A network model to assess base-filter combinations

Granular filters retain base material within the narrowest constrictions of their void network. A direct comparison of the base material particle size distribution (PSD) and the filter constriction size distribution (CSD) cannot easily be used to assess fi lter - base compatibility. Here a conceptually simple random - walk network model using a filter CSD derived from discrete element modelling and base PSD is used to assess filter - base compatibility. Following verification using experimental data the model is a pplied to assess empirical ratios between filter and base characteristic diameters. The effects of filter density, void connectivity and blocking in the first few filter layers are highlighted

Empirical assessment of the critical time increment in explicit particulate discrete element method simulations

This contribution considers the critical time increment (〖∆t〗_crit) to achieve stable simulations using particulate discrete element method (DEM) codes that adopt a Verlet-type time integration scheme. The 〖∆t〗_crit is determined by considering the maximum vibration frequency of the system. Based on a series of parametric studies, 〖∆t〗_crit is shown to depend on the particle mass (m), the maximum contact stiffness (Kmax), and the maximum particle coordination number (CN,max). Empirical expressions relating 〖∆t〗_crit to m, Kmax, and CN,max are presented; while strictly only valid within the range of simulation scenarios considered here, these can inform DEM analysts selecting appropriate 〖∆t〗_crit values

Contact based void partitioning to assess filtration properties in DEM simulations

Discrete element method (DEM) simulations model the behaviour of a granular material by explicitly considering the individual particles. In principle, DEM analyses then provide a means to relate particle scale mechanisms with the overall, macro-scale response. However, interpretative algorithms must be applied to gain useful scientific insight using the very large amount of data available from DEM simulations. The particle and contact coordinates as well as the contact orientations can be directly obtained from a DEM simulation and the application of measures such as the coordination number and the fabric tensor to describe these data is now well-established. However, a granular material has two phases and a full description of the material also requires consideration of the voids. Quantitative analysis of the void space can give further insight into directional fabric and is also useful in assessing the filtration characteristics of a granular material. The void topology is not directly given by the DEM simulation data; rather it must be inferred from the geometry of particle phase. The current study considers the use of the contact coordinates to partition the void space for 3D DEM simulation datasets and to define individual voids as well as the boundaries or constrictions between the voids. The measured constriction sizes are comparable to those calculated using Delaunay-triangulation based methods, and the contact-based method has the advantage of being less subjective. In an example application, the method was applied to DEM models of reservoir sandstones to establish the relationship between particle and constriction sizes as well as the relationship between the void topology and the coordination number and the evolution of these properties during shearing

Empirical assessment of the critical time increment in explicit particulate discrete element method simulations

This contribution considers the critical time increment (〖∆t〗_crit) to achieve stable simulations using particulate discrete element method (DEM) codes that adopt a Verlet-type time integration scheme. The 〖∆t〗_crit is determined by considering the maximum vibration frequency of the system. Based on a series of parametric studies, 〖∆t〗_crit is shown to depend on the particle mass (m), the maximum contact stiffness (Kmax), and the maximum particle coordination number (CN,max). Empirical expressions relating 〖∆t〗_crit to m, Kmax, and CN,max are presented; while strictly only valid within the range of simulation scenarios considered here, these can inform DEM analysts selecting appropriate 〖∆t〗_crit values

Influence of the coefficient of uniformity on the size and frequency of constrictions in sand filters

Constrictions between voids control the filtration and permeability properties of granular materials. This study uses high-resolution microcomputed tomography images and discrete-element modelling to analyse two important characteristics of constrictions in granular filters: (a) the constriction size distribution (CSD) and (b) the constriction density per unit volume. The results demonstrate the importance of the particle size distribution (PSD) and void ratio of the granular material in determining the constriction density, with more widely graded materials having more densely spaced constrictions. The PSD is shown to be the main determinant of the CSD, in agreement with previous studies. The data are used to examine proposed approaches to estimate constriction spacing or void size

Contamination in RIT Processing

Contamination during processing in RIT’S cleanroom facility is a leading cause in the failure of fabricated devices and circuits. Detection of these contaminants is possible using an ESTEK WIS-600 surface inspection system. Before and after particle counts were taken using this system when processing wafers in many common RIT procedures. It was found that contamination levels were significant in most areas, and cleaning procedures were useful only to remove large particles

Constriction size distributions of granular filters: a numerical study

The retention capability of granular filters is controlled by the narrow constrictions connecting the voids within the filter. The theoretical justification for empirical filter rules used in practice includes consideration of an idealised soil fabric in which constrictions form between co-planar combinations of spherical filter particles. This idealised fabric has not been confirmed by experimental or numerical observations of real constrictions. This paper reports the results of direct, particle-scale measurement of the constriction size distribution (CSD) within virtual samples of granular filters created using the discrete-element method (DEM). A previously proposed analytical method that predicts the full CSD using inscribed circles to estimate constriction sizes is found to poorly predict the CSD for widely graded filters due to an over-idealisation of the soil fabric. The DEM data generated are used to explore quantitatively the influence of the coefficient of uniformity, particle size distribution and relative density of the filter on the CSD. For a given relative density CSDs form a narrow band of similarly shaped curves when normalised by characteristic filter diameters. This lends support to the practical use of characteristic diameters to assess filter retention capability

Using DEM to assess the influence of stress and fabric inhomogeneity and anisotropy on susceptibility to suffusion

Underfilled and gap-graded soils are known to be susceptible to suffusion; a form of internal instability in which the finer fraction of a soil is washed out from the coarser matrix under the action of seepage. This phenomenon poses a risk to embankment dams and flood embankments. The processes and mechanisms operate at the particle scale, and insight can be gained via the particulate discrete element method (DEM). Vir-tual samples can be created using DEM and simulation results can provide information on particle stresses, as well as quantitative information on the fabric of the particulate material. This is important as the amount of stress carried by the finer particles is thought to govern the susceptibility of a given material to suffusion. DEM modelling can also provide information on variation in properties within samples as well as the detailed data needed to quantify the material fabric. DEM models are, however, an idealization of reality and con-strained in particular by the number of particles used and sample preparation method. This study examines key issues relating to the development of virtual samples for use in DEM analysis and also examines the proportion of the applied stress that is carried by the finer particles

Quantifying the evolution of soil fabric during shearing using directional parameters

Over the past 50 years, experimental studies have repeatedly demonstrated that the mechanical behaviour of sand is sensitive to the material fabric, that is, the arrangement of the grains. Up until now there have been relatively few attempts to describe this fabric quantitatively. Much of our understanding of the link between the particle movements and interactions and the macro-scale response of granular materials, including sand, comes from discrete-element modelling and experiments on ‘analogue’ sands with simple, idealised shapes. This paper investigates methods of quantifying the directional fabric of a real sand and its evolution under loading. Statistical analyses of the distribution of fabric directional data in terms of particle, contact normal, branch vector and void orientations were carried out at different stages of shearing deformation. The data show that the initial particle orientation fabric that develops during the deposition of the material tends to persist during shearing, while in the post-peak regime the contact normals seem to be reoriented along the direction of the major principal stress. Different patterns were observed within the shear band, as both the particles and the contact normal vectors appeared to rotate along the shear plane

Study on the gender dimension of trafficking in human beings

The purpose of this study is to contribute to the identification and understanding of what it means to be ‘taking into account the gender perspective, to strengthen the prevention of this crime and protection of the victims there-of’, as required in Article 1 of European Union (EU) Directive 2011/36/EU on Preventing and Combating Trafficking in Human Beings and Protecting its Victims in the context of the EU Strategy (COM(2012) 286 final) Towards the Eradication of Trafficking in Human Beings. The study contributes to Priority E Action 2 of the Strategy, which states that ‘the Commission will develop knowledge on the gender dimensions of human trafficking, including the gender consequences of the various forms of trafficking and potential differences in the vulnerability of men and women to victimisation and its impact on them.’ Its specific objectives and tasks are to address: the ‘gender dimension of vulnerability, recruitment, and victimisation’; ‘gender issues related to traffickers and to those creating demand’; and ‘an examination of law and policy responses on trafficking in human beings from a gender perspective’. The study addresses the five priorities of the EU Strategy: identifying, protecting, and assisting victims of traf-ficking; stepping up the prevention of trafficking in human beings; better law enforcement; enhanced coordination and cooperation among key actors and policy coherence; and increased knowledge of an effective response to emerging concerns. This study, according to its terms of reference, aims to look specifically at the gender dimension of trafficking for the purpose of sexual exploitation. This follows evidence from statistical data from Eurostat, as well as da-ta from The European Police Office (Europol) and the United Nations Office on Drugs and Crime (UNODC), accord-ing to which the most reported form of exploitation of victims is that of sexual exploitation and its strong gen-der dimension (96 % women and girls). It further addresses recommendations addressed in the Resolution of the European Parliament of 26 February 2014 on sexual exploitation and prostitution and its impact on gender equality (2013/2103(INI)) urging the European Commission to evaluate the impact that the European legal frame-work designed to eliminate trafficking for sexual exploitation has had to date and to undertake further research on patterns of prostitution, on human trafficking for the purpose of sexual exploitation and on the increased lev-el of sex tourism in the EU, with particular reference to minors, and to promote the exchange of best practices among the Member States. The study identifies and draws on EU law and policy competence in gender equality in its identification of the gen-der dimensions of trafficking. The gender dimensions are clustered into five issues: gender specificity and equal treatment; gender expertise, gender balance in decision-making and gender mainstreaming; the relationship be-tween prostitution and trafficking; gendered policy fields and strategic priorities; gendered systems and the the-ory of prevention