Paul Langston

Validation tests on a distinct element model of vibrating cohesive particle systems

Abstract The discrete element method (DEM) is becoming widely used to simulate granular flows. This method simulates the individual dynamics of all particles in an assembly by numerically integrating their acceleration resulting from all the contact forces. It is generally recognised that such a complex model must be validated by comparison with experimental results. Indeed the authors have addressed this in earlier publications given in the references. However, one important aspect is often given little attention, and that is testing the code to ensure that the computer program executes the model specification correctly. This paper describes in detail a DEM program for cohesive particle vibration and shows some simple simulations that have helped to verify the code. It is concluded that these ‘mathematical tests’ on artificial situations can uncover bugs in programs that appear to be running correctly, even if they appear to simulate real experiments reasonably well. These tests are published with the aim of helping others validate their programs in similar applications. The paper also discusses the widely used simplification of particle–particle interactions by Hookes law and suggests that its validity depends upon the application. The main limitation of the DEM technique is the computational (CPU) time required. Techniques to minimise this are described, in particular in relation to particle referencing and array sizes. The runtimes shown illustrate the importance of optimising array sizes for the neighbourhood lists.

Cooperation versus competition in a mass emergency evacuation: A new laboratory simulation and a new theoretical model

Virtual reality technology is argued to be suitable to the simulation study of mass evacuation behavior, because of the practical and ethical constraints in researching this field. This article describes three studies in which a new virtual reality paradigm was used, in which participants had to escape from a burning underground rail station. Study 1 was carried out in an immersion laboratory and demonstrated that collective identification in the crowd was enhanced by the (shared) threat embodied in emergency itself. In Study 2, high-identification participants were more helpful and pushed less than did low-identification participants. In Study 3, identification and group size were experimentally manipulated, and similar results were obtained. These results support a hypothesis according to which (emergent) collective identity motivates solidarity with strangers. It is concluded that the virtual reality technology developed here represents a promising start, although more can be done to embed it in a traditional psychology laboratory setting.

Particle and droplet size analysis from chord distributions

Two methods of estimating particle or droplet size distributions from chord measurements are presented and evaluated. The first is a probability apportioning method (PAM). This assumes that particles are cut randomly and calculates a diameter probability distribution from each chord size detected. The distribution is then accumulated over all chord measurements. PAM has proved to be a fairly robust method when the particle diameters are known. The second is a finite element method (FEM) that has been found to be more generally applicable where there is a wide range of unknown particle diameters. Both methods have been rigorously tested on sets of chord data developed from either the ideal probability functions, a distinct element simulation or true data from a chord measuring instrument (Par-Tec).

Integration schemes and damping algorithms in distinct element models

Abstract Computational run times in distinct element method (DEM) simulations of granular flow can be large and limit the size of the system being modeled. Hence, it is important to use efficient numerical integration schemes. This paper investigates some numerical integration schemes for their accuracy, stability and computational efficiency. It also investigates the effect of different particle contact damping algorithms on the model mathematical accuracy and stability. It is shown that the half-step leapfrog Verlet algorithm is the best integration scheme, while Euler is poor in terms of accuracy. Non-linear damping has been shown in the literature to be more realistic in terms of experimental data on particle impact coefficient of restitution. This was reproduced here. This paper also shows that no n-linear damping reduces the mathematical error in the integration scheme because the force change is less discontinuous. However, in particle assembly simulations, filling a hopper, the non-linear damping model was less stable, probably because less energy is dissipated at low velocities.

Microstructural simulation and imaging of granular flows in two- and three-dimensional hoppers

Abstract Distinct element simulations of granular flows in two- and three-dimensional hoppers are compared with imaging data from conventional photography and gamma-ray tomography where information of the order of the particle size can be extracted. A novel feature of these comparisons is that both particle and vessel dimensions are matched exactly between the experiments and the computer simulations, thereby leaving little scope for speculation regarding śscale effects’ which are often used to justify scepticism over the validity of simulation predictions. Another novel feature of the work is that quantitative comparisons are provided during the entire period of filling and discharge events rather than selecting an arbitrary ‘snapshot’ in time, as is often the case in such simulation studies. Microstructural inspection of two-dimensional photographs of systems with large disc particles provides quantitative information which shows good agreement with simulation in terms of packing height, static and flowing voidage, stagnant/flow boundaries in funnel flow and heap/repose angles. Three-dimensional solids fraction data from packed beds of 7 mm diameter maple peas obtained by transmission gamma-ray tomography show encouraging agreement with simulation. An important result of these investigations is the degree of correlation between the flowing voidage and flow velocity of particles which are individually both affected by variations in particle size and shape but are mutually compensating in their effects on the simulated and measured discharge rates. In general, the simulations produce a less dilated assembly moving at smaller velocities.

Formation of stable clusters in colloidal suspensions.

The experimental evidence and theoretical explanations of stable cluster formation in colloidal suspensions are reviewed. The clusters form in the intermediate range between a stable suspension built up by singlets and the irreversible coagulation or gelation of the suspension. The stable clusters develop as a result of a balance between competing short range attraction and long range repulsion between colloidal particles or due to reversible flocculation in the shallow secondary potential well. Heteroaggregation in binary colloids can also result in formation of stable clusters.

Waste electric and electronic equipment in Jordan: willingness and generation rates

Waste generated from electric and electronic equipment (WEEE) is increasing due to the demand for information and communication technologies (ICT), rapid product obsolescence, coupled with rapid economic growth, urbanisation and technology advancement. Developed countries have reacted actively to manage this waste, while developing countries are still in the early stages of recognising this problem. This paper gauges Jordanian household WEEE awareness levels, their electric and electronic equipment (EEE) consumption patterns, and estimates the lifetime of EEE using questionnaire and interview methods. Based on these findings, the WEEE generation rate in Jordan is predicted.

The reduction of extractive agent in extractive distillation and auto-extractive distillation

Abstract Methanol–acetone, methanol–methyl acetate and methanol–chloroform were used as binary systems in extractive distillation and water–methanol–acetone, water–methanol–methyl acetate and water–methanol–chloroform were used as ternary systems in auto-extractive distillation, to study the possibility of reducing the specific consumption of extractive agent ( q ) and therefore, to reduce energy consumption of the separation process. Water was used in both binary and ternary systems as the extractive agent. Increasing the distance between the mixture feed and extractive agent feed along the separation column led to a 35–40% reduction in the specific consumption of extractive agent, while splitting the extractive agent feed into two branches on the column led to a further reduction of 25–30%. Computer simulation with HYSYS supported the conclusions for the binary methanol–acetone system.

Energy monitoring in distinct element models of particle systems

Abstract This paper describes techniques for monitoring energy in discrete element method (DEM) simulations of granular flow. They have been applied to DMX a three-dimensional model of polydisperse cohesive spheres flowing into a cylindrical vessel, settling and then subject to vibration. The model takes account of gravitational potential energy, linear and tangential ‘particle spring’ potential energies and net work done by the particles, normal and angular kinetic energies, dissipated energies due to linear and tangential damping and friction, and the work done by the vibrating vessel on the particle system. Energy monitoring enhances understanding of the physics and further validates the program code. It was found that the numerical technique inherently introduces artificial energy components, but that these can be explicitly monitored. Energy conservation was thus verified and the artificial components explained. Simulations of various particle types and sizes were performed monitoring all the energy components with time. The results show that explicit dissipated energy calculation is required and cannot be simplified as the remainder term of total minus potential and kinetic energies, and that energy is dissipated mainly in normal damping and gross sliding. Total energy dissipation is not sensitive to particle stiffness, but moderately sensitive to damping and friction. However, the maximum rate of energy dissipation is significantly affected by the damping coefficient and the particle stiffness, and only negligibly by the friction coefficient. Initial studies showed that in some low energy vibration the artificial energy component is not negligible and its effect must be considered in some DEM applications.

Comparison of least-squares method and Bayes’ theorem for deconvolution of mixture composition

Abstract A method of de-convoluting a mixture composition from a set of property data for the mixture is presented and evaluated. This is the probability apportioning method (PAM version 4). It is compared with the well-known spectral analysis method of least squares (LSQ). Both these methods assume that the component data is combined in the mixture in a linearly independent manner. PAM starts with an assumed composition and uses Bayes’ theorem on conditional probabilities in an iterative procedure to recalculate the prediction. LSQ is shown to be generally more applicable, but PAM is less susceptible to noise where there are components in the mixture with similar property data (spectra).