Free-flowing Materials Research Articles

A software-agnostic benchmark for DEM simulation of cohesive and non-cohesive materials

Although DEM (Discrete Element Method) was introduced >40 years ago, there are still various challenges in applying it with a certain level of confidence. To develop a realistic material model, calibration, validation, particle shape representation and scaling are well-known challenges that have been studied by various researchers over the past two decades. In addition, once a realistic DEM material model is developed and published in line with open science principles, it is unknown to what extent the calibrated values can be used across software packages. Although a benchmark between 8 open source codes was recently published, it did not cover cohesive materials, rolling friction models, realistic calibrated material behaviour, and commercial software that is widely used in industries.The aim of this study is to investigate the portability of input parameters between different codes and to identify if software independent calibration is possible. We propose a framework to assist DEM users in industry and academia to reach a software independent DEM simulation when required.To compare the results between software packages, the framework considers the following aspects: 1) identical implementation of contact models, 2) single contact modelling, 3) bulk level simulation, and 4) identical post-processing. This framework is demonstrated for two contact models (Hertz-Mindlin and Edinburgh Elasto Plastic Adhesive) with rolling friction model and two commercial software packages (EDEM and PFC) but is applicable for any other (open-source) software. Moreover, two realistic calibrated material models are included to cover a range of material characteristics, from free-flowing incompressible materials to cohesive compressible materials. This study shows that individual particle level simulations give identical results, bulk level simulations show differences for both contact models. In general, users should use parameter values from other software packages with caution, especially where critical or sensitive applications are modelled. This paper also highlights the use of novel computation techniques, such as the GPU engine, to achieve practical computation times when modelling industrial applications.

Photoresponsive Type III Porous Liquids.

Porous materials are the subject of extensive research because of potential applications in areas such as gas adsorption and molecular separations. Until recently, most porous materials were solids, but there is now an emerging class of materials known as porous liquids. The incorporation of intrinsic porosity or cavities in a liquid can result in free-flowing materials that are capable of gas uptakes that are significantly higher than conventional non-porous liquids. A handful of porous liquids have also been investigated for gas separations. Until now, the release of gas from porous liquids has relied on molecular displacement (e.g., by adding small solvent molecules), pressure or temperature swings, or sonication. Here, we explore a new method of gas release which involves photoisomerisable porous liquids comprising a photoresponsive MOF dispersed in an ionic liquid. This results in the selective uptake of CO2 over CH4 and allows gas release to be controlled by using UV light.

ПРОЄКТ КОНСТРУКЦІЇ ПРИЗМАТИЧНОГО БУНКЕРА ТА ОЦІНКА ЕФЕКТИВНОСТІ ПРИЙНЯТИХ РІШЕНЬ

The ensuring of the minimum metal consumption of the structure, the reducing the labour-intensiveness of its manufacture and assembly in the construction of steel prismatic bunkers, designed for storage and unloading of free-flowing materials, are entirely determined by the quality of the design structure. The disadvantage of bunker designs is the connection of the side panels at right angles, which causes the unevenness of the stress-strain state and, as a consequence, the occurrence of emergencies. Some studies suggest that one of the main specific causes of bunker breakdowns and damages is related to incorrect determination of structural loads. A number of authors propose the use of multilayer complex design with the utilization of corrugatedsteel sheets, which directed to address the issue of reducing metal consumption. Today there is still no complex approach to solving the problem of ensuring the reliable operation of the structure owing to searching the best structural forms, technologies for its manufacture and assembly.In this connection, the purpose of this work is to develop the design of the prismatic bunker, which allows to ensure operational reliability, production manufacturability and minimal attended costs associated with the construction of the struc-ture on the building site. By means of modeling the stress state in the structures of bunker tanks, the optimal constructive design options were determined, particularly, dimensional parameters and layout of vertical and stiffeningplates.To ensure the operational reliability of bunker tanks, a solution has been developed that provides for the use of stiffening plates in the design of bunker funnel, which ensure the combined work of its sheet part with the stiffening frame. Use of the block principle of construction of bunker tanks allows to increase a degree of factorial readiness of dispatch elements and to reduce labourintensiveness on assembly sites.

Using chaotic advection for facile high-throughput fabrication of ordered multilayer micro- and nanostructures: continuous chaotic printing

This paper introduces the concept of continuous chaotic printing, i.e. the use of chaotic flows for deterministic and continuous extrusion of fibers with internal multilayered micro- or nanostructures. Two free-flowing materials are coextruded through a printhead containing a miniaturized Kenics static mixer (KSM) composed of multiple helicoidal elements. This produces a fiber with a well-defined internal multilayer microarchitecture at high-throughput (>1.0 m min−1). The number of mixing elements and the printhead diameter determine the number and thickness of the internal lamellae, which are generated according to successive bifurcations that yield a vast amount of inter-material surface area (∼102 cm2 cm−3) at high resolution (∼10 µm). This creates structures with extremely high surface area to volume ratio (SAV). Comparison of experimental and computational results demonstrates that continuous chaotic 3D printing is a robust process with predictable output. In an exciting new development, we demonstrate a method for scaling down these microstructures by 3 orders of magnitude, to the nanoscale level (∼150 nm), by feeding the output of a continuous chaotic 3D printhead into an electrospinner. The simplicity and high resolution of continuous chaotic printing strongly supports its potential use in novel applications, including—but not limited to—bioprinting of multi-scale layered biological structures such as bacterial communities, living tissues composed of organized multiple mammalian cell types, and fabrication of smart multi-material and multilayered constructs for biomedical applications.

Development of a standard calibration procedure for the DEM parameters of cohesionless bulk materials – Part II: Efficient optimization-based calibration

The numerical complexity of Discrete Element Method (DEM) simulations generally forces an idealisation of DEM models, making the calibration process the key to realistic simulation results. When calibrating cohesionless, free-flowing bulk materials, individual simple experiments are commonly used as reference for the calibration, such as the angle of repose in various test methods. Regardless of the experiment, the calibration is regularly performed by trial and error, systematic variation of the parameters, or using optimization algorithms until a suitable combination of parameters is found. This paper deals with the development and test of a highly efficient optimization-based calibration procedure. First, a brief overview of various optimization methods is given and explains why the use of surrogate models seems the best choice for DEM calibration. Subsequently, a new modular algorithm called “generalized surrogate modeling-based calibration” (GSMC) is presented. For testing the new algorithm, a modified draw down test is used.

A new split cell for the measurement of the horizontal to vertical stress ratio of powders

According to the standardized procedures for silo design, the horizontal to vertical stress ratio is a key parameter for the correct estimate of stresses acting on silo walls. To date no standardized equipment is available for the measurement of such ratio. In this paper the state of the art is presented and discussed. A new split cell for the measurement of the horizontal to vertical stress ratio was built. It is a significant improvement of another apparatus, previously developed by the same authors, with the aim of simplifying as much as possible the design of the cell and the procedure to be applied to carry out the measurement. This new version of the cell can overcome the effects of the friction on the cell lid and on the cell bottom, without the use of the twisting action that was necessary in the former cell. The new cell was tested with three different non-compressible powders and three different compressible powders in wide range of vertical stresses between 1 and 16 kPa. Results indicate that the cell can provide repeatable and consistent results with standard deviations within few points percent for vertical stresses in the most usual range between 3 and 16 kPa. Larger standard deviations of results are found at lower stresses, in particular with the most free-flowing materials, but measurements still provide internal consistency of results. Experiments show no significant effect of the twisting action on the lid at high consolidation stresses, while some effect of the twisting is visible a low powder consolidation levels. Results of the vertical to horizontal stress ratio are compared with the most used theoretical and empirical equations to relate the ratio to the angle of internal friction. It is shown that incompressible materials are closer to the internal state of incipient shear during the tests and that, consistently, the theoretical expression relating the stress ratio to the effective angle of internal friction provides satisfactory results. Instead, compressible powders are apparently away from a condition of internal shear and, therefore, empirical and more conservative expression for the stress ratio provide better predictions.

Development of a standard calibration procedure for the DEM parameters of cohesionless bulk materials – part I: Solving the problem of ambiguous parameter combinations

The numerical complexity of Discrete Element Method (DEM) simulations generally forces an idealisation of DEM models, making the calibration process the key to realistic simulation results. When calibrating cohesionless, free-flowing bulk materials, individual simple experiments are commonly used as reference for the calibration, such as the angle of repose in various test methods. Regardless of the experiment, the calibration is regularly performed by trial and error, systematic variation of the parameters, or using optimisation algorithms until a suitable combination of parameters is found. A problem of the calibration, which is often ignored, is the ambiguity of these parameter combinations. Thus, there are usually a variety of contact parameters that can map the same macroscopic reference value. This paper deals in detail with the ambiguity of parameter combinations during the calibration process. It shows which standard tests can be used to generate different experimental reference values for the calibration. The results of simulations with systematic parameter variation highlight the problem of ambiguity. Subsequently it will be shown how the combination of different tests can significantly reduce the acceptable parameter combinations. A modified draw down test will be presented as a calibration test which can deliver different reference values in a single test. Hence, this calibration test allows to obtain an almost unique set of DEM parameter.

Use of additive technologies to increase effectiveness of design and use of a vacuum gripping devices for flexible containers

In an effort to increase efficiency of the process of handling of an air-penetrating flexible container during its’ filling with free-flowing materials a devise has been designed for automatic gripping, opening, holding and sealing of flexible container, containing vacuum gripping devise, pneumatic drive for its’ transfer and a control system based on programmable logical controller. Vacuum gripping device can be produced using the method of additive technologies. It has a modular structure, while each module contains random or structured vacuum chambers located on work surface area, connected to a system of internally branched channels with the source of vacuum. Large number of small vacuum chambers in place allows to minimize leakage of air though pores and openings on the surface of the mouth of weaved flexible container, which increases reliability of holding and reduces energy consumption during the process of handling air-penetrating packaging during its’ filling.

Intelligent system for automatic control of powerful rotating furnaces, designed for sintering of free-flowing metallurgical materials using associative knowledge bases

Intelligent system for automatic control of powerful rotating furnaces, designed for sintering of free-flowing metallurgical materials using associative knowledge bases

The Problems of Dynamics of Wheeled Vehicles with Flow Able Building Cargo

The issues of the dynamics of the wheeled vehicles transporting free-flowing construction materials (concrete, mortar, bitumen and other construction mixes) are considered. It is proved that for the purpose of safety, the oscillations of free-flowing construction materials during the vehicle movement should be taken into account. So, in general, free-flowing construction materials are non-Newtonian fluids, i.e. their flow rheograms are non-linear. If, however, we take into account the fact that during the oscillations the deformations of non-Newtonian fluids do not occur in one plane but they do in all four coordinate planes, then the hysteresis behaviour of these rheograms is natural, which is confirmed experimentally. Then the area of ​​the hysteresis loop allows us to perform the linearization of the problem when calculating the oscillations of the dynamic system "road-vehicle-free-flowing construction materials". The oscillation analysis methods of the wheeled vehicles with free-flowing construction materials both in transversal and longitudinal planes have been developed, as well as the recommendations for increasing the stability of the wheeled vehicles against rollover and for reducing structural loads in transient motion mode of the vehicle have been given. The research allows us to consider the dynamics of the wheeled vehicles both with free flowing and solid construction materials in general. What is more, the case with a free-flowing construction material is comprehensive and the one with a solid material is specific. The developed methodology made it possible to implement a number of design solutions which improve the structural safety of the wheeled vehicles transporting free-flowing construction materials.

A semi-implicit material point method for the continuum simulation of granular materials

We present a new continuum-based method for the realistic simulation of large-scale free-flowing granular materials. We derive a compact model for the rheology of the material, which accounts for the exact nonsmooth Drucker-Prager yield criterion combined with a varying volume fraction. Thanks to a semi-implicit time-stepping scheme and a careful spatial discretization of our rheology built upon the Material-Point Method, we are able to preserve at each time step the exact coupling between normal and tangential stresses, in a stable way. This contrasts with previous approaches which either regularize or linearize the yield criterion for implicit integration, leading to unrealistic behaviors or visible grid artifacts. Remarkably, our discrete problem turns out to be very similar to the discrete contact problem classically encountered in multibody dynamics, which allows us to leverage robust and efficient nonsmooth solvers from the literature. We validate our method by successfully capturing typical macroscopic features of some classical experiments, such as the discharge of a silo or the collapse of a granular column. Finally, we show that our method can be easily extended to accommodate more complex scenarios including two-way rigid body coupling as well as anisotropic materials.

Model of Quasifaceted Cells and Possibility of its Application to Free-Flowing Materials

A model of the ordered packing structure of free-flowing media (spheres) with cells in the form of conventionally cut-out (quasifaceted) parallelepipeds, whose vertices are located at the center of eight neighboring granules- spheres, is studied. Power-law and exponential dependences of the coordination number of the granules- spheres on the packing density γ and porosity ω of the medium are presented. More precise linear and nonlinear dependences are presented for comparatively narrow ranges of γ and ω which are characteristic for filling-media.

An Intermittent Arch Model for Predicting Dynamic Pressures during Discharge in Grain Storage Bins

Abstract. Arching is a common phenomenon in bulk solids storage structures during discharge. It is hypothesized that dynamic pressures during discharge result from the formation of intermittent arches in grain bins. For free-flowing materials such as agricultural grains, these arches are not strong enough to stop the material flow, but they may slow down the moving material in some locations in the bin. When the moving material is slowed down, an inertia force caused by deceleration will be induced. The arch will also carry some of the material weight above it. Since the arch is supported by the bin wall, both the inertia force and the additional material weight carried by the arch will be transferred through the arch to the bin wall, causing the pressure to increase on the bin wall. Based on this force transfer mechanism, a predictive model was developed for predicting dynamic pressures on the walls of grain bins during discharge. A mass-spring system was proposed to represent the grain bulk in calculating the inertia force caused by deceleration. The predicted dynamic pressure is in good agreement with the experimental data reported in the literature for wheat, and the predicted overpressure factor is in close agreement with that recommended in ASABE Standards for bin design.

Methods and Means of Reducing the Power Requirements of Ventilation Systems in the Transfer of Free-Flowing Materials

Theoretical prerequisites for reducing the power requirements of ventilation systems are considered. A classification of methods and means of minimizing the flow rates of air entering a ventilation shelter through leaks and reducing the volumes of air entering the shelter through a charging chute is developed.

A device for the measurement of the horizontal to vertical stress ratio in powders

A new experimental device for the measurement of the hzorizontal to vertical stress ratio was developed. The tester is designed to conduct the required measurements using a single standard load cell without the need to apply strain gauges on custom built parts. Three different procedures were tested with free-flowing incompressible powders to determine the most precise procedure. This optimal procedure included the preliminary twisting of the cell lid to pre-shear the sample and was modified to conduct experiments with cohesive compressible powders. The results obtained from the best procedure were compared with those from commonly used estimating equations for the horizontal to vertical stress ratio as a function of the angle of internal friction. The obtained results were consistent with the Koenen equation (Koenen in Centralblatt der Bauverwaltung 16:446–449, 1896) for free-flowing materials and with the DIN 1055 equation (DIN 1055 Teil 6, Lastannahmen fur Bauten, Lasten in Silozellen, 1987) for cohesive materials.

Experimental study of thermophysical properties of Shubarkol coal

The thermophysical properties of Shubarkol coal are studied using the quasi-steady thermal-state method. The temperature dependencies of the heat capacity and heat conductivity of the coal are obtained, and the heat effect of the coking reaction is also shown and their comparative analysis vis-a-vis the temperature dependencies of a number of similar Kuzhetsk coals is performed. The experimental thermophysical results are essential for study of the feasibility of the use and treatment of Shubarkol coals, and for investigation of the thermophysical properties of any solid, free-flowing bulk materials.

Preparation of mixture of free-flowing materials in double-shaft paddle mixers

Results are presented for investigation of the movement of free-flowing materials in double-shaft paddle mixers. A method of reducing the mixing times of these free-flowing materials, and the design of the doubleshaft paddle mixer for this reduction, which provides for intensification of mixing due to an additional loop for particle movement are examined. Paddle mixers are widely used in various branches of industry. A number of investigations relative to mixing of freeflowing materials have been conducted in these mixers for the purpose of shortening the time required for preparation of the mixture, and improvement of its quality [1, 2]. Let us isolate two basic types of double-shaft paddle mixers: without intersection of the rotational trajectories of the paddles of the parallel shafts (Fig. 1a); and, with intersecting rotational trajectories of the paddles of the parallel shafts (Fig. 2a). It is proposed to shorten the preparation time of the mixture in the mixers with allowance for charging rules by rapid equalization of the initial components along the tiers of the effective members, intensification of convective and diffusion mixing [3, 4], and organization of large-flow discharge of the mixture without loss of its uniformity due to variation in the rotational direction of the shafts and variation of the position of the paddles on these shafts. Two methods of charging the initial components into the mixer are: uniform charging in order of decreasing sizes (the concentration of initial components in each tier of effective members is essentially the same); and, simultaneous charging via different devices (positioning of the initial components is strictly isolated with respect to the tiers of effective members) [4]. Let us examine a mixer (see Fig. 1a) in the housing of which paddles 2 are located on shafts 1 such that the material is mixed from one terminal section of the mixer in one direction, and to the other terminal section in the other direction, while at the end, one or two pairs of paddles are mounted at the opposite angle for agitation of the mixture in the opposite direction. The rotational trajectory of paddles 2 on shafts 1 do not intersect. The interaxial distance between the shafts a 1 > 2R (R is the radius of the rotational trajectory of the paddles). For the method of simultaneous loading of the initial components, a largeflow counter-movement of particles 3 is formed from one terminal section to the other in the initial stage to equalize their concentration along the shafts in each tier of effective members, and intensify convective mixing (see Fig. 1b). Shafts 1 rotate in opposite directions, displacing the free-flowing material from the center of the mixer toward the lateral walls of the housing. Formation of zones of material slip (A) is possible near the terminal walls of the housing in the mixer, resulting in a nonuniform load over the length of the shaft, and slowing of the process of uniform movement of the material. Movement of material is insignificant between the effective zones of the shafts; this renders equalization of the concentration of initial components difficult within the entire reaction space (see Fig. 1b). Charging requires organization within the central zone along the mixer. The charging consumes from 0.1 to 0.3 of the total time required for mixing. To intensify diffusion mixing, the direction of rotation of the shafts is reversed, mixing the material from the lateral walls of the housing toward the center of the mixer. Active exchange of particles occurs between the opposing flows in the central portion of the mixer (see Fig. 1c).

Fractal theory of dome formation

This study presents a theoretical model of dome formation during nonsteady flow of polydisperse cohesive free-flowing materials using fractal structures simulating deterministic relationships. A procedure is proposed for analysis of dome shapes based on fractal theory of nonsteady flow. Experimental data on determination of dome shape for a model free-flowing material, which have demonstrated higher accuracy of the proposed method as compared with existing ones, are cited.

Influence of the distance between a spectrometer and sample on the intensity of X-ray fluorescence

Expressions for calculating the extreme point coordinates of focal spots of X-ray tubes and a detector’s sensitive area are found. These coordinates are used in calculations of the sample area. The dependence of the X-ray fluorescence intensity on the distance between a spectrometer and an analyzed sample is studied experimentally. The intensities of the FeKα spectral lines in iron-containing materials are calculated at different geometric parameters of a spectrometer. It is found that the distance between a sample and detector at which the maximum of the measured intensity is observed depends on the size and position of the detector and X-ray tube collimators, as well as on the relative position of the X-ray tube and detector. Recommendations on how to achieve the maximum intensity of X-ray fluorescence are proposed. The results of the present work can be used for the development of X-ray fluorescence analysis techniques applicable for free-flowing materials directly in technological processes.

Free-flowing granular materials with two-way solid coupling

We present a novel continuum-based model that enables efficient simulation of granular materials. Our approach fully solves the internal pressure and frictional stresses in a granular material, thereby allows visually noticeable behaviors of granular materials to be reproduced, including freely dispersing splashes without cohesion, and a global coupling between friction and pressure. The full treatment of internal forces in the material also enables two-way interaction with solid bodies. Our method achieves these results at only a very small fraction of computational costs of the comparable particle-based models for granular flows.