Álvaro Ramírez-Gómez

Three dimensional discrete element models for simulating the filling and emptying of silos: Analysis of numerical results

Abstract The discrete element method (DEM) is a promising technique that allows the mechanical behaviour of the material stored in silos and hoppers to be studied. The present work analyses the numerical results obtained by two three-dimensional DEM models that simulate the filling and discharge of a silo for two materials: glass beads or maize grains. The aim of the present work was to assess the capacity of these models to predict the behaviour of the studied materials. To guarantee the maximum representativeness of the results, many of the simplifications usually used in DEM models were avoided. The results analysed included the vertical distributions of the normal pressure, tangential pressure and mobilised friction, the horizontal distribution of normal pressure, velocity profiles and the spatial distribution of the bulk density. The results of this analysis highlight the potential of DEM models for studying the behaviour of granular materials in silos and hoppers, provided that simplifications are minimized.

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.

Research needs on biomass characterization to prevent handling problems and hazards in industry

ABSTRACT Biomass comes from different sources and can be used for different purposes, for electricity generation, for transport fuels, for heating, as well as for the manufacturing of bio-based products. Nowadays, the European framework for energy calls for the use of biomass in electricity, heat, and transport (fuels), to reach 20% of all energy use from renewable sources by 2020. Therefore, the use of biomass should roughly have to double. If this is the case, large amounts of biomass will be mobilized and handling problems are expected to grow aligned with the increased use of biomass. Biomass is characterized because it does not flow well, packs easily, it can knit together, it is very dusty, and it is prone to self-heating and self-ignition, among others. If more research is not conducted on the characterization of the behavioral properties of biomasses, the sustainability of supply chains could be then severely affected.

Determining the shape of agricultural materials using spherical harmonics

Display Omitted Box modelling or 3D scanning are fast techniques to obtain initial geometrical data.A methodology to accurately describe particle shapes has been proposed.Spherical harmonics can be successfully used with agricultural grains.The main limitation of SH was overtaken dividing into regions the particle surface.The use of SH presents a significant potential for DEM modelling in the near future. Determining the shape of agricultural products is important in many areas of the food industry, such as for classification or quality inspection. The characterization methods used to describe the behaviour of particulate systems during handling may also benefit from an accurate description of the particle shapes. Several methods for particle shape representation have been proposed, including those using super-quadric equations, polygon formulations, or composite particles. However, it has been proved that these methods are not accurate enough to represent complex-shaped particles. The use of spherical harmonics has recently received increasing attention for this purpose. In this paper, spherical harmonics are used to obtain the shapes of three agricultural grains, namely bean, chickpea, and maize, which differ in complexity. Once it has been proved that spherical harmonics can accurately describe the shapes of agricultural grains, the advantages and disadvantages of this technique are discussed. Furthermore, the relationship between spherical harmonics and the discrete element method for the simulation of particle systems is also discussed.

Semi-analytical model for non-spherical particles

The problem of the particle shape arising in modelling granular materials by the Discrete Element Method (DEM) is investigated. A semi-analytical modelling concept for arbitrarily shaped particles is presented and with the aim of implementing it in DEM. Two types of surfaces, namely, reference and actual surfaces, are considered. The reference surface is assumed to be the ‘exact surface’, and the actual surface, which is an approximated surface of the particle used for modeling purposes. A corn kernel surface is considered to be the reference surface. The considered semi-analytical method uses spherical harmonics and the obtained results demonstrate the increase in the accuracy of the model with the increase in the number of spherical harmonics used.

Scaling Laws in Granular Flow and Pedestrian Flow

We use particle-based simulations to examine the flow of particles through an exit. Simulations involve both gravity-driven particles (representing granular material) and velocity-driven particles (mimicking pedestrian dynamics). Contact forces between particles include elastic, viscous, and frictional forces; and simulations use bunker geometry. Power laws are observed in the relation between flow rate and exit width. Simulations of granular flow showed that the power law has little dependence on the coefficient of friction. Polydisperse granular systems produced higher flow rates than those produced by monodisperse ones. We extend the particle model to include the main features of pedestrian dynamics: thoracic shape, shoulder rotation, and desired velocity oriented towards the exit. Higher desired velocity resulted in higher flow rate. Granular simulations always give higher flow rate than pedestrian simulations, despite the values of aspect ratio of the particles. In terms of force distribution, pedestrians and granulates share similar properties with the non-democratic distribution of forces that poses high risks of injuries in a bottleneck situation.

Effects of different snow load arrangements on steel silo roof structures

Large diameter steel silos usually require a beam structure to support rooftop inspection gangways and resist loads derived from the snow and wind actions. The existence of localized overloads caused by drifted snow on roofs as a consequence of the wind action has been reported in the literature. European standard EN 1991-1-3 also considers the need of taking into account asymmetric patterns for snow loads calculation. However, conical roofs are not included in the specific list of cases considered by this standard. The present work compares the normal stresses and displacements produced in a conical steel silo roof structure by applying balanced loads distributed on the whole roof and unbalanced loads applied on a roof sector. Experimental measurements and a three-dimensional beam model developed by the authors have been used to predict the stresses and vertical displacements of a metal silo roof structure measuring 18.34 m in diameter. The results show that the existence of an asymmetric load pattern produces higher normal stresses (up to 23%) and vertical displacements (up to 50%) than those derived from balanced loads, for any similar load per beam considered.

International Conference for Conveying and Handling of Particulate Solids (CHoPS2015)

We are pleased to introduce invited papers selected from authors who presented at the 8th International Conference for Conveying and Handling of Particulate Solids (CHoPS2015) held in Tel Aviv, Israel, during 3rd to 7th May 2015. All papers have been rigorously and independently peer reviewed and we extend our thanks to the authors for submitting work of such high quality. Over the past 20 years, CHoPS events have become leading conferences on the handling and processing of bulk solids. CHoPS2015 was, as the organizers promised, “a great scientific event but also a most pleasant and memorable experience” as will be the next CHoPS conference to be held in London, UK, on 10th to 14th September 2018.

Discrete element analysis on the discharge rate of elongated particles

The Discrete Element Method (DEM) has been broadly used for investigating many different silo phenomena, such as pressure distributions, flow patterns or segregation processes. In this research work has been used to analyze the discharge rate of elongated particles (simulated biomass briquettes) in a hopper with different inclinations of the walls and outlet diameters. Numerical results have been compared with analytical results in order to assess its validity for this type of biomass materials.

A Discrete Spheropolygon Model for Calculation of Stress in Crowd Dynamics

Several models have been presented to evaluate flow rates in pedestrian dynamics, yet very few focus on the calculation of the stress experienced by pedestrians under high density. With this aim, a pedestrian dynamics model is implemented to calculate the stress developed under crowd conditions. The model is based on an extension of a granular dynamics model to account contact forces, ground reaction forces and torques in the pedestrians. Contact stiffness is obtained from biomedical journal articles, and coefficient of restitution is obtained by direct observations of energy loss in collisions. Existing rotational equations of motion are modified to incorporate a rotational viscous component, which allows pedestrians to come to a comfortable stop after a collision rather than rotating indefinitely. The shape of the pedestrian is obtained from a bird’s eye, cross sectional view of the human chest cavity and arms, which was edited to produce an enclosed shape. This shape is them approximated by a spheropolygon, which is a mathematical object that allows real-time simulation of complex-shape particles. The proposed method provides real benefits to the accuracy on particle shape representation, and rotational dynamics of pedestrians at micro-simulation level. It provides a new tool to calculate the risk of injuries and asphyxiation when people are trapped in dense crowds that lead to development of high pressure.