Darius Markauskas

Parallel discrete element simulation of poly-dispersed granular material

The paper presents parallel 3D DEM simulation of poly-dispersed material described by the normal size distribution. Static domain decomposition and message passing inter-processor communication have been implemented in the DEM code. A novel algorithm for moving particles that exchange processors has been incorporated in the domain decomposition framework. Parallel performance of the developed algorithm and software has been investigated by a series of benchmark tests conducting tri-axial compaction of material with different numbers of particles, heterogeneity ratios and compaction durations. The speed-up equal to 8.81 has been obtained on 10 processors of the distributed memory PC cluster. It has been shown that a drastic increase of computational expenses of simulation for the poly-dispersed material in terms of CPU time is associated with the increase of its heterogeneity. A contribution of the temporal evolution of microscopic behaviour has also been illustrated.

COMPACTING OF PARTICLES FOR BIAXIAL COMPRESSION TEST BY THE DISCRETE ELEMENT METHOD

Numerical simulation of the compacting of particles for the biaxial compression test using the discrete element method is presented. Compacting is considered as the first independent step required for a proper simulation of the entire compression process. In terms of the continuum approach, compacting is regarded as generation of the initial conditions. Three different compacting scenarios with differently manipulated loading history on the boundaries, namely, compacting by using the moving rigid walls, by the static pressure using flexible membranes as well as combining the above two methods are considered. Discrete element methodology and basic relations, as well as formulation of the compacting problem and computational aspects of compacting are presented in detail. Each of the scenarios is illustrated by the numerical results. It has been found that the combined compacting scenario yields the required initial conditions exhibiting the best physically adjustable state of particles.

Maize grain shape approaches for DEM modelling

Display Omitted The shape of a grain of maize was approached using the multi-sphere method.Models with single-spherical particles and with rolling friction were also used.Results from two DEM software codes were compared.Recommendations on the shape approach for DEM modelling were provided. Granular materials are commonly stored in silos, and usually presented in a wide range of shapes and sizes. To understand its behaviour simulations based on the Discrete Element Method (DEM) are becoming widely used nowadays. The strength of this method lies in its ability to capture the discrete nature of particle assemblies in comparison with other methods that are based in continuum approaches. However, one of the challenges that still need to be tackled is the approximation of the shape of real particles in order to achieve more realistic simulations. In this paper, the shape of a grain of maize has been approached using the so-called multi-sphere method, and also as single-spherical particles with rolling friction. Furthermore, as it is already known that the use of different software codes can show differences, although good agreement in general, results from two DEM software codes were compared.

The comparison of two domain repartitioning methods used for parallel discrete element computations of the hopper discharge

Two methods are employed for dynamic domain decomposition of hopper discharge.Implementation of the k-way graph partitioning method in DEM codes is more complex.A higher speed-up is measured, applying the recursive coordinate bisection method.Parallel efficiency of 0.87 is attained simulating 5.1i?106 particles on 2048 cores. The paper presents an application of two domain repartitioning methods to solving hopper discharge problem simulated by the discrete element method. Quantitative comparison of parallel speed-up obtained by using the multilevel k-way graph partitioning method and the recursive coordinate bisection method is presented. The detailed investigation of load balance, interprocessor communication and repartitioning is performed. Speed-up of the parallel computations based on the dynamic domain decomposition is investigated by a series of benchmark tests simulating the granular visco-elastic frictional media in hoppers containing 0.3i?106 and 5.1i?106 spherical particles. A soft-particle approach is adopted, when the simulation is performed by small time increments and the contact forces between the particles are calculated using the contact law. The parallel efficiency of 0.87 was achieved on 2048 cores, modelling the hopper filled with 5.1i?106 particles.

Computation and visualization of discrete particle systems on gLite-based grid

Three-dimensional simulation of discrete particle systems is performed by the discrete element method (DEM) software on the gLite-based BalticGrid infrastructure. The performance of a parallel algorithm for particles exchanging processors is investigated by using a number of benchmarks. Polydispersed particle systems are visualized by a novel grid e-service VizLitG designed for convenient access and interactive visualization of remote data files located on the grid. Partial dataset transfer from the storage element is implemented in the visualization e-service. The efficiency tests of VizLitG are performed on the datasets of different sizes. Two granular problems associated with triaxial compaction and hopper discharge are solved.

Discrete element simulating the hydrodynamic effects in acoustic agglomeration of micron-sized particles

ABSTRACT Numerical simulation of the acoustic agglomeration of micron-sized aerosol particles by a discrete element method (DEM) is demonstrated. The conventional DEM technique used in granular dynamics is modified for simulation of the acoustically induced attractive motion of particles in an incompressible fluid. The problem-specific orthokinetic collision, acoustic wake, and mutual radiation pressure effects yielding binary attraction and sticking of the particles are considered within the DEM approach. The acoustically induced agglomeration of two aerosol particles and 3D particles’ system is illustrated by numerical results. Numerical values of the agglomeration time of two particles obtained for a wide range of acoustic frequencies are analyzed. Comparison of various hydrodynamic effects with available experimental data indicates an overestimated contribution of the mutual radiation pressure model. The performance of the DEM technique and specific features concerning long-range interactions between particles are demonstrated by simulating 3D particles’ systems. The obtained numerical results illustrating the variation of number concentration with time are compared to available experimental data of coal-fired fly ash particles’ agglomeration; a relatively good agreement with the acoustic wake mechanism is observed.

SENSITIVITY OF DYNAMIC BEHAVIOUR OF THE FE MODEL: CASE STUDY FOR THE IGNALINA NPP REACTOR BUILDING

Abstract The 3D thin‐walled finite element model of Ignalina NPP Unit 2 reactor building was developed aimed at the evaluation of the global dynamic behaviour with a focus on the seismic response. The model comprises description of the monolithic structures, while prefabricated frame structures are ignored and replaced by external masses. Sensitivity study of the selected dynamic characteristics of the model with respect to data uncertainties is considered. Uncertainty of the model is considered in terms of masses of removed structures and wall stiffness. Seismic input is represented by the site specific free‐field ground response acceleration spectra. The sensitivity study concerns variations of frequencies and acceleration of in‐structure horizontal response spectra at specified points. Maximal bending moments are also considered. It was obtained that the reactor level is not sensitive to the uncertainties considered, while discernable sensitivity was detected at the top level of the structure.

Comparative study on mesh-based and mesh-less coupled CFD-DEM methods to model particle-laden flow

A comparative study on mesh-based and mesh-less Computational Fluid Dynamics (CFD) approaches coupled with the Discrete Element Method (DEM) is presented. As the mesh-based CFD approach a Finite Volume Method (FVM) is used. A Smoothed Particle Hydrodynamics (SPH) method represents mesh-less CFD. The unresolved fluid model is governed by the locally averaged Navier-Stokes equations. A newly developed model for applying boundary conditions in the SPH is described and validation tests are performed. With the help of the presented comparative tests, the similarities and differences of DEM-FVM and DEM-SPH methods are discussed. Three test cases, comprised of a single solid particle sedimentation test, flow through a porous block and sedimentation of a porous block, are performed using both methods. Drag forces acting on solid particles highly depend on local fluid fractions. For comparative reasons, the size of a cell in FVM is chosen such that fluid fractions match those computed in SPH. In general, DEM-FVM and DEM-SPH methods exhibit good agreement with analytic reference results. Differences between DEM-SPH and DEM-FVM approaches were found mostly due to differences in computed local fluid fractions.

Adapting the discrete element method to simulation of acoustic agglomeration of aerosol particles

Adaptation of the Discrete Element Method (DEM) for simulation of acoustic agglomeration of aerosol particles is considered. The aerosol is assumed to be a system of dispersed spherical particles embedded in incompressible medium. The modelled processes are considered on the scale of the oscillatory motion of particles. Under the action of a monochromatic sound wave agglomeration of smaller particles, which are sequentially formed into larger particles, is initiated. The effect of the orthokinetic and acoustic wake mechanisms is taken into account. This report presents some theoretical aspects of acoustic agglomeration, as well as the overall computational framework and evaluation of the acoustic forces on the particles resulting from oscillatory fluid motion. Theoretical models are illustrated by the results obtained in numerical simulation of a multiparticle system.

Visualization of cracks by using the local Voronoi decompositions and distributed software

Local Voronoi decompositions are applied to extraction of crack surfaces.Cracks propagate in mono-dispersed particulate media simulated by the DEM.The geometry of crack surfaces is accurately defined by faces of the decomposition.Generation of local decompositions rather than global meshes reduces execution time.The sufficiently high speed-up of distributed visualization software is achieved. The paper presents a novel visualization technique for cracks propagating in mono-dispersed particulate material. The proposed technique is based on local space decompositions generated in fractured areas. The contact surfaces of the neighboring particles are defined by the local Voronoi decomposition generated according to the lattice topology employed in computations of the discrete element method. The visual model validation helps to indicate the regions of a highly deformed lattice, where the defects detected between the pairs of the neighboring particles on the lattice connections cannot be directly mapped onto the relevant edges of the Voronoi diagram. The parallel implementation of the visualization technique is based on the domain decomposition and two layers of ghost vertices and connections. The technique is implemented in the distributed visualization software VisPartDEM. Datasets of the elastic solid problem exhibiting non-uniform distribution of fracture force values are considered to validate the performance of the proposed technique. The parallel speed-up of the visualization software is investigated. The superior performance of the applied local technique is compared to the performance observed by using the standard global Voronoi algorithm.