Publications
224 results found
Farsi A, Xiang J, Latham JP, et al., 2017, Does shape matter? FEMDEM estimations of strength and post failure behaviour of catalyst supports, 5th International Conference on Particle-Based Methods
Gao K, Harrison J, Lei Q, et al., 2017, Investigating the relationship between far-field stress and local values of the stress tensor, Procedia Engineering, Vol: 191, Pages: 536-542, ISSN: 1877-7058
In situ stress is an important parameter in rock mechanics, thus robust estimation of far–field stress to be used as boundary loadings for further rock engineering analysis based on the local in situ stress data seems indispensable. Here, as part of a preliminary investigation into this problem, we use the combined finite–discrete element method to examine how the mean of local stress tensors is related to the far–field stress. We have conducted a series of stress simulations on a model of a fractured rock mass subjected to various boundary loadings, and calculated the Euclidean mean of the stress data and compared them with the boundary loadings. The results shows that the Euclidean mean and boundary loadings are approximately equal, which gives us an indication that the Euclidean mean of the stress data can be a reasonable estimation of the far–field stress.
Farsi A, Pullen AD, Latham JP, et al., 2017, Full deflection profile calculation and Young's modulus optimisation for engineered high performance materials, Scientific Reports, Vol: 7, Pages: 1-13, ISSN: 2045-2322
New engineered materials have critical applications in different fields in medicine, engineering and technology but their enhanced mechanical performances are significantly affected by the microstructural design and the sintering process used in their manufacture. This work introduces (i) a methodology for the calculation of the full deflection profile from video recordings of bending tests, (ii) an optimisation algorithm for the characterisation of Young’s modulus, (iii) a quantification of the effects of optical distortions and (iv) a comparison with other standard tests. The results presented in this paper show the capabilities of this procedure to evaluate the Young’s modulus of highly stiff materials with greater accuracy than previously possible with bending tests, by employing all the available information from the video recording of the tests. This methodology extends to this class of materials the possibility to evaluate both the elastic modulus and the tensile strength with a single mechanical test, without the need for other experimental tools.
Obeysekara, Lei Q, Salinas P, et al., 2017, Modelling the evolution of a fracture network under excavation-induced unloading and seepage effects based on a fully coupled fluid-solid simulation, 51st US Rock Mechanics/Geomechanics Symposium
Latham J-P, Yang P, Lei Q, et al., 2017, Blast fragmentation in rock with discontinuities using an equation of state gas model coupled to a transient dynamics fracturing and fragmenting FEMDEM code, 51st US Rock Mechanics/Geomechanics Symposium
Guo L, Latham J-P, Xiang J, 2017, A numerical study of fracture spacing and through-going fracture formation in layered rocks, International Journal of Solids and Structures, Vol: 110-111, Pages: 44-57, ISSN: 0020-7683
Naturally fractured reservoirs are an important source of hydrocarbons. Computational models capable of generating fracture geometries according to geomechanical principles offer a means to create a numerical representation of a more realistic rock mass structure. In this work, the combined finite-discrete element method is applied to investigate fracture patterns in layered rocks. First, a three-layer model undergoing layer normal compression is simulated with the aim of examining the controls on fracture spacing in layered rocks. Second, a seven-layer model with low competence contrast is modelled under direct tension parallel to the layering and bending conditions with the focus on investigating through-going fracture formation across layer interfaces. The numerical results give an insight into the understanding of various mechanisms that contribute to fracture pattern development in layered rocks.
Latham J, Xiang J, Obeysekara A, et al., 2017, Modelling hydro-geomechanical behaviour of fractured and fracturing rock masses: application to tunnel excavation-induced damage, 16th Conference on the Mechanics and Engineering of Rock, MIR
Lei Q, Latham J-P, Tsang C-H, 2017, The use of discrete fracture networks for modelling coupled geomechanical and hydrological behaviour of fractured rocks, Computers and Geotechnics, Vol: 85, Pages: 151-176, ISSN: 1873-7633
We present a discussion of the state-of-the-art on the use of discrete fracture networks (DFNs) for modelling geometrical characteristics, geomechanical evolution and hydromechanical (HM) behaviour of natural fracture networks in rock. The DFN models considered include those based on geological mapping, stochastic generation and geomechanical simulation. Different types of continuum, discontinuum and hybrid geomechanical models that integrate DFN information are summarised. Numerical studies aiming at investigating geomechanical effects on fluid flow in DFNs are reviewed. The paper finally provides recommendations for advancing the modelling of coupled HM processes in fractured rocks through more physically-based DFN generation and geomechanical simulation.
Xiang J, Latham JP, Farsi A, 2017, Algorithms and capabilities of solidity to simulate interactions and packing of complex shapes, DEM 7, Pages: 139-149, ISSN: 0930-8989
© Springer Science+Business Media Singapore 2017. A number of numerical algorithms for simulation of particle packing have been proposed and used in a wide range of industries: mining, chemical engineering, pharmaceuticals, agriculture and food handling, etc. However, most of them can only deal with simple and regular shapes due to the complex and expensive numerical algorithms needed to simulate complex shapes. In this paper, a FEMDEM code, Solidity, is used to more accurately capture the influence of complex shape. It combines deformable fracturing arbitrary-shaped particle interactions modelled by FEM with discrete particulate motion modelled by DEM. This paper will cover recent code optimisation for the contact force calculation with arbitrary body shape, parallelisation performance and discussion of results showing both deformable and rigid body versions of the code in different application scenarios. Solidity also provides post-processing tools to analyse the particle packing structure in terms of local porosity and orientation distributions, contact forces, and coordination number, etc. Some examples of Platonic and Archimedean body packs are presented.
Joulin C, Xiang J, Latham J-P, et al., 2017, A New Finite Discrete Element Approach for Heat Transfer in Complex Shaped Multi Bodied Contact Problems, 7th International Conference on Discrete Element Methods (DEM), Publisher: SPRINGER-VERLAG SINGAPORE PTE LTD, Pages: 311-327, ISSN: 0930-8989
Vagnon F, Ferrero AM, Latham JP, et al., 2017, Benchmarking of debris flow experimental tests using combined finite-discrete element method, FEMDEM, Pages: 739-752
Numerical simulations of debris flow events provide a useful tool for investigating, within realistic geological contexts, the dynamics of these phenomena. One application of such numerical models is to evaluate and forecast impact loading on protection barriers. Most of these models are uncoupled methods: this means that it is necessary to use different codes for analysing the motion phase and for evaluating impact forces. In this way, the uncertainties related to the barrier design are increased and difficult to quantify. In this paper the combined finite-discrete element method, FEMDEM is employed to back-analyse experimental impact tests of debris flow interacting against rigid and waterproof barrier. This methodology allows the simultaneous determination of flow characteristics (velocity and thickness) and impact load on the barrier structure. Two different numerical set-ups were adopted to reproduce laboratory experiments. In the first case, we defined a priori the size and the shape of the impacting particles. In the second case, the unstable mass was hypothesised as a unique block with fracture joint elements behaviour, cohesionless, and very low tensile strength. In this way the gravity and friction forces led the particles flowing into the flume. The results were compared in order to quantify the effects of the set-up schemes and material characteristics on the simulations. Limitations and future developments on the application of FEMDEM methodology to this type of geotechnical problem are discussed.
Latham, Obeysekara, Xiang J, et al., 2016, Modelling hydro-geomechanical behaviour of fractured and fracturing rock masses: application to tunnel excavation-induced damage, Conferenze di Meccanica e Ingegneria delle Rocce
Farsi A, Xiang J, Latham JP, et al., 2016, Simulation and characterisation of packed columns for cylindrical catalyst supports and other complex-shaped bodies, Proceedings of the 7th International Conference on Discrete Element Methods, Publisher: Springer Singapore, Pages: 397-406, ISBN: 978-981-10-1926-5
Catalyst pellets are packed in reactor beds and the shape and mechanicalproperties have a major influence on the reactor performance by virtue of (i)the detailed topology of the void space and grain surface area and (ii) the fragility of the pack to withstand in-service stresses within the solid skeleton – often through thermal and cyclic stressing. The paper highlights the features of the FEMDEM code used to simulate these performance-related properties of the pack. The local porosity, packing structure, bulk porosity and orientation distributions of the resulting bodies making up the pack of pellets will be presented. The generic methodology illustrated is shown to be suitable for shape optimisation of industrial packing processes.
Yang P, Xiang J, Chen M, et al., 2016, The immersed-body gas-solid interaction model for blast analysis in fractured solid media, International Journal of Rock Mechanics and Mining Sciences, Vol: 91, Pages: 119-132, ISSN: 1873-4545
Blast-induced fractures are simulated by a novel gas-solid interaction model, which combines an immersed-body method and a cohesive zone fracture model. The approach employs a finite element fluid model and a combined finite-discrete element solid model. This model is fully coupled and simulates the whole blasting process including gas pressure impulse, shock wave propagation, gas expansion, fragmentation and burden movement phases. In the fluid model, the John-Wilkins-Lee equation of state is introduced to resolve the relationship between pressure and density of the highly compressible gas in blasts and explosions. A Q-scheme is used to stabilise the model when solving extremely high pressure situations. Two benchmark tests, blasting cylinder and projectile fire, are used to validate this coupled model. The results of these tests are in good agreement with experimental data. To demonstrate the potential of the proposed method, a blasting engineering simulation with shock waves, fracture propagation, gas-solid interaction and flying fragments is simulated.
Anastasaki E, Latham J-P, Xiang J, 2016, Numerical test for single concrete armour layer on breakwaters, Proceedings of the Institution of Civil Engineers - Maritime Engineering, Vol: 169, Pages: 174-187, ISSN: 1741-7597
The ability of concrete armour units for breakwaters to interlock and form an integral single layer is important for withstanding severe wave conditions. In reality, displacements take place under wave loading, whether they are small and insignificant or large and representing serious structural damage. In this work, a code that combines finite- and discrete-element methods which can simulate motion and interaction among units was used to conduct a numerical investigation. Various concrete armour layer structures were built using a carefully researched placement technique and then subjected to a boundary vibration. By analysing the displacements and assessing the number of units that were displaced by more than one-third their nominal size, the numerical test programme indicated clearly that the initial build packing density was the most important parameter influencing the stability of concrete armour layers under vibration. The size of the underlayer rock and the type of unit also affected the numerical performance of the single-layer concrete armour systems under vibration. The results presented are for full-scale systems and therefore add further insights into simple laboratory ‘shake tests’, although the oscillatory loading in this study is acknowledged to be profoundly different to wave action.
Latham JP, Xiang J, Farsi A, 2016, Accurate modelling of particle shape effects in packed granular structures: a class of particulate problems solved by FEMDEM, 7th International Conference on Discrete Element Methods
Lei Q, Latham J-P, Xiang J, 2016, Implementation of an empirical joint constitutive model into finite-discrete element analysis of the geomechanical behaviour of fractured rocks, Rock Mechanics and Rock Engineering, Vol: 49, Pages: 4799-4816, ISSN: 1434-453X
An empirical joint constitutive model (JCM) thatcaptures the rough wall interaction behaviour of individualfractures associated with roughness characteristicsobserved in laboratory experiments is combined with thesolid mechanical model of the finite-discrete elementmethod (FEMDEM). The combined JCM-FEMDEM formulationgives realistic fracture behaviour with respect toshear strength, normal closure, and shear dilatancy andincludes the recognition of fracture length influence as seenin experiments. The validity of the numerical model isdemonstrated by a comparison with the experimentallyestablished empirical solutions. A 2D plane strain geomechanicalsimulation is conducted using an outcrop-basednaturally fractured rock model with far-field stresses loadedin two consecutive phases, i.e. take-up of isotropicstresses and imposition of two deviatoric stress conditions.The modelled behaviour of natural fractures in response tovarious stress conditions illustrates a range of realisticbehaviour including closure, opening, shearing, dilatancy,and new crack propagation. With the increase in stressratio, significant deformation enhancement occurs in thevicinity of fracture tips, intersections, and bends, wherelarge apertures can be generated. The JCM-FEMDEMmodel is also compared with conventional approaches thatneglect the scale dependency of joint properties or theroughness-induced additional frictional resistance. Theresults of this paper have important implications forunderstanding the geomechanical behaviour of fracturedrocks in various engineering activities
Obeysekara A, Lei Q, Salinas P, et al., 2016, A fluid-solid coupled approach for numerical modeling of near-wellbore hydraulic fracturing and flow dynamics with adaptive mesh refinement, 50th US Rock Mechanics/Geomechanics Symposium
Gao K, Harrison JP, Lei Q, et al., 2016, Influence of boundary constraint stiffness on stress heterogeneity modelling, 50th US Rock Mechanics / Geomechanics Symposium
Lei Q, Wang X, Xiang J, et al., 2016, Influence of stress on the permeability of a three-dimensional fractured sedimentary layer, 50th US Rock Mechanics/Geomechanics Symposium
Yang P, Xiang J, Fang F, et al., 2016, Modelling of fluid–structure interaction with multiphase viscous flows using an immersed-body method, Journal of Computational Physics, Vol: 321, Pages: 571-592, ISSN: 1090-2716
An immersed-body method is developed here to model fluid–structure interaction for multiphase viscous flows. It does this by coupling a finite element multiphase fluid model and a combined finite–discrete element solid model. A coupling term containing the fluid stresses is introduced within a thin shell mesh surrounding the solid surface. The thin shell mesh acts as a numerical delta function in order to help apply the solid–fluid boundary conditions. When used with an advanced interface capturing method, the immersed-body method has the capability to solve problems with fluid–solid interfaces in the presence of multiphase fluid–fluid interfaces. Importantly, the solid–fluid coupling terms are treated implicitly to enable larger time steps to be used. This two-way coupling method has been validated by three numerical test cases: a free falling cylinder in a fluid at rest, elastic membrane and a collapsing column of water moving an initially stationary solid square. A fourth simulation example is of a water–air interface with a floating solid square being moved around by complex hydrodynamic flows including wave breaking. The results show that the immersed-body method is an effective approach for two-way solid–fluid coupling in multiphase viscous flows.
Guo L, Xiang J, Latham J-P, et al., 2016, A numerical investigation of mesh sensitivity for a new three-dimensional fracture model within the combined finite-discrete element method, Engineering Fracture Mechanics, Vol: 151, Pages: 70-91, ISSN: 1873-7315
Recently a new three-dimensional fracture model has been developed in the context of the combined finite-discrete element method. In order to provide quantitative guidance for engineering applications, mesh size and orientation sensitivity are investigated by specially designed numerical tests. The mesh size sensitivity is analysed by modelling a single tensile fracture propagation problem and three-point bending tests using a series of models with the same geometry but different structured mesh sizes. The mesh orientation sensitivity is investigated by diametrically compressing a disc specimen of unstructured meshes from different angles. The computational efficiency of the three-dimensional fracture model is also studied.
Farsi A, Xiang J, Latham J, et al., 2015, An application of the finite-discrete element method in the simulation of ceramic breakage: methodology for a validation study for alumina specimens, IV International Conference on Particle-based Methods, Publisher: International Center for Numerical Methods in Engineering (CIMNE), Pages: 921-932
ABSTRACT: Alumina (aluminum oxide, Al 2 O 3) particles are pelletised and fired to produce high porosity catalyst pellets of complex shapes. These pellets fill cylindrical reactor columns with particulate packing structures that are key to the in-service performance, but will suffer breakages which impact on catalyst performance. The combined Finite-Discrete Element Method (FEMDEM) is ideally suited to the simulation of both the multi-body pellet dynamic packing and quasi-static interactions as well as the stress field of each individual pellet, its deformations and fragmentation. The application of FEMDEM fracture modelling to a fine-grained brittle and porous material is novel. This paper presents a methodology for a validation study through comparison with three point-bending and Brazilian tests and discusses FEMDEM's potential in modelling multi-body fragile systems.
Anastasaki E, Latham J-P, Xiang J, 2015, Numerical modelling of armour layers with reference to Core-Loc units and their placement acceptance criteria, OCEAN ENGINEERING, Vol: 104, Pages: 204-218, ISSN: 0029-8018
Lei Q, Latham JP, Xiang J, 2015, Modelling rock mass failure around a repository excavation in a fractured crystalline rock using the finite-discrete element method, The Geological Society of London: The Geology of Geomechanics
Lei Q, Latham J-P, Tsang C-F, et al., 2015, A new approach to upscaling fracture network models while preserving geostatistical and geomechanical characteristics, Journal of Geophysical Research. Solid Earth, Vol: 120, Pages: 4784-4807, ISSN: 2169-9356
A new approach to upscaling two-dimensional fracture network models is proposed for preserving geostatistical and geomechanical characteristics of a smaller-scale “source” fracture pattern. First, the scaling properties of an outcrop system are examined in terms of spatial organization, lengths, connectivity, and normal/shear displacements using fractal geometry and power law relations. The fracture pattern is observed to be nonfractal with the fractal dimension D ≈ 2, while its length distribution tends to follow a power law with the exponent 2 < a < 3. To introduce a realistic distribution of fracture aperture and shear displacement, a geomechanical model using the combined finite-discrete element method captures the response of a fractured rock sample with a domain size L = 2 m under in situ stresses. Next, a novel scheme accommodating discrete-time random walks in recursive self-referencing lattices is developed to nucleate and propagate fractures together with their stress- and scale-dependent attributes into larger domains of up to 54 m × 54 m. The advantages of this approach include preserving the nonplanarity of natural cracks, capturing the existence of long fractures, retaining the realism of variable apertures, and respecting the stress dependency of displacement-length correlations. Hydraulic behavior of multiscale growth realizations is modeled by single-phase flow simulation, where distinct permeability scaling trends are observed for different geomechanical scenarios. A transition zone is identified where flow structure shifts from extremely channeled to distributed as the network scale increases. The results of this paper have implications for upscaling network characteristics for reservoir simulation.
Lei Q, Latham J-P, Xiang J, et al., 2015, Polyaxial stress-induced variable aperture model for persistent 3D fracture networks, Geomechanics for Energy and the Environment, Vol: 1, Pages: 34-47, ISSN: 2352-3808
This paper presents a stress-induced variable aperture model to characterise the effect of polyaxial stress conditions on the fluid flow in three-dimensional (3D) persistent fracture networks. Geomechanical modelling of the fractured rock is achieved by the finite-discrete element method (FEMDEM), which can capture deformability of matrix blocks, heterogeneity of stress fields as well as sliding and opening of pre-existing fractures. Propagation of new cracks is not required for this study of persistent fracture systems. The deformed fracture network topologies include details of dilation, opening and closing of fracture apertures, from which the local variations in hydraulic apertures are derived. Stress-controlled distribution of fracture apertures is modelled with both fracture-scale and network-scale effects considered. Under a geomechanical condition with low differential stress ratio, fracture porosity is dominated by the fracture-scale roughness. However, with the increase of stress ratio, some favourably oriented fractures are reactivated for shearing, and matrix blocks are promoted to rotate and generate large openings along their boundaries, which tend to be the key contributors to the aperture field in such persistent systems. The flow behaviour is then considered for these stressed but static solid skeletons and is investigated using a finite element solution to the Laplace problem of single-phase fluid flow. The equivalent permeability tensor of each cube-shaped rock mass is computed based on a series of flow simulations under a macroscopic pressure differential applied at opposite model boundaries with no-flow conditions on the remaining boundaries. Components of the permeability tensor are found to vary more than three orders of magnitude with respect to the change of stress ratio. Large aperture channels formed under a critical stress state accommodate significant localisation features in the flow structure of the network. The results of this study h
Su K, Latham J-P, Pavlidis D, et al., 2015, Multiphase flow simulation through porous media with explicitly resolved fractures, Geofluids, Vol: 15, Pages: 592-607, ISSN: 1468-8123
Accurate simulation of multiphase flow in fractured porous media remains a challenge. An important problem is the representation of the discontinuous or near discontinuous behaviour of saturation in real geological formations. In the classical continuum approach, a refined mesh is required at the interface between fracture and porous media to capture the steep gradients in saturation and saturation-dependent transport properties. This dramatically increases the computational load when large numbers of fractures are present in the numerical model. A discontinuous finite element method is reported here to model flow in fractured porous media. The governing multiphase porous media flow equations are solved in the adaptive mesh computational fluid dynamics code IC-FERST on unstructured meshes. The method is based on a mixed control volume – discontinuous finite element formulation. This is combined with the PN+1DG-PNDG element pair, which has discontinuous (order N+1) representation for velocity and discontinuous (order N) representation for pressure. A number of test cases are used to evaluate the method's ability to model fracture flow. The first is used to verify the performance of the element pair on structured and unstructured meshes of different resolution. Multiphase flow is then modelled in a range of idealised and simple fracture patterns. Solutions with sharp saturation fronts and computational economy in terms of mesh size are illustrated.
Latham JP, Xiang J, Higuera P, 2015, Numerical Modelling of the Stability of Breakwater Armour Systems, Pages: 687-698
Rubble-mound breakwaters using granular cover layers of rock or concrete units are mainly designed using empirical equations, scaled hydraulics laboratory test models and precedent practice. Multi-body solids numerical models can now represent the geometry and contact forces in such armour systems, when at rest or in a dynamic state of motion. The challenge tackled in this paper is to introduce oscillatory wave disturbance forces to such solids models with sufficient realism so that useful new numerical model information on armour stability can be used by designers. This is achieved using advanced 3D numerical wave tank simulations of wave-induced fluid flows inside the armour and under layers. Pressure-time histories of 2D sea states are also modelled enabling longer runs and entire storm sea states to be investigated for the equivalent rubble-mound structure. The correctly calibrated pressure-time storm history for the entire 3D domain heralds the introduction of a new 'wave proxy' method, based on the multi-body solids FEMDEM solver, Solidity. The integrated surface pressures acting on units yield the required oscillatory hydraulic and buoyancy forces. These drive the potential instability and movements and are superimposed on each unit in combination with the constantly updated contact, inertia and body forces. Results of this 'one-way' coupling method are briefly illustrated. We compare the effect of two different extreme storm conditions on the stability of a Core-Loc structure.
Guo L, Xiang J, Latham JP, et al., 2015, Numerical simulation of hydraulic fracturing using a three-dimensional fracture model coupled with an adaptive mesh fluid model, Pages: 2170-2179
A three-dimensional fracture model developed in the context of the combined finite-discrete element method is incorporated into a two-way fluid-solid coupling model. The fracture model is capable of simulating the whole fracturing process. It includes pre-peak hardening deformation, post-peak strain softening, transition from continuum to discontinuum, and the explicit interaction between discrete fracture surfaces, for both tensile and shear fracture initiation and propagation. The fluid-solid coupling model can simulate the interactions between moving fluids and multi-body solids. By incorporating the fracture model into the coupling model, a methodology of using the new coupling model to capture fracturing behaviour of solids in fluid-solid coupling simulations is proposed. To solve the problem in the coupling model of having adaptive continuous meshes being used by the fluid code and discontinuous meshes in the solid code, a scheme to convert different meshes is developed. A single fracture propagation driven by fluid pressures is simulated and the results show that the modelling obtains the correct critical load and propagation direction for fluid-driven fracturing. Several important phenomena, such as stress concentration ahead of the fracture tip, adaptive refinement of fluid mesh as a response to the fracture propagation and fluids flowing into fractures, are properly captured.
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