Optimal parameters for a hierarchical grid data structure for contact detection in arbitrarily polydisperse particle systems.

The objective of this paper is to find the optimum number of hierarchy levels and their cell sizes for contact detection algorithms based on a versatile hierarchical grid data structure, for polydisperse particle systems with arbitrary distribution of particle radii. These algorithms perform as fast as O(N) for N particles, but the prefactor can be as large as N for a given system, depending on the algorithm parameters chosen, making a recipe for choosing these parameters necessary. We estimate theoretically the calculation time of two distinct algorithms for particle systems with various packing fractions, where the sizes of the particles are modelled by an arbitrary probability density function. We suggest several methods for choosing the number of hierarchy levels and the respective cell sizes, based on truncated power-law radii distributions with different exponents and widths. The theoretical estimations are then compared with simulation results for particle systems with up to one million particles. The proposed recipe for selecting the optimal hierarchical grid parameters allows to find contacts in arbitrarily polydisperse particle systems as fast as the commonly-used linked-cell method in purely monodisperse particle systems, i.e., extra work is avoided in presence of polydispersity. Furthermore, the contact detection time per particle even decreases slightly with increasing polydispersity or decreasing particle packing fraction

A review of recent work on the discrete particle method at the University of Twente: an introduction to the open-source package MercuryDPM

In this paper we review some recent advances in DEM (DPM) modelling undertaken at the University of Twente. We introduce the new open-source package MercuryDPM that we have been developing over the last few years.\ud MercuryDPM is an object-oriented program with a simple C++ implementation and includes: support for moving and complex walls, such as polyhedra or screw-threads; state-of-the-art granular contact models; multi-species support; specialised classes, allowing the easy implementation of common geometries like chutes, hoppers, etc.; common handler interfaces for particles, walls and boundaries (so all type of objects are changed using the same interfaces); restarting; large self-test suite and numerous simple demos; and, visualisation support, both internal and using Visual Molecular Dynamics.\ud Additionally to these features, MercuryDPM has two major components that, to the best of our knowledge, cannot be found in other DPM packages. Firstly, it uses a novel advanced contact detection method that is able of dealing with multiple distinct granular components with sizes ranging over many orders of magnitude: the hierarchical grid. We explain how this algorithm works and demonstrate the speedup gained over the traditional linked cell approach. This algorithm has lower complexity for poly-dispersed ows which means for the first time large simulations with extremely wide size distributions are feasible.\ud Secondly, we present a novel way to extract continuum fields from discrete particle systems that is applicable to mixtures as well as boundaries and interfaces. The particle data is coarse grained in a way that is by construction compatible with the continuum equations of mass-, momentum-, and energy balance. Boundary interaction forces are taken into account in a self-consistent way and thus allow the construction of a continuous stress field even within one particle radius of the boundaries. The method does not require temporal averaging and thus can be used to investigate time-dependent flows as well as static and steady situations. This coarse-graining method is available from MercuryDPM either as a post-processing tool or it can be run in real time. In real-time mode, it not only reduces the data which has to be stored but also allows boundary conditions etc. to be updated depending on the current macroscopic state of the system, e.g. allowing the creation of a pressure-release wall.\ud Finally, we illustrate these tools and a selection of other features of MercuryDPM via various problems including size-driven segregation in chute flow, rotating drums, and screw-conveyer

2D cyclic pure shear of granular materials, simulations and model

Discrete particle simulations of granular materials under 2D, isochoric, cyclic pure shear have been performed and are compared to a recently developed constitutive model involving a deviatoric yield stress, dilatant stresses and structural anisotropy. The original model shows the cyclic response qualitatively, but suffers from an artificial drift in pressure. With a small modification in the definition of the stress anisotropy and an additional limit-pressure term in the evolution equation for the pressure, it is able to show the transient as well as the limit cycles. The overall goal – beyond the scope of the present study – is to develop a local constitutive model that is able to predict real life, large scale granular systems

Reprint of "Experiments and discrete element simulation of the dosing of cohesive powders in a simplified geometry"

(Reprinted from Powder Technoly, vol 287, pg 108-120