the traditional friction and contact forces that operate in granular materials, with the addition of nanometer-scale forces between gels, including van der Waals and electric double-layer forces. The contact normal forces were based on Hertz contact law. The van der Waals attractive forces were calculated on the basis of Hamakers equation. The electric double-layer forces, generated from the negative charges on the C-S-H gel surface and the ion species in the pore solution, were calculated by using Monte Carlo simulations. The particles are spherical with diameters of approximately 5 nm. Both elastic modulus and hardness, calculated from the DEM, were much smaller than the results from nanoindentation experiments. The effects of interparticle forces on the elastic modulus and hardness were studied to explore possible reasons for the differences. The simulations give insight into the morphology of C-S-H nanoparticles and the interparticle forces between C-S-H nanoparticles.