Discrete Modeling Techniques Research Articles

Rate sensitivity in discrete dislocation plasticity in hexagonal close-packed crystals

The origin of the rate-sensitive behaviour of plasticity over strain rate regimes from 10−5 to 105 s−1 has been assessed with reference to three key mechanisms: dislocation nucleation, time of flight (dislocation mobility) and thermally activated escape of pinned dislocations. A new mechanistic formalism for incorporating thermally activated dislocation escape into discrete dislocation plasticity modelling techniques is presented. It is shown that nucleation and dislocation mobility explain rate-sensitive behaviour for strain rates in the range 102 to 105 s−1, but cannot do so for significantly lower strain rates, for which thermally-activated dislocation escape becomes the predominant rate-controlling mechanism. At low strain rates, and for a model Ti alloy considered at 20 °C, the strong experimentally observed rate-sensitive behaviour manifested as stress relaxation and creep is shown to be captured well by the new thermal activation discrete dislocation plasticity model, which otherwise simply cannot be captured by nucleation or mobility arguments. Increasing activation energy leads to a higher energy barrier and as a consequence, a higher dislocation escape time. Conversely, increasing obstacle spacing tends to diminish the thermal activation time.

Sustainable travel modes in Saudi Arabia: is there a potential for formal car-sharing scheme?

Purpose– The purpose of this paper is to investigate the actual demand on formal car-sharing scheme in Saudi Arabia as a means of public modes of transport in order to assess the potential future of this mode in Saudi Arabia as a policy measure.Design/methodology/approach– In this context, this paper investigates the potential feasibility of introducing formal car-sharing scheme as a means of public modes of transport in Tabuk city. Investigation of the characteristics of trip makers, which impact on the decision of selecting the currently available informal car-sharing as a mode of travel, is presented. The paper investigates the factors which affect the decisions of car-sharing in the Kingdom of Saudi Arabia using the household survey data. Discrete choice modelling techniques have been adopted in this research to investigate behaviour and attitudes to car-sharing and the binary logit model has been utilised.Findings– From the results, in the case of Tabuk, the main factor in selection of the transport mode is the social role, and specifically the role inside the family. Results showed that about 49 per cent of total respondents drive to work, while about 20 per cent reported that they use car-sharing. About 8 per cent of respondents reported that they use a private driver, while other 8 per cent reported that they use a contracted driver.Originality/value– Investigation of the potential of introducing formal car-sharing scheme in Saudi Arabia as a means of public modes of transport in Saudi Arabia which is a more sustainable mode of transport.

Adaptive Remeshing Technique in Discrete Models of Random Geometry

The adaptive discretization technique for static discrete models of random geometry based on Voronoi cells is developed here. It is based on adding randomly located nodes into the highly stressed regions and updating the discretization based on those nodes before the crack may initiate in or propagate through the regions.

A Finite Element Simulation of the Electrochemical Growth of a Single Hemispherical Silver Nucleus

Understanding the early stages of electrochemical nucleation and growth is the cornerstone for nanoscale electrodeposition. Although studied since decades, the process is not yet fully understood. In this paper, we introduce a new modelling approach to study the growth of a single hemispherical nucleus: Multi-Ion Transport and Reaction Model (MITReM). This approach takes into account the transport driven by diffusion and migration of all species in the electrolyte together with the electrochemical reactions at the electrode boundary. A Finite Element Method (FEM) is used to solve the balance equations for the concentration of all the active species and the electrolyte potential. In contrast to analytical models or discrete scale modelling techniques, the strength of this approach is that no assumptions on the diffusional or kinetic limitations have to be made. In addition, this novel platform allows to add further levels of complexity, such as multiple nuclei, adatom surface diffusion, aggregation, particle detachment, etc. The simulation results prove that, the initial growth stage of a 10 nm single hemispherical silver nucleus always starts under kinetic control, regardless of concentration and electrode potential. Later on, a transition from kinetic to diffusion control takes place. The time of transition depends on the imposed concentration and electrode potential. Moreover, the simulations clearly show that the growth rate is strongly affected by the imposed concentration and electrode potential, as it has been proven experimentally in countless occasions. Numerical simulation by MITReM proves to be of great interest to gain knowledge towards unravelling the early stages of electrochemical nucleation and growth processes.

Guiding and modelling quality improvement in higher education institutions

The article considers the process of creating quality improvement in higher education institutions from the point of view of current organisational theory and social-science modelling techniques. The author considers the higher education institution as a functioning complex of rules, norms and other organisational features and reviews the social mechanisms and processes through which agencies can stimulate quality improvement. The article provides a few examples of how these social processes might be modelled using social simulation techniques, including agent-based models, discrete event simulation and other modelling techniques developed for representing complex social processes of coordination and cooperation. A better representation of universities as complexes of organisations will support more effective quality improvement by higher education leaders and external agencies.

Defining the effect of sweep tillage tool cutting edge geometry on tillage forces using 3D discrete element modelling

The energy required for tillage processes accounts for a significant proportion of total energy used in crop production. In many tillage processes decreasing the draft and upward vertical forces is often desired for reduced fuel use and improved penetration, respectively. Recent studies have proved that the discrete element modelling (DEM) can effectively be used to model the soil–tool interaction. In his study, Fielke (1994) [1] examined the effect of the various tool cutting edge geometries, namely; cutting edge height, length of underside rub, angle of underside clearance, on draft and vertical forces. In this paper the experimental parameters of Fielke (1994) [1] were simulated using 3D discrete element modelling techniques. In the simulations a hysteretic spring contact model integrated with a linear cohesion model that considers the plastic deformation behaviour of the soil hence provides better vertical force prediction was employed. DEM parameters were determined by comparing the experimental and simulation results of angle of repose and penetration tests. The results of the study showed that the simulation results of the soil-various tool cutting edge geometries agreed well with the experimental results of Fielke (1994) [1]. The modelling was then used to simulate a further range of cutting edge geometries to better define the effect of sweep tool cutting edge geometry parameters on tillage forces. The extra simulations were able to show that by using a sharper cutting edge with zero vertical cutting edge height the draft and upward vertical force were further reduced indicating there is benefit from having a really sharp cutting edge. The extra simulations also confirmed that the interpolated trends for angle of underside clearance as suggested by Fielke (1994) [1] where correct with a linear reduction in draft and upward vertical force for angle of underside clearance between the ranges of −25 and −5°, and between −5 and 0°. The good correlations give confidence to recommend further investigation of the use of DEM to model the different types of tillage tools.

Molecular level biodegradation of phenol and its derivatives through dmp operon of Pseudomonas putida: A bio-molecular modeling and docking analysis

Participation of Pseudomonas putida-derived methyl phenol (dmp) operon and DmpR protein in the biodegradation of phenol or other harmful, organic, toxic pollutants was investigated at a molecular level. Documentation documents that P. putida has DmpR protein which positively regulates dmp operon in the presence of inducers; like phenols. From the operon, phenol hydroxylase encoded by dmpN gene, participates in degrading phenols after dmp operon is expressed. For the purpose, the 3-D models of the four domains from DmpR protein and of the DNA sequences from the two Upstream Activation Sequences (UAS) present at the promoter region of the operon were demonstrated using discrete molecular modeling techniques. The best modeled structures satisfying their stereo-chemical properties were selected in each of the cases. To stabilize the individual structures, energy optimization was performed. In the presence of inducers, probable interactions among domains and then the two independent DNA structures with the fourth domain were perused by manifold molecular docking simulations. The complex structures were made to be stable by minimizing their overall energy. Responsible amino acid residues, nucleotide bases and binding patterns for the biodegradation, were examined. In the presence of the inducers, the biodegradation process is initiated by the interaction of phe50 from the first protein domain with the inducers. Only after the interaction of the last domain with the DNA sequences individually, the operon is expressed. This novel residue level study is paramount for initiating transcription in the operon; thereby leading to expression of phenol hydroxylase followed by phenol biodegradation.

Inexpensive discrete atomistic model technique for studying excitations on infinite disordered media: The case of orientational glass ArN2

Inexpensive discrete atomistic model technique for studying excitations on infinite disordered media: The case of orientational glass ArN₂

A NEW METHOD FOR CALCULATION OF EXCHANGE AREAS IN HIGH TEMPERATURE FURNACES, BASED ON DISCRET TRANSFER MODELLING

A new method based on discrete transfer modeling technique for calculating the direct exchange areas (DEA) in zonal method is presented. The key property of this method is fast DEA matrix evaluation. The computational time was found to be small in comparison to other methods for direct exchange areas evaluations based on numerical quadrature integration. The accuracy of the procedure is established by comparing the predictions with those based on the numerical integration on test case (IFRF furnace).

Model selection and misspecification in discrete choice welfare analysis

This study extends the work by Herriges and Kling (1997) to further evaluate the impact of discrete choice modelling techniques on welfare measures. Particularly, we evaluate the performance of the increasingly popular mixed logit model and the computational strategy for deriving discrete choice welfare measures. Our simulation results show that model misspecification can have profound effects on welfare measures. In general, the flexible mixed logit model performs relatively well in the presence of misspecification. However, when the nesting structure can be appropriately identified (via statistical tests and a priori knowledge/experience), the nested logit model provides more reliable welfare measures than the mixed logit model.

A multi-criteria policy set optimisation framework for large-scale simulation models

Simulation modelling is an analysis approach utilised in nearly every domain for analysis of large and complex systems. Synchronous data flow (SDF) is used to model systems whose data does not follow the predetermined global schedule of discrete event simulation modelling techniques. A policy set optimisation (PSO) problem for any simulation model is the selection of a small set of controllable inputs for manipulation by a decision maker (DM) in order to achieve a desired goal. This is a multi-criteria decision making problem and a large-scale SDF simulation model creates a complex mathematical model for solution. Our PSO framework and associated procedure aims to generate the policies that will provide an estimation of the Pareto optimal solutions for the simulation model using only pre-processed model sampling. Our solution methodologies for the PSO problem aims to minimise the computation time required from the point at which a DM selects the outcomes of interest, to when they receive solution policies to choose from. This paper provides a sample problem and a discussion about the quality of the solution found.

A method for calculating the surface area of numerically simulated aggregates

The success of many industrial processes largely depends on the structural characteristics of aggregates. In intensive aerobic digestion process for wastewater treatment applications, the structural characteristics namely aggregate shape, size and therefore the aggregate surface area strongly influence the transfer of dissolved oxygen from the aeration process to aggregates of harmful contaminants/microorganisms.The aim of this study was to apply Discrete Element Modelling (DEM) techniques to the aggregation of suspended particles (microorganisms) to quantify the available surface area for convection and diffusion as a function of particles number concentration and surface charge. The simulation inputs included particle and fluid characteristics such as particle size and density, solid concentration, suspension pH and ionic strength. A post processing method based on the Go-chess concept was developed to quantify the surface area of aggregate structure. The simulation results showed that whilst an increase in connection points increases the total surface area of the aggregate, this does not necessarily translate into an increase in the surface area available for oxygen transfer as combinations of open and close pores are formed. Aggregate surface area was directly determined by aggregate structural characteristics, and increased rapidly when the coordination number was below 3.5 and the fractal dimension was less than 1.5. A correlation for prediction of aggregate external surface area was also proposed as a function of aggregate structural characteristics in terms of fractal dimension and coordination number.

Spectral decomposition based approach for DC–DC converters modeling

The modeling process presents a key step on the analysis and the control of DC–DC converters that can exhibit complicated nonlinear phenomena. In order to describe accurately the converter behavior, a discrete modeling technique is proposed; it allows the exact description of the system with a minimum of simplifying assumptions and validity hypothesis of the mode. The validation of the proposed method is achieved through simulation and comparative study results using some criteria such as the approximation error, computing time and allocated memory for data processing and storage.

Pore orientation of granular materials during biaxial compression

The local pore spaces in granular materials tend to be aligned parallel to the major principal stress direction upon particle mobilization. Manifestation of this response has been numerically validated in our previous studies with the aid of discrete element method modeling and image processing techniques during creep and shearing. We now extend the modeling of pore geometry, constructed with spherical particles, to assemblies of particle clumps. Two-dimensional simulations are performed for both loose and dense assemblies of spherical particles and particle clumps. Each particle packing is bound by rigid or flexible walls and subjected to biaxial compression and the particle mobilization effect on the evolution of pore orientation is explored. Randomly shaped pores surrounded by adjacent particles are geometrically quantified by Delaunay tessellation and fitted with ellipses. Results show that localization is apparent in dense assemblies, in particular for clumped particle packing, while loose assemblies exhibit diffusive failure. Small pores within well-defined shear bands tend to align either parallel to the direction of the shear band or perpendicular to the major principal stress. On the other hand, small pores within the blocks and large pores have a tendency to become elongate towards the major principal stress direction. This study reveals for the first time that pore orientation is dependent upon particle shape, pore size, and assembly conditions on the pore and global scales.

MASONRY-INFILLED RC FRAMES SUBJECTED TO COMBINED IN-PLANE AND OUT-OF-PLANE LOADING

The structural responses of infilled frames subjected to combined in-plane and out-of-plane loadings are usually analyzed by separately applying in-plane and out-of-plane loads. The interaction effect of in-plane and out-of-plane loads on the structural behavior of the frames is ignored; thus errors in predicting the actual force-transfer mechanisms and modes of failure of the structures can be incurred. To solve the problem, this paper presents a discrete finite element modeling technique, which employs a damage-based cohesive crack representation of fracture behavior of masonry infills, followed by a study on the force-transfer mechanisms and failure modes of the anchored and unanchored infilled reinforced concrete (RC) frames subjected to interactive in-plane and out-of-plane loads. The analysis indicates that under out-of-plane loading the diagonal compressive thrust of masonry-infill walls, which is induced by in-plane lateral loading and acts on the walls, may reduce the in-plane load capacity of the RC frame by up to 50% and cause buckling of infill walls. On the other hand, the anchorage can effectively prevent the separation of infill walls from the bounding frame and provide stabilizing forces to the walls against buckling.

A clinically oriented introduction and review on finite element models of the human cochlea.

Due to the inaccessibility of the inner ear, direct in vivo information on cochlear mechanics is difficult to obtain. Mathematical modelling is a promising way to provide insight into the physiology and pathology of the cochlea. Finite element method (FEM) is one of the most popular discrete mathematical modelling techniques, mainly used in engineering that has been increasingly used to model the cochlea and its elements. The aim of this overview is to provide a brief introduction to the use of FEM in modelling and predicting the behavior of the cochlea in normal and pathological conditions. It will focus on methodological issues, modelling assumptions, simulation of clinical scenarios, and pathologies.

Fourier-based incremental homogenisation of coupled unilateral damage–plasticity model for masonry structures

In structural analysis of large masonry structures, nondemanding computation effort, numerical stability and simplified model assembly and meshing often have a higher priority over precise details of local stress or strain responses. This paper presents the development of a Fourier-based incremental homogenisation technique, where the macro–micro transformations of mechanical variables are derived by incremental variational problems to minimise the potential energy in representative volume elements (RVEs) with respect to local fluctuating displacement fields expanded in Fourier series. In addition to the proposed homogenisation technique, a unilateral damage–plasticity constitutive model for mortar joints in the RVE is developed within the framework of thermomechanics, which accounts for the stiffness and strength degradation (or recovery) due to the transverse crack opening/closing in the mortar joints. The numerical solution for the homogenisation problem and the performances of the proposed coupled-damage plastic mode and Fourier-based homogenisation scheme verified by detailed case studies are presented. It has been shown that the computational effort of the analysis with the proposed modelling technique can be considerably reduced by more than 20% as compared with that of the discrete modelling technique.

Modeling tensile response of fiber‐reinforced polymer composites using discrete element method

An advanced discrete element method (DEM), coupled with imaging techniques, of the tensile response of carbon fiber‐reinforced composite materials is presented in this article. DEM was developed using the image‐based shape structural model to determine the composites' elastic modulus, stress–strain response, and tensile strength. The developed model utilizes the microfabric micromechanical discrete element modeling technique. Clusters of very small bonded discrete elements were used to model the two composite constituents (matrix and reinforcement). The microparameters of each discrete element were determined from the macrocharacteristics of each constituent. The results from the developed model were compared with the results from an experimental case study. The results obtained from DEM simulations are within the coefficient of variation of the experimental values. The comparison indicates that the image‐based DEM micromechanical model accurately determines the elastic modulus and tensile strength of the molded carbon fiber‐reinforced polymer composite. POLYM. COMPOS., 34:877–886, 2013. © 2013 Society of Plastics Engineers

HOW DOES THE SPREAD OF PRIMARY AND SECONDARY SCHOOLING INFLUENCE THE FERTILITY TRANSITION? EVIDENCE FROM RURAL NEPAL

From 1996 to 2006, Nepal experienced a substantial fertility decline, with the total fertility rate dropping from 4.6 to 3.1 births per woman. This study examines the associations between progress towards universal primary and secondary schooling and fertility decline in rural Nepal. Several hypotheses regarding mechanisms through which education affects current fertility behaviour are tested, including: the school environment during women's childhood; current availability of schools; knowledge of educational costs; and women's own educational attainment. Data for the analysis come from the 2003-04 Nepal Living Standards Survey, a nationally representative random sample of households, which includes detailed data on fertility, household expenditure, educational attainment, demographic characteristics and the use of social services. Census and administrative data are also used to construct district-level gross enrolment ratios for primary and secondary schools during the women's childhood. Discrete dependent variable modelling techniques are used to estimate the effects of the following variables on the probability of women giving birth in a given year: district-level gross enrolment ratios for primary and secondary schools during women's childhood; having had a child previously in school; women's own educational level; current school availability; and other covariates. Separate models are estimated for the overall sample of rural women of reproductive age, and for parity-specific sub-samples. The results suggest that district-level gross enrolment ratios for secondary schools and, in some instances, having had a previous child enrolled in school are significant determinants of fertility in rural areas. These results are highly independent of women's own educational levels. Overall, the results suggest that, in the rural Nepal context, mass schooling influences the fertility transition through both community- and household-level pathways.

Fretting damage modeling of liner-bearing interaction by combined finite element – discrete element method

This paper presents a methodology to study the fretting damage behavior by combined finite element-discrete element method based on FFD method and cut boundary displacement method. A discrete element modeling technique is developed. An inter-element contact constitutive model and its microscopic parameters are determined and calibrated to reproduce continuous and discontinuous behaviors of material in DEM. A crack visualization technique is developed to display cracks according to their failure mode and time dependency. The effect of fretting condition on crack initiation and propagation as well as fretting damage is studied. The mechanism of fretting wear is investigated.