Publications
112 results found
Lei Q, Latham, Xiang J, et al., 2017, Role of natural fractures in damage evolution around tunnel excavation in fractured rocks, Engineering Geology, Vol: 231, Pages: 100-113, ISSN: 0013-7952
This paper studies the role of pre-existing fractures in the damage evolution around tunnel excavation in fractured rocks. The length distribution of natural fractures can be described by a power law model, whose exponent a defines the relative proportion of large and small fractures in the system. The larger a is, the higher proportion of small fractures is. A series of two-dimensional discrete fracture networks (DFNs) associated with different length exponent a and fracture intensity P21 is generated to represent various scenarios of distributed pre-existing fractures in the rock. The geomechanical behaviour of the fractured rock embedded with DFN geometry in response to isotropic/anisotropic in-situ stress conditions and excavation-induced perturbations is simulated using the hybrid finite-discrete element method (FEMDEM), which can capture the deformation of intact rocks, the interaction of matrix blocks, the displacement of natural fractures, and the propagation of new cracks. An excavation damaged zone (EDZ) develops around the man-made opening as a result of reactivation of pre-existing fractures and propagation of wing cracks. The simulation results show that when a is small, the system which is dominated by large fractures can remain stable after excavation given that P21 is not very high; however, intensive structurally-governed kinematic instability can occur if P21 is sufficiently high and the fracture spacing is much smaller than the tunnel size. With the increase of a, the system becomes more dominated by small fractures, and the EDZ is mainly created by the coalescence of small fractures near the tunnel boundary. The results of this study have important implications for designing stable underground openings for radioactive waste repositories as well as other engineering facilities that are intended to generate minimal damage in the host rock mass.
Lei Q, Wang X, Xiang J, et al., 2017, Polyaxial stress-dependent permeability of a three-dimensional fractured rock layer, Hydrogeology Journal, Vol: 25, Pages: 2251-2262, ISSN: 1435-0157
A study about the influence of polyaxial (true-triaxial) stresses on the permeability of a three-dimensional (3D) fractured rock layer is presented. The 3D fracture system is constructed by extruding a two-dimensional (2D) outcrop pattern of a limestone bed that exhibits a ladder structure consisting of a “through-going” joint set abutted by later-stage short fractures. Geomechanical behaviour of the 3D fractured rock in response to in-situ stresses is modelled by the finite-discrete element method, which can capture the deformation of matrix blocks, variation of stress fields, reactivation of pre-existing rough fractures and propagation of new cracks. A series of numerical simulations is designed to load the fractured rock using various polyaxial in-situ stresses and the stress-dependent flow properties are further calculated. The fractured layer tends to exhibit stronger flow localisation and higher equivalent permeability as the far-field stress ratio is increased and the stress field is rotated such that fractures are preferentially oriented for shearing. The shear dilation of pre-existing fractures has dominant effects on flow localisation in the system, while the propagation of new fractures has minor impacts. The role of the overburden stress suggests that the conventional 2D analysis that neglects the effect of the out-of-plane stress (perpendicular to the bedding interface) may provide indicative approximations but not fully capture the polyaxial stress-dependent fracture network behaviour. The results of this study have important implications for understanding the heterogeneous flow of geological fluids (e.g. groundwater, petroleum) in subsurface and upscaling permeability for large-scale assessments.
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
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
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
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
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
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.
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.
Xiao D, Yang P, Fang F, et al., 2016, A non-intrusive reduced-order model for compressible fluid and fractured solid coupling and its application to blasting, Journal of Computational Physics, Vol: 330, Pages: 221-224, ISSN: 0021-9991
This work presents the first application of a non-intrusive reduced order method to model solid interacting with compressible fluid flows to simulate crack initiation and propagation. In the high fidelity model, the coupling process is achieved by introducing a source term into the momentum equation, which represents the effects of forces of the solid on the fluid. A combined single and smeared crack model with the Mohr–Coulomb failure criterion is used to simulate crack initiation and propagation. The non-intrusive reduced order method is then applied to compressible fluid and fractured solid coupled modelling where the computational cost involved in the full high fidelity simulation is high. The non-intrusive reduced order model (NIROM) developed here is constructed through proper orthogonal decomposition (POD) and a radial basis function (RBF) multi-dimensional interpolation method.The performance of the NIROM for solid interacting with compressible fluid flows, in the presence of fracture models, is illustrated by two complex test cases: an immersed wall in a fluid and a blasting test case. The numerical simulation results show that the NIROM is capable of capturing the details of compressible fluids and fractured solids while the CPU time is reduced by several orders of magnitude. In addition, the issue of whether or not to subtract the mean from the snapshots before applying POD is discussed in this paper. It is shown that solutions of the NIROM, without mean subtracted before constructing the POD basis, captured more details than the NIROM with mean subtracted from snapshots.
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.
Takabatake K, Sun X, Sakai M, et al., 2016, Numerical study on a heat transfer model in a Lagrangian fluid dynamics simulation, International Journal of Heat and Mass Transfer, Vol: 103, Pages: 635-645, ISSN: 0017-9310
Phenomena related to phase change heat transfer are often encountered in engineering. These phenomena are regarded to be complex, since not only phase transition from solid to liquid occurs but also movement of fluid interface has to be taken into consideration. Detailed numerical modeling of these complex systems is essential to better understand them and optimize industrial designs. Lagrangian methods are promising for simulating such complex systems. The Moving Particle Semi-implicit (MPS) method, which is one of the Lagrangian methods, is employed here to simulate the free surface fluid flows involving heat transfer and phase change. On the other hand, the existing MPS method could not apply Neumann boundary condition such as heat flux in the heat transfer simulations. This is because the surface direction could not be readily defined on the surface of the spherical fluid particles in the MPS method. Hence, prescribing the heat fluxes becomes problematic in the existing MPS method. To solve this problem, a new heat flux model is developed, where the divergence operator is applied in the heat transfer simulation. Simple verification tests are performed to demonstrate the heat flux model, where the calculation results are compared against analytically derived solutions. In addition, application of the signed distance function is also investigated in the heat transfer simulation for arbitrary shaped boundary. In simple verification tests, the computation results are shown to agree well with the analytical solutions. Consequently, adequacy of the novel heat transfer model developed here is shown in the Lagrangian fluid dynamics simulation.
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
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
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
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.
Xiao D, Yang P, Fang F, et al., 2016, Non-intrusive reduced order modelling of fluid-structure interactions, Computer Methods in Applied Mechanics and Engineering, Vol: 303, Pages: 35-54, ISSN: 0045-7825
A novel non-intrusive reduced order model (NIROM) for fluid–structure interaction (FSI) has been developed. The model is based on proper orthogonal decomposition (POD) and radial basis function (RBF) interpolation method. The method is independent of the governing equations, therefore, it does not require modifications to the source code. This is the first time that a NIROM was constructed for FSI phenomena using POD and RBF interpolation method. Another novelty of this work is the first implementation of the FSI NIROM under the framework of an unstructured mesh finite element multi-phase model (Fluidity) and a combined finite-discrete element method based solid model (Y2D).The capability of this new NIROM for FSI is numerically illustrated in three coupling simulations: a one-way coupling case (flow past a cylinder), a two-way coupling case (a free-falling cylinder in water) and a vortex-induced vibration of an elastic beam test case. It is shown that the FSI NIROM results in a large CPU time reduction by several orders of magnitude while the dominant details of the high fidelity model are captured.
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.
Zheng J, Zhu J, Wang Z, et al., 2015, Towards a new multiscale air quality transport model using the fully unstructured anisotropic adaptive mesh technology of Fluidity (version 4.1.9), Geoscientific Model Development, ISSN: 1991-9603
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.
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
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 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
Vire A, Xiang J, Pain CC, 2015, An immersed-shell method for modelling fluid-structure interactions, PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 373, ISSN: 1364-503X
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