221 results found
Adesina P, O'Sullivan C, Wang T, 2023, Understanding the interplay between particle shape, grading and sample density on the behaviour of granular assemblies: A DEM approach, Granular Matter, ISSN: 1434-5021
Che H, Windows-Yule K, O'Sullivan C, et al., 2023, A novel semi-resolved CFD-DEM method with two-grid mapping:methodology and verification, AIChE Journal, ISSN: 0001-1541
Morimoto T, O'Sullivan C, Taborda DMG, 2023, Capturing particle-fluid heat transfer in thermo-hydro-mechanical analyses using DEM coupled with a pore network model, Powder Technology, Vol: 429, ISSN: 0032-5910
Numerical simulation of fluid-saturated granular materials subject to temporal or spatial variations in temperature is important for a range of industrial applications and to understand a number of natural processes. Amongst the available fluid-coupling options for Discrete Element Method (DEM), pore network models (PNMs) are attractive as the local heterogeneities in the flow field can be captured with a relatively low computational cost. Any coupled DEM-PNM framework that considers thermal behaviour must account for heat transfer between the solid particles and the fluid phase. This contribution develops a novel expression for the Nusselt number for use in coupled DEM-PNM formulations. Previous studies considering Nusselt number formulations for flow in pipes formed the basis for the proposed model. Finite volume simulations of laminar fluid flow through assemblies of spheres were used to assess the model's efficacy and to calibrate the empirical parameters.
Salomon J, Morimoto T, O'Sullivan C, et al., 2023, Quantifying shear-induced permeability changes in medium loose sands, Journal of Geotechnical and Geoenvironmental Engineering, ISSN: 0733-9410
Chavda M, Zeeshan M, O'Sullivan C, et al., 2023, Magnetic-Induction-Based Positioning System Using Dual Multiplexing Technique, IEEE SENSORS LETTERS, Vol: 7, ISSN: 2475-1472
Adesina P, O'Sullivan C, Wang T, 2023, DEM study on the effect of particle shape on the shear behaviour of granular materials, Computational Particle Mechanics, ISSN: 2196-4378
This study investigates the effects of particle convexity, sphericity and aspect ratio (AR) on the behaviour of sheared granular materials using two-dimensional discrete element method (DEM) simulations. Isotropic, dense and loose assemblies with different particle shapes were prepared and subjected to drained shearing via biaxial compression until the critical state was reached. Macroscopic characteristics such as strength and dilatancy are presented. The factors underlying the macroscopic behaviour are then investigated by considering the coordination number, fabric anisotropy, particle moment, friction mobilization at contacts and particle rotation. For the range of shapes considered here, the data indicate that the shear strength decreases as particle convexity and sphericity increases while the shear strength increases with increasing AR. The shear strength and convexity are weakly correlated, however a stronger correlation is observed between AR and strength. The volumetric strain at large strains tends to increase with increasing AR. There is a stronger correlation between the critical state strength and both the critical state coordination number and the critical state mechanical void ratio than there is between the critical state void ratio and the critical state strength. The contact fabric anisotropy, the magnitude of the moment transmitted by particles and the friction mobilised at the contacts are important factors underlying strength. The critical state strength increases as both the mean particle moment and the mean mobilised friction increased. Analysis of particle rotation provides insights into the response of the granular materials to shearing.
Patino-Ramirez F, O'Sullivan C, 2023, Optimal tip shape for minimum drag and lift during horizontal penetration in granular media, Acta Geotechnica, ISSN: 1861-1125
Horizontal penetration in granular media is ubiquitous, from tunneling and geotechnical site investigation, to root growthand the locomotion of burrowing animals in nature. This contribution couples the discrete element method (DEM) with antcolony optimisation, a heuristic optimisation algorithm, to find the optimal tip shapes to minimise drag and lift forcesduring horizontal penetration. The tip which minimizes drag has a slender profile with a low tip curvature, to give a dragforce that is 15:6% lower compared to a conventional CPT intruder, however this shape induces a downwards force thatincreases with intruder depth. Conversely, the tip that minimizes lift is blunt, with a high tip curvature and short width, andreduces the drag and lift forces by 4:5% and 30:8% (respectively) compared to the CPT. The lift and drag forces arecompeting optimisation objectives, thus the tip shape with the optimal trade-off between drag and lift forces can beestablished using Pareto optimality. The Pareto optimal tip shape reduces the drag and lift forces by 10:7% and 19:4%respectively, and is strikingly similar to the profile of a sandfish. This contribution shows that when a common goal exists,bio-inspired solutions can offer an optimal solution to engineering applications. We also show the potential to integrateDEM simulations within an optimization framework to develop innovative design solutions.
Patino-Ramirez F, O'Sullivan C, Dini D, 2023, Percolating contacts network and force chains during interface shear in granular media, Granular Matter, Vol: 25, ISSN: 1434-5021
The concept of force chains transmitting stress through granular materials is well established; however identification of individual force chains and the associated quantitative analysis is non-trivial. This paper proposes two algorithms to (1) find the network of percolating contacts that control the response of loaded granular media, and (2) decompose this network into the individual force chains that comprise it. The new framework is demonstrated considering data from discrete element method simulations of a ribbed interface moving against a granular sample. The subset of contacts in the material that transfers load across the sample, namely the percolating contact network (G perc), is found using the maximum flow algorithm. The resulting network is fully-connected and its maximum flow value corresponds to the force percolating the system in the direction normal to the ribbed wall. G perc re-orientates in response to the ribbed interface movement and transmits 85–95% of the stress, with only 40–65% of the contacts in the sample. Then, is split into individual force chains using a novel implementation of the widest path problem. Results show that denser materials with increased force-chain centrality promote a higher density of force chains, which results in a higher macro-scale strength during interface shearing. The contribution of force chains in the network is revealed to be highly centralized, composed by a small set of strong and long-lived force chains, plus a large set of weak and short-lived force chains.
Sassel T, Patino-Ramirez F, Hanley K, et al., 2023, Linking the macro-scale response of granular materials during drained cyclic loading to the evolution of micro-structure, contact network and energy components, Granular Matter, Vol: 25, Pages: 1-19, ISSN: 1434-5021
This study has considered the behaviour of granular materials subjected to drained cyclic loading under constant mean effective stress. Using the discrete element method, cubical, isotropically compressed samples were subjected to 50 loading cycles at different values of mean stress (p′= 100, 200, 300 kPa) and different loading amplitudes (ζ= 5%, 10% and 20% of p′). At low cycle numbers, the deformation mechanism is controlled by contractive volumetric strains, before transitioning to the ratcheting regime, characterised by the persistent accumulation of plastic strains. An energy/work analysis showed that the volumetric work per cycle decreased as hysteresis loops tighten. During ratcheting, most boundary work was dissipated by contact sliding. The mechanical response was controlled by ζ, with little to no influence of p′. For ζ=5%, deformations were confined to the elastic range, with no increase in secant stiffness Gsec or shear strength after cyclic loading. For ζ=10%, Gsec and the shear strength increased after cyclic loading, although no significant expansion of the yield surfaces was observed. The largest loading amplitude (ζ=20%) induced yielding at low cycles, leading to significant changes in the fabric, volume and yield surfaces of the samples, and a significant increase of shear strength and Gsec. At the micro-scale, graph theory was used to quantify the evolution of the contact network. After ∼20 loading cycles, the network reached a steady-state of constant but persistent topology changes in the material, with most of the topology retained between loading cycles.
Schnaider Bortolotto M, Taborda D, O'Sullivan C, 2023, A systematic approach for conducting and interpreting hydraulic conductivity tests on granular soils under non-isothermal conditions, 8th International Symposium on Deformation Characteristics of Geomaterials
Liu D, O'Sullivan C, Carraro JAH, 2023, The influence of particle size distribution on the stress distribution in granular materials, Géotechnique, Vol: 73, Pages: 250-264, ISSN: 0016-8505
This study systematically explores the effect of the shape of the particle size distribution on stress transmission in granular materials using three-dimensional discrete element method simulations. Extending prior studies that have focussed on bi-modal mixtures of coarser and finer grains, a broad range of isotropically compressed specimens with spherical particles and linear, fractal and gap-graded particle size distributions are considered. Considering isotropic stress conditions the nature of stress distribution was analysed by determining the mean effective particle stresses and considering the proportion of this stress transmitted by particles with different sizes. For gap-graded materials a contact-based perspective was adopted to consider the stress transmission both within and between the different size fractions. A clear correlation emerged between the cumulative distribution of particle sizes by volume and the cumulative distribution of particle sizes by mean effective stress for specimens with continuous PSDs. This correlation does not hold universally for gap-graded materials. In gap-graded materials the distribution of effective stress between the different size fractions depends upon the size ratio and the percentage of finer grains in the specimen. In contrast to specimens with continuous gradings, in the gap-graded specimens the distribution of effective stress amongst the different size fractions exhibited a marked sensitivity to density. Basic network analysis is shown to provide useful insight into effective stress transmission in the bimodal gap-graded materials.
Bandera S, O'Sullivan C, Tangney P, et al., 2023, Response to the discussion on ?Coarse-grained molecular dynamics of clay compression?, COMPUTERS AND GEOTECHNICS, Vol: 155, ISSN: 0266-352X
Sanvitale N, Zhao B, Bowman E, et al., 2023, Particle-scale observation of seepage flow in granular soils using PIV and CFD, Geotechnique: international journal of soil mechanics, Vol: 73, Pages: 71-88, ISSN: 0016-8505
Seepage-induced instabilities pose a challenge in many geotechnical applications. Particle-scale mechanisms govern the initiation of instability. However, current understanding is based on a macro-scale perspective that draws on continuum mechanics. Recent developments in imaging and numerical analysis can provide the particle-scale fundamental perspective needed to develop a comprehensive insight. This contribution demonstrates the value of combining particle-scale experimental and numerical studies. The experiments consider transparent soil samples created using refractive image matching and monitored by particle image velocimetry (PIV). Three-dimensional pore topology is extracted from a series of 2D images and imported into computational fluid dynamics (CFD)simulations. Permeability is estimated by three distinct approaches: using flow rate, PIV-and CFD-generated data. The flow fields obtained from PIV and CFD are in good agreement considering both flow rate contour plots and flow rate distributions; this demonstrates the successful reconstruction of three-dimensional pore structure and flow-field analysis. The comparison also reveals that the side boundary effects in CFD simulations are constrained within a limited region. The multi-plane results characterize the variance of flow velocity with the three-dimensional pore topology. Finally, the fluid-particle interactions obtained from CFD results show a larger variance in the angular particle packings.
Chavda M, Chandola A, Malik S, et al., 2023, Development of Time-Multiplexed Magnetic-Induction Based Ranging Systems
Ranging and locating systems are necessary currently. Localization of robots operating underground, miners and machinery in mining environments, and interior environments are few applications that demand a reliable and accurate positioning system. Magnetic-induction (MI) based ranging and positioning system is widely used in such applications because the magnetic permeability of different mediums are almost same. However, the MI based system are complex since multiple transmitters are needed for accurate ranging and positioning. In this paper, we present a time division multiplexing MI based ranging system. A single signal source is needed to excite the magnetic coil sequentially. A prototype is developed and tested for a range of 1 meter with an error less than ± 2 cm.
O'Sullivan C, Arson C, Coasne B, 2022, A perspective on Darcy's law across the scales: from physical foundations to particulate mechanics, Journal of Engineering Mechanics, Vol: 148, ISSN: 0733-9399
This paper puts forward a perspective or opinion that we can demonstrate Darcy’s law is valid at any scale where fluid can be modelled/analyzed as a continuum. Darcy’s law describes the flow of a fluid through a porous medium by a linear relationship between the flow rate and the pore pressuregradient through the permeability tensor. We show that such a linear relationship can be established at any scale, so long as the permeability tensor is expressed as a function of adequate parameters that describe the pore space geometry, fluid properties and physical phenomena. Analytical models at pore scale provide essential information on the key variables that permeability depends on under different flow regimes. Upscaling techniques based on the Lippman-Schwinger equation, pore network models orEshelby’s homogenization theory make it possible to predict fluid flow beyond the pore scale. One of the key challenges to validate these techniques is to characterize microstructure and measure transport properties at multiple scales. Recent developments in imaging, multi-scale modeling and advanced computing offer new possibilities to address some of these challenges.
Liu D, O'Sullivan C, Harb Carraro JA, 2022, Use of combined static and dynamic testing to quantify the participation of particles in stress transmission, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 148, ISSN: 0733-9410
A number of research studies have recognised that not all particles in a sand specimen are active in stress transmission, particularly in the case of gap-graded soils. This has implications for the use of the global void ratio (e), and variables/parameters that depend upon e, to predict the influence of the state on the mechanical behaviour of gap-graded soils. This study explores the possibility of comparing the shear stiffness determined in dynamic wave propagation tests (G_dyn) with stiffness values determined in small-strain probes (G_sta) to assess the extent to which all the particles in a specimen are actively engaged in stress transmission. The idea is initially developed using three-dimensional discrete element method simulations and considering ideal specimens. The practical application of this approach is then tested in a series of drained triaxial compression tests. The numerical studies considered both specimens with continuous, linear particle size distributions as well as gap-graded soils. The results show that the ratio G_sta/G_dyn may be associated with the ratio between the bulk density calculated considering only the stress transmitting particles, ρ_m, and the bulk density calculated considering all the particles, ρ. The ratio ρ_m/ρ is determined by the proportion of inactive particles, which varies with the proportion by mass of finer particles in the soil (F_finer) for gap-graded soils. The relationship between G_sta/G_dyn (e) and F_finer enables a qualitative assessment of the proportion of inactive particles. The corresponding experimental test results show a similar but weaker correlation between the ratio of G_sta/G_dyn and F_finer, this weaker correlation may be attributed to the differences between the simulations and the experimental conditions. However, despite the challenges with experimental implementation, the data presented here support the idea that it may be possible to qualitatively estimate the proportion by volum
Morimoto T, Zhao B, Taborda D, et al., 2022, Critical appraisal of pore network models to simulate fluid flow through assemblies of spherical particles, Computers and Geotechnics, Vol: 150, Pages: 1-20, ISSN: 0266-352X
Coupled numerical models considering fluid flow and particle movement enable fundamental analyses of a variety of phenomena in geomechanics including seepage-induced instabilities. Amongst the various CFD (Computational Fluid Dynamics)-DEM (Discrete Element Method) coupled frameworks which have been proposed, Pore Network Models (PNMs) have the potential to simulate fluid flow in granular materials accurately with a low computational cost to enable simulations on Representative Volume Elements (RVEs). However, the current models of the local conductance between the connected pores are very simple, limiting the accuracy of PNMs. This study develops novel local conductance models by detailed analysisof existing analytical studies of fluid flow through different 3D lattice packings of uniform spheres. The performance of these new models relative to existing, simpler models is demonstrated using CFD simulations in which the flow in the pore space of random assemblies of polydisperse spheres is accurately resolved. The analyses show that the new models proposed here can more accurately predict the local and global permeabilities of specimens with a wide range of void ratios and polydispersities. These models do not require any optimisation via merging pores so that they can efficiently simulate systems with an evolving pore space topology.
Malik S, O'Sullivan C, Reddyhoff T, et al., 2022, An acoustic 3D positioning system for robots operating underground, IEEE Sensors Letters, Vol: 6, Pages: 1-4, ISSN: 2475-1472
Underground robots are potentially helpful in many application domains, including geotechnical engineering, agriculture, and archaeology. One of the critical challenges in developing underground robotics is the accurate estimation of the positions of the robots. Acoustic-based positioning systems have been explored for developing an underground 3D positioning system. However, the positioning range is limited due to attenuation in the medium. This letter proposes an underground positioning system that utilizes a novel and easy-to-implement electronic approach for measuringthe acoustic propagation times between multiple transmitters and a receiver. We demonstrate a prototype using four transmitters at the surface and a single buried acoustic sensor as a proof-of-concept. The times of arrival for signals emitted by the different sources are measured by correlating the transmitted and received signals. The distances between the multiple transmitters and a receiver are estimated, and a tri-linearization algorithm is used to estimate the position of the buried sensor in 3D with respect to reference coordinates. The system is tested in a soil tank. The experimental results show that the proposed system is able to estimate the 3D position of buried sensors with an error of less than ±2.5 cm within a measurement field of size 50 cm × 50 cm × 35 cm in X, Y, and Z (width × length × depth). The proposed electronic synchronization approach allows increasing the positioning range of the system by increasing the number of transmittersat the surface. This paves the way for the development of a positioning system for robots operating underground.
Adesina P, Morimoto T, Otsubo M, et al., 2022, Determining a representative element volume for DEM simulations of samples with non-circular particles, Particuology: science and technology of particles, Vol: 68, Pages: 29-43, ISSN: 1672-2515
Numerical studies on the number of particles or system size required to attain a representative element volume (REV) for discrete element method (DEM) simulations of granular materials have almost always considered samples with spherical or circular particles. This study considers how many particles are needed to attain a REV for 2D samples of 2-disc cluster particles where the particle aspect ratio (AR) was systematically varied. Dense and loose assemblies of particles were simulated. The minimum REV was assessed both by considering the repeatability of static packing characteristics and the shearing behaviour in biaxial compression tests, and by investigating the effect of sample size on the measured characteristics and observed shearing behaviour. The repeatability of the data considered generally improved with increasing sample size. The packing characteristics of the dense samples were more repeatable suggesting that the minimum REV reduces with increasing packing density. The minimum REV was observed to be sensitive to the characteristic measured. Although the overall responses of the samples during shear deformation were similar irrespective of the sample sizes, the smaller the sample size, the higher the fluctuations observed in the responses. Analysis of the coefficient of variation of the fluctuations around the critical state stress ratio can provide insight as to whether a REV is attained. The particle AR influences the effect of sample size on shearing characteristics and thus the minimum number of particles required to attain a REV; this can be explained by the influence of AR on the number of contacts within the samples.
Adesina P, O'Sullivan C, Morimoto T, et al., 2022, Determining a representative element volume for DEM simulations of samples with non-circular particles, PARTICUOLOGY, Vol: 68, Pages: 29-43, ISSN: 1674-2001
Otsubo M, Kuwano R, O'Sullivan C, et al., 2022, Using geophysical data to quantify stress-transmission in gap-graded granular materials, Geotechnique: international journal of soil mechanics, Vol: 72, Pages: 565-582, ISSN: 0016-8505
The behaviour of gap-graded granular materials, i.e. mixtures of coarse and cohesionless finer grains having a measurable difference in particle size, does not always confirm to established frameworks of sand behaviour. Prior research has revealed that the role of the finer particles on the stress-strain response, liquefaction resistance, and internal stability of non-cohesive gap-graded soils is significant and complex, and highly dependent on both the volumetric proportion of finer particles in the material and the coarse-particle to finer-particle size ratio. Quantifying the participation of the finer particles on the stress transmission and overall behaviour is central to understanding the behaviour of these materials. However, no experimental technique that can directly quantify the contribution of finer particles to the overall behaviour has hitherto been proposed. This paper explores to what extent the participation of finer particles can be assessed using laboratory geophysics, recognizing that granular materials act as a filter to remove the high frequency components of applied seismic / sound waves. Discrete element method simulations are performed to understand the link between particle-scale stress transmission and the overall response observed during shear wave propagation. When the proportion of finer particles is increased systematically both the shear wave velocity (VS) and low-pass frequency (flp) increase sharply once a significant amount of the applied stress is transferred via the finer particles. This trend is also observed in equivalent laboratory experiments. Consequently, the flp–VS relationship can provide useful insights to assess whether the finer particles contribute to stress transmission and hence the mechanical behaviour of the gap-graded materials.
Liu D, Morimoto T, Carraro JAH, et al., 2022, A semi-empirical re-evaluation of the influence of state on elastic stiffness in granular materials, Granular Matter, Vol: 24, Pages: 1-22, ISSN: 1434-5021
This study uses data acquired from three-dimensional discrete element method simulations to reconsider what measure of state can be used to predict stiffness in granular materials. A range of specimens with linear and gap-graded particle size distributions are considered and stiffness is measured using small amplitude strain probes. Analysis of the data firstly confirms that the void ratio, which is typically used as a measure of state in experimental soil mechanics, does not correlate well with shear stiffness. However, the empirical expressions developed by Hardin and his colleagues can capture variations in stiffness, provided an appropriate state variable is used. The study then highlights that the contribution of individual contacts to the overall stiffness is highly variable, depending on both the contact force transmitted and the particle size. Analyses explore how the stress transmission both within and between the different size fractions affects the overall stiffness. This heterogeneity in stiffness relates to the heterogeneity in the stress transmission amongst the different fractions. By considering the heterogeneity of stress distribution amongst different particle size fractions, a new semi-empirical stress-based state variable is proposed that provides insight into the factors that influence stiffness.
Yu M, Reddyhoff T, Dini D, et al., 2022, Acoustic emission enabled particle size estimation via low stress-varied axial interface shearing, IEEE Transactions on Instrumentation and Measurement, Vol: 71, ISSN: 0018-9456
Acoustic emission (AE) refers to a rapid release of localized stress energy that propagates as a transient elastic wave and is typically used in geotechnical applications to study stick-slip during shearing, and breakage and fracture of particles. This article develops a novel method of estimating the particle size, an important characteristic of granular materials, using axial interface shearing-induced AE signals. Specifically, a test setup that enables axial interface shearing between a one-dimensional compression granular deposit and a smooth shaft surface is developed. The interface sliding speed (up to 3mm/s), the compression stress (0-135kPa), and the particle size (150μm-5mm) are varied to test the acoustic response. The start and end moments of a shearing motion, between which a burst of AE data is produced, are identified through the variation of the AE count rates, before key parameters can be extracted from the bursts of interests. Linear regression models are then built to correlate the AE parameters with particle size, where a comprehensive evaluation and comparison in terms of estimation errors is performed. For granular samples with a single size, it is found that both the AE energy related parameters and AE counts, obtained using an appropriate threshold voltage, are effective in differentiating the particle size, exhibiting low fitting errors. The value of this technique lies in its potential application to field testing, for example as an add-on to cone penetration test systems and to enable in-situ characterization of geological deposits.
Kalderon M, Smith E, O'Sullivan C, 2022, Comparative analysis of porosity coarse-graining techniques for discrete element simulations of dense particulate systems, Computational Particle Mechanics, Vol: 9, Pages: 199-219, ISSN: 2196-4378
The discrete element method (DEM) is a well-established approach to study granular materials in numerous fields of application; each granular particle is modelled individually to predict the overall behaviour. This behaviour can be then extracted by averaging, or coarse graining, the sample using a suitable method. The choice of appropriate coarse-graining method entails a compromise between accuracy and computational cost, especially in the large-scale simulations typically required by industry. A number of coarse-graining methods have been proposed in the literature, and these are reviewed and categorized in this work. Within this contribution, two novel porosity coarse-graining strategies are proposed including a voxel method where a secondary dense grid of “pixel cells” is implemented adopting a binary logic for the coarse graining and a hybrid method where both analytical formulas and pixels are utilized. The proposed methods are compared with four coarse-graining schemes that have been documented in the literature, including the particle centroid method, an analytical method, a method which solves the diffusion equation and an approach which employs averaging using kernels. The novel methods are validated for problems in both two and three dimensions through comparison with the “accurate” analytical method. It is shown that, once validated, both the proposed schemes can approximate the exact solutions quite accurately; however, there is a high computational cost associated with the voxel method. The accuracy of both methods can be adjusted allowing the user to decide between accuracy and computational time. A detailed comparison is then presented for all six schemes considering “accuracy”, “smoothness” and “computational cost”. Optimal parameters are obtained for all six methods, and recommendations for coarse-graining DEM samples are discussed.
Su TC, O'Sullivan C, Yasuda H, et al., 2022, Understanding the rheological transitions in semi-solid alloys by a combined <i>In Situ</i> Imaging and granular micromechanics modeling approach, Solid State Phenomena, Vol: 327, Pages: 127-132, ISSN: 1012-0394
To gain better understanding of rheological transitions from suspension flow to granular deformation and shear cracking, this research conducted shear-deformation on globular semi-solid Al-Cu alloys to study the rheological behavior of semi-solid as a function of solid fraction (38% - 85%) and shear rate (10<jats:sup>-4</jats:sup> – 10<jats:sup>-1</jats:sup> s<jats:sup>-1</jats:sup>) under real-time synchrotron radiography observation. By analyzing 17 X-ray imaging datasets, we define three rheological transitions: (i) the critical solid fraction from a suspension to a loosely percolating assembly; (ii) from the net contraction of a loose assembly to the net dilation of a densely packed assembly, and (iii) to shear cracking at high solid fraction and shear rate. Inspired by in-situ observations of semi-solid deformation showing a disordered assembly of percolating crystals in partially-cohesive contact with liquid flow, we reproduced a two-phase sample using the coupled lattice Boltzmann method-discrete element method (LBM-DEM) simulation approach for granular micromechanical modeling. In DEM, each globular Al grain is represented by a discrete element, and the flow of interstitial liquid is solved by LBM. The LBM-DEM simulations show quantitative agreement of semi-solid strain localization with the experiments and are used to explore the components involved in the shear rate dependence of the transitions, and the role of liquid pressure on the initiation of shear cracking.
Zhao B, O'Sullivan C, 2022, Fluid particle interaction in packings of monodisperse angular particles, Powder Technology, Vol: 395, Pages: 133-148, ISSN: 0032-5910
Understanding fluid flow in granular materials is essential for many engineering applications, including petroleum recovery, groundwater movement and embankment stability. This study investigates the influence of particle angularity on permeability and fluid-particle interaction forces. A random shape generator based on spherical harmonics is used to create irregular-shaped particles with different levels of angularity. Granular packings of uniformly sized (monodisperse) particles are then constructed with the discrete element method (DEM), and pore scale computational fluid dynamics (CFD) simulations are used to determine the flow fields and the resulted fluid-particle interaction. The more angular particle assemblies thus generated are less permeable, and their fluid-particle interaction forces are higher. However, angularity has limited influence on flow rate distribution and flow tortuosity. The influence of angularity is localized. An increase in angularity generates a larger variance of the pressure distribution on the particle surfaces, thus increasing the pressure component of the fluid-particle interaction force.
Morimoto T, O'Sullivan C, Taborda D, 2021, Exploiting DEM to Link Thermal Conduction and Elastic Stiffness in Granular Materials, Journal of Engineering Mechanics, Vol: 148, ISSN: 0733-9399
Estimating the effective thermal conductivity (ETC) of granular materials is important in various engineering disciplines. The ETC of a granular material is not unique, rather it depends upon the material's packing characteristics, i.e. porosity and coordination number. Directly measuring the ETC of granular materials with a particular packing density and subjected to specific stress conditions is experimentally challenging. There is a need to develop reliable, indirect experimental methods to measure the ETC of granular materials. Here we explore the possibility of linking the ETC of granular materials to their elastic moduli.This study used a thermal pipe network model implemented in a Discrete Element Method (DEM) code to generate ETC data for ideal, virtual two-phase granular samples with stagnant pore fluid. Parametric studies considered the sensitivity of the ETC to the sample packing. Data from small deformation probes were used to explore links between the samples' elastic moduli and their ETCs. The results provide a theoretical background for the development of an indirect experimental method to predict the ETC or trends in the variation in the ETC by considering stiffness data which are relatively straightforward to acquire. The study shows how DEM can be used as a sophisticated thought experiment to explore novel ideas for developing experimental procedures.
Yu M, Reddyhoff T, Dini D, et al., 2021, Using ultrasonic reflection resonance to probe stress wave velocity in assemblies of spherical particles, IEEE Sensors Journal, Vol: 21, Pages: 22489-22498, ISSN: 1530-437X
A high-sensitivity method to measure acousticwave speed in soils by analyzing the reflected ultrasonic signalfrom a resonating layered interface is proposed here.Specifically, an ultrasonic transducer which can be used to bothtransmit and receive signals is installed on a low-high acousticimpedance layered structure of hard PVC and steel, which in turnis placed in contact with the soil deposit of interest. The acousticimpedance of the soil (the product of density and wave velocity)is deduced from analysis of the waves reflected back to thetransducer. A system configuration design is enabled bydeveloping an analytical model that correlates the objectivewave speed with the measurable reflection coefficient spectrum.The physical viability of this testing approach is demonstratedby means of a one-dimensional compression device that probesthe stress-dependence of compression wave velocity of differentsizes of glass ballotini particles. Provided the ratio of thewavelength of the generated wave to the soil particle size issufficiently large the data generated are in agreement with dataobtained using conventional time-of-flight measurements. Inprinciple, this high-sensitivity approach avoids the need for thewave to travel a long distance between multiple transmitterreceiver sensors as is typically the case in geophysical testingof soil. Therefore it is particularly suited to in-situ observation ofsoil properties in a highly compact setup, where only a single transducer is required. Furthermore, high spatialresolution of local measurements can be achieved, and the data are unaffected by wave attenuation as transmitted insoil.
Bandera S, Angioletti-Uberti S, Tangney P, et al., 2021, Coarse-grained molecular dynamics simulations of clay compression, Computers and Geotechnics, Vol: 138, Pages: 1-18, ISSN: 0266-352X
This paper outlines a framework for using molecular dynamics to simulate the compression of kaolinite saturated at alkaline pH (=8) in a low (1 mM) concentration solution. The particles are modelled as flat (3D) ellipsoids and their interactions are described by a modified form of the Gay-Berne potential, calibrated against DLVO theory. The LAMMPS software was used to generate monodisperse and slightly polydisperse samples, and to simulate isotropic compression to 100 kPa. The influences of sample size and strain rate on the void ratio and the arrangement of particles within the samples were investigated via parametric studies. It is useful to consider the extent to which the system temperature (a measure of the average kinetic energy) is controlled when assessing whether the applied strain rate is appropriate. It is found that the number of particles that can be considered a representative element volume is orders of magnitude larger than the number simulated in earlier studies and that larger number of particles are required in polydisperse samples than in the monodisperse case. A comparison between the results obtained and those of published experimental studies show that the methodology proposed can deliver sensible results for the material considered.
O'Sullivan C, Cheng H, Zhao J, 2021, Use of DEM in geomechanics: Special issue associated with the DEM 8 conference, Computers and Geotechnics, Vol: 137, Pages: 1-4, ISSN: 0266-352X
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