A 2D Electromechanical Model of Human Atrial Tissue Using the Discrete Element Method
Brocklehurst, Paul; Adeniran, Ismail; Yang, Dongmin; Sheng, Yong; Zhang, Henggui; Ye, Jianqiao
2015-01-01
Cardiac tissue is a syncytium of coupled cells with pronounced intrinsic discrete nature. Previous models of cardiac electromechanics often ignore such discrete properties and treat cardiac tissue as a continuous medium, which has fundamental limitations. In the present study, we introduce a 2D electromechanical model for human atrial tissue based on the discrete element method (DEM). In the model, single-cell dynamics are governed by strongly coupling the electrophysiological model of Courtemanche et al. to the myofilament model of Rice et al. with two-way feedbacks. Each cell is treated as a viscoelastic body, which is physically represented by a clump of nine particles. Cell aggregations are arranged so that the anisotropic nature of cardiac tissue due to fibre orientations can be modelled. Each cell is electrically coupled to neighbouring cells, allowing excitation waves to propagate through the tissue. Cell-to-cell mechanical interactions are modelled using a linear contact bond model in DEM. By coupling cardiac electrophysiology with mechanics via the intracellular Ca2+ concentration, the DEM model successfully simulates the conduction of cardiac electrical waves and the tissue's corresponding mechanical contractions. The developed DEM model is numerically stable and provides a powerful method for studying the electromechanical coupling problem in the heart. PMID:26583141
A framework for grand scale parallelization of the combined finite discrete element method in 2d
NASA Astrophysics Data System (ADS)
Lei, Z.; Rougier, E.; Knight, E. E.; Munjiza, A.
2014-09-01
Within the context of rock mechanics, the Combined Finite-Discrete Element Method (FDEM) has been applied to many complex industrial problems such as block caving, deep mining techniques (tunneling, pillar strength, etc.), rock blasting, seismic wave propagation, packing problems, dam stability, rock slope stability, rock mass strength characterization problems, etc. The reality is that most of these were accomplished in a 2D and/or single processor realm. In this work a hardware independent FDEM parallelization framework has been developed using the Virtual Parallel Machine for FDEM, (V-FDEM). With V-FDEM, a parallel FDEM software can be adapted to different parallel architecture systems ranging from just a few to thousands of cores.
2D resistivity inversion using conjugate gradients for a finite element discretization
NASA Astrophysics Data System (ADS)
Bortolozo, C. A.; Santos, F. M.; Porsani, J. L.
2014-12-01
In this work we present a DC 2D inversion algorithm using conjugate gradients relaxation to solve the maximum likelihood inverse equations. We apply, according to Zhang (1995), the maximum likelihood inverse theory developed by Tarantola and Valette (1982) to our 2D resistivity inversion. This algorithm was chosen to this research because it doesn't need to calculate the field's derivatives. Since conjugate gradient techniques only need the results of the sensitivity matrix Ã or its transpose ÃT multiplying a vector, the actual computation of the sensitivity matrix are not performed, according to the methodology described in Zhang (1995). In Zhang (1995), the terms Ãx and ÃTy, are dependent of the stiffness matrix K and its partial derivative ∂K⁄∂ρ. The inversion methodology described in Zhang (1995) is for the case of 3D electrical resistivity by finite differences discretization. So it was necessary to make a series of adjustments to obtain a satisfactory result for 2D electrical inversion using finite element method. The difference between the modeling of 3D resistivity with finite difference and the 2D finite element method are in the integration variable, used in the 2D case. In the 2D case the electrical potential are initially calculated in the transformed domain, including the stiffness matrix, and only in the end is transformed in Cartesian domain. In the case of 3D, described by Zhang (1995) this is done differently, the calculation is done directly in the Cartesian domain. In the literature was not found any work describing how to deal with this problem. Because the calculations of Ãx and ÃTy must be done without having the real stiffness matrix, the adaptation consist in calculate the stiffness matrix and its partial derivative using a set of integration variables. We transform those matrix in the same form has in the potential case, but with different sets of variables. The results will be presented and are very promising.
Bailey, T S; Adams, M L; Chang, J H
2008-10-01
We present a new spatial discretization of the discrete-ordinates transport equation in two-dimensional cylindrical (RZ) geometry for arbitrary polygonal meshes. This discretization is a discontinuous finite element method that utilizes the piecewise linear basis functions developed by Stone and Adams. We describe an asymptotic analysis that shows this method to be accurate for many problems in the thick diffusion limit on arbitrary polygons, allowing this method to be applied to radiative transfer problems with these types of meshes. We also present numerical results for multiple problems on quadrilateral grids and compare these results to the well-known bi-linear discontinuous finite element method.
NASA Astrophysics Data System (ADS)
Igami, M.; Shibazaki, B.; Nakama, Y.
2002-12-01
Particle based simulations such as the lattice solid modeling (Mora and Place, 1994; Abe et al., 2002) and the modeling using the discrete element method (Morgan and Boettcher, 1999) are very useful for investigating frictional behavior of the fault zone. We investigate the fault behavior using the discrete element method considering the effect of the time-dependent increase of contact area between particles. In our model the tangential force due to the frictional contact is assumed to be SA, where S is the shear stress within microcontacts and A is the contact area. For stationary contact, the contact area is assumed to increase with time following the equation A(t)=A0}(1+k{BT/E ln (1+t/t0)) (Brechet and Estrin, 1994), where t0 is an increasing function of temperature T. On the other hand, when sliding velocity V is not equal to 0, t is replaced with D c/V. Based on the elastic contact theory, A0 is assumed to be in proportion to Fn3/2, where Fn is the normal force that acts on each grain. As a test, we perform velocity step experiments. We consider the particle size distribution of r max/r min=2, where r max and r min represent maximum and minimum particle size, respectively. We found that stability of the fault zone is controlled by T. For small T or t0, velocity weakening behavior was observed. When T or t0 is large, however, no velocity weakening was observed. Our model is able to include the increase of contact area due to solution-transfer proposed by Hickman and Evans (1992). We also report the results of numerical simulation using the functional form of contact area when the solution-transfer is at work within microcontacts.
Morris, J; Johnson, S
2007-12-03
The Distinct Element Method (also frequently referred to as the Discrete Element Method) (DEM) is a Lagrangian numerical technique where the computational domain consists of discrete solid elements which interact via compliant contacts. This can be contrasted with Finite Element Methods where the computational domain is assumed to represent a continuum (although many modern implementations of the FEM can accommodate some Distinct Element capabilities). Often the terms Discrete Element Method and Distinct Element Method are used interchangeably in the literature, although Cundall and Hart (1992) suggested that Discrete Element Methods should be a more inclusive term covering Distinct Element Methods, Displacement Discontinuity Analysis and Modal Methods. In this work, DEM specifically refers to the Distinct Element Method, where the discrete elements interact via compliant contacts, in contrast with Displacement Discontinuity Analysis where the contacts are rigid and all compliance is taken up by the adjacent intact material.
2-d Finite Element Code Postprocessor
Energy Science and Technology Software Center (ESTSC)
1996-07-15
ORION is an interactive program that serves as a postprocessor for the analysis programs NIKE2D, DYNA2D, TOPAZ2D, and CHEMICAL TOPAZ2D. ORION reads binary plot files generated by the two-dimensional finite element codes currently used by the Methods Development Group at LLNL. Contour and color fringe plots of a large number of quantities may be displayed on meshes consisting of triangular and quadrilateral elements. ORION can compute strain measures, interface pressures along slide lines, reaction forcesmore » along constrained boundaries, and momentum. ORION has been applied to study the response of two-dimensional solids and structures undergoing finite deformations under a wide variety of large deformation transient dynamic and static problems and heat transfer analyses.« less
A discrete simulation of 2-D fluid flow on TERASYS
Mullins, P.G.; Krolak, P.D.
1995-12-01
A discrete simulation of two-dimensional (2-D) fluid flow, on a recently designed novel architecture called TERASYS is presented. The simulation uses a cellular automaton approach, implemented in a new language called data-parallel bit C (dbC). A performance comparison between our implementation on TERASYS and an implementation on the Connection Machine is discussed. We comment briefly on the suitability of the TERASYS system for modeling fluid flow using cellular automata.
NASA Technical Reports Server (NTRS)
Gelinas, R. J.; Doss, S. K.; Vajk, J. P.; Djomehri, J.; Miller, K.
1983-01-01
The mathematical background regarding the moving finite element (MFE) method of Miller and Miller (1981) is discussed, taking into account a general system of partial differential equations (PDE) and the amenability of the MFE method in two dimensions to code modularization and to semiautomatic user-construction of numerous PDE systems for both Dirichlet and zero-Neumann boundary conditions. A description of test problem results is presented, giving attention to aspects of single square wave propagation, and a solution of the heat equation.
2-D Finite Element Heat Conduction
Energy Science and Technology Software Center (ESTSC)
1989-10-30
AYER is a finite element program which implicitly solves the general two-dimensional equation of thermal conduction for plane or axisymmetric bodies. AYER takes into account the effects of time (transient problems), in-plane anisotropic thermal conductivity, a three-dimensional velocity distribution, and interface thermal contact resistance. Geometry and material distributions are arbitrary, and input is via subroutines provided by the user. As a result, boundary conditions, material properties, velocity distributions, and internal power generation may be mademore » functions of, e.g., time, temperature, location, and heat flux.« less
Application of the 2-D discrete-ordinates method to multiple scattering of laser radiation
Zardecki, A.; Gerstl, S.A.W.; Embury, J.F.
1983-05-01
The discrete-ordinates finite-element radiation transport code twotran is applied to describe the multiple scattering of a laser beam from a reflecting target. For a model scenario involving a 99% relative humidity rural aerosol we compute the average intensity of the scattered radiation and correction factors to the Beer-Lambert law arising from multiple scattering. As our results indicate, 2-D x-y and r-z geometry modeling can reliably describe a realistic 3-D scenario. Specific results are presented for the two visual ranges of 1.52 and 0.76 km which show that, for sufficiently high aerosol concentrations (e.g., equivalent to V = 0.76 km), the target signature in a distant detector becomes dominated by multiply scattered radiation from interactions of the laser light with the aerosol environment. The merits of the scaling group and the delta-M approximation for the transfer equation are also explored.
Discrete Element Modelling of Floating Debris
NASA Astrophysics Data System (ADS)
Mahaffey, Samantha; Liang, Qiuhua; Parkin, Geoff; Large, Andy; Rouainia, Mohamed
2016-04-01
Flash flooding is characterised by high velocity flows which impact vulnerable catchments with little warning time and as such, result in complex flow dynamics which are difficult to replicate through modelling. The impacts of flash flooding can be made yet more severe by the transport of both natural and anthropogenic debris, ranging from tree trunks to vehicles, wheelie bins and even storage containers, the effects of which have been clearly evident during recent UK flooding. This cargo of debris can have wide reaching effects and result in actual flood impacts which diverge from those predicted. A build-up of debris may lead to partial channel blockage and potential flow rerouting through urban centres. Build-up at bridges and river structures also leads to increased hydraulic loading which may result in damage and possible structural failure. Predicting the impacts of debris transport; however, is difficult as conventional hydrodynamic modelling schemes do not intrinsically include floating debris within their calculations. Subsequently a new tool has been developed using an emerging approach, which incorporates debris transport through the coupling of two existing modelling techniques. A 1D hydrodynamic modelling scheme has here been coupled with a 2D discrete element scheme to form a new modelling tool which predicts the motion and flow-interaction of floating debris. Hydraulic forces arising from flow around the object are applied to instigate its motion. Likewise, an equivalent opposing force is applied to fluid cells, enabling backwater effects to be simulated. Shock capturing capabilities make the tool applicable to predicting the complex flow dynamics associated with flash flooding. The modelling scheme has been applied to experimental case studies where cylindrical wooden dowels are transported by a dam-break wave. These case studies enable validation of the tool's shock capturing capabilities and the coupling technique applied between the two numerical
A multispeed Discrete Boltzmann Model for transcritical 2D shallow water flows
NASA Astrophysics Data System (ADS)
La Rocca, Michele; Montessori, Andrea; Prestininzi, Pietro; Succi, Sauro
2015-03-01
In this work a Discrete Boltzmann Model for the solution of transcritical 2D shallow water flows is presented and validated. In order to provide the model with transcritical capabilities, a particular multispeed velocity set has been employed for the discretization of the Boltzmann equation. It is shown that this particular set naturally yields a simple and closed procedure to determine higher order equilibrium distribution functions needed to simulate transcritical flow. The model is validated through several classical benchmarks and is proven to correctly and accurately simulate both 1D and 2D transitions between the two flow regimes.
Discrete elements method of neutral particle transport
Mathews, K.A.
1983-01-01
A new discrete elements (L/sub N/) transport method is derived and compared to the discrete ordinates S/sub N/ method, theoretically and by numerical experimentation. The discrete elements method is more accurate than discrete ordinates and strongly ameliorates ray effects for the practical problems studied. The discrete elements method is shown to be more cost effective, in terms of execution time with comparable storage to attain the same accuracy, for a one-dimensional test case using linear characteristic spatial quadrature. In a two-dimensional test case, a vacuum duct in a shield, L/sub N/ is more consistently convergent toward a Monte Carlo benchmark solution than S/sub N/, using step characteristic spatial quadrature. An analysis of the interaction of angular and spatial quadrature in xy-geometry indicates the desirability of using linear characteristic spatial quadrature with the L/sub N/ method.
A novel sliding window algorithm for 2D discrete Fourier transform
NASA Astrophysics Data System (ADS)
Dong, Zhifang; Wu, Jiasong; Gui, Jiyong
2015-12-01
Discrete Fourier transform (DFT) is one of the most wildly used tools for signal processing. In this paper, a novel sliding window algorithm is presented for fast computing 2D DFT when sliding window shifts more than one-point. The propose algorithm computing the DFT of the current window using that of the previous window. For fast computation, we take advantage of the recursive process of 2D SDFT and butterfly-based algorithm. So it can be directly applied to 2D signal processing. The theoretical analysis shows that the computational complexity is equal to 2D SDFT when one sample comes into current window. As well, the number of additions and multiplications of our proposed algorithm are less than those of 2D vector radix FFT when sliding window shifts mutiple-point.
Discrete Element Modeling of Drop Tests
NASA Astrophysics Data System (ADS)
Wang, Yuannian; Tonon, Fulvio
2012-09-01
A discrete element code with impact model has been developed and calibrated to simulate the dynamic behavior of rock materials, with special regard to rock fragmentation upon impact during rock-fall analysis. The paper summarizes the discrete element code, the calibration algorithms developed to identify the model microparameters, and the impact model. Experimental work on drop tests is then used to validate the code on modeling impact fragmentation. It has been found that the developed discrete element code and impact model can reasonably simulate rock fragmentation in drop tests. The use of the discrete element code and impact model can provide good reference results in evaluating impact fragmentation in rock-fall analysis.
ELLIPT2D: A Flexible Finite Element Code Written Python
Pletzer, A.; Mollis, J.C.
2001-03-22
The use of the Python scripting language for scientific applications and in particular to solve partial differential equations is explored. It is shown that Python's rich data structure and object-oriented features can be exploited to write programs that are not only significantly more concise than their counter parts written in Fortran, C or C++, but are also numerically efficient. To illustrate this, a two-dimensional finite element code (ELLIPT2D) has been written. ELLIPT2D provides a flexible and easy-to-use framework for solving a large class of second-order elliptic problems. The program allows for structured or unstructured meshes. All functions defining the elliptic operator are user supplied and so are the boundary conditions, which can be of Dirichlet, Neumann or Robbins type. ELLIPT2D makes extensive use of dictionaries (hash tables) as a way to represent sparse matrices.Other key features of the Python language that have been widely used include: operator over loading, error handling, array slicing, and the Tkinter module for building graphical use interfaces. As an example of the utility of ELLIPT2D, a nonlinear solution of the Grad-Shafranov equation is computed using a Newton iterative scheme. A second application focuses on a solution of the toroidal Laplace equation coupled to a magnetohydrodynamic stability code, a problem arising in the context of magnetic fusion research.
Finite Element Analysis of 2-D Elastic Contacts Involving FGMs
NASA Astrophysics Data System (ADS)
Abhilash, M. N.; Murthy, H.
2014-05-01
The response of elastic indenters in contact with Functionally Graded Material (FGM) coated homogeneous elastic half space has been presented in the current paper. Finite element analysis has been used due to its ability to handle complex geometry, material, and boundary conditions. Indenters of different typical surface profiles have been considered and the problem has been idealized as a two-dimensional (2D) plane strain problem considering only normal loads. Initially, indenters were considered to be rigid and the results were validated with the solutions presented in the literature. The analysis has then been extended to the case of elastic indenters on FGM-coated half spaces and the results are discussed.
Tensor representation of color images and fast 2D quaternion discrete Fourier transform
NASA Astrophysics Data System (ADS)
Grigoryan, Artyom M.; Agaian, Sos S.
2015-03-01
In this paper, a general, efficient, split algorithm to compute the two-dimensional quaternion discrete Fourier transform (2-D QDFT), by using the special partitioning in the frequency domain, is introduced. The partition determines an effective transformation, or color image representation in the form of 1-D quaternion signals which allow for splitting the N × M-point 2-D QDFT into a set of 1-D QDFTs. Comparative estimates revealing the efficiency of the proposed algorithms with respect to the known ones are given. In particular, a proposed method of calculating the 2r × 2r -point 2-D QDFT uses 18N2 less multiplications than the well-known column-row method and method of calculation based on the symplectic decomposition. The proposed algorithm is simple to apply and design, which makes it very practical in color image processing in the frequency domain.
2D-3D hybrid stabilized finite element method for tsunami runup simulations
NASA Astrophysics Data System (ADS)
Takase, S.; Moriguchi, S.; Terada, K.; Kato, J.; Kyoya, T.; Kashiyama, K.; Kotani, T.
2016-09-01
This paper presents a two-dimensional (2D)-three-dimensional (3D) hybrid stabilized finite element method that enables us to predict a propagation process of tsunami generated in a hypocentral region, which ranges from offshore propagation to runup to urban areas, with high accuracy and relatively low computational costs. To be more specific, the 2D shallow water equation is employed to simulate the propagation of offshore waves, while the 3D Navier-Stokes equation is employed for the runup in urban areas. The stabilized finite element method is utilized for numerical simulations for both of the 2D and 3D domains that are independently discretized with unstructured meshes. The multi-point constraint and transmission methods are applied to satisfy the continuity of flow velocities and pressures at the interface between the resulting 2D and 3D meshes, since neither their spatial dimensions nor node arrangements are consistent. Numerical examples are presented to demonstrate the performance of the proposed hybrid method to simulate tsunami behavior, including offshore propagation and runup to urban areas, with substantially lower computation costs in comparison with full 3D computations.
2D-3D hybrid stabilized finite element method for tsunami runup simulations
NASA Astrophysics Data System (ADS)
Takase, S.; Moriguchi, S.; Terada, K.; Kato, J.; Kyoya, T.; Kashiyama, K.; Kotani, T.
2016-05-01
This paper presents a two-dimensional (2D)-three-dimensional (3D) hybrid stabilized finite element method that enables us to predict a propagation process of tsunami generated in a hypocentral region, which ranges from offshore propagation to runup to urban areas, with high accuracy and relatively low computational costs. To be more specific, the 2D shallow water equation is employed to simulate the propagation of offshore waves, while the 3D Navier-Stokes equation is employed for the runup in urban areas. The stabilized finite element method is utilized for numerical simulations for both of the 2D and 3D domains that are independently discretized with unstructured meshes. The multi-point constraint and transmission methods are applied to satisfy the continuity of flow velocities and pressures at the interface between the resulting 2D and 3D meshes, since neither their spatial dimensions nor node arrangements are consistent. Numerical examples are presented to demonstrate the performance of the proposed hybrid method to simulate tsunami behavior, including offshore propagation and runup to urban areas, with substantially lower computation costs in comparison with full 3D computations.
Sebastian Schunert; Yousry Y. Azmy; Damien Fournier
2011-05-01
We present a comprehensive error estimation of four spatial discretization schemes of the two-dimensional Discrete Ordinates (SN) equations on Cartesian grids utilizing a Method of Manufactured Solution (MMS) benchmark suite based on variants of Larsen’s benchmark featuring different orders of smoothness of the underlying exact solution. The considered spatial discretization schemes include the arbitrarily high order transport methods of the nodal (AHOTN) and characteristic (AHOTC) types, the discontinuous Galerkin Finite Element method (DGFEM) and the recently proposed higher order diamond difference method (HODD) of spatial expansion orders 0 through 3. While AHOTN and AHOTC rely on approximate analytical solutions of the transport equation within a mesh cell, DGFEM and HODD utilize a polynomial expansion to mimick the angular flux profile across each mesh cell. Intuitively, due to the higher degree of analyticity, we expect AHOTN and AHOTC to feature superior accuracy compared with DGFEM and HODD, but at the price of potentially longer grind times and numerical instabilities. The latter disadvantages can result from the presence of exponential terms evaluated at the cell optical thickness that arise from the semianalytical solution process. This work quantifies the order of accuracy and the magnitude of the error of all four discretization methods for different optical thicknesses, scattering ratios and degrees of smoothness of the underlying exact solutions in order to verify or contradict the aforementioned intuitive expectation.
Modeling and 2-D discrete simulation of dislocation dynamics for plastic deformation of metal
NASA Astrophysics Data System (ADS)
Liu, Juan; Cui, Zhenshan; Ou, Hengan; Ruan, Liqun
2013-05-01
Two methods are employed in this paper to investigate the dislocation evolution during plastic deformation of metal. One method is dislocation dynamic simulation of two-dimensional discrete dislocation dynamics (2D-DDD), and the other is dislocation dynamics modeling by means of nonlinear analysis. As screw dislocation is prone to disappear by cross-slip, only edge dislocation is taken into account in simulation. First, an approach of 2D-DDD is used to graphically simulate and exhibit the collective motion of a large number of discrete dislocations. In the beginning, initial grains are generated in the simulation cells according to the mechanism of grain growth and the initial dislocation is randomly distributed in grains and relaxed under the internal stress. During the simulation process, the externally imposed stress, the long range stress contribution of all dislocations and the short range stress caused by the grain boundaries are calculated. Under the action of these forces, dislocations begin to glide, climb, multiply, annihilate and react with each other. Besides, thermal activation process is included. Through the simulation, the distribution of dislocation and the stress-strain curves can be obtained. On the other hand, based on the classic dislocation theory, the variation of the dislocation density with time is described by nonlinear differential equations. Finite difference method (FDM) is used to solve the built differential equations. The dislocation evolution at a constant strain rate is taken as an example to verify the rationality of the model.
HPLC analysis of discrete haptoglobin isoform N-linked oligosaccharides following 2D-PAGE isolation.
He, Zhicong; Aristoteli, Lina P; Kritharides, Leonard; Garner, Brett
2006-05-01
Glycosylation is a common but variable modification that regulates glycoprotein structure and function. We combined small format 2D-PAGE with HPLC to analyse discrete human haptoglobin isoform N-glycans. Seven major and several minor haptoglobin isoforms were detected by 2D-PAGE. N-Glycans released from Coomassie-stained gel spots using PNGase were labeled at their reducing termini with 2-aminobenzamide. HPLC analysis of selected major isoform N-glycans indicated that sialic acid composition determined their separation by isoelectric focussing. N-Glycans from two doublets of quantitatively minor isoforms were also analysed. Although separation of each pair of doublets was influenced by sialylation, individual spots within each doublet contained identical N-glycans. Thus, heterogeneity in minor haptoglobin isoforms was due to modifications distinct from N-glycan structure. These studies describe a simple method for analysing low abundance protein N-glycans and provide details of discrete haptoglobin isoform N-glycan structures which will be useful in proteomic analysis of human plasma samples. PMID:16546121
Discrete elements for 3D microfluidics
Bhargava, Krisna C.; Thompson, Bryant; Malmstadt, Noah
2014-01-01
Microfluidic systems are rapidly becoming commonplace tools for high-precision materials synthesis, biochemical sample preparation, and biophysical analysis. Typically, microfluidic systems are constructed in monolithic form by means of microfabrication and, increasingly, by additive techniques. These methods restrict the design and assembly of truly complex systems by placing unnecessary emphasis on complete functional integration of operational elements in a planar environment. Here, we present a solution based on discrete elements that liberates designers to build large-scale microfluidic systems in three dimensions that are modular, diverse, and predictable by simple network analysis techniques. We develop a sample library of standardized components and connectors manufactured using stereolithography. We predict and validate the flow characteristics of these individual components to design and construct a tunable concentration gradient generator with a scalable number of parallel outputs. We show that these systems are rapidly reconfigurable by constructing three variations of a device for generating monodisperse microdroplets in two distinct size regimes and in a high-throughput mode by simple replacement of emulsifier subcircuits. Finally, we demonstrate the capability for active process monitoring by constructing an optical sensing element for detecting water droplets in a fluorocarbon stream and quantifying their size and frequency. By moving away from large-scale integration toward standardized discrete elements, we demonstrate the potential to reduce the practice of designing and assembling complex 3D microfluidic circuits to a methodology comparable to that found in the electronics industry. PMID:25246553
A 2-D Interface Element for Coupled Analysis of Independently Modeled 3-D Finite Element Subdomains
NASA Technical Reports Server (NTRS)
Kandil, Osama A.
1998-01-01
Over the past few years, the development of the interface technology has provided an analysis framework for embedding detailed finite element models within finite element models which are less refined. This development has enabled the use of cascading substructure domains without the constraint of coincident nodes along substructure boundaries. The approach used for the interface element is based on an alternate variational principle often used in deriving hybrid finite elements. The resulting system of equations exhibits a high degree of sparsity but gives rise to a non-positive definite system which causes difficulties with many of the equation solvers in general-purpose finite element codes. Hence the global system of equations is generally solved using, a decomposition procedure with pivoting. The research reported to-date for the interface element includes the one-dimensional line interface element and two-dimensional surface interface element. Several large-scale simulations, including geometrically nonlinear problems, have been reported using the one-dimensional interface element technology; however, only limited applications are available for the surface interface element. In the applications reported to-date, the geometry of the interfaced domains exactly match each other even though the spatial discretization within each domain may be different. As such, the spatial modeling of each domain, the interface elements and the assembled system is still laborious. The present research is focused on developing a rapid modeling procedure based on a parametric interface representation of independently defined subdomains which are also independently discretized.
Predicting Fracture Using 2D Finite Element Modeling
MacNeil, J.A.M.; Adachi, J.D; Goltzman, D; Josse, R.G; Kovacs, C.S; Prior, J.C; Olszynski, W; Davison, K.S.; Kaiser, S.M
2013-01-01
A decrease in bone density at the hip or spine has been shown to increase the risk of fracture. A limitation of the bone mineral density (BMD) measurement is that it provides only a measure of a bone samples average density when projected onto a 2D surface. Effectively, what determines bone fracture is whether an applied load exceeds ultimate strength, with both bone tissue material properties (can be approximated through bone density), and geometry playing a role. The goal of this project was to use bone geometry and BMD obtained from radiographs and DXA measurements respectively to estimate fracture risk, using a two-dimensional finite element model (FEM) of the sagittal plane of lumbar vertebrae. The Canadian Multicenter Osteoporosis Study (CaMos) data was used for this study. There were 4194 men and women over the age of 50 years, with 786 having fractures. Each subject had BMD testing and radiographs of their lumbar vertebrae. A single two dimensional FEM of the first to fourth lumbar vertebra was automatically generated for each subject. Bone tissue stiffness was assigned based on the BMD of the individual vertebrae, and adjusted for patient age. Axial compression boundary conditions were applied with a force proportional to body mass. The resulting overall strain from the applied force was found. Men and women were analyzed separately. At baseline, the sensitivity of BMD to predict fragility fractures in women and men was 3.77 % and 0.86 %, while the sensitivity of FEM to predict fragility fractures for women and men was 10.8 % and 11.3 %. The FEM ROC curve demonstrated better performance compared to BMD. The relative risk of being considered at high fracture risk using FEM at baseline, was a better predictor of 5 year incident fragility fracture risk compared to BMD. PMID:21959170
FPGA implementation of 2-D discrete cosine transforms algorithm using systemC
NASA Astrophysics Data System (ADS)
Liu, Yifei; Ding, Mingyue
2007-12-01
Discrete Cosine Transform (DCT) is widely applied in image and video compression. This paper presented the software and hardware co-design method based on SystemC. As a case of study, a two dimension (2D) DCT Algorithm was implemented on Programmable Gate Arrays (FPGAs) chip. The short simulation time and verification process greatly increases the design efficiency of SystemC, making the product designed by SystemC more quickly into the market. The design effect using SystemC is compared between the expertise hardware designer and the software designer with little hardware knowledge. The result shows SystemC is an excellent and high efficiency hardware design method for an expertise hardware designer.
Robust H(∞) control for a class of 2-D discrete delayed systems.
Ye, Shuxia; Li, Jianzhen; Yao, Juan
2014-09-01
In this paper, we deal with the problem of robust H∞ control for a class of 2-D discrete uncertain systems with delayed perturbations described by the Roesser state-space model (RM). The problem to be addressed is the design of robust controllers via state feedback such that the stability of the resulting closed-loop system is guaranteed and a prescribed H∞ performance level is ensured for all delayed perturbations. By utilizing the Lyapunov method and some results, H∞ controllers are given. The results are delay-dependent and can be expressed in terms of linear matrix inequalities (LMIs). Finally, some numerical examples are given to illustrate the effectiveness of the proposed results. PMID:24411024
A new stationary gridline artifact suppression method based on the 2D discrete wavelet transform
Tang, Hui; Tong, Dan; Dong Bao, Xu; Dillenseger, Jean-Louis
2015-04-15
Purpose: In digital x-ray radiography, an antiscatter grid is inserted between the patient and the image receptor to reduce scattered radiation. If the antiscatter grid is used in a stationary way, gridline artifacts will appear in the final image. In most of the gridline removal image processing methods, the useful information with spatial frequencies close to that of the gridline is usually lost or degraded. In this study, a new stationary gridline suppression method is designed to preserve more of the useful information. Methods: The method is as follows. The input image is first recursively decomposed into several smaller subimages using a multiscale 2D discrete wavelet transform. The decomposition process stops when the gridline signal is found to be greater than a threshold in one or several of these subimages using a gridline detection module. An automatic Gaussian band-stop filter is then applied to the detected subimages to remove the gridline signal. Finally, the restored image is achieved using the corresponding 2D inverse discrete wavelet transform. Results: The processed images show that the proposed method can remove the gridline signal efficiently while maintaining the image details. The spectra of a 1D Fourier transform of the processed images demonstrate that, compared with some existing gridline removal methods, the proposed method has better information preservation after the removal of the gridline artifacts. Additionally, the performance speed is relatively high. Conclusions: The experimental results demonstrate the efficiency of the proposed method. Compared with some existing gridline removal methods, the proposed method can preserve more information within an acceptable execution time.
Discrete element modeling of subglacial sediment deformation
NASA Astrophysics Data System (ADS)
Damsgaard, Anders; Egholm, David L.; Piotrowski, Jan A.; Tulaczyk, Slawek; Larsen, Nicolaj K.; Tylmann, Karol
2013-12-01
The Discrete Element Method (DEM) is used in this study to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to those at the bed of warm-based glaciers. Complementary to analog experiments, the numerical approach allows a detailed analysis of the material dynamics and the shear zone development during progressive shear strain. The geometry of the heterogeneous stress network is visible in the form of force-carrying grain bridges and adjacent, volumetrically dominant, inactive zones. We demonstrate how the shear zone thickness and dilation depend on the level of normal (overburden) stress, and we show how high normal stress can mobilize material to great depths. The particle rotational axes tend to align with progressive shear strain, with rotations both along and reverse to the shear direction. The results from successive laboratory ring-shear experiments on simple granular materials are compared to results from similar numerical experiments. The simulated DEM material and all tested laboratory materials deform by an elastoplastic rheology under the applied effective normal stress. These results demonstrate that the DEM is a viable alternative to continuum models for small-scale analysis of sediment deformation. It can be used to simulate the macromechanical behavior of simple granular sediments, and it provides an opportunity to study how microstructures in subglacial sediments are formed during progressive shear strain.
Discrete Element Modeling for Mobility and Excavation
NASA Astrophysics Data System (ADS)
Knuth, M. A.; Hopkins, M. A.
2011-12-01
The planning and completion of mobility and excavation efforts on the moon requires a thorough understanding of the planetary regolith. In this work, a discrete element method (DEM) model is created to replicate those activities in the laboratory and for planning mission activities in the future. The crux of this work is developing a particle bed that best replicates the regolith tool/wheel interaction seen in the laboratory. To do this, a DEM geotechnical triaxial strength cell was created allowing for comparison of laboratory JSC-1a triaxial tests to DEM simulated soils. This model relies on a triangular lattice membrane covered triaxial cell for determining the macroscopic properties of the modeled granular material as well as a fast and efficient contact detection algorithm for a variety of grain shapes. Multiple grain shapes with increasing complexity (ellipsoid, poly-ellipsoid and polyhedra) have been developed and tested. This comparison gives us a basis to begin scaling DEM grain size and shape to practical values for mobility and excavation modeling. Next steps include development of a DEM scoop for percussive excavation testing as well as continued analysis of rover wheel interactions using a wide assortment of grain shape and size distributions.
Discrete Element Modeling of Triboelectrically Charged Particles
NASA Technical Reports Server (NTRS)
Hogue, Michael D.; Calle, Carlos I.; Weitzman, Peter S.; Curry, David R.
2008-01-01
Tribocharging of particles is common in many processes including fine powder handling and mixing, printer toner transport and dust extraction. In a lunar environment with its high vacuum and lack of water, electrostatic forces are an important factor to consider when designing and operating equipment. Dust mitigation and management is critical to safe and predictable performance of people and equipment. The extreme nature of lunar conditions makes it difficult and costly to carry out experiments on earth which are necessary to better understand how particles gather and transfer charge between each other and with equipment surfaces. DEM (Discrete Element Modeling) provides an excellent virtual laboratory for studying tribocharging of particles as well as for design of devices for dust mitigation and for other purposes related to handling and processing of lunar regolith. Theoretical and experimental work has been performed pursuant to incorporating screened Coulombic electrostatic forces into EDEM, a commercial DEM software package. The DEM software is used to model the trajectories of large numbers of particles for industrial particulate handling and processing applications and can be coupled with other solvers and numerical models to calculate particle interaction with surrounding media and force fields. While simple Coulombic force between two particles is well understood, its operation in an ensemble of particles is more complex. When the tribocharging of particles and surfaces due to frictional contact is also considered, it is necessary to consider longer range of interaction of particles in response to electrostatic charging. The standard DEM algorithm accounts for particle mechanical properties and inertia as a function of particle shape and mass. If fluid drag is neglected, then particle dynamics are governed by contact between particles, between particles and equipment surfaces and gravity forces. Consideration of particle charge and any tribocharging and
Discrete element modelling of subglacial sediment deformation
NASA Astrophysics Data System (ADS)
Christensen, A. D.; Egholm, D. L.; Piotrowski, J. A.; Tulaczyk, S.
2012-04-01
Soft, deformable sediments are often present under glaciers. Subglacial sediments deform under the differential load of the ice, and this causes the overlying glacier to accelerate its motion. Understanding the rheology of subglacial sediment is therefore important for models of glacial dynamics. Previous studies of the mechanical behaviour of subglacial sediments have primarily relied on analytical considerations and laboratory shearing experiments. As a novel approach, the Discrete Element Method (DEM) is used to explore the highly nonlinear dynamics of a granular bed that is exposed to stress conditions comparable to subglacial environments. The numerical approach allows close monitoring of the mechanical and rheological behaviour under a range of conditions. Of special interest is bed shear strength, strain distribution and -localization, mode of deformation, and role of effective normal pressure during shearing. As a calibration benchmark, results from laboratory ring-shear experiments on granular material are compared to similar numerical experiments. The continuously recorded stress dynamics in the laboratory shear experiments are compared to DEM experiments, and the micro-mechanical parameters in the contact model of the DEM code are calibrated to match the macroscopic Mohr-Coulomb failure criteria parameters, constrained from successive laboratory shear tests under a range of normal pressures. The data-parallel nature of the basic DEM formulation makes the problem ideal for utilizing the high arithmetic potential of modern general-purpose GPUs. Using the Nvidia Cuda C toolkit, the algorithm is formulated for spherical particles in three dimensions with a soft-body contact model. Scene rendering is performed using a custom Cuda ray-tracing algorithm. Efforts on optimization of the particle algorithm are discussed, and future plans of expansion are presented.
Generation of Random Particle Packings for Discrete Element Models
NASA Astrophysics Data System (ADS)
Abe, S.; Weatherley, D.; Ayton, T.
2012-04-01
An important step in the setup process of Discrete Element Model (DEM) simulations is the generation of a suitable particle packing. There are quite a number of properties such a granular material specimen should ideally have, such as high coordination number, isotropy, the ability to fill arbitrary bounding volumes and the absence of locked-in stresses. An algorithm which is able to produce specimens fulfilling these requirements is the insertion based sphere packing algorithm originally proposed by Place and Mora, 2001 [2] and extended in this work. The algorithm works in two stages. First a number of "seed" spheres are inserted into the bounding volume. In the second stage the gaps between the "seed" spheres are filled by inserting new spheres in a way so they have D+1 (i.e. 3 in 2D, 4 in 3D) touching contacts with either other spheres or the boundaries of the enclosing volume. Here we present an implementation of the algorithm and a systematic statistical analysis of the generated sphere packings. The analysis of the particle radius distribution shows that they follow a power-law with an exponent ≈ D (i.e. ≈3 for a 3D packing and ≈2 for 2D). Although the algorithm intrinsically guarantees coordination numbers of at least 4 in 3D and 3 in 2D, the coordination numbers realized in the generated packings can be significantly higher, reaching beyond 50 if the range of particle radii is sufficiently large. Even for relatively small ranges of particle sizes (e.g. Rmin = 0.5Rmax) the maximum coordination number may exceed 10. The degree of isotropy of the generated sphere packing is also analysed in both 2D and 3D, by measuring the distribution of orientations of vectors joining the centres of adjacent particles. If the range of particle sizes is small, the packing algorithm yields moderate anisotropy approaching that expected for a face-centred cubic packing of equal-sized particles. However, once Rmin < 0.3Rmax a very high degree of isotropy is demonstrated in
Discrete Element Modeling of Complex Granular Flows
NASA Astrophysics Data System (ADS)
Movshovitz, N.; Asphaug, E. I.
2010-12-01
Granular materials occur almost everywhere in nature, and are actively studied in many fields of research, from food industry to planetary science. One approach to the study of granular media, the continuum approach, attempts to find a constitutive law that determines the material's flow, or strain, under applied stress. The main difficulty with this approach is that granular systems exhibit different behavior under different conditions, behaving at times as an elastic solid (e.g. pile of sand), at times as a viscous fluid (e.g. when poured), or even as a gas (e.g. when shaken). Even if all these physics are accounted for, numerical implementation is made difficult by the wide and often discontinuous ranges in continuum density and sound speed. A different approach is Discrete Element Modeling (DEM). Here the goal is to directly model every grain in the system as a rigid body subject to various body and surface forces. The advantage of this method is that it treats all of the above regimes in the same way, and can easily deal with a system moving back and forth between regimes. But as a granular system typically contains a multitude of individual grains, the direct integration of the system can be very computationally expensive. For this reason most DEM codes are limited to spherical grains of uniform size. However, spherical grains often cannot replicate the behavior of real world granular systems. A simple pile of spherical grains, for example, relies on static friction alone to keep its shape, while in reality a pile of irregular grains can maintain a much steeper angle by interlocking force chains. In the present study we employ a commercial DEM, nVidia's PhysX Engine, originally designed for the game and animation industry, to simulate complex granular flows with irregular, non-spherical grains. This engine runs as a multi threaded process and can be GPU accelerated. We demonstrate the code's ability to physically model granular materials in the three regimes
Discrete element modelling of bedload transport
NASA Astrophysics Data System (ADS)
Loyer, A.; Frey, P.
2011-12-01
Discrete element modelling (DEM) has been widely used in solid mechanics and in granular physics. In this type of modelling, each individual particle is taken into account and intergranular interactions are modelled with simple laws (e.g. Coulomb friction). Gravity and contact forces permit to solve the dynamical behaviour of the system. DEM is interesting to model configurations and access to parameters not directly available in laboratory experimentation, hence the term "numerical experimentations" sometimes used to describe DEM. DEM was used to model bedload transport experiments performed at the particle scale with spherical glass beads in a steep and narrow flume. Bedload is the larger material that is transported on the bed on stream channels. It has a great geomorphic impact. Physical processes ruling bedload transport and more generally coarse-particle/fluid systems are poorly known, arguably because granular interactions have been somewhat neglected. An existing DEM code (PFC3D) already computing granular interactions was used. We implemented basic hydrodynamic forces to model the fluid interactions (buoyancy, drag, lift). The idea was to use the minimum number of ingredients to match the experimental results. Experiments were performed with one-size and two-size mixtures of coarse spherical glass beads entrained by a shallow turbulent and supercritical water flow down a steep channel with a mobile bed. The particle diameters were 4 and 6mm, the channel width 6.5mm (about the same width as the coarser particles) and the channel inclination was typically 10%. The water flow rate and the particle rate were kept constant at the upstream entrance and adjusted to obtain bedload transport equilibrium. Flows were filmed from the side by a high-speed camera. Using image processing algorithms made it possible to determine the position, velocity and trajectory of both smaller and coarser particles. Modelled and experimental particle velocity and concentration depth
Discrete element modelling of bed load transport
NASA Astrophysics Data System (ADS)
Maurin, Raphael; Chareyre, Bruno; Chauchat, Julien; Frey, Philippe
2013-04-01
Discrete element method (DEM) is a numerical method to simulate an assembly of particles, which has been widely used in mechanics (soil, rock) and granular physics. DEM consists in considering undeformable particles and modelling the intergranular interactions with simple laws (e.g. linear elastic and Coulomb friction law). The expression of the equation of motion on each particle considering the nearest neighbor interactions allows then to solve the dynamical behavior of the system explicitely. Since its introduction more than thirty years ago, this type of model has proven its ability to well describe the behavior of granular media in several different situations, from quasi-static system to flow of granular media. Bedload transport in streams is characterized by particle transport restricted to the interface between fluid flow and immerged granular media, where particles are rolling, sliding or in saltation over the bed. This situation corresponds to the larger particles transported on the bed in stream channels and has a great influence on geomorphology. Physical mechanisms and processes ruling bedload transport and more generally coarse-particle/fluid systems are poorly known. This is partly due to the small attention given to the role of granular interactions. Starting from these considerations, we used DEM to reproduce experiments carried out with spherical glass beads in an experimental steep and narrow flume. This was done in order to focus on granular interactions and to have access to parameters not available in the experiment. DEM open-source code Yade was coupled with a simplified fluid model, taking into account the different hydrodynamical interactions (buoyancy, drag, lift...) experienced by the particles. Numerical results obtained from the simulation are compared with an experimental data set established previously at the laboratory. It consists in monodisperse and bidisperse mixtures of coarse spherical glass beads entrained by a shallow
Discrete element modeling of subglacial sediment deformation
NASA Astrophysics Data System (ADS)
Damsgaard, A.; Egholm, D. L.; Piotrowski, J. A.; Tulaczyk, S. M.; Larsen, N. K.
2013-12-01
The Discrete Element Method (DEM) is used to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to those at the bed of warm-based glaciers. In the DEM, the material is simulated on a grain-by-grain basis, and defining the micromechanical properties of the inter-particle contacts parameterizes the model. For validating the numerical approach, the macromechanical behavior of the numerical material is compared to the results from successive laboratory ring-shear experiments. Overall, there is a good agreement between the geotechnical behavior of the real granular materials and the numerical results. The materials deform by an elasto-plastic rheology under the applied effective normal stress and horizontal shearing. The peak and ultimate shear strengths depend linearly on the magnitude of the normal stress by the Mohr-Coulomb constitutive relationship. The numerical approach allows for a detailed analysis of the material dynamics and shear zone development during progressive shear strain. We demonstrate how the shear zone thickness and dilation increase with the magnitude of the normal stress. The stresses are distributed heterogeneously through the granular material along stress-carrying force chains. Between the force chains are the volumetrically dominant inactive zones. Overall, the force chain orientation is parallel to the maximum compressive stress. The data-parallel nature of the basic DEM formulation makes the problem ideal for utilizing the high arithmetic potential of modern general-purpose GPUs. Using the Nvidia CUDA C toolkit, the algorithm is formulated for spherical particles in three dimensions with a linear-elastic soft-body contact model. We have coupled the DEM model to a model for porewater flow, and we present early results of particle-porewater interactions. The two-way mechanical coupling is used to investigate pore-pressure feedbacks, which may be very important for the dynamics of soft
Discrete elements method of neutral particle transport. Doctoral thesis
Mathews, K.A.
1983-10-01
A new 'discrete elements' (LN) transport method is derived and compared to the discrete ordinates SN method, theoretically and by numerical experimentation. The discrete elements method is more accurate than discrete ordinates and strongly ameliorates ray effects for the practical problems studied. The discrete elements method is shown to be more cost effective in terms of execution time with comparable storage to attain the same accuracy, for a one-dimensional test case using linear characteristic spatial quadrature. In a two-dimensional test case, a vacuum duct in a shield, LN is more consistently convergent toward a Monte Carlo benchmark solution than SN, using step characteristic spatial quadrature. An analysis of the interaction of angular and spatial quadrature in xy-geometry indicates the desirability of using linear characteristic spatial quadrature with the LN method. The discrete elements method is based on discretizing the Boltzmann equation over a set of elements of angle. The zeroth and first angular moments of the directional flux, over each element, are estimated by numerical quadrature and yield a flux-weighted average streaming direction for the element. (Data for this estimation are fluxes in fixed directions calculated as in SN.)
Setting up virgin stress conditions in discrete element models
Rojek, J.; Karlis, G.F.; Malinowski, L.J.; Beer, G.
2013-01-01
In the present work, a methodology for setting up virgin stress conditions in discrete element models is proposed. The developed algorithm is applicable to discrete or coupled discrete/continuum modeling of underground excavation employing the discrete element method (DEM). Since the DEM works with contact forces rather than stresses there is a need for the conversion of pre-excavation stresses to contact forces for the DEM model. Different possibilities of setting up virgin stress conditions in the DEM model are reviewed and critically assessed. Finally, a new method to obtain a discrete element model with contact forces equivalent to given macroscopic virgin stresses is proposed. The test examples presented show that good results may be obtained regardless of the shape of the DEM domain. PMID:27087731
Wheat mill stream properties for discrete element method modeling
Technology Transfer Automated Retrieval System (TEKTRAN)
A discrete phase approach based on individual wheat kernel characteristics is needed to overcome the limitations of previous statistical models and accurately predict the milling behavior of wheat. As a first step to develop a discrete element method (DEM) model for the wheat milling process, this s...
Automatic differentiation of the TACO2D finite element code using ADIFOR
Carle, A.; Fagan, M.
1996-04-01
The need for sensitivities in particular applications is becoming increasingly important in problems such as optimal design or control. In this study, the authors use ADIFOR to generate derivative code for TACO2D, a finite element heat transfer code. The study of TACO2D indicates that ADIFOR-generated derivatives yield accurate derivatives at a fraction of the time requirements of finite difference approximations, and space requirements proportional to the number of variables. The primary focus on TACO2D was for the design of chemical vapor deposition reactors.
Hallquist, J.O.
1982-02-01
This revised report provides an updated user's manual for DYNA2D, an explicit two-dimensional axisymmetric and plane strain finite element code for analyzing the large deformation dynamic and hydrodynamic response of inelastic solids. A contact-impact algorithm permits gaps and sliding along material interfaces. By a specialization of this algorithm, such interfaces can be rigidly tied to admit variable zoning without the need of transition regions. Spatial discretization is achieved by the use of 4-node solid elements, and the equations-of motion are integrated by the central difference method. An interactive rezoner eliminates the need to terminate the calculation when the mesh becomes too distorted. Rather, the mesh can be rezoned and the calculation continued. The command structure for the rezoner is described and illustrated by an example.
Modeling rammed earth wall using discrete element method
NASA Astrophysics Data System (ADS)
Bui, T.-T.; Bui, Q.-B.; Limam, A.; Morel, J.-C.
2016-03-01
Rammed earth is attracting renewed interest throughout the world thanks to its "green" characteristics in the context of sustainable development. Several research studies have thus recently been carried out to investigate this material. Some of them attempted to simulate the rammed earth's mechanical behavior by using analytical or numerical models. Most of these studies assumed that there was a perfect cohesion at the interface between earthen layers. This hypothesis proved to be acceptable for the case of vertical loading, but it could be questionable for horizontal loading. To address this problem, discrete element modeling seems to be relevant to simulate a rammed earth wall. To our knowledge, no research has been conducted thus far using discrete element modeling to study a rammed earth wall. This paper presents an assessment of the discrete element modeling's robustness for rammed earth walls. Firstly, a brief description of the discrete element modeling is presented. Then the parameters necessary for discrete element modeling of the material law of the earthen layers and their interfaces law following the Mohr-Coulomb model with a tension cut-off and post-peak softening were given. The relevance of the model and the material parameters were assessed by comparing them with experimental results from the literature. The results showed that, in the case of vertical loading, interfaces did not have an important effect. In the case of diagonal loading, model with interfaces produced better results. Interface characteristics can vary from 85 to 100% of the corresponding earthen layer's characteristics.
Effective Temperature of 2D Dusty Plasma Liquids at the Discrete Level
Io, C.-W.; Chan, C.-L.; I Lin
2007-07-13
Fluctuation-dissipation theory has been used to measure the effective temperature of non-equilibrium system. In this work, using a 2D dusty plasma liquid formed by the negatively charged fine particles suspending in weakly ionized discharges and sheared by two CW counter parallel laser beams, we measure the micro-transport at the kinetic level. The effective temperatures Teff at different time scales are obtained through the Stokes-Einstein relation which relates the diffusion coefficient (D) and the viscosity ({eta}). The external energy is cascaded from the slow hopping modes to the fast caging modes through mutual coupling, which leads to the higher effective temperature of the slow hopping modes.
NASA Astrophysics Data System (ADS)
Mei, Hong-Xin; Zhang, Ting; Huang, Hua-Qi; Huang, Rong-Bin; Zheng, Lan-Sun
2016-03-01
Three mix-ligand Ag(I) coordination compounds, namely, {[Ag10(tpyz) 5(L1) 5(H2 O)2].(H2 O)4}n (1, tpyz = 2,3,4,5-tetramethylpyrazine, H2 L1 = phthalic acid), [Ag4(tpyz) 2(L2) 2(H2 O)].(H2 O)5}n (2, H2 L2 = isophthalic acid) {[Ag2(tpyz) 2(L3) (H2 O)4].(H2 O)8}n (3, H2 L3 = terephthalic acid), have been synthesized and characterized by elemental analysis, IR, PXRD and X-ray single-crystal diffraction. 1 exhibits a 2D layer which can be simplified as a (4,4) net. 2 is a 3D network which can be simplified as a (3,3)-connected 2-nodal net with a point symbol of {102.12}{102}. 3 consists of linear [Ag(tpyz) (H2 O)2]n chain. Of particular interest, discrete hexamer water clusters were observed in 1 and 2, while a 2D L10(6) water layer exists in 3. The results suggest that the benzene dicarboxylates play pivotal roles in the formation of the different host architectures as well as different water aggregations. Moreover, thermogravimetric analysis (TGA) and emissive behaviors of these compounds were investigated.
Bailey, T S; Adams, M L; Yang, B; Zika, M R
2005-07-15
We develop a piecewise linear (PWL) Galerkin finite element spatial discretization for the multi-dimensional radiation diffusion equation. It uses piecewise linear weight and basis functions in the finite element approximation, and it can be applied on arbitrary polygonal (2D) or polyhedral (3D) grids. We show that this new PWL method gives solutions comparable to those from Palmer's finite-volume method. However, since the PWL method produces a symmetric positive definite coefficient matrix, it should be substantially more computationally efficient than Palmer's method, which produces an asymmetric matrix. We conclude that the Galerkin PWL method is an attractive option for solving diffusion equations on unstructured grids.
Spiral waves are stable in discrete element models of two-dimensional homogeneous excitable media
NASA Technical Reports Server (NTRS)
Feldman, A. B.; Chernyak, Y. B.; Cohen, R. J.
1998-01-01
The spontaneous breakup of a single spiral wave of excitation into a turbulent wave pattern has been observed in both discrete element models and continuous reaction-diffusion models of spatially homogeneous 2D excitable media. These results have attracted considerable interest, since spiral breakup is thought to be an important mechanism of transition from the heart rhythm disturbance ventricular tachycardia to the fatal arrhythmia ventricular fibrillation. It is not known whether this process can occur in the absence of disease-induced spatial heterogeneity of the electrical properties of the ventricular tissue. Candidate mechanisms for spiral breakup in uniform 2D media have emerged, but the physical validity of the mechanisms and their applicability to myocardium require further scrutiny. In this letter, we examine the computer simulation results obtained in two discrete element models and show that the instability of each spiral is an artifact resulting from an unphysical dependence of wave speed on wave front curvature in the medium. We conclude that spiral breakup does not occur in these two models at the specified parameter values and that great care must be exercised in the representation of a continuous excitable medium via discrete elements.
Dynamic Analysis of 2D Electromagnetic Resonant Optical Scanner Using 3D Finite Element Method
NASA Astrophysics Data System (ADS)
Hirata, Katsuhiro; Hong, Sara; Maeda, Kengo
The optical scanner is a scanning device in which a laser beam is reflected by a mirror that can be rotated or oscillated. In this paper, we propose a new 2D electromagnetic resonant optical scanner that employs electromagnets and leaf springs. Torque characteristics and resonance characteristics of the scanner are analyzed using the 3D finite element method. The validity of the analysis is shown by comparing the characteristics inferred from the analysis with the characteristics of the prototype. Further, 2D resonance is investigated by introducing a superimposed-frequency current in a single coil.
DelGrande, J. Mark; Mathews, Kirk A.
2001-09-15
Conventional discrete ordinates transport calculations often produce negative fluxes due to unphysical negative scattering cross sections and/or as artifacts of spatial differencing schemes such as diamond difference. Inherently nonnegative spatial methods, such as the nonlinear, exponential characteristic spatial quadrature, eliminate negative fluxes while providing excellent accuracy, presuming the group-to-group, ordinate-to-ordinate cross sections are all nonnegative. A hybrid approach is introduced in which the flow from spatial cell to spatial cell uses discrete ordinates spatial quadratures, while anisotropic scattering of flux from one energy-angle bin (energy group and discrete element of solid angle) to another such bin is modeled using a Monte Carlo simulation to evaluate the bin-to-bin cross sections. The directional elements tile the sphere of directions; the ordinates for the spatial quadrature are at the centroids of the elements. The method is developed and contrasted with previous schemes for positive cross sections. An algorithm for evaluating the Monte Carlo (MC)-discrete elements (MC-DE) cross sections is described, and some test cases are presented. Transport calculations using MC-DE cross sections are compared with calculations using conventional cross sections and with MCNP calculations. In this testing, the new method is about as accurate as the conventional approach, and often is more accurate. The exponential characteristic spatial quadrature, using the MC-DE cross sections, is shown to provide useful results where linear characteristic and spherical harmonics provide negative scalar fluxes in every cell in a region.
Nonconforming mortar element methods: Application to spectral discretizations
NASA Technical Reports Server (NTRS)
Maday, Yvon; Mavriplis, Cathy; Patera, Anthony
1988-01-01
Spectral element methods are p-type weighted residual techniques for partial differential equations that combine the generality of finite element methods with the accuracy of spectral methods. Presented here is a new nonconforming discretization which greatly improves the flexibility of the spectral element approach as regards automatic mesh generation and non-propagating local mesh refinement. The method is based on the introduction of an auxiliary mortar trace space, and constitutes a new approach to discretization-driven domain decomposition characterized by a clean decoupling of the local, structure-preserving residual evaluations and the transmission of boundary and continuity conditions. The flexibility of the mortar method is illustrated by several nonconforming adaptive Navier-Stokes calculations in complex geometry.
NASA Astrophysics Data System (ADS)
Maginot, Peter G.; Morel, Jim E.; Ragusa, Jean C.
2012-08-01
We present a new nonlinear spatial finite-element method for the linearized Boltzmann transport equation with Sn angular discretization in 1-D and 2-D Cartesian geometries. This method has two central characteristics. First, it is equivalent to the linear-discontinuous (LD) Galerkin method whenever that method yields a strictly non-negative solution. Second, it always satisfies both the zeroth and first spatial moment equations. Because it yields the LD solution when that solution is non-negative, one might interpret our method as a classical fix-up to the LD scheme. However, fix-up schemes for the LD equations derived in the past have given up solution of the first moment equations when the LD solution is negative in order to satisfy positivity in a simple manner. We present computational results comparing our method in 1-D to the strictly non-negative linear exponential-discontinuous method and to the LD method. We present computational results in 2-D comparing our method to a recently developed LD fix-up scheme and to the LD scheme. It is demonstrated that our method is a valuable alternative to existing methods.
Identification of micro parameters for discrete element simulation of agglomerates
NASA Astrophysics Data System (ADS)
Palis, Stefan; Antonyuk, Sergiy; Dosta, Maksym; Heinrich, Stefan
2013-06-01
The mechanical behaviour of solid particles like agglomerates, granules or crystals strongly depends on their micro structure, e.g. structural defects and porosity. In order to model the mechanical behaviour of these inhomogeneous media the discrete element method has been proven to be an appropriate tool. The model parameters used are typically micro parameters like bond stiffness, particle-particle contact stiffness, strength of the bonds. Due to the lack of general methods for a direct micro parameter determination, normally laborious parameter adaptation has to be done in order to fit experiment and simulation. In this contribution a systematic and automatic way for parameter adaptation using real experiments is proposed. Due to the fact, that discrete element models are typically systems of differential equations of very high order, gradient based methods are not suitable. Hence, the focus will be on derivative free methods.
Justification for a 2D versus 3D fingertip finite element model during static contact simulations.
Harih, Gregor; Tada, Mitsunori; Dolšak, Bojan
2016-10-01
The biomechanical response of a human hand during contact with various products has not been investigated in details yet. It has been shown that excessive contact pressure on the soft tissue can result in discomfort, pain and also cumulative traumatic disorders. This manuscript explores the benefits and limitations of a simplified two-dimensional vs. an anatomically correct three-dimensional finite element model of a human fingertip. Most authors still use 2D FE fingertip models due to their simplicity and reduced computational costs. However we show that an anatomically correct 3D FE fingertip model can provide additional insight into the biomechanical behaviour. The use of 2D fingertip FE models is justified when observing peak contact pressure values as well as displacement during the contact for the given studied cross-section. On the other hand, an anatomically correct 3D FE fingertip model provides a contact pressure distribution, which reflects the fingertip's anatomy. PMID:26856769
NASA Astrophysics Data System (ADS)
Yan, Bo; Li, Yuguo; Liu, Ying
2016-07-01
In this paper, we present an adaptive finite element (FE) algorithm for direct current (DC) resistivity modeling in 2-D generally anisotropic conductivity structures. Our algorithm is implemented on an unstructured triangular mesh that readily accommodates complex structures such as topography and dipping layers and so on. We implement a self-adaptive, goal-oriented grid refinement algorithm in which the finite element analysis is performed on a sequence of refined grids. The grid refinement process is guided by an a posteriori error estimator. The problem is formulated in terms of total potentials where mixed boundary conditions are incorporated. This type of boundary condition is superior to the Dirichlet type of conditions and improves numerical accuracy considerably according to model calculations. We have verified the adaptive finite element algorithm using a two-layered earth with azimuthal anisotropy. The FE algorithm with incorporation of mixed boundary conditions achieves high accuracy. The relative error between the numerical and analytical solutions is less than 1% except in the vicinity of the current source location, where the relative error is up to 2.4%. A 2-D anisotropic model is used to demonstrate the effects of anisotropy upon the apparent resistivity in DC soundings.
Bailey, Teresa S. Adams, Marvin L. Yang, Brian Zika, Michael R.
2008-04-01
We develop a piecewise linear (PWL) Galerkin finite element spatial discretization for the multi-dimensional radiation diffusion equation. It uses recently introduced piecewise linear weight and basis functions in the finite element approximation and it can be applied on arbitrary polygonal (2D) or polyhedral (3D) grids. We first demonstrate some analytical properties of the PWL method and perform a simple mode analysis to compare the PWL method with Palmer's vertex-centered finite-volume method and with a bilinear continuous finite element method. We then show that this new PWL method gives solutions comparable to those from Palmer's. However, since the PWL method produces a symmetric positive-definite coefficient matrix, it should be substantially more computationally efficient than Palmer's method, which produces an asymmetric matrix. We conclude that the Galerkin PWL method is an attractive option for solving diffusion equations on unstructured grids.
NASA Astrophysics Data System (ADS)
Zou, B.; Li, D. F.; Hu, H. J.; Zhang, H. W.; Lou, L. H.; Chen, M.; Lv, Z. Y.
Based on the verified two dimensional(2D) finite element model for river flow simulation, the effect of estuary training levees on the water flow and sediment movement in the Yellow River estuary is analyzed. For disclosing the effect of setting the two training levees on the flow and sediment motion, the calculation and analysis for the two projects, (one is no levees, the other is setting up two no levees) are given. The results show that when setting up two training levees, water flow is bound by levees and the water flows become more concentrated. As a result, velocity increases in the main channel, sediment carrying capacity of water flow increases correspondingly.
A Discrete-Element Approach for Blood Cell Adhesion
NASA Astrophysics Data System (ADS)
Chesnutt, Jennifer; Marshall, Jeffrey
2006-11-01
An efficient computational model for simulation of the individual dynamics of adhering blood cells is discussed. Each cell is represented as a discrete particle so that the model can extend existing discrete-element approaches for dense particulate fluid flows to account for receptor-ligand binding of particles, elliptical particle shape, and deformation of the particles due to shear forces. Capabilities of the method in simulating large numbers of particles are illustrated through simulations of the formation of red blood cell rouleaux in shear flow. The effects of several factors, such as aspect ratio of the elliptical particle, shear rate, strength of the cell adhesion force, and hematocrit are investigated. Comparison of the discrete-element results with results of a level-set approach which computes the entire flow field about a small number of cells is used to develop an improved model of the effect of nearby red blood cells on the cell drag force expression. The method is also being applied to examine the influence of red blood cells on other components of the blood, such as platelet dispersion and activation in high shear regions.
Discrete Element Modeling of Landslides in Valles Marineris, Mars
NASA Astrophysics Data System (ADS)
Smart, K. J.; Hooper, D. M.; Sims, D. W.
2010-12-01
High-resolution MOC, THEMIS, HiRISE, and HRSC image data and geomorphologic characterization based on MOLA-derived topography are being used as input for discrete element modeling to simulate slope failure in Valles Marineris. Two landslides have been selected for detailed analysis. The first landslide, in Coprates Chasma, has a strongly arcuate and recessional 4-km-high main scarp, and a runout length of approximately 70 km to the opposing canyon wall. The landslide deposit or transported material has a hummocky topography. The second landslide, in Ganges Chasma, has a 3-4 km high main scarp, a complex rupture surface with a displaced block, and a runout length of approximately 25 km. The landslide deposit is characterized by longitudinal ridges and furrows. The main scarp and displaced material of a landslide provide insight into the mechanical nature of the surface and shallow subsurface of Mars. We use two-dimensional discrete element models oriented parallel with the slide direction to examine the effects of mechanical layering upon the morphology of slip surfaces, scarps, and transported deposits that form as a result of slope failure on Mars. The initial geometry of the models is designed to replicate the height and length of each study site and to capture the observed and interpreted mechanical stratigraphy. Discrete element particle diameters range from approximately 30-60 m; a compromise between model fidelity and computation time. Bond properties (i.e., bond stiffness and strength), which control the macroscale behavior, are adjusted between layers to produce variable mechanical stratigraphic configurations. Our models were conducted under Mars gravity (3.71 m/s2) using a pre-slide free surface that dips 60°. Model results show that an initial slip surface forms some distance from the lateral free surface and subsequently migrates away from the free surface in discrete increments producing a well-developed main scarp. The models also show rotated blocks
Moving finite elements in 2-D. Technical progress report, year 3
Gelinas, R.J.
1984-04-03
The moving finite element (MFE) method has emerged as a potentially potent and interesting method for solving partial differential equations (PDE's) with large gradients. The principal feature of the MFE method is that the grid node co-ordinates, themselves, are dependent variables and are calculated at each time step so as to minimize a PDE residual in some norm. This has the effect of moving the grid nodes continuously and systematically to those positions which minimize PDE numerical solution errors. Research on the MFE method to this time has been advanced by a relatively small number of groups and individual investigators. Of these, the presently proposing group at Science Applications, Inc. (SAI), in Pleasanton, California, has pursued simultaneously developments of the basic theory, numerical analysis, and real-world applications under sponsorship of the DOE and others. The results of our MFE research to date in both 1-D and 2-D transient PDE systems have been quite positive, as well as laden with indicators for further advancements. We report the progress of this third year of 2-D MFE research and indicate those research tasks which should now be pursued into their next logical stages of advancement for large-gradient PDE problems in 2-D.
A 2D finite element wave equation solver based on triangular base elements
NASA Astrophysics Data System (ADS)
Van Eester, D.; Lerche, E.; Evrard, M.
2009-11-01
A finite element method based on the subdivision of the physical domain in triangular sub-domains in which simple local 'areale' coordinates are adopted is explored. The advantage of the method is that it straightforwardly allows grid refinement in regions where higher precision is required. The plasma model was kept simple for this 'proof-of-principle' exercise. Rather than accounting for the actual differential or integro-differential dielectric tensor, its locally uniform plasma equivalent was adopted for 3 possible choices: the cold plasma response, the full hot Stix/Swanson plasma tensor retaining all orders in finite Larmor radius (FLR) and the more common hot tensor, truncated at terms of second order in the Larmor radius.
A 2D finite element wave equation solver based on triangular base elements
Van Eester, D.; Lerche, E.; Evrard, M.
2009-11-26
A finite element method based on the subdivision of the physical domain in triangular sub-domains in which simple local 'areale' coordinates are adopted is explored. The advantage of the method is that it straightforwardly allows grid refinement in regions where higher precision is required. The plasma model was kept simple for this 'proof-of-principle' exercise. Rather than accounting for the actual differential or integro-differential dielectric tensor, its locally uniform plasma equivalent was adopted for 3 possible choices: the cold plasma response, the full hot Stix/Swanson plasma tensor retaining all orders in finite Larmor radius (FLR) and the more common hot tensor, truncated at terms of second order in the Larmor radius.
Discrete Element Method Simulation of Nonlinear Viscoelastic Stress Wave Problems
NASA Astrophysics Data System (ADS)
Tang, Zhiping; Horie, Y.; Wang, Wenqiang
2002-07-01
A DEM(Discrete Element Method) simulation of nonlinear viscoelastic stress wave problems is carried out. The interaction forces among elements are described using a model in which neighbor elements are linked by a nonlinear spring and a certain number of Maxwell components in parallel. By making use of exponential relaxation moduli, it is shown that numerical computation of the convolution integral does not require storing and repeatedly calculating strain history, so that the computational cost is dramatically reduced. To validate the viscoelastic DM2 code1, stress wave propagation in a Maxwell rod with one end subjected to a constant stress loading is simulated. Results excellently fit those from the characteristics calculation. The code is then used to investigate the problem of meso-scale damage in a plastic-bonded explosive under shock loading. Results not only show "compression damage", but also reveal a complex damage evolution. They demonstrate a unique capability of DEM in modeling heterogeneous materials.
Discrete Element Method Simulation of Nonlinear Viscoelastic Stress Wave Problems
NASA Astrophysics Data System (ADS)
Wang, Wenqiang; Tang, Zhiping; Horie, Y.
2002-07-01
A DEM(Discrete Element Method) simulation of nonlinear viscoelastic stress wave problems is carried out. The interaction forces among elements are described using a model in which neighbor elements are linked by a nonlinear spring and a certain number of Maxwell components in parallel. By making use of exponential relaxation moduli, it is shown that numerical computation of the convolution integral does not require storing and repeatedly calculating strain history, so that the computational cost is dramatically reduced. To validate the viscoelastic DM2 code[1], stress wave propagation in a Maxwell rod with one end subjected to a constant stress loading is simulated. Results excellently fit those from the characteristics calculation. The code is then used to investigate the problem of meso-scale damage in a plastic-bonded explosive under shock loading. Results not only show "compression damage", but also reveal a complex damage evolution. They demonstrate a unique capability of DEM in modeling heterogeneous materials.
The low frequency 2D vibration sensor based on flat coil element
Djamal, Mitra; Sanjaya, Edi; Islahudin; Ramli
2012-06-20
Vibration like an earthquake is a phenomenon of physics. The characteristics of these vibrations can be used as an early warning system so as to reduce the loss or damage caused by earthquakes. In this paper, we introduced a new type of low frequency 2D vibration sensor based on flat coil element that we have developed. Its working principle is based on position change of a seismic mass that put in front of a flat coil element. The flat coil is a part of a LC oscillator; therefore, the change of seismic mass position will change its resonance frequency. The results of measurements of low frequency vibration sensor in the direction of the x axis and y axis gives the frequency range between 0.2 to 1.0 Hz.
Scattering of elastic waves by a 2-D crack using the Indirect Boundary Element Method (IBEM)
NASA Astrophysics Data System (ADS)
Iturrarán-Viveros, Ursula; Vai, Rossana; Sánchez-Sesma, Francisco J.
2005-09-01
The scattering of elastic waves by cracks is an old problem and various ways to solve it have been proposed in the last decades. One approach is using dual integral equations, another useful and common formulation is the Boundary Element Method (BEM). With the last one, the boundary conditions of the crack lead to hyper-singularities and particular care should be taken to regularize and solve the resulting integral equations. In this work, instead, the Indirect Boundary Element Method (IBEM) is applied to study problems of zero-thickness 2-D cracks. The IBEM yields the Crack Opening Displacement (COD) which is used to evaluate the solution away from the crack. We use a multiregional approach which consists of splitting a boundary S into two identical boundaries S+ and S- chosen such that the cracks lie in the interface. The resulting integral equations are not hyper-singular and wave propagation within media that contain zero-thickness cracks can be rigorously solved. In order to validate the method, we deal with the scalar case, namely the scattering of antiplane SH waves by a 2-D crack. We compare results against a recently published analytic solution, obtaining an excellent agreement. This comparison gives us confidence to study cases where no analytic solutions exist. Some examples of incidence of P- or SV waves are depicted and the salient aspects of the method are also discussed.
Preece, D.S. Perkins, E.D.
1999-02-10
Techniques for modeling oil well sand production have been developed using the formulations for superquadric discrete elements and Darcy fluid flow. Discrete element models are generated using the new technique of particle cloning. Discrete element sources and sinks allow simulation of sand production from the initial state through the transition to an equilibrium state where particles are created and removed at the same rate.
NASA Astrophysics Data System (ADS)
Steinke, R. C.
2015-12-01
Discretizing 1-D vadose zone simulations in the moisture content domain, such as is done in the Talbot-Ogden method, provides some advantages over discretizing in depth, such as is done in Richards' Equation. These advantages include inherent mass conservation and lower computational cost. However, doing so presents a difficulty for integration with 2-D groundwater interflow simulations. The equations of motion of the bins of discrete moisture content take the depth of the water table as an input. They do not produce it as an output. Finding the correct water table depth so that the groundwater recharge from the 1-D vadose zone simulation mass balances with the lateral flows from the 2-D groundwater interflow simulation was a previously unsolved problem. In this paper we present a net-groundwater-recharge method to solve to this problem and compare it with the source-term method used with Richards' Equation.
Discrete Element Modeling (DEM) of Triboelectrically Charged Particles: Revised Experiments
NASA Technical Reports Server (NTRS)
Hogue, Michael D.; Calle, Carlos I.; Curry, D. R.; Weitzman, P. S.
2008-01-01
In a previous work, the addition of basic screened Coulombic electrostatic forces to an existing commercial discrete element modeling (DEM) software was reported. Triboelectric experiments were performed to charge glass spheres rolling on inclined planes of various materials. Charge generation constants and the Q/m ratios for the test materials were calculated from the experimental data and compared to the simulation output of the DEM software. In this paper, we will discuss new values of the charge generation constants calculated from improved experimental procedures and data. Also, planned work to include dielectrophoretic, Van der Waals forces, and advanced mechanical forces into the software will be discussed.
A Review of Discrete Element Method Research on Particulate Systems
NASA Astrophysics Data System (ADS)
Mahmood, A. A.; Elektorowicz, M.
2016-07-01
This paper summarizes research done using the Discrete Element Method (DEM) and explores new trends in its use on Particulate systems. The rationale for using DEM versus the traditional continuum-based approach is explained first. Then, DEM application is explored in terms of geotechnical engineering and mining engineering materials, since particulate media are mostly associated with these two disciplines. It is concluded that no research to date had addressed the issue of using the DEM to model the strength and weathering characteristics of peaty soil-slag-Portland cement-fly ash combinations.
From discrete elements to continuum fields: Extension to bidisperse systems
NASA Astrophysics Data System (ADS)
Tunuguntla, Deepak R.; Thornton, Anthony R.; Weinhart, Thomas
2015-11-01
Micro-macro transition methods can be used to, both, calibrate and validate continuum models from discrete data obtained via experiments or simulations. These methods generate continuum fields such as density, momentum, stress, etc., from discrete data, i.e. positions, velocity, orientations and forces of individual elements. Performing this micro-macro transition step is especially challenging for non-uniform or dynamic situations. Here, we present a general method of performing this transition, but for simplicity we will restrict our attention to two-component scenarios. The mapping technique, presented here, is an extension to the micro-macro transition method, called coarse-graining, for unsteady two-component flows and can be easily extended to multi-component systems without any loss of generality. This novel method is advantageous; because, by construction the obtained macroscopic fields are consistent with the continuum equations of mass, momentum and energy balance. Additionally, boundary interaction forces can be taken into account in a self-consistent way and thus allow for the construction of continuous stress fields even within one element radius of the boundaries. Similarly, stress and drag forces can also be determined for individual constituents of a multi-component mixture, which is critical for several continuum applications, e.g. mixture theory-based segregation models. Moreover, the method does not require ensemble-averaging and thus can be efficiently exploited to investigate static, steady and time-dependent flows. The method presented in this paper is valid for any discrete data, e.g. particle simulations, molecular dynamics, experimental data, etc.; however, for the purpose of illustration we consider data generated from discrete particle simulations of bidisperse granular mixtures flowing over rough inclined channels. We show how to practically use our coarse-graining extension for both steady and unsteady flows using our open-source coarse
From discrete elements to continuum fields: Extension to bidisperse systems
NASA Astrophysics Data System (ADS)
Tunuguntla, Deepak R.; Thornton, Anthony R.; Weinhart, Thomas
2016-07-01
Micro-macro transition methods can be used to, both, calibrate and validate continuum models from discrete data obtained via experiments or simulations. These methods generate continuum fields such as density, momentum, stress, etc., from discrete data, i.e. positions, velocity, orientations and forces of individual elements. Performing this micro-macro transition step is especially challenging for non-uniform or dynamic situations. Here, we present a general method of performing this transition, but for simplicity we will restrict our attention to two-component scenarios. The mapping technique, presented here, is an extension to the micro-macro transition method, called coarse-graining, for unsteady two-component flows and can be easily extended to multi-component systems without any loss of generality. This novel method is advantageous; because, by construction the obtained macroscopic fields are consistent with the continuum equations of mass, momentum and energy balance. Additionally, boundary interaction forces can be taken into account in a self-consistent way and thus allow for the construction of continuous stress fields even within one element radius of the boundaries. Similarly, stress and drag forces can also be determined for individual constituents of a multi-component mixture, which is critical for several continuum applications, e.g. mixture theory-based segregation models. Moreover, the method does not require ensemble-averaging and thus can be efficiently exploited to investigate static, steady and time-dependent flows. The method presented in this paper is valid for any discrete data, e.g. particle simulations, molecular dynamics, experimental data, etc.; however, for the purpose of illustration we consider data generated from discrete particle simulations of bidisperse granular mixtures flowing over rough inclined channels. We show how to practically use our coarse-graining extension for both steady and unsteady flows using our open-source coarse
NASA Astrophysics Data System (ADS)
Liu, Wenyan; Tang, Z. P.; Liu, Yunxin
2000-04-01
In recent years, more attention has been paid to a better understanding of the failure behavior and mechanism of heterogeneous materials at the meso-scale level. In this paper, the crack initiation and development in epoxy composites reinforced with short steel fibers under dynamic loading were simulated and analyzed with the 2D Discrete Meso-Element Dynamic Method. Results show that the damage process depends greatly on the binding property between matrix and fibers.
3D Discrete Element Model with 1 Million Particles: an Example of Hydro-fracturing
NASA Astrophysics Data System (ADS)
Liu, C.; Pollard, D. D.
2013-12-01
The Discrete Element Method (DEM) permits large relative motion and breakage of elements, and does not require re-meshing, for example as would the Finite Element Method. DEM has a wide range of applications in the fields of solid-earth geophysics, geomechanics, mining engineering, and structural geology. However, due to the computational cost, particle numbers of discrete element models are generally less than a few tens of thousands, which limits the applications. A new 3D DEM system 'MatDEM' can complete dynamic simulations of one million particles. The conversion formulas between particle parameters and model mechanical properties were derived, and the conversion of energy in DEM can be simulated. In a recent paper (Liu et al., 2013, JGR), the analytical solutions of elastic properties and failure modes of a 2D close-packed discrete element model were proposed. Based on these theoretical results, it is easy to create materials using DEM, which have similar mechanical properties to rock. Given the mechanical properties and state of stress, geologists and engineers can investigate the characteristics of rock deformation and failure under different conditions. MatDEM provides an alternative way to study the micro-macro relationships of rock and soil, and the evolution of geologic structures. As an example, MatDEM was used to investigate the generation and development of fluid driven fractures around a micro pore. The simulation result of fractures of an anisotropic 3D model, which includes 1 million particles, is demonstrated. Via parallel computing technology, MatDEM may handle tens of millions of particles in near future. Left: Fluid pressure is applied in the pore to generate fractures. Right: Simulation results (black segments represent fractures).
Discrete Element Method Simulations of Ice Floe Dynamics
NASA Astrophysics Data System (ADS)
Calantoni, J.; Bateman, S. P.; Shi, F.; Orzech, M.; Veeramony, J.
2014-12-01
Ice floes were modeled using LIGGGHTS, an open source discrete element method (DEM) software, where individual elements were bonded together to make floes. The bonds were allowed to break with a critical stress calibrated to existing laboratory measurements for the compressive, tensile, and flexural strength of ice floes. The DEM allows for heterogeneous shape and size distributions of the ice floes to evolve over time. We simulated the interaction between sea ice and ocean waves in the marginal ice zone using a coupled wave-ice system. The waves were modeled with NHWAVE, a non-hydrostatic wave model that predicts instantaneous surface elevation and the three-dimensional flow field. The ice floes and waves were coupled through buoyancy and drag forces. Preliminary comparisons with field and laboratory measurements for coupled simulations will be presented.
Discrete-element modeling of particulate aerosol flows
NASA Astrophysics Data System (ADS)
Marshall, J. S.
2009-03-01
A multiple-time step computational approach is presented for efficient discrete-element modeling of aerosol flows containing adhesive solid particles. Adhesive aerosol particulates are found in numerous dust and smoke contamination problems, including smoke particle transport in the lungs, particle clogging of heat exchangers in construction vehicles, industrial nanoparticle transport and filtration systems, and dust fouling of electronic systems and MEMS components. Dust fouling of equipment is of particular concern for potential human occupation on dusty planets, such as Mars. The discrete-element method presented in this paper can be used for prediction of aggregate structure and breakup, for prediction of the effect of aggregate formation on the bulk fluid flow, and for prediction of the effects of small-scale flow features (e.g., due to surface roughness or MEMS patterning) on the aggregate formation. After presentation of the overall computational structure, the forces and torques acting on the particles resulting from fluid motion, particle-particle collision, and adhesion under van der Waals forces are reviewed. The effect of various parameters of normal collision and adhesion of two particles are examined in detail. The method is then used to examine aggregate formation and particle clogging in pipe and channel flow.
Discrete-element modeling of particulate aerosol flows
Marshall, J.S.
2009-03-20
A multiple-time step computational approach is presented for efficient discrete-element modeling of aerosol flows containing adhesive solid particles. Adhesive aerosol particulates are found in numerous dust and smoke contamination problems, including smoke particle transport in the lungs, particle clogging of heat exchangers in construction vehicles, industrial nanoparticle transport and filtration systems, and dust fouling of electronic systems and MEMS components. Dust fouling of equipment is of particular concern for potential human occupation on dusty planets, such as Mars. The discrete-element method presented in this paper can be used for prediction of aggregate structure and breakup, for prediction of the effect of aggregate formation on the bulk fluid flow, and for prediction of the effects of small-scale flow features (e.g., due to surface roughness or MEMS patterning) on the aggregate formation. After presentation of the overall computational structure, the forces and torques acting on the particles resulting from fluid motion, particle-particle collision, and adhesion under van der Waals forces are reviewed. The effect of various parameters of normal collision and adhesion of two particles are examined in detail. The method is then used to examine aggregate formation and particle clogging in pipe and channel flow.
The Wavelet Element Method. Part 2; Realization and Additional Features in 2D and 3D
NASA Technical Reports Server (NTRS)
Canuto, Claudio; Tabacco, Anita; Urban, Karsten
1998-01-01
The Wavelet Element Method (WEM) provides a construction of multiresolution systems and biorthogonal wavelets on fairly general domains. These are split into subdomains that are mapped to a single reference hypercube. Tensor products of scaling functions and wavelets defined on the unit interval are used on the reference domain. By introducing appropriate matching conditions across the interelement boundaries, a globally continuous biorthogonal wavelet basis on the general domain is obtained. This construction does not uniquely define the basis functions but rather leaves some freedom for fulfilling additional features. In this paper we detail the general construction principle of the WEM to the 1D, 2D and 3D cases. We address additional features such as symmetry, vanishing moments and minimal support of the wavelet functions in each particular dimension. The construction is illustrated by using biorthogonal spline wavelets on the interval.
A linear analytical boundary element method (BEM) for 2D homogeneous potential problems
NASA Astrophysics Data System (ADS)
Friedrich, Jürgen
2002-06-01
The solution of potential problems is not only fundamental for geosciences, but also an essential part of related subjects like electro- and fluid-mechanics. In all fields, solution algorithms are needed that should be as accurate as possible, robust, simple to program, easy to use, fast and small in computer memory. An ideal technique to fulfill these criteria is the boundary element method (BEM) which applies Green's identities to transform volume integrals into boundary integrals. This work describes a linear analytical BEM for 2D homogeneous potential problems that is more robust and precise than numerical methods because it avoids numerical schemes and coordinate transformations. After deriving the solution algorithm, the introduced approach is tested against different benchmarks. Finally, the gained method was incorporated into an existing software program described before in this journal by the same author.
An implicit finite element method for discrete dynamic fracture
Jobie M. Gerken
1999-12-01
A method for modeling the discrete fracture of two-dimensional linear elastic structures with a distribution of small cracks subject to dynamic conditions has been developed. The foundation for this numerical model is a plane element formulated from the Hu-Washizu energy principle. The distribution of small cracks is incorporated into the numerical model by including a small crack at each element interface. The additional strain field in an element adjacent to this crack is treated as an externally applied strain field in the Hu-Washizu energy principle. The resulting stiffness matrix is that of a standard plane element. The resulting load vector is that of a standard plane element with an additional term that includes the externally applied strain field. Except for the crack strain field equations, all terms of the stiffness matrix and load vector are integrated symbolically in Maple V so that fully integrated plane stress and plane strain elements are constructed. The crack strain field equations are integrated numerically. The modeling of dynamic behavior of simple structures was demonstrated within acceptable engineering accuracy. In the model of axial and transverse vibration of a beam and the breathing mode of vibration of a thin ring, the dynamic characteristics were shown to be within expected limits. The models dominated by tensile forces (the axially loaded beam and the pressurized ring) were within 0.5% of the theoretical values while the shear dominated model (the transversely loaded beam) is within 5% of the calculated theoretical value. The constant strain field of the tensile problems can be modeled exactly by the numerical model. The numerical results should therefore, be exact. The discrepancies can be accounted for by errors in the calculation of frequency from the numerical results. The linear strain field of the transverse model must be modeled by a series of constant strain elements. This is an approximation to the true strain field, so some
NASA Astrophysics Data System (ADS)
Banton, J.; Villard, P.; Jongmans, D.; Scavia, C.
2009-11-01
Application of the discrete element method (DEM) to model avalanches of granular materials requires determining the correct geometric and rheological parameters for and between the particles as well as for the basal surface. The use of spherical (circular in 2-D) particles enhances particle rolling, yielding excessive runout values. The solution usually adopted to correct this effect is to introduce a drag force which artificially slows down the particle velocities. The aim of this study is to test the capability of the DEM to simulate well-controlled unsteady channelized granular flows, considering the measured properties of the particles and of the basal surface which naturally contribute to dissipate energy. We first performed a parametrical analysis on a simple 2-D model in order to estimate the influence of particle shape, friction parameters, and restitution coefficients on the dynamics of the flow and on the deposit geometry. We then simulated three channelized laboratory experiments performed with two materials and two bed linings. Using the geometrical layout and the values of the mechanical parameters provided by the authors, we obtained a remarkable agreement between the observed and 2-D simulated deposit shapes for the three experiments. Also, the computed mass evolution with time was very consistent with the experimental snapshots in all cases. These results highlight the capability of the DEM technique for modeling avalanche of granular material when the particle shape as well as the friction and restitution coefficients are properly considered.
Predicting the behavior of microfluidic circuits made from discrete elements
Bhargava, Krisna C.; Thompson, Bryant; Iqbal, Danish; Malmstadt, Noah
2015-01-01
Microfluidic devices can be used to execute a variety of continuous flow analytical and synthetic chemistry protocols with a great degree of precision. The growing availability of additive manufacturing has enabled the design of microfluidic devices with new functionality and complexity. However, these devices are prone to larger manufacturing variation than is typical of those made with micromachining or soft lithography. In this report, we demonstrate a design-for-manufacturing workflow that addresses performance variation at the microfluidic element and circuit level, in context of mass-manufacturing and additive manufacturing. Our approach relies on discrete microfluidic elements that are characterized by their terminal hydraulic resistance and associated tolerance. Network analysis is employed to construct simple analytical design rules for model microfluidic circuits. Monte Carlo analysis is employed at both the individual element and circuit level to establish expected performance metrics for several specific circuit configurations. A protocol based on osmometry is used to experimentally probe mixing behavior in circuits in order to validate these approaches. The overall workflow is applied to two application circuits with immediate use at on the bench-top: series and parallel mixing circuits that are modularly programmable, virtually predictable, highly precise, and operable by hand. PMID:26516059
Predicting the behavior of microfluidic circuits made from discrete elements
NASA Astrophysics Data System (ADS)
Bhargava, Krisna C.; Thompson, Bryant; Iqbal, Danish; Malmstadt, Noah
2015-10-01
Microfluidic devices can be used to execute a variety of continuous flow analytical and synthetic chemistry protocols with a great degree of precision. The growing availability of additive manufacturing has enabled the design of microfluidic devices with new functionality and complexity. However, these devices are prone to larger manufacturing variation than is typical of those made with micromachining or soft lithography. In this report, we demonstrate a design-for-manufacturing workflow that addresses performance variation at the microfluidic element and circuit level, in context of mass-manufacturing and additive manufacturing. Our approach relies on discrete microfluidic elements that are characterized by their terminal hydraulic resistance and associated tolerance. Network analysis is employed to construct simple analytical design rules for model microfluidic circuits. Monte Carlo analysis is employed at both the individual element and circuit level to establish expected performance metrics for several specific circuit configurations. A protocol based on osmometry is used to experimentally probe mixing behavior in circuits in order to validate these approaches. The overall workflow is applied to two application circuits with immediate use at on the bench-top: series and parallel mixing circuits that are modularly programmable, virtually predictable, highly precise, and operable by hand.
Predicting the behavior of microfluidic circuits made from discrete elements.
Bhargava, Krisna C; Thompson, Bryant; Iqbal, Danish; Malmstadt, Noah
2015-01-01
Microfluidic devices can be used to execute a variety of continuous flow analytical and synthetic chemistry protocols with a great degree of precision. The growing availability of additive manufacturing has enabled the design of microfluidic devices with new functionality and complexity. However, these devices are prone to larger manufacturing variation than is typical of those made with micromachining or soft lithography. In this report, we demonstrate a design-for-manufacturing workflow that addresses performance variation at the microfluidic element and circuit level, in context of mass-manufacturing and additive manufacturing. Our approach relies on discrete microfluidic elements that are characterized by their terminal hydraulic resistance and associated tolerance. Network analysis is employed to construct simple analytical design rules for model microfluidic circuits. Monte Carlo analysis is employed at both the individual element and circuit level to establish expected performance metrics for several specific circuit configurations. A protocol based on osmometry is used to experimentally probe mixing behavior in circuits in order to validate these approaches. The overall workflow is applied to two application circuits with immediate use at on the bench-top: series and parallel mixing circuits that are modularly programmable, virtually predictable, highly precise, and operable by hand. PMID:26516059
Determining Trajectory of Triboelectrically Charged Particles, Using Discrete Element Modeling
NASA Technical Reports Server (NTRS)
2008-01-01
The Kennedy Space Center (KSC) Electrostatics and Surface Physics Laboratory is participating in an Innovative Partnership Program (IPP) project with an industry partner to modify a commercial off-the-shelf simulation software product to treat the electrodynamics of particulate systems. Discrete element modeling (DEM) is a numerical technique that can track the dynamics of particle systems. This technique, which was introduced in 1979 for analysis of rock mechanics, was recently refined to include the contact force interaction of particles with arbitrary surfaces and moving machinery. In our work, we endeavor to incorporate electrostatic forces into the DEM calculations to enhance the fidelity of the software and its applicability to (1) particle processes, such as electrophotography, that are greatly affected by electrostatic forces, (2) grain and dust transport, and (3) the study of lunar and Martian regoliths.
Adaptive model reduction for nonsmooth discrete element simulation
NASA Astrophysics Data System (ADS)
Servin, Martin; Wang, Da
2016-03-01
A method for adaptive model order reduction for nonsmooth discrete element simulation is developed and analysed in numerical experiments. Regions of the granular media that collectively move as rigid bodies are substituted with rigid bodies of the corresponding shape and mass distribution. The method also support particles merging with articulated multibody systems. A model approximation error is defined and used to derive conditions for when and where to apply reduction and refinement back into particles and smaller rigid bodies. Three methods for refinement are proposed and tested: prediction from contact events, trial solutions computed in the background and using split sensors. The computational performance can be increased by 5-50 times for model reduction level between 70-95 %.
Senapati, Rajeev; Zhang Jianmei
2010-02-22
Advanced ceramic materials have been extensively applied in aerospace, automobile and other industries. However, the reliability of the advanced ceramics is a major concern because of the brittle nature of the materials. In this paper, combination of nondestructive testing and numerical modeling Discrete Element Method is proposed to identify the fracture origin in ceramics. The nondestructive testing--laser scattering technology is first performed on the ceramic components to reveal the machining-induced damage such as cracks and the material-inherent flaws such as voids, then followed by the four point bending test. Discrete Element software package PFC{sup 2D} is used to simulate the four point bending test and try to identify where the fractures start. The numerical representation of the ceramic materials is done by generating a densely packed particle system using the specimen genesis procedure and then applying the suitable microparameters to the particle system. Simulation of four point bending test is performed on materials having no defects, materials having manufacturing-induced defects like cracks, and materials having material-inherent flaws like voids. The initiation and propagation of defects is modeled and the mean contact force on the loading ball is also plotted. The simulation prediction results are well in accordance with the nondestructive testing results.
Discrete Meso-Element Simulation of Failure Behavior of Short-Fiber Composites under Shock Loading
NASA Astrophysics Data System (ADS)
Tang, Z. P.; Liu, Wenyan; Liu, Yunxin
1999-06-01
Recent years, it was paid more attention to better understanding the failure behavior and mechanism of heterogeneous materials at meso- scale level. In this paper, the crack initiation and development in epoxy composite reinforced with short steel fibre under dynamic loading were simulated and analyzed with 2D Discrete Meso-Element Dynamic Method. Results show that cracks initiate at the tips of fibres on the Loading side where stress concentrates. The effective strength of the composite sample is related to shape, orientation, weight percentage of the fibres, and particularly, the bonding strength between fibre and matrix. In the case of low bonding strength, the crack will propagate along the fibre and finally penetrate the whole sample. The differences compared with static loading are also discussed.
NASA Astrophysics Data System (ADS)
Bouclier, R.; Elguedj, T.; Combescure, A.
2013-11-01
This work deals with the development of 2D solid shell non-uniform rational B-spline elements. We address a static problem, that can be solved with a 2D model, involving a thin slender structure under small perturbations. The plane stress, plane strain and axisymmetric assumption can be made. projection and reduced integration techniques are considered to deal with the locking phenomenon. The use of the approach leads to the implementation of two strategies insensitive to locking: the first strategy is based on a 1D projection of the mean strain across the thickness; the second strategy undertakes to project all the strains onto a suitably chosen 2D space. Conversely, the reduced integration approach based on Gauss points is less expensive, but only alleviates locking and is limited to quadratic approximations. The performance of the various 2D elements developed is assessed through several numerical examples. Simple extensions of these techniques to 3D are finally performed.
Discrete Element Modeling Results of Proppant Rearrangement in the Cooke Conductivity Cell
Earl Mattson; Hai Huang; Michael Conway; Lisa O'Connell
2014-02-01
The study of propped fracture conductivity began in earnest with the development of the Cooke cell which later became part of the initial API standard. Subsequent developments included a patented multicell design to conduct 4 tests in a press at the same time. Other modifications have been used by various investigators. Recent studies by the Stim-Lab proppant consortium have indicated that the flow field across a Cooke proppant conductivity testing cell may not be uniform as initially believed which resulted is significantly different conductivity results. Post test analysis of low temperature metal alloy injections at the termination of proppant testing prior to the release of the applied stress suggest that higher flow is to be expected along the sides and top of the proppant pack than compared to the middle of the pack. To evaluate these experimental findings, a physics-based two-dimensional (2-D) discrete element model (DEM) was developed and applied to simulate proppant rearrangement during stress loading in the Cooke conductivity cell and the resulting porosity field. Analysis of these simulations are critical to understanding the impact of modification to the testing cell as well as understanding key proppant conductivity issues such as how these effects are manifested in proppant concentration testing results. The 2-D DEM model was constructed to represent a realistic cross section of the Cooke cell with a distribution of four material properties, three that represented the Cooke cell (steel, sandstone,square rings), and one representing the proppant. In principle, Cooke cell materials can be approximated as assemblies of independent discrete elements (particles) of various sizes and material properties that interact via cohesive interactions, repulsive forces, and frictional forces. The macroscopic behavior can then be modeled as the collective behavior of many interacting discrete elements. This DEM model is particularly suitable for modeling proppant
2D Distinct Element Method (DEM) models of the initiation, propagation and saturation of rock joints
NASA Astrophysics Data System (ADS)
Arslan, A.; Schöpfer, M. P.; Walsh, J. J.; Childs, C.
2009-12-01
In layered sequences, rock joints usually best develop within the more brittle layers and commonly display a regular spacing that scales with layer thickness. A variety of conceptual and mechanical models have been developed for these observations. A limitation of previous approaches, however, is that fracture initiation and associated interface slip are not explicitly simulated; instead, fractures were predefined and interfaces were welded. To surmount this problem, we have modelled the formation and growth of joints in layered sequences by using the two-dimensional Distinct Element Method (DEM) as implemented in the Particle Flow Code (PFC-2D). In PFC-2D, rock is represented by an assemblage of circular particles that are bonded at particle-particle contacts. Failure occurs if either the tensile or shear strength of a bond is exceeded. The models comprise a central brittle layer with high Young’s modulus, which is embedded in a low Young’s modulus matrix. The interfaces between the layers are defined by ‘smooth joint’ contacts, a modelling feature that eliminates interparticle bumpiness and associated interlocking friction. Consequently, this feature allows the user to assign macroscopic properties such as friction and cohesion along layer interfaces in a controlled manner. Layer parallel extension is applied by assigning a velocity to particles at the lateral boundaries of the model while maintaining a constant vertical confining pressure. Models were extended until joint saturation in the central layer was reached. We thereby explored the impact of confining pressure and interface properties (friction, cohesion) on joint spacing. A number of important conclusions can be drawn from our models: (i) The distributions of average horizontal normal stress within the layer and of shear stress at the interface are consistent with analytical solutions (stress-transfer theory). (ii) At low interfacial shear strength, new joints form preferentially midway between
Impact and Penetration of Granular Materials by Discrete Element Simulations
NASA Astrophysics Data System (ADS)
Garvin, Justin W.; Lechman, Jeremy B.; Lane, J. Matthew D.
2008-03-01
Granular material response to impact is important in a range of fields, from munitions delivery, to meteorite collision and crater formation. Recently a model for the force experienced on a penetrator has been proposed [L.S. Tsimring and D. Volfson, Powders and Grains 2005, 1215-1223] and shown to fit experimental data well [H. Katsuragi and D.J. Durian, Nature Physics, Vol. 3, June 2007]. This model describes two components of the force: i) a velocity dependent, depth independent term related to the inertial force required to mobilize a volume of grains in front of the penetrator; and ii) a velocity independent, depth dependent, Coulomb friction-like term. In the current study, massively parallel, discrete element simulations have been performed to study the penetration of a large spherical impactor into a multi-million particle bed of granular material. Results agree with previous work for slow impact speeds (< 400cm/s). In addition, the current work extends the comparison with the proposed model to higher speeds (˜1000cm/s). The physics of the phenomenon is discussed along with the challenges for modeling and simulation in the even higher velocity regime.
Discrete element analysis of powder processing: Fill and compaction
NASA Astrophysics Data System (ADS)
Kong, Consuelo Margarita
The production of various ceramic components is often achieved by processing powder into the desired shape and dimensions. The process includes filling a die with powder, which then is compacted. Even today, most parameters are adjusted by trial-and-error. The quality of the product is associated with its density homogeneity. This is a function of powder properties, die geometry, filling method and compacting cycle. The goal of the present work is to understand the parameters that affect dimension and quality of the compacted part. A discrete element model is proved suitable to simulate the powders' behavior during fill and compaction. This model suggests that application of accepted radius ratio rules for die filling have an effect opposite to that intended, because fines catalyze bridge formation instead of filling voids. Our results provide strong support that compaction is clearly localized before and during the I-II transition prior to propagating in a wave-like fashion throughout the rest of the compact. The changes in local density associated to the Stage I-II transition on the compaction curves signifies a change in the direction of the transmitted pressure through the uppermost layer, from dispersed to joined. Cyclic compaction allows for the periodic release of stress that homogenizes the granular matter immediately in front of the wave, producing a locally uniform propagation of pressure during succeeding cycles. The model is proven a valuable tool in predicting improvements in die design.
Discrete element crowd model for pedestrian evacuation through an exit
NASA Astrophysics Data System (ADS)
Peng, Lin; Jian, Ma; Siuming, Lo
2016-03-01
A series of accidents caused by crowds within the last decades evoked a lot of scientific interest in modeling the movement of pedestrian crowds. Based on the discrete element method, a granular dynamic model, in which the human body is simplified as a self-driven sphere, is proposed to simulate the characteristics of crowd flow through an exit. In this model, the repulsive force among people is considered to have an anisotropic feature, and the physical contact force due to body deformation is quantified by the Hertz contact model. The movement of the human body is simulated by applying the second Newton’s law. The crowd flow through an exit at different desired velocities is studied and simulation results indicated that crowd flow exhibits three distinct states, i.e., smooth state, transition state and phase separation state. In the simulation, the clogging phenomenon occurs more easily when the desired velocity is high and the exit may as a result be totally blocked at a desired velocity of 1.6 m/s or above, leading to faster-to-frozen effect. Project supported by the National Natural Science Foundation of China (Grant Nos. 71473207, 51178445, and 71103148), the Research Grant Council, Government of Hong Kong, China (Grant No. CityU119011), and the Fundamental Research Funds for the Central Universities, China (Grant Nos. 2682014CX103 and 2682014RC05).
NASA Technical Reports Server (NTRS)
Buczek, M. B.; Gregory, M. A.; Herakovich, C. T.
1983-01-01
CLFE2D is a two dimensional generalized plane strain finite element code, using a linear, four node, general quadrilateral, isoparametric element. The program is developed to calculate the displacements, strains, stresses, and strain energy densities in a finite width composite laminate. CLFE2D offers any combination of the following load types: nodal displacements, nodal forces, uniform normal strain, or hygrothermal. The program allows the user to input one set of three dimensional orthotropic material properties. The user can then specify the angle of material principal orientation for each element in the mesh. Output includes displacements, stresses, strains and strain densities at points selected by the user. An option is also available to plot the underformed and deformed finite element meshes.
SIMULATIONS OF 2D AND 3D THERMOCAPILLARY FLOWS BY A LEAST-SQUARES FINITE ELEMENT METHOD. (R825200)
Numerical results for time-dependent 2D and 3D thermocapillary flows are presented in this work. The numerical algorithm is based on the Crank-Nicolson scheme for time integration, Newton's method for linearization, and a least-squares finite element method, together with a matri...
Dual Element Intercalation into 2D Layered Bi₂Se₃ Nanoribbons.
Chen, Karen P; Chung, Frank R; Wang, Mengjing; Koski, Kristie J
2015-04-29
We demonstrate the intercalation of multiple zero-valent atomic species into two-dimensional (2D) layered Bi2Se3 nanoribbons. Intercalation is performed chemically through a stepwise combination of disproportionation redox reactions, hydrazine reduction, or carbonyl decomposition. Traditional intercalation is electrochemical thus limiting intercalant guests to a single atomic species. We show that multiple zero-valent atoms can be intercalated through this chemical route into the host lattice of a 2D crystal. Intermetallic species exhibit unique structural ordering demonstrated in a variety of superlattice diffraction patterns. We believe this method is general and can be used to achieve a wide variety of new 2D materials previously inaccessible. PMID:25851420
Lapchuk, A; Pashkevich, G A; Prygun, O V; Yurlov, V; Borodin, Y; Kryuchyn, A; Korchovyi, A A; Shylo, S
2015-10-01
The quasi-spiral 2D diffractive optical element (DOE) based on M-sequence of length N=15 is designed and manufactured. The speckle suppression efficiency by the DOE rotation is measured. The speckle suppression coefficients of 10.5, 6, and 4 are obtained for green, violet, and red laser beams, respectively. The results of numerical simulation and experimental data show that the quasi-spiral binary DOE structure can be as effective in speckle reduction as a periodic 2D DOE structure. The numerical simulation and experimental results show that the speckle suppression efficiency of the 2D DOE structure decreases approximately twice at the boundaries of the visible range. It is shown that a replacement of this structure with the bilateral 1D DOE allows obtaining the maximum speckle suppression efficiency in the entire visible range of light. PMID:26479664
A discrete element modelling approach for block impacts on trees
NASA Astrophysics Data System (ADS)
Toe, David; Bourrier, Franck; Olmedo, Ignatio; Berger, Frederic
2015-04-01
These past few year rockfall models explicitly accounting for block shape, especially those using the Discrete Element Method (DEM), have shown a good ability to predict rockfall trajectories. Integrating forest effects into those models still remain challenging. This study aims at using a DEM approach to model impacts of blocks on trees and identify the key parameters controlling the block kinematics after the impact on a tree. A DEM impact model of a block on a tree was developed and validated using laboratory experiments. Then, key parameters were assessed using a global sensitivity analyse. Modelling the impact of a block on a tree using DEM allows taking into account large displacements, material non-linearities and contacts between the block and the tree. Tree stems are represented by flexible cylinders model as plastic beams sustaining normal, shearing, bending, and twisting loading. Root soil interactions are modelled using a rotation stiffness acting on the bending moment at the bottom of the tree and a limit bending moment to account for tree overturning. The crown is taken into account using an additional mass distribute uniformly on the upper part of the tree. The block is represented by a sphere. The contact model between the block and the stem consists of an elastic frictional model. The DEM model was validated using laboratory impact tests carried out on 41 fresh beech (Fagus Sylvatica) stems. Each stem was 1,3 m long with a diameter between 3 to 7 cm. Wood stems were clamped on a rigid structure and impacted by a 149 kg charpy pendulum. Finally an intensive simulation campaign of blocks impacting trees was done to identify the input parameters controlling the block kinematics after the impact on a tree. 20 input parameters were considered in the DEM simulation model : 12 parameters were related to the tree and 8 parameters to the block. The results highlight that the impact velocity, the stem diameter, and the block volume are the three input
Discrete Element Modeling of Impact Damage on Thermal Barrier Coatings
NASA Astrophysics Data System (ADS)
Minor, Peter Michel
Natural gas turbines have become an increasingly important part of the energy landscape in the United States, currently accounting for 19% of all electricity production. Efforts to increase thermal efficiency in gas turbines has led to the adoption of highly porous ceramic thermal barrier coatings (TBCs), which are susceptible to erosion and foreign object impact damage. Despite significant investment to improve the design of TBCs, few numerical tools exist which are capable of both accurately capturing the specific failure mechanisms inherent to TBCs and iterating design parameters without the requirement for coupled experimental data. To overcome these limitations, a discrete element model (DEM) was created to simulate the microstructure of a TBC using a large-scale assembly of bonded particles. Acting as Lagrangian nodes, the particles can be combined to create accurate representations of TBC geometry and porosity. The inclusion of collision-driven particle dynamics and bonds derived from displacement-dependent force functions endow the microstructure model with the ability to deform and reproduce damage in a highly physical manner. Typical TBC damage mechanisms such as compaction, fracture and spallation occur automatically, without having to tune the model based on experimental observation. Therefore, the first order performance of novel TBC designs and materials can be determined numerically, greatly decreasing the cost of development. To verify the utility and effectiveness of the proposed damage model framework, a nanoindentation materials test simulation was developed to serve as a test case. By varying model parameters, such as the porosity of the TBC and maximum applied indenter force, nanoindentation data from more than one hundred distinct permutations was gathered and analyzed. This data was used to calculate the elastic modulus (E) and hardness (H) of the simulated microstructure, which could then be compared to known experimental material property
Parallel Finite Element Electron-Photon Transport Analysis on 2-D Unstructured Mesh
Drumm, C.R.
1999-01-01
A computer code has been developed to solve the linear Boltzmann transport equation on an unstructured mesh of triangles, from a Pro/E model. An arbitriwy arrangement of distinct material regions is allowed. Energy dependence is handled by solving over an arbitrary number of discrete energy groups. Angular de- pendence is treated by Legendre-polynomial expansion of the particle cross sections and a discrete ordinates treatment of the particle fluence. The resulting linear system is solved in parallel with a preconditioned conjugate-gradients method. The solution method is unique, in that the space-angle dependence is solved si- multaneously, eliminating the need for the usual inner iterations. Electron cross sections are obtained from a Goudsrnit-Saunderson modifed version of the CEPXS code. A one-dimensional version of the code has also been develop@ for testing and development purposes.
Coupled 2D-3D finite element method for analysis of a skin panel with a discontinuous stiffener
NASA Technical Reports Server (NTRS)
Wang, J. T.; Lotts, C. G.; Davis, D. D., Jr.; Krishnamurthy, T.
1992-01-01
This paper describes a computationally efficient analysis method which was used to predict detailed stress states in a typical composite compression panel with a discontinuous hat stiffener. A global-local approach was used. The global model incorporated both 2D shell and 3D brick elements connected by newly developed transition elements. Most of the panel was modeled with 2D elements, while 3D elements were employed to model the stiffener flange and the adjacent skin. Both linear and geometrically nonlinear analyses were performed on the global model. The effect of geometric nonlinearity induced by the eccentric load path due to the discontinuous hat stiffener was significant. The local model used a fine mesh of 3D brick elements to model the region at the end of the stiffener. Boundary conditions of the local 3D model were obtained by spline interpolation of the nodal displacements from the global analysis. Detailed in-plane and through-the-thickness stresses were calculated in the flange-skin interface near the end of the stiffener.
Nosich, Andrey A; Gandel, Yuriy V; Magath, Thore; Altintas, Ayhan
2007-09-01
Considered is the beam wave guidance and scattering by 2D quasi-optical reflectors modeling the components of beam waveguides. The incident field is taken as the complex-source-point field to simulate a finite-width beam generated by a small-aperture source. A numerical solution is obtained from the coupled singular integral equations (SIEs) for the surface currents on reflectors, discretized by using the recently introduced Nystrom-type quadrature formulas. This analysis is applied to study what effect the edge illumination has on the performance of a chain of confocal elliptic reflectors. We also develop a semianalytical approach for shaped reflector synthesis after a prescribed near-field pattern. Here a new point is the use of auxiliary SIEs of the same type as in the scattering analysis problem, however, for the gradient of the objective function. Sample results are presented for the synthesis of a reflector-type beam splitter. PMID:17767252
NASA Astrophysics Data System (ADS)
Choi, S.-J.; Giraldo, F. X.; Kim, J.; Shin, S.
2014-11-01
The non-hydrostatic (NH) compressible Euler equations for dry atmosphere were solved in a simplified two-dimensional (2-D) slice framework employing a spectral element method (SEM) for the horizontal discretization and a finite difference method (FDM) for the vertical discretization. By using horizontal SEM, which decomposes the physical domain into smaller pieces with a small communication stencil, a high level of scalability can be achieved. By using vertical FDM, an easy method for coupling the dynamics and existing physics packages can be provided. The SEM uses high-order nodal basis functions associated with Lagrange polynomials based on Gauss-Lobatto-Legendre (GLL) quadrature points. The FDM employs a third-order upwind-biased scheme for the vertical flux terms and a centered finite difference scheme for the vertical derivative and integral terms. For temporal integration, a time-split, third-order Runge-Kutta (RK3) integration technique was applied. The Euler equations that were used here are in flux form based on the hydrostatic pressure vertical coordinate. The equations are the same as those used in the Weather Research and Forecasting (WRF) model, but a hybrid sigma-pressure vertical coordinate was implemented in this model. We validated the model by conducting the widely used standard tests: linear hydrostatic mountain wave, tracer advection, and gravity wave over the Schär-type mountain, as well as density current, inertia-gravity wave, and rising thermal bubble. The results from these tests demonstrated that the model using the horizontal SEM and the vertical FDM is accurate and robust provided sufficient diffusion is applied. The results with various horizontal resolutions also showed convergence of second-order accuracy due to the accuracy of the time integration scheme and that of the vertical direction, although high-order basis functions were used in the horizontal. By using the 2-D slice model, we effectively showed that the combined spatial
NASA Astrophysics Data System (ADS)
Nissen-Meyer, Tarje; Fournier, Alexandre; Dahlen, F. A.
2008-09-01
We portray a dedicated spectral-element method to solve the elastodynamic wave equation upon spherically symmetric earth models at the expense of a 2-D domain. Using this method, 3-D wavefields of arbitrary resolution may be computed to obtain Fréchet sensitivity kernels, especially for diffracted arrivals. The meshing process is presented for varying frequencies in terms of its efficiency as measured by the total number of elements, their spacing variations and stability criteria. We assess the mesh quantitatively by defining these numerical parameters in a general non-dimensionalized form such that comparisons to other grid-based methods are straightforward. Efficient-mesh generation for the PREM example and a minimum-messaging domain decomposition and parallelization strategy lay foundations for waveforms up to frequencies of 1 Hz on moderate PC clusters. The discretization of fluid, solid and respective boundary regions is similar to previous spectral-element implementations, save for a fluid potential formulation that incorporates the density, thereby yielding identical boundary terms on fluid and solid sides. We compare the second-order Newmark time extrapolation scheme with a newly implemented fourth-order symplectic scheme and argue in favour of the latter in cases of propagation over many wavelengths due to drastic accuracy improvements. Various validation examples such as full moment-tensor seismograms, wavefield snapshots, and energy conservation illustrate the favourable behaviour and potential of the method.
A design study for the addition of higher order parametric discrete elements to NASTRAN
NASA Technical Reports Server (NTRS)
Stanton, E. L.
1972-01-01
The addition of discrete elements to NASTRAN poses significant interface problems with the level 15.1 assembly modules and geometry modules. Potential problems in designing new modules for higher-order parametric discrete elements are reviewed in both areas. An assembly procedure is suggested that separates grid point degrees of freedom on the basis of admissibility. New geometric input data are described that facilitate the definition of surfaces in parametric space.
A Review of Discrete Element Method (DEM) Particle Shapes and Size Distributions for Lunar Soil
NASA Technical Reports Server (NTRS)
Lane, John E.; Metzger, Philip T.; Wilkinson, R. Allen
2010-01-01
As part of ongoing efforts to develop models of lunar soil mechanics, this report reviews two topics that are important to discrete element method (DEM) modeling the behavior of soils (such as lunar soils): (1) methods of modeling particle shapes and (2) analytical representations of particle size distribution. The choice of particle shape complexity is driven primarily by opposing tradeoffs with total number of particles, computer memory, and total simulation computer processing time. The choice is also dependent on available DEM software capabilities. For example, PFC2D/PFC3D and EDEM support clustering of spheres; MIMES incorporates superquadric particle shapes; and BLOKS3D provides polyhedra shapes. Most commercial and custom DEM software supports some type of complex particle shape beyond the standard sphere. Convex polyhedra, clusters of spheres and single parametric particle shapes such as the ellipsoid, polyellipsoid, and superquadric, are all motivated by the desire to introduce asymmetry into the particle shape, as well as edges and corners, in order to better simulate actual granular particle shapes and behavior. An empirical particle size distribution (PSD) formula is shown to fit desert sand data from Bagnold. Particle size data of JSC-1a obtained from a fine particle analyzer at the NASA Kennedy Space Center is also fitted to a similar empirical PSD function.
Coupling finite and boundary element methods for 2-D elasticity problems
NASA Technical Reports Server (NTRS)
Krishnamurthy, T.; Raju, I. S.; Sistla, R.
1993-01-01
A finite element-boundary element (FE-BE) coupling method for two-dimensional elasticity problems is developed based on a weighted residual variational method in which a portion of the domain of interest is modeled by FEs and the remainder of the region by BEs. The performance of the FE-BE coupling method is demonstrated via applications to a simple 'patch test' problem and three-crack problems. The method passed the patch tests for various modeling configurations and yielded accurate strain energy release rates for the crack problems studied.
TOPAZ - a finite element heat conduction code for analyzing 2-D solids
Shapiro, A.B.
1984-03-01
TOPAZ is a two-dimensional implicit finite element computer code for heat conduction analysis. This report provides a user's manual for TOPAZ and a description of the numerical algorithms used. Sample problems with analytical solutions are presented. TOPAZ has been implemented on the CRAY and VAX computers.
2-D Time-Dependent Fuel Element, Thermal Analysis Code System.
Energy Science and Technology Software Center (ESTSC)
2001-09-24
Version 00 WREM-TOODEE2 is a two dimensional, time-dependent, fuel-element thermal analysis program. Its primary purpose is to evaluate fuel-element thermal response during post-LOCA refill and reflood in a pressurized water reactor (PWR). TOODEE2 calculations are carried out in a two-dimensional mesh region defined in slab or cylindrical geometry by orthogonal grid lines. Coordinates which form order pairs are labeled x-y in slab geometry, and those in cylindrical geometry are labeled r-z for the axisymmetric casemore » and r-theta for the polar case. Conduction and radiation are the only heat transfer mechanisms assumed within the boundaries of the mesh region. Convective and boiling heat transfer mechanisms are assumed at the boundaries. The program numerically solves the two-dimensional, time-dependent, heat conduction equation within the mesh region. KEYWORDS: FUEL MANAGEMENT; HEAT TRANSFER; LOCA; PWR« less
Flow transition with 2-D roughness elements in a 3-D channel
NASA Technical Reports Server (NTRS)
Liu, Zhining; Liu, Chaoquin; Mccormick, Stephen F.
1993-01-01
We develop a new numerical approach to study the spatially evolving instability of the streamwise dominant flow in the presence of roughness elements. The difficulty in handling the flow over the boundary surface with general geometry is removed by using a new conservative form of the governing equations and an analytical mapping. The numerical scheme uses second-order backward Euler in time, fourth-order central differences in all three spatial directions, and boundary-fitted staggered grids. A three-dimensional channel with multiple two-dimensional-type roughness elements is employed as the test case. Fourier analysis is used to decompose different Fourier modes of the disturbance. The results show that surface roughness leads to transition at lower Reynolds number than for smooth channels.
Finite element nonlinear flutter and fatigue life of 2-D panels with temperature effects
NASA Technical Reports Server (NTRS)
Mei, Chuh; Xue, David Y.
1991-01-01
A frequency domain method for two-dimensional nonlinear panel flutter with thermal effects obtained from a consistent finite element formulation is presented. The von Karman nonlinear strain-displacement relation is used to account for large deflections, and the quasi-steady first-order piston theory is employed for aerodynamic loading. The finite element frequency domain results are compared with analytical time domain solutions. In a limit-cycle motion, the panel frequency and stress can be determined, thus fatigue life can be predicted. The influence of temperature and dynamic pressure on panel fatigue life is presented. An endurance dynamic pressure can be established at a given temperature from the present method.
Using Multithreading for the Automatic Load Balancing of 2D Adaptive Finite Element Meshes
NASA Technical Reports Server (NTRS)
Heber, Gerd; Biswas, Rupak; Thulasiraman, Parimala; Gao, Guang R.; Bailey, David H. (Technical Monitor)
1998-01-01
In this paper, we present a multi-threaded approach for the automatic load balancing of adaptive finite element (FE) meshes. The platform of our choice is the EARTH multi-threaded system which offers sufficient capabilities to tackle this problem. We implement the question phase of FE applications on triangular meshes, and exploit the EARTH token mechanism to automatically balance the resulting irregular and highly nonuniform workload. We discuss the results of our experiments on EARTH-SP2, an implementation of EARTH on the IBM SP2, with different load balancing strategies that are built into the runtime system.
Using Multi-threading for the Automatic Load Balancing of 2D Adaptive Finite Element Meshes
NASA Technical Reports Server (NTRS)
Heber, Gerd; Biswas, Rupak; Thulasiraman, Parimala; Gao, Guang R.; Saini, Subhash (Technical Monitor)
1998-01-01
In this paper, we present a multi-threaded approach for the automatic load balancing of adaptive finite element (FE) meshes The platform of our choice is the EARTH multi-threaded system which offers sufficient capabilities to tackle this problem. We implement the adaption phase of FE applications oil triangular meshes and exploit the EARTH token mechanism to automatically balance the resulting irregular and highly nonuniform workload. We discuss the results of our experiments oil EARTH-SP2, on implementation of EARTH on the IBM SP2 with different load balancing strategies that are built into the runtime system.
A simple adaptive mesh generator for 2-D finite element calculations
Fernandez, F.A.; Yong, Y.C.; Ettinger, R.D. )
1993-03-01
A strategy for adaptive mesh generation is proposed. The method consists of the use of a suitably defined density function', which can either be defined by the user or be calculated from a previous approximate solution, to guide the generation of a new mesh. This new mesh is built starting from a minimal number of triangular elements which are then in several sweeps, repeatedly refined according to the density function. The Delaunay algorithm is used in each stage to keep the shape of the triangles as equilateral as possible.
ZONE - a finite element mesh generator. [2-D, for CDC 7600
Burger, M.J.
1980-03-12
The ZONE computer program is a finite element mesh generator that produces the nodes and element description of any two-dimensional geometry. The geometry is subdivided into a mesh of quadrilateral and triangular zones arranged sequentially in an ordered march through the geometry. The order of march can be chosen so that the minimum bandwidth is obtained. The node points are defined in terms of the x and y coordinates in a global rectangular coordinate system. The zones generated are quadrilaterals or triangles defined by four node points in a counterclockwise sequence. Node points defining the outside boundary are generated for slide lines and to describe pressure boundary conditions. The mesh that is generated can be used as input to any two dimensional as well as any axisymmetrical structure program. The following points are taken up: program concept and characteristics; regions; layers; meridians (offset, circular arc, ellipse); rays; common characterstics - rays and meridians, ZONE input description; output files; examples; and program availability. Also generated is the input to the program PLOT. 15 figures. (RWR)
Mixed-RKDG Finite Element Methods for the 2-D Hydrodynamic Model for Semiconductor Device Simulation
Chen, Zhangxin; Cockburn, Bernardo; Jerome, Joseph W.; Shu, Chi-Wang
1995-01-01
In this paper we introduce a new method for numerically solving the equations of the hydrodynamic model for semiconductor devices in two space dimensions. The method combines a standard mixed finite element method, used to obtain directly an approximation to the electric field, with the so-called Runge-Kutta Discontinuous Galerkin (RKDG) method, originally devised for numerically solving multi-dimensional hyperbolic systems of conservation laws, which is applied here to the convective part of the equations. Numerical simulations showing the performance of the new method are displayed, and the results compared with those obtained by using Essentially Nonoscillatory (ENO) finite difference schemes. Frommore » the perspective of device modeling, these methods are robust, since they are capable of encompassing broad parameter ranges, including those for which shock formation is possible. The simulations presented here are for Gallium Arsenide at room temperature, but we have tested them much more generally with considerable success.« less
Stress analysis of a rectangular implant in laminated composites using 2-D and 3-D finite elements
NASA Technical Reports Server (NTRS)
Chow, Wai T.; Graves, Michael J.
1992-01-01
An analysis method using the FEM based on the Hellinger-Reissner variation principle has been developed to determine the 3-D stresses and displacements near a rectangular implant inside a laminated composite material. Three-dimensional elements are employed in regions where the interlaminar stress is considered to be significant; 2-D elements are used in other areas. Uniaxially loaded graphite-epoxy laminates have been analyzed; the implant was modeled as four plies of 3501/6 epoxy located in the middle of the laminate. It is shown that the interlaminar stresses are an order of magnitude lower than the stress representing the applied far-field load. The stress concentration factors of both the interlaminar and in-plane stresses depend on the stacking sequence of the laminate.
A simple discrete-element-model of Brazilian test
NASA Astrophysics Data System (ADS)
Kundu, Sumanta; Stroisz, Anna; Pradhan, Srutarshi
2016-05-01
We present a statistical model which is able to capture some interesting features exhibited in the Brazilian test of rock samples. The model is based on elements which break irreversibly when the force experienced by the elements exceed their own load capacity. If an element breaks the load capacity of the neighboring elements are decreased by a certain amount, assuming weakening effect around the defected zone. From the model we numerically investigate the stress-strain behavior, the strength of the system, how it scales with the system size and also its fluctuation for both uniform and Weibull distribution of breaking thresholds in the system. To check the validity of our statistical model we perform few Brazilian tests on Sandstone and Chalk samples. The stress-strain curve from model results agree qualitatively well with the lab-test data. Also, the damage profile right at the point when the stress-strain curve reaches its maximum is seen to mimic the crack patterns observed in our Brazilian test experiments.
Partition of the contact force network obtained in discrete element simulations of element tests
NASA Astrophysics Data System (ADS)
Huang, Xin; O'Sullivan, Catherine; Hanley, Kevin J.; Kwok, Chung-Yee
2016-01-01
The transmission of stress within a granular material composed of rigid spheres is explored using the discrete element method. The contribution of contacts to both deviatoric stress and structural anisotropy is investigated. The influences of five factors are considered: inter-particle friction coefficient, loading regime, packing density, contact model, and boundary conditions. The data generated indicate that using the above-average normal contact force criterion to decompose the contact force network into two subsets with distinct contributions to stress transmission and structural anisotropy is not robust. The characteristic normal contact forces marking the transition from negative to positive contribution to the overall deviatoric stress and structural anisotropy are not unique values but vary during shearing. Once the critical state is attained (i.e., once shearing continues at a constant deviator stress and solid fraction), the characteristic normal contact force remains approximately constant and this critical state characteristic normal force is observed to decrease with increasing inter-particle friction. The characteristic normal contact force considering the contribution to deviatoric stress has a power-law relationship with the mean effective stress at the critical state.
The Combined Finite-Discrete Element Method applied to the Study of Rock Fracturing Behavior in 3D
Rougier, Esteban; Bradley, Christopher R.; Broom, Scott T.; Knight, Earl E.; Munjiza, Ante; Sussman, Aviva J.; Swift, Robert P.
2011-01-01
Since its introduction the combined finite-discrete element method (FEM/DEM), has become an excellent tool to address a wide range of problems involving fracturing and fragmentation of solids. Within the context of rock mechanics, the FEM/DEM method has been applied to many complex industrial problems such as block caving, deep mining techniques, rock blasting, seismic waves, packing problems, rock crushing problems, etc. In the real world most of the problems involving fracture and fragmentation of solids are three dimensional problems. With the aim of addressing these problems an improved 2D/3D FEM/DEM capability has been developed at Los Alamos National Laboratory (LANL). These capabilities include state of the art 3D contact detection, contact interaction, constitutive material models, and fracture models. In this paper, Split Hopkinson Pressure Bar (SHPB) Brazilian experiments are simulated using this improved 2D/3D FEM/DEM approach which is implemented in LANL's MUNROU (Munjiza-Rougier) code. The results presented in this work show excellent agreement with both the SHPB experiments and previous 2D numerical simulations performed by other FEM/DEM research groups.
Accurate 2d finite element calculations for hydrogen in magnetic fields of arbitrary strength
NASA Astrophysics Data System (ADS)
Schimeczek, C.; Wunner, G.
2014-02-01
Recent observations of hundreds of hydrogen-rich magnetic white dwarf stars with magnetic fields up to 105 T (103 MG) have called for more comprehensive and accurate databases for wavelengths and oscillator strengths of the H atom in strong magnetic fields for all states evolving from the field-free levels with principal quantum numbers n≤10. We present a code to calculate the energy eigenvalues and wave functions of such states which is capable of covering the entire regime of field strengths B=0 T to B˜109 T. We achieve this high flexibility by using a two-dimensional finite element expansion of the wave functions in terms of B-splines in the directions parallel and perpendicular to the magnetic field, instead of using asymptotically valid basis expansions in terms of spherical harmonics or Landau orbitals. We have paid special attention to the automation of the program such that the data points for the magnetic field strengths at which the energy of a given state are calculated can be selected automatically. Furthermore, an elaborate method for varying the basis parameters is applied to ensure that the results reach a pre-selected precision, which also can be adjusted freely. Energies and wave functions are stored in a convenient format for further analysis, e.g. for the calculation of transition energies and oscillator strengths. The code has been tested to work for 300 states with an accuracy of better than 10-6 Rydberg across several symmetry subspaces over the entire regime of magnetic field strengths.
Discrete element thermomechanical modelling of rock cutting with valuation of tool wear
NASA Astrophysics Data System (ADS)
Rojek, Jerzy
2014-05-01
The paper presents a thermomechanical discrete element model of rock cutting process. The thermomechanical formulation of the discrete element method considers mechanical and thermal phenomena and their reciprocal influence. The thermal model developed for transient heat conduction problems takes into account conductive heat transfer at the contact between particles and convection on the free surface. The thermal and mechanical problems are coupled by consideration of: (1) heat generated due to friction which is calculated in the mechanical problem and passed to the thermal solution, (2) influence of thermal expansion on mechanical interaction between particles. Estimation of temperature dependent wear has been included into the contact model. The coupled problem is solved using the staggered scheme.The thermomechanical algorithm has been implemented in a discrete element program and applied to simulation of rock cutting with single pick of a dredge cutter head. Numerical results confirm good performance of the developed algorithm.
Numerical integration techniques for curved-element discretizations of molecule-solvent interfaces.
Bardhan, Jaydeep P; Altman, Michael D; Willis, David J; Lippow, Shaun M; Tidor, Bruce; White, Jacob K
2007-07-01
Surface formulations of biophysical modeling problems offer attractive theoretical and computational properties. Numerical simulations based on these formulations usually begin with discretization of the surface under consideration; often, the surface is curved, possessing complicated structure and possibly singularities. Numerical simulations commonly are based on approximate, rather than exact, discretizations of these surfaces. To assess the strength of the dependence of simulation accuracy on the fidelity of surface representation, here methods were developed to model several important surface formulations using exact surface discretizations. Following and refining Zauhar's work [J. Comput.-Aided Mol. Des. 9, 149 (1995)], two classes of curved elements were defined that can exactly discretize the van der Waals, solvent-accessible, and solvent-excluded (molecular) surfaces. Numerical integration techniques are presented that can accurately evaluate nonsingular and singular integrals over these curved surfaces. After validating the exactness of the surface discretizations and demonstrating the correctness of the presented integration methods, a set of calculations are presented that compare the accuracy of approximate, planar-triangle-based discretizations and exact, curved-element-based simulations of surface-generalized-Born (sGB), surface-continuum van der Waals (scvdW), and boundary-element method (BEM) electrostatics problems. Results demonstrate that continuum electrostatic calculations with BEM using curved elements, piecewise-constant basis functions, and centroid collocation are nearly ten times more accurate than planar-triangle BEM for basis sets of comparable size. The sGB and scvdW calculations give exceptional accuracy even for the coarsest obtainable discretized surfaces. The extra accuracy is attributed to the exact representation of the solute-solvent interface; in contrast, commonly used planar-triangle discretizations can only offer improved
Level set discrete element method for three-dimensional computations with triaxial case study
NASA Astrophysics Data System (ADS)
Kawamoto, Reid; Andò, Edward; Viggiani, Gioacchino; Andrade, José E.
2016-06-01
In this paper, we outline the level set discrete element method (LS-DEM) which is a discrete element method variant able to simulate systems of particles with arbitrary shape using level set functions as a geometric basis. This unique formulation allows seamless interfacing with level set-based characterization methods as well as computational ease in contact calculations. We then apply LS-DEM to simulate two virtual triaxial specimens generated from XRCT images of experiments and demonstrate LS-DEM's ability to quantitatively capture and predict stress-strain and volume-strain behavior observed in the experiments.
Quasi-Optimal Schwarz Methods for the Conforming Spectral Element Discretization
NASA Technical Reports Server (NTRS)
Casarin, Mario
1996-01-01
Fast methods are proposed for solving the system K(sub N)x = b resulting from the discretization of self-adjoint elliptic equations in three dimensional domains by the spectral element method. The domain is decomposed into hexahedral elements, and in each of these elements the discretization space is formed by polynomials of degree N in each variable. Gauss-Lobatto-Legendre (GLL) quadrature rules replace the integrals in the Galerkin formulation. This system is solved by the preconditioned conjugate gradients method. The conforming finite element space on the GLL mesh consisting of piecewise Q(sub 1) elements produces a stiffness matrix K(sub h) that is spectrally equivalent to the spectral element stiffness matrix K(sub N). The action of the inverse of K(sub h) is expensive for large problems, and is therefore replaced by a Schwarz preconditioner B(sub h) of this finite element stiffness matrix. The preconditioned operator then becomes B(sub h)(exp -l)K(sub N). The technical difficulties stem from the nonregularity of the mesh. Tools to estimate the convergence of a large class of new iterative substructuring and overlapping Schwarz preconditioners are developed. This technique also provides a new analysis for an iterative substructuring method proposed by Pavarino and Widlund for the spectral element discretization.
NASA Astrophysics Data System (ADS)
Noji, H.
This study investigates the losses in a two conducting-layer REBCO cable fabricated by researchers at Furukawa Electric Co. Ltd. The losses were calculated using a combination of my electric circuit (EC) model with a two-dimensional finite element method (2D FEM). The helical pitches of the tapes in each layer, P1 and P2, were adjusted to equalize the current in both cable layers, although the loss calculation assumed infinite helical pitches and the same current in each layer at first. The results showed that the losses depended on the relative tape-position angle between the layers (θ/θ'), because the vertical field between adjacent tapes in the same layer varied with θ/θ'. When simulating the real cable, the helical pitches were adjusted and the layer currents were calculated by the EC model. These currents were input to the 2D FEM to compute the losses. The losses changed along the cable length because the difference between P1 and P2 altered the θ/θ' along this direction. The average angle-dependent and position-dependent losses were equal and closely approximated the measured losses. As an example to reduce the loss in this cable, the angle and the helical pitches were fixed at θ/θ' = 0.5 and P1 = P2 = 100 mm (S-direction). The calculation with these conditions indicated that the loss is about one order of magnitude lower than the measurement.
Ye, Xingwei; Zhang, Fangzheng; Pan, Shilong
2016-09-01
A hardware-compressive optical true time delay architecture for 2D beam steering in a planar phased array antenna is proposed using fiber-Bragg-grating-based tunable dispersive elements (TDEs). For an M×N array, the proposed system utilizes N TDEs and M wavelength-fixed optical carriers to control the time delays. Both azimuth and elevation beam steering are realized by programming the settings of the TDEs. An experiment is carried out to demonstrate the delay controlling in a 2×2 array, which is fed by a wideband pulsed signal. Radiation patterns calculated from the experimentally measured waveforms at the four antennas match well with the theoretical results. PMID:27607946
Siku: A Sea Ice Discrete Element Method Model on a Spherical Earth
NASA Astrophysics Data System (ADS)
Kulchitsky, A. V.; Hutchings, J. K.; Johnson, J.
2014-12-01
Offshore oil and gas exploration and production activities in the Beaufort and Chukchi Seas can be significantly and adversely affected by sea ice. In the event of an oil spill, sea ice complicates the tracking of ice/oil trajectories and can hinder cleanup operations. There is a need for a sea ice dynamics model that can accurately simulate ice pack deformation and failure to improve the ability to track ice/oil trajectories and support oil response operations. A discrete element method (DEM) model, where each ice floe is represented by discrete elements that are initially bonded (frozen) together will be used to address the difficulty continuum modeling approaches have with representing discrete phenomena in sea ice, such as the formation of leads and ridges. Each discrete element in the DEM is a rigid body driven by environmental forcing (wind, current and Coriolis forces) and interaction forces with other discrete elements (compression, shear, tension, bond rupture and regrowth). We introduce a new DEM model ``Siku'', currently under development, to simulate ice drift of an ice floe on a spherical Earth. We will present initial free-drift results. Siku is focused on improving sea ice interaction mechanics and providing an accurate geometrical representation needed for basin scale and regional simulations. Upon completion, Siku will be an open source GNU GPL licensed user friendly program with embedded python capability for setting up simulations "scenarios" and coupling with other models to provide forcing fields. We use a unique quaternion representation for position and orientation of polygon sea-ice elements that use a second order integration scheme of sea-ice element motion on the Earth's sphere that does not depend on the location of the element and, hence, avoids numerical problems near the pole.
The dual variable method for finite element discretizations of Navier/Stokes equations
NASA Astrophysics Data System (ADS)
Hall, C. A.; Peterson, J. S.; Porsching, T. A.; Sledge, F. R.
1985-05-01
The dual-variable method of Amit et al. (1981) and Hall et al. (1980) is applied to the numerical solution of the transient Navier-Stokes equations for two-dimensional incompressible flows. The basic procedures of the method are reviewed, including determining the rank of the discrete divergence matrix, obtaining a particular solution of the discrete continuity equation, and defining the null space of the discrete divergence operator. Finite-element algorithms based on quadrilateral piecewise-bilateral-velocity/constant-pressure elements are developed and demonstrated for Poiseuille flow, a lid-driven cavity, and flow past a semicircular obstacle. The results are presented in tables and graphs and compared with those of a primitive-variable method, and the dual-variable approach is found to yield significant savings in dynamic memory and computation time.
Applications of discrete element method in modeling of grain postharvest operations
Technology Transfer Automated Retrieval System (TEKTRAN)
Grain kernels are finite and discrete materials. Although flowing grain can behave like a continuum fluid at times, the discontinuous behavior exhibited by grain kernels cannot be simulated solely with conventional continuum-based computer modeling such as finite-element or finite-difference methods...
Particle models for discrete element modeling of bulk grain properties of wheat kernels
Technology Transfer Automated Retrieval System (TEKTRAN)
Recent research has shown the potential of discrete element method (DEM) in simulating grain flow in bulk handling systems. Research has also revealed that simulation of grain flow with DEM requires establishment of appropriate particle models for each grain type. This research completes the three-p...
NASA Astrophysics Data System (ADS)
Schaa, R.; Gross, L.; du Plessis, J.
2016-04-01
We present a general finite-element solver, escript, tailored to solve geophysical forward and inverse modeling problems in terms of partial differential equations (PDEs) with suitable boundary conditions. Escript’s abstract interface allows geoscientists to focus on solving the actual problem without being experts in numerical modeling. General-purpose finite element solvers have found wide use especially in engineering fields and find increasing application in the geophysical disciplines as these offer a single interface to tackle different geophysical problems. These solvers are useful for data interpretation and for research, but can also be a useful tool in educational settings. This paper serves as an introduction into PDE-based modeling with escript where we demonstrate in detail how escript is used to solve two different forward modeling problems from applied geophysics (3D DC resistivity and 2D magnetotellurics). Based on these two different cases, other geophysical modeling work can easily be realized. The escript package is implemented as a Python library and allows the solution of coupled, linear or non-linear, time-dependent PDEs. Parallel execution for both shared and distributed memory architectures is supported and can be used without modifications to the scripts.
NASA Astrophysics Data System (ADS)
Zhang, Qi-Hua
2015-10-01
Finite element generation of complicated fracture networks is the core issue and source of technical difficulty in three-dimensional (3-D) discrete fracture network (DFN) flow models. Due to the randomness and uncertainty in the configuration of a DFN, the intersection lines (traces) are arbitrarily distributed in each face (fracture and other surfaces). Hence, subdivision of the fractures is an issue relating to subdivision of two-dimensional (2-D) domains with arbitrarily-distributed constraints. When the DFN configuration is very complicated, the well-known approaches (e.g. Voronoi Delaunay-based methods and advancing-front techniques) cannot operate properly. This paper proposes an algorithm to implement end-to-end connection between traces to subdivide 2-D domains into closed loops. The compositions of the vertices in the common edges between adjacent loops (which may belong to a single fracture or two connected fractures) are thus ensured to be topologically identical. The paper then proposes an approach for triangulating arbitrary loops which does not add any nodes to ensure consistency of the meshes at the common edges. In addition, several techniques relating to tolerance control and improving code robustness are discussed. Finally, the equivalent permeability of the rock mass is calculated for some very complicated DFNs (the DFN may contain 1272 fractures, 633 connected fractures, and 16,270 closed loops). The results are compared with other approaches to demonstrate the veracity and efficiency of the approach proposed in this paper.
Application of an enhanced discrete element method to oil and gas drilling processes
NASA Astrophysics Data System (ADS)
Ubach, Pere Andreu; Arrufat, Ferran; Ring, Lev; Gandikota, Raju; Zárate, Francisco; Oñate, Eugenio
2016-03-01
The authors present results on the use of the discrete element method (DEM) for the simulation of drilling processes typical in the oil and gas exploration industry. The numerical method uses advanced DEM techniques using a local definition of the DEM parameters and combined FEM-DEM procedures. This paper presents a step-by-step procedure to build a DEM model for analysis of the soil region coupled to a FEM model for discretizing the drilling tool that reproduces the drilling mechanics of a particular drill bit. A parametric study has been performed to determine the model parameters in order to maintain accurate solutions with reduced computational cost.
Comparative Results from a CFD Challenge Over a 2D Three-Element High-Lift Airfoil
NASA Technical Reports Server (NTRS)
Klausmeyer, Steven M.; Lin, John C.
1997-01-01
A high-lift workshop was held in May of 1993 at NASA Langley Research Center. A major part of the workshop centered on a blind test of various computational fluid dynamics (CFD) methods in which the flow about a two- dimensional (2D) three-element airfoil was computed without prior knowledge of the experimental data. The results of this 'blind' test revealed: (1) The Reynolds Averaged Navier-Stokes (RANS) methods generally showed less variability among codes than did potential/Euler solvers coupled with boundary-layer solution techniques. However, some of the coupled methods still provided excellent predictions. (2) Drag prediction using coupled methods agreed more closely with experiment than the RANS methods. Lift was more accurately predicted than drag for both methods. (3) The CFD methods did well in predicting lift and drag changes due to changes in Reynolds number, however, they did not perform as well when predicting lift and drag increments due to changing flap gap, (4) Pressures and skin friction compared favorably with experiment for most of the codes. (5) There was a large variability in most of the velocity profile predictions. Computational results predict a stronger siat wake than measured suggesting a missing component in turbulence modeling, perhaps curvature effects.
Anssari-Benam, Afshin; Bucchi, Andrea; Bader, Dan L
2015-09-18
Discrete element models have often been the primary tool in investigating and characterising the viscoelastic behaviour of soft tissues. However, studies have employed varied configurations of these models, based on the choice of the number of elements and the utilised formation, for different subject tissues. This approach has yielded a diverse array of viscoelastic models in the literature, each seemingly resulting in different descriptions of viscoelastic constitutive behaviour and/or stress-relaxation and creep functions. Moreover, most studies do not apply a single discrete element model to characterise both stress-relaxation and creep behaviours of tissues. The underlying assumption for this disparity is the implicit perception that the viscoelasticity of soft tissues cannot be described by a universal behaviour or law, resulting in the lack of a unified approach in the literature based on discrete element representations. This paper derives the constitutive equation for different viscoelastic models applicable to soft tissues with two characteristic times. It demonstrates that all possible configurations exhibit a unified and universal behaviour, captured by a single constitutive relationship between stress, strain and time as: σ+Aσ̇+Bσ¨=Pε̇+Qε¨. The ensuing stress-relaxation G(t) and creep J(t) functions are also unified and universal, derived as [Formula: see text] and J(t)=c2+(ε0-c2)e(-PQt)+σ0Pt, respectively. Application of these relationships to experimental data is illustrated for various tissues including the aortic valve, ligament and cerebral artery. The unified model presented in this paper may be applied to all tissues with two characteristic times, obviating the need for employing varied configurations of discrete element models in preliminary investigation of the viscoelastic behaviour of soft tissues. PMID:26232814
NASA Technical Reports Server (NTRS)
Hua, Chongyu; Volakis, John L.
1990-01-01
AUTOMESH-2D is a computer program specifically designed as a preprocessor for the scattering analysis of two dimensional bodies by the finite element method. This program was developed due to a need for reproducing the effort required to define and check the geometry data, element topology, and material properties. There are six modules in the program: (1) Parameter Specification; (2) Data Input; (3) Node Generation; (4) Element Generation; (5) Mesh Smoothing; and (5) Data File Generation.
NASA Astrophysics Data System (ADS)
Batoz, Jean-Louis; Katili, Irwan
1992-11-01
In this paper the formulation of a new triangular element based on the Reissner/Mindlin plate theory is presented. The element has three nodes and three d.o.f. per node only. It is based on constant bending modes plus incompatible energy orthogonal higher order bending modes. The transverse shear effects are represented using the moment equilibrium and the constitutive equations. Discrete (collocation) shear constraints are considered on each side to relate the kinematical and the independent shear strains. The element has a proper rank, is completely locking free, passes all constant patch-tests exactly. The detailed numerical evaluation shows that the element, called DST-BK, is a robust and high-performance element for thick and thin plates.
Application of the control volume mixed finite element method to a triangular discretization
Naff, R.L.
2012-01-01
A two-dimensional control volume mixed finite element method is applied to the elliptic equation. Discretization of the computational domain is based in triangular elements. Shape functions and test functions are formulated on the basis of an equilateral reference triangle with unit edges. A pressure support based on the linear interpolation of elemental edge pressures is used in this formulation. Comparisons are made between results from the standard mixed finite element method and this control volume mixed finite element method. Published 2011. This article is a US Government work and is in the public domain in the USA. ?? 2012 John Wiley & Sons, Ltd. This article is a US Government work and is in the public domain in the USA.
3-D and quasi-2-D discrete element modeling of grain commingling in a bucket elevator boot system
Technology Transfer Automated Retrieval System (TEKTRAN)
Unwanted grain commingling impedes new quality-based grain handling systems and has proven to be an expensive and time consuming issue to study experimentally. Experimentally validated models may reduce the time and expense of studying grain commingling while providing additional insight into detail...
NASA Astrophysics Data System (ADS)
Wang, Yajun; Liu, Yang; Li, Hong; Wang, Jinfeng
2016-03-01
In this article, a Galerkin finite element method combined with second-order time discrete scheme for finding the numerical solution of nonlinear time fractional Cable equation is studied and discussed. At time t_{k-α/2} , a second-order two step scheme with α -parameter is proposed to approximate the first-order derivative, and a weighted discrete scheme covering second-order approximation is used to approximate the Riemann-Liouville fractional derivative, where the approximate order is higher than the obtained results by the L1-approximation with order (2-α in the existing references. For the spatial direction, Galerkin finite element approximation is presented. The stability of scheme and the rate of convergence in L^2 -norm with O(Δ t^2+(1+Δ t^{-α})h^{m+1}) are derived in detail. Moreover, some numerical tests are shown to support our theoretical results.
Load identification for a rolling disc: finite element discretization and virtual calibration
NASA Astrophysics Data System (ADS)
Ronasi, Hamed; Johansson, Håkan; Larsson, Fredrik
2012-02-01
For many applications, direct measurement of forces in mechanical systems is difficult or even impossible, and indirect measurement involving inverse analysis must be adopted. One such application of major industrial relevance is the accurate measurement of contact forces acting on rolling bodies. In this paper, a newly proposed strategy for load identification is applied to an example problem of a rolling disc. Based on strain gauge measurements, the contact force is estimated using finite element analysis. A virtual calibration procedure is introduced in order to reduce the dependency of the results on the spatial discretization. In particular, the sensitivity of the results with respect to finite element discretization, sensor placement and noise is discussed. Numerical results based on synthetic data illustrate the behavior and accuracy of the proposed strategy.
Evaluation of discretization procedures for transition elements in adaptive mesh refinement
NASA Technical Reports Server (NTRS)
Park, K. C.; Levit, Itzak; Stanley, Gary M.
1991-01-01
Three transition interpolation schemes for use in h-or r-refinement have been analyzed in terms of accuracy, implementation ease and extendability. They include blending-function interpolation, displacement averaging, and strain matching at discrete points along the transition edge lines. The results suggest that the choice of matching depends strongly on the element formulations, (viz. displacement or assumed strain, etc.) and mesh refinement criteria employed, and to a lesser extent the choice of computer architecture (serial vs. parallel) and the equation solution procedures. A recommended pairing of some of the elements with the choice factors is suggested.
NASA Astrophysics Data System (ADS)
Korneev, V. G.
2012-09-01
BPS is a well known an efficient and rather general domain decomposition Dirichlet-Dirichlet type preconditioner, suggested in the famous series of papers Bramble, Pasciak and Schatz (1986-1989). Since then, it has been serving as the origin for the whole family of domain decomposition Dirichlet-Dirichlet type preconditioners-solvers as for h so hp discretizations of elliptic problems. For its original version, designed for h discretizations, the named authors proved the bound O(1 + log2 H/ h) for the relative condition number under some restricting conditions on the domain decomposition and finite element discretization. Here H/ h is the maximal relation of the characteristic size H of a decomposition subdomain to the mesh parameter h of its discretization. It was assumed that subdomains are images of the reference unite cube by trilinear mappings. Later similar bounds related to h discretizations were proved for more general domain decompositions, defined by means of coarse tetrahedral meshes. These results, accompanied by the development of some special tools of analysis aimed at such type of decompositions, were summarized in the book of Toselli and Widlund (2005). This paper is also confined to h discretizations. We further expand the range of admissible domain decompositions for constructing BPS preconditioners, in which decomposition subdomains can be convex polyhedrons, satisfying some conditions of shape regularity. We prove the bound for the relative condition number with the same dependence on H/ h as in the bound given above. Along the way to this result, we simplify the proof of the so called abstract bound for the relative condition number of the domain decomposition preconditioner. In the part, related to the analysis of the interface sub-problem preconditioning, our technical tools are generalization of those used by Bramble, Pasciak and Schatz.
Comparison of 3-D finite element model of ashlar masonry with 2-D numerical models of ashlar masonry
NASA Astrophysics Data System (ADS)
Beran, Pavel
2016-06-01
3-D state of stress in heterogeneous ashlar masonry can be also computed by several suitable chosen 2-D numerical models of ashlar masonry. The results obtained from 2-D numerical models well correspond to the results obtained from 3-D numerical model. The character of thermal stress is the same. While using 2-D models the computational time is reduced more than hundredfold and therefore this method could be used for computation of thermal stresses during long time periods with 10 000 of steps.
Mesoscale dynamic coupling of finite- and discrete-element methods for fluid-particle interactions.
Srivastava, S; Yazdchi, K; Luding, S
2014-08-01
A new method for two-way fluid-particle coupling on an unstructured mesoscopically coarse mesh is presented. In this approach, we combine a (higher order) finite-element method (FEM) on the moving mesh for the fluid with a soft sphere discrete-element method for the particles. The novel feature of the proposed scheme is that the FEM mesh is a dynamic Delaunay triangulation based on the positions of the moving particles. Thus, the mesh can be multi-purpose: it provides (i) a framework for the discretization of the Navier-Stokes equations, (ii) a simple tool for detecting contacts between moving particles, (iii) a basis for coarse-graining or upscaling, and (iv) coupling with other physical fields (temperature, electromagnetic, etc.). This approach is suitable for a wide range of dilute and dense particulate flows, because the mesh resolution adapts with particle density in a given region. Two-way momentum exchange is implemented using semi-empirical drag laws akin to other popular approaches; for example, the discrete particle method, where a finite-volume solver on a coarser, fixed grid is used. We validate the methodology with several basic test cases, including single- and double-particle settling with analytical and empirical expectations, and flow through ordered and random porous media, when compared against finely resolved FEM simulations of flow through fixed arrays of particles. PMID:24982251
NASA Astrophysics Data System (ADS)
Bürger, Raimund; Kumar, Sarvesh; Ruiz-Baier, Ricardo
2015-10-01
The sedimentation-consolidation and flow processes of a mixture of small particles dispersed in a viscous fluid at low Reynolds numbers can be described by a nonlinear transport equation for the solids concentration coupled with the Stokes problem written in terms of the mixture flow velocity and the pressure field. Here both the viscosity and the forcing term depend on the local solids concentration. A semi-discrete discontinuous finite volume element (DFVE) scheme is proposed for this model. The numerical method is constructed on a baseline finite element family of linear discontinuous elements for the approximation of velocity components and concentration field, whereas the pressure is approximated by piecewise constant elements. The unique solvability of both the nonlinear continuous problem and the semi-discrete DFVE scheme is discussed, and optimal convergence estimates in several spatial norms are derived. Properties of the model and the predicted space accuracy of the proposed formulation are illustrated by detailed numerical examples, including flows under gravity with changing direction, a secondary settling tank in an axisymmetric setting, and batch sedimentation in a tilted cylindrical vessel.
Finite Elements Analysis of a Composite Semi-Span Test Article With and Without Discrete Damage
NASA Technical Reports Server (NTRS)
Lovejoy, Andrew E.; Jegley, Dawn C. (Technical Monitor)
2000-01-01
AS&M Inc. performed finite element analysis, with and without discrete damage, of a composite semi-span test article that represents the Boeing 220-passenger transport aircraft composite semi-span test article. A NASTRAN bulk data file and drawings of the test mount fixtures and semi-span components were utilized to generate the baseline finite element model. In this model, the stringer blades are represented by shell elements, and the stringer flanges are combined with the skin. Numerous modeling modifications and discrete source damage scenarios were applied to the test article model throughout the course of the study. This report details the analysis method and results obtained from the composite semi-span study. Analyses were carried out for three load cases: Braked Roll, LOG Down-Bending and 2.5G Up-Bending. These analyses included linear and nonlinear static response, as well as linear and nonlinear buckling response. Results are presented in the form of stress and strain plots. factors of safety for failed elements, buckling loads and modes, deflection prediction tables and plots, and strainage prediction tables and plots. The collected results are presented within this report for comparison to test results.
NASA Astrophysics Data System (ADS)
Pennec, Fabienne; Alzina, Arnaud; Tessier-Doyen, Nicolas; Naitali, Benoit; Smith, David S.
2012-11-01
This work is about the calculation of thermal conductivity of insulating building materials made from plant particles. To determine the type of raw materials, the particle sizes or the volume fractions of plant and binder, a tool dedicated to calculate the thermal conductivity of heterogeneous materials has been developped, using the discrete element method to generate the volume element and the finite element method to calculate the homogenized properties. A 3D optical scanner has been used to capture plant particle shapes and convert them into a cluster of discret elements. These aggregates are initially randomly distributed but without any overlap, and then fall down in a container due to the gravity force and collide with neighbour particles according to a velocity Verlet algorithm. Once the RVE is built, the geometry is exported in the open-source Salome-Meca platform to be meshed. The calculation of the effective thermal conductivity of the heterogeneous volume is then performed using a homogenization technique, based on an energy method. To validate the numerical tool, thermal conductivity measurements have been performed on sunflower pith aggregates and on packed beds of the same particles. The experimental values have been compared satisfactorily with a batch of numerical simulations.
Discrete adaptive zone light elements (DAZLE): a new approach to adaptive imaging
NASA Astrophysics Data System (ADS)
Kellogg, Robert L.; Escuti, Michael J.
2007-09-01
New advances in Liquid Crystal Spatial Light Modulators (LCSLM) offer opportunities for large adaptive optics in the midwave infrared spectrum. A light focusing adaptive imaging system, using the zero-order diffraction state of a polarizer-free liquid crystal polarization grating modulator to create millions of high transmittance apertures, is envisioned in a system called DAZLE (Discrete Adaptive Zone Light Elements). DAZLE adaptively selects large sets of LCSLM apertures using the principles of coded masks, embodied in a hybrid Discrete Fresnel Zone Plate (DFZP) design. Issues of system architecture, including factors of LCSLM aperture pattern and adaptive control, image resolution and focal plane array (FPA) matching, and trade-offs between filter bandwidths, background photon noise, and chromatic aberration are discussed.
NASA Astrophysics Data System (ADS)
Casas, Guillermo; Mukherjee, Debanjan; Celigueta, Miguel Angel; Zohdi, Tarek I.; Onate, Eugenio
2015-11-01
A modular discrete element framework is presented for large-scale simulations of industrial grain-handling systems. Our framework enables us to simulate a markedly larger number of particles than previous studies, thereby allowing for efficient and more realistic process simulations. This is achieved by partitioning the particle dynamics into distinct regimes based on their contact interactions, and integrating them using different time-steps, while exchanging phase-space data between them. The framework is illustrated using numerical experiments based on fertilizer spreader applications. The model predictions show very good qualitative and quantitative agreement with available experimental data. Valuable insights are developed regarding the role of lift vs drag forces on the particle trajectories in-flight, and on the role of geometric discretization errors for surface meshing in governing the emergent behavior of a system of particles.
NASA Astrophysics Data System (ADS)
Crone, Joshua C.; Chung, Peter W.; Leiter, Kenneth W.; Knap, Jaroslaw; Aubry, Sylvie; Hommes, Gregg; Arsenlis, Athanasios
2014-04-01
Discrete dislocation dynamics (DD) approaches have proven useful in modeling the dynamics of large ensembles of dislocations. Continuing interest in finite body effects via image stresses has extended DD numerical approaches to improve the handling of surfaces. However, a physically accurate, yet computationally scalable, implementation has been elusive. This paper presents a new framework and implementation of a finite element-based discrete DD code that (1) treats arbitrarily shaped non-convex surfaces through image tractions, (2) allows for systematic refinement of the finite element mesh both in the bulk and on the surface and (3) provides a platform to scale to relatively larger and lengthier simulations. The approach is based on the capabilities of the Parallel Dislocation Simulator coupled through a distributed shared memory implementation for the calculation of large numbers of dislocation segments interacting with an independently large number of surface finite elements. Surface tracking approaches enable topological features at surfaces to be modeled. We verify the computed results via comparisons with analytical solutions for an infinite screw dislocation and prismatic loop near a surface and examine surface effects on a Frank-Read source. Convergence of the image force error with h- and p-refinement is shown to indicate the computational robustness. Additionally, through larger numerical experiments, we demonstrate the new capabilities in a three-dimensional elastic body of finite extent.
A high-order staggered finite-element vertical discretization for non-hydrostatic atmospheric models
Guerra, Jorge E.; Ullrich, Paul A.
2016-06-01
Atmospheric modeling systems require economical methods to solve the non-hydrostatic Euler equations. Two major differences between hydrostatic models and a full non-hydrostatic description lies in the vertical velocity tendency and numerical stiffness associated with sound waves. In this work we introduce a new arbitrary-order vertical discretization entitled the staggered nodal finite-element method (SNFEM). Our method uses a generalized discrete derivative that consistently combines the discontinuous Galerkin and spectral element methods on a staggered grid. Our combined method leverages the accurate wave propagation and conservation properties of spectral elements with staggered methods that eliminate stationary (2Δx) modes. Furthermore, high-order accuracy alsomore » eliminates the need for a reference state to maintain hydrostatic balance. In this work we demonstrate the use of high vertical order as a means of improving simulation quality at relatively coarse resolution. We choose a test case suite that spans the range of atmospheric flows from predominantly hydrostatic to nonlinear in the large-eddy regime. Lastly, our results show that there is a distinct benefit in using the high-order vertical coordinate at low resolutions with the same robust properties as the low-order alternative.« less
Svyatskiy, Daniil; Shashkov, Mikhail; Kuzmin, D
2008-01-01
A new approach to the design of constrained finite element approximations to second-order elliptic problems is introduced. This approach guarantees that the finite element solution satisfies the discrete maximum principle (DMP). To enforce these monotonicity constrains the sufficient conditions for elements of the stiffness matrix are formulated. An algebraic splitting of the stiffness matrix is employed to separate the contributions of diffusive and antidiffusive numerical fluxes, respectively. In order to prevent the formation of spurious undershoots and overshoots, a symmetric slope limiter is designed for the antidiffusive part. The corresponding upper and lower bounds are defined using an estimate of the steepest gradient in terms of the maximum and minimum solution values at surrounding nodes. The recovery of nodal gradients is performed by means of a lumped-mass L{sub 2} projection. The proposed slope limiting strategy preserves the consistency of the underlying discrete problem and the structure of the stiffness matrix (symmetry, zero row and column sums). A positivity-preserving defect correction scheme is devised for the nonlinear algebraic system to be solved. Numerical results and a grid convergence study are presented for a number of anisotropic diffusion problems in two space dimensions.
A high-order staggered finite-element vertical discretization for non-hydrostatic atmospheric models
NASA Astrophysics Data System (ADS)
Guerra, Jorge E.; Ullrich, Paul A.
2016-06-01
Atmospheric modeling systems require economical methods to solve the non-hydrostatic Euler equations. Two major differences between hydrostatic models and a full non-hydrostatic description lies in the vertical velocity tendency and numerical stiffness associated with sound waves. In this work we introduce a new arbitrary-order vertical discretization entitled the staggered nodal finite-element method (SNFEM). Our method uses a generalized discrete derivative that consistently combines the discontinuous Galerkin and spectral element methods on a staggered grid. Our combined method leverages the accurate wave propagation and conservation properties of spectral elements with staggered methods that eliminate stationary (2Δx) modes. Furthermore, high-order accuracy also eliminates the need for a reference state to maintain hydrostatic balance. In this work we demonstrate the use of high vertical order as a means of improving simulation quality at relatively coarse resolution. We choose a test case suite that spans the range of atmospheric flows from predominantly hydrostatic to nonlinear in the large-eddy regime. Our results show that there is a distinct benefit in using the high-order vertical coordinate at low resolutions with the same robust properties as the low-order alternative.
A high-order staggered finite-element vertical discretization for non-hydrostatic atmospheric models
Guerra, Jorge E.; Ullrich, Paul A.
2016-06-01
Atmospheric modeling systems require economical methods to solve the non-hydrostatic Euler equations. Two major differences between hydrostatic models and a full non-hydrostatic description lies in the vertical velocity tendency and numerical stiffness associated with sound waves. In this work we introduce a new arbitrary-order vertical discretization entitled the staggered nodal finite-element method (SNFEM). Our method uses a generalized discrete derivative that consistently combines the discontinuous Galerkin and spectral element methods on a staggered grid. Our combined method leverages the accurate wave propagation and conservation properties of spectral elements with staggered methods that eliminate stationary (2Δx) modes. Furthermore, high-order accuracy alsomore » eliminates the need for a reference state to maintain hydrostatic balance. In this work we demonstrate the use of high vertical order as a means of improving simulation quality at relatively coarse resolution. We choose a test case suite that spans the range of atmospheric flows from predominantly hydrostatic to nonlinear in the large-eddy regime. Our results show that there is a distinct benefit in using the high-order vertical coordinate at low resolutions with the same robust properties as the low-order alternative.« less
Prediction of Fracture Behavior in Rock and Rock-like Materials Using Discrete Element Models
NASA Astrophysics Data System (ADS)
Katsaga, T.; Young, P.
2009-05-01
The study of fracture initiation and propagation in heterogeneous materials such as rock and rock-like materials are of principal interest in the field of rock mechanics and rock engineering. It is crucial to study and investigate failure prediction and safety measures in civil and mining structures. Our work offers a practical approach to predict fracture behaviour using discrete element models. In this approach, the microstructures of materials are presented through the combination of clusters of bonded particles with different inter-cluster particle and bond properties, and intra-cluster bond properties. The geometry of clusters is transferred from information available from thin sections, computed tomography (CT) images and other visual presentation of the modeled material using customized AutoCAD built-in dialog- based Visual Basic Application. Exact microstructures of the tested sample, including fractures, faults, inclusions and void spaces can be duplicated in the discrete element models. Although the microstructural fabrics of rocks and rock-like structures may have different scale, fracture formation and propagation through these materials are alike and will follow similar mechanics. Synthetic material provides an excellent condition for validating the modelling approaches, as fracture behaviours are known with the well-defined composite's properties. Calibration of the macro-properties of matrix material and inclusions (aggregates), were followed with the overall mechanical material responses calibration by adjusting the interfacial properties. The discrete element model predicted similar fracture propagation features and path as that of the real sample material. The path of the fractures and matrix-inclusion interaction was compared using computed tomography images. Initiation and fracture formation in the model and real material were compared using Acoustic Emission data. Analysing the temporal and spatial evolution of AE events, collected during the
Discrete element method for emergency flow of pedestrian in S-type corridor.
Song, Gyeongwon; Park, Junyoung
2014-10-01
Pedestrian flow in curved corridor should be modeled before design because this type of corridor can be most dangerous part during emergency evacuation. In this study, this flow is analyzed by Discrete Element Method with psychological effects. As the turning slope of corridor increases, the evacuation time is linearly increases. However, in the view of crashed death accident, the case with 90 degree turning slope can be dangerous because there are 3 dangerous points. To solve this matter, the pedestrian gathering together in curved part should be dispersed. PMID:25942811
Coupled discrete element and smoothed particle hydrodynamics simulations of the die filling process
NASA Astrophysics Data System (ADS)
Breinlinger, Thomas; Kraft, Torsten
2015-08-01
Die filling is an important part of the powder compaction process chain, where defects in the final part can be introduced—or prevented. Simulation of this process is therefore a goal for many part producers and has been studied by some researchers already. In this work, we focus on the influence of the surrounding air on the powder flow. We demonstrate the implementing and coupling of the discrete element method for the granular powder and the smoothed particle hydrodynamics method for the gas flow. Application of the method to the die filling process is demonstrated.
Preliminary discrete element modeling of a falling particle curtain for CSP central tower receivers
NASA Astrophysics Data System (ADS)
Zanino, R.; Ho, C. K.; Romano, D.; Savoldi, L.
2016-05-01
Current methods used to simulate the curtain thickness in a falling particle receiver lead to a poor agreement with the experiments. Here the Discrete Element Method (DEM) is proposed to address the problem, including both the top hopper and the interactions between particles in the model. Some first promising results are presented, showing an acceptable agreement between simulation and experiment for an ad-hoc set of input parameters. A sensitivity study provides a first assessment of the effects of the main input parameters of the model (boundary conditions at the release, particle Young's modulus, restitution coefficients and effective particle diameter) on the predicted curtain thickness.
Electro-impulse de-icing electrodynamic solution by discrete elements
NASA Technical Reports Server (NTRS)
Bernhart, W. D.; Schrag, R. L.
1988-01-01
This paper describes a technique for analyzing the electrodynamic phenomena associated with electro-impulse deicing. The analysis is done in the time domain and utilizes a discrete element formulation concept expressed in state variable form. Calculated results include coil current, eddy currents in the target (aircraft leading edge skin), pressure distribution on the target, and total force and impulse on the target. Typical results are presented and described. Some comparisons are made between calculated and experimental results, and also between calculated values from other theoretical approaches. Application to the problem of a nonrigid target is treated briefly.
Damping of rotating beams with particle dampers: Discrete element method analysis
NASA Astrophysics Data System (ADS)
Els, D. N. J.
2013-06-01
The performance of particle dampers (PDs) under centrifugal loads was investigated. A test bench consisting of a rotating cantilever beam with a particle damper at the tip was developed (D. N. J. Els, AIAA Journal 49, 2228-2238 (2011)). Equal mass containers with different depths, filled with a range of uniform-sized steel ball bearings, were used as particle dampers. The experiments were duplicated numerically with a discrete element method (DEM) model, calibrated against the experimental data. The DEM model of the rotating beam with a PD at the tip captured the performance of the PD very well over a wide range of tests with different configurations and rotation velocities.
Cleary, Paul W; Prakash, Mahesh
2004-09-15
Particle-based simulation methods, such as the discrete-element method and smoothed particle hydrodynamics, have specific advantages in modelling complex three-dimensional (3D) environmental fluid and particulate flows. The theory of both these methods and their relative advantages compared with traditional methods will be discussed. Examples of 3D flows on realistic topography illustrate the environmental application of these methods. These include the flooding of a river valley as a result of a dam collapse, coastal inundation by a tsunami, volcanic lava flow and landslides. Issues related to validation and quality data availability are also discussed. PMID:15306427
Discrete Element Method Simulation of a Boulder Extraction From an Asteroid
NASA Technical Reports Server (NTRS)
Kulchitsky, Anton K.; Johnson, Jerome B.; Reeves, David M.; Wilkinson, Allen
2014-01-01
The force required to pull 7t and 40t polyhedral boulders from the surface of an asteroid is simulated using the discrete element method considering the effects of microgravity, regolith cohesion and boulder acceleration. The connection between particle surface energy and regolith cohesion is estimated by simulating a cohesion sample tearing test. An optimal constant acceleration is found where the peak net force from inertia and cohesion is a minimum. Peak pulling forces can be further reduced by using linear and quadratic acceleration functions with up to a 40% reduction in force for quadratic acceleration.
Hsu, Sen-Ming; Chang, Hung-Chun
2007-11-26
A full-vectorial finite element method based eigenvalue algorithm is developed to analyze the band structures of two-dimensional (2D) photonic crystals (PCs) with arbitray 3D anisotropy for in-planewave propagations, in which the simple transverse-electric (TE) or transverse-magnetic (TM) modes may not be clearly defined. By taking all the field components into consideration simultaneously without decoupling of the wave modes in 2D PCs into TE and TM modes, a full-vectorial matrix eigenvalue equation, with the square of the wavenumber as the eigenvalue, is derived. We examine the convergence behaviors of this algorithm and analyze 2D PCs with arbitrary anisotropy using this algorithm to demonstrate its correctness and usefulness by explaining the numerical results theoretically. PMID:19550864
Multiple-contact discrete-element model for simulating dense granular media.
Brodu, Nicolas; Dijksman, Joshua A; Behringer, Robert P
2015-03-01
This article presents a new force model for performing quantitative simulations of dense granular materials. Interactions between multiple contacts (MC) on the same grain are explicitly taken into account. Our readily applicable MC-DEM method retains all the advantages of discrete-element method simulations and does not require the use of costly finite-element methods. The new model closely reproduces our recent experimental measurements, including contact force distributions in full 3D, at all compression levels of the packing up to the experimental maximum limit of 13%. Comparisons with classic simulations using the nondeformable spheres approach, as well as with alternative models for interactions between multiple contacts, are provided. The success of our model, compared to these alternatives, demonstrates that interactions between multiple contacts on each grain must be included for dense granular packings. PMID:25871097
NASA Astrophysics Data System (ADS)
Zohdi, T. I.
2016-03-01
In industry, particle-laden fluids, such as particle-functionalized inks, are constructed by adding fine-scale particles to a liquid solution, in order to achieve desired overall properties in both liquid and (cured) solid states. However, oftentimes undesirable particulate agglomerations arise due to some form of mutual-attraction stemming from near-field forces, stray electrostatic charges, process ionization and mechanical adhesion. For proper operation of industrial processes involving particle-laden fluids, it is important to carefully breakup and disperse these agglomerations. One approach is to target high-frequency acoustical pressure-pulses to breakup such agglomerations. The objective of this paper is to develop a computational model and corresponding solution algorithm to enable rapid simulation of the effect of acoustical pulses on an agglomeration composed of a collection of discrete particles. Because of the complex agglomeration microstructure, containing gaps and interfaces, this type of system is extremely difficult to mesh and simulate using continuum-based methods, such as the finite difference time domain or the finite element method. Accordingly, a computationally-amenable discrete element/discrete ray model is developed which captures the primary physical events in this process, such as the reflection and absorption of acoustical energy, and the induced forces on the particulate microstructure. The approach utilizes a staggered, iterative solution scheme to calculate the power transfer from the acoustical pulse to the particles and the subsequent changes (breakup) of the pulse due to the particles. Three-dimensional examples are provided to illustrate the approach.
Combined Finite-Discrete Element Method for Simulation of Hydraulic Fracturing
NASA Astrophysics Data System (ADS)
Yan, Chengzeng; Zheng, Hong; Sun, Guanhua; Ge, Xiurun
2016-04-01
Hydraulic fracturing is widely used in the exploitation of unconventional gas (such as shale gas).Thus, the study of hydraulic fracturing is of particular importance for petroleum industry. The combined finite-discrete element method (FDEM) proposed by Munjiza is an innovative numerical technique to capture progressive damage and failure processes in rock. However, it cannot model the fracturing process of rock driven by hydraulic pressure. In this study, we present a coupled hydro-mechanical model based on FDEM for the simulation of hydraulic fracturing in complex fracture geometries, where an algorithm for updating hydraulic fracture network is proposed. The algorithm can carry out connectivity searches for arbitrarily complex fracture networks. Then, we develop a new combined finite-discrete element method numerical code (Y-flow) for the simulation of hydraulic fracturing. Finally, several verification examples are given, and the simulation results agree well with the analytical or experimental results, indicating that the newly developed numerical code can capture hydraulic fracturing process correctly and effectively.
NASA Astrophysics Data System (ADS)
Martin, Hugo; Mangeney, Anne; Farin, Maxime; Richard, Patrick
2016-04-01
The mechanical behavior of granular flows is still an open issue. In particular, quantitative agreement between the detailed dynamics of the flow and laboratory experiments is necessary to better constrain the performance and limits of the models. We propose here to compare quantitatively the flow profiles and the force during granular column collapse simulated using Discrete Element Models and laboratory experiments. These small scale experiments are performed with dry granular material released initially from a cylinder on a sloping plane. The flow profiles and the acoustic signal generated by the granular impacts and stresses on the plane are recorded systematically [Farin et al., 2015]. These experiments are simulated using the Discrete Element Method Modys [Richard et al., 2000]. We show that the effect of the removing gate should be taken into account in the model in order to quantatively reproduce the flow dynamics. Furthermore we compare the simulated and observed acoustic signals that are generated by the fluctuating stresses exerted by the grains on the substrate in different frequency bands. [1] P. Richard et Luc Oger. 2000 Etude de la géométrie de milieux granulaires modèles tridimensionnels par simulation numérique. [2] Farin, M., Mangeney, A., Toussaint, R., De Rosny, J., Shapiro, N., Dewez, T., Hibert, C., Mathon, C., Sedan, O., Berger. 2015, Characterization of rockfalls from seismic signal: insights from laboratory experiments
Coupled discrete element and finite volume solution of two classical soil mechanics problems
Chen, Feng; Drumm, Eric; Guiochon, Georges A
2011-01-01
One dimensional solutions for the classic critical upward seepage gradient/quick condition and the time rate of consolidation problems are obtained using coupled routines for the finite volume method (FVM) and discrete element method (DEM), and the results compared with the analytical solutions. The two phase flow in a system composed of fluid and solid is simulated with the fluid phase modeled by solving the averaged Navier-Stokes equation using the FVM and the solid phase is modeled using the DEM. A framework is described for the coupling of two open source computer codes: YADE-OpenDEM for the discrete element method and OpenFOAM for the computational fluid dynamics. The particle-fluid interaction is quantified using a semi-empirical relationship proposed by Ergun [12]. The two classical verification problems are used to explore issues encountered when using coupled flow DEM codes, namely, the appropriate time step size for both the fluid and mechanical solution processes, the choice of the viscous damping coefficient, and the number of solid particles per finite fluid volume.
Novel Discrete Element Method for 3D non-spherical granular particles.
NASA Astrophysics Data System (ADS)
Seelen, Luuk; Padding, Johan; Kuipers, Hans
2015-11-01
Granular materials are common in many industries and nature. The different properties from solid behavior to fluid like behavior are well known but less well understood. The main aim of our work is to develop a discrete element method (DEM) to simulate non-spherical granular particles. The non-spherical shape of particles is important, as it controls the behavior of the granular materials in many situations, such as static systems of packed particles. In such systems the packing fraction is determined by the particle shape. We developed a novel 3D discrete element method that simulates the particle-particle interactions for a wide variety of shapes. The model can simulate quadratic shapes such as spheres, ellipsoids, cylinders. More importantly, any convex polyhedron can be used as a granular particle shape. These polyhedrons are very well suited to represent non-rounded sand particles. The main difficulty of any non-spherical DEM is the determination of particle-particle overlap. Our model uses two iterative geometric algorithms to determine the overlap. The algorithms are robust and can also determine multiple contact points which can occur for these shapes. With this method we are able to study different applications such as the discharging of a hopper or silo. Another application the creation of a random close packing, to determine the solid volume fraction as a function of the particle shape.
Veijola, Timo; Råback, Peter
2007-01-01
We present a straightforward method to solve gas damping problems for perforated structures in two dimensions (2D) utilising a Perforation Profile Reynolds (PPR) solver. The PPR equation is an extended Reynolds equation that includes additional terms modelling the leakage flow through the perforations, and variable diffusivity and compressibility profiles. The solution method consists of two phases: 1) determination of the specific admittance profile and relative diffusivity (and relative compressibility) profiles due to the perforation, and 2) solution of the PPR equation with a FEM solver in 2D. Rarefied gas corrections in the slip-flow region are also included. Analytic profiles for circular and square holes with slip conditions are presented in the paper. To verify the method, square perforated dampers with 16–64 holes were simulated with a three-dimensional (3D) Navier-Stokes solver, a homogenised extended Reynolds solver, and a 2D PPR solver. Cases for both translational (in normal to the surfaces) and torsional motion were simulated. The presented method extends the region of accurate simulation of perforated structures to cases where the homogenisation method is inaccurate and the full 3D Navier-Stokes simulation is too time-consuming.
Seismic evaluation of lead caves using no-tension discrete model with interface elements
Khaleel, M.A.; Deibler, J.E.; Koontz, D.A.
1995-07-01
This paper investigates quasi-static behavior of lead cave walls radiation shields made by stacking lead bricks. The bricks have high stiffness, whereas the joints are weak and incapable of supporting tension. Global behavior of this kind of wall is strongly influenced by size friction coefficient of the brick elements. The general finite element code ANSYS was used for the analysis of the lead caves. A series of 2-D models that spanned the range of height-to-width aspect ratios of the cave wall were constructed. Two types of contact elements were incorporated in the model. The point-to-point contact element was used to represent contact in the horizontal direction. This element permits either compression in the direction normal to the surfaces or opening of a gap. The point-to-surface contact element was chosen to represent contact in the vertical direction. This element allows sliding in addition to the compression or gap formation normal to the surface. A series of static analyses were performed for each model. A l-g. vertical acceleration representing gravity was applied. The lateral acceleration was increased until the solution would not converge. This acceleration is defined as the critical lateral acceleration. This was achieved with a set of load steps with increasing lateral load. The critical acceleration was found to depend on the wall aspect ratio. For a wall with an aspect ratio up to three, the maximum acceleration is above the required 0.1 g. The wall failure mechanisms were also identified based on the numerical results. The two failure modes are the rotation and loss of interlocking among the blocks or silding of upper layers of the wall.
Shouchun Deng; Robert Podgorney; Hai Huang
2011-02-01
Key challenges associated with the EGS reservoir development include the ability to reliably predict hydraulic fracturing and the deformation of natural fractures as well as estimating permeability evolution of the fracture network with time. We have developed a physics-based rock deformation and fracture propagation simulator by coupling a discrete element model (DEM) for fracturing with a network flow model. In DEM model, solid rock is represented by a network of discrete elements (often referred as particles) connected by various types of mechanical bonds such as springs, elastic beams or bonds that have more complex properties (such as stress-dependent elastic constants). Fracturing is represented explicitly as broken bonds (microcracks), which form and coalesce into macroscopic fractures when external and internal load is applied. The natural fractures are represented by a series of connected line segments. Mechanical bonds that intersect with such line segments are removed from the DEM model. A network flow model using conjugate lattice to the DEM network is developed and coupled with the DEM. The fluid pressure gradient exerts forces on individual elements of the DEM network, which therefore deforms the mechanical bonds and breaks them if the deformation reaches a prescribed threshold value. Such deformation/fracturing in turn changes the permeability of the flow network, which again changes the evolution of fluid pressure, intimately coupling the two processes. The intimate coupling between fracturing/deformation of fracture networks and fluid flow makes the meso-scale DEM- network flow simulations necessary in order to accurately evaluate the permeability evolution, as these methods have substantial advantages over conventional continuum mechanical models of elastic rock deformation. The challenges that must be overcome to simulate EGS reservoir stimulation, preliminary results, progress to date and near future research directions and opportunities will be
NASA Astrophysics Data System (ADS)
Derakhshani, S. M.; Schott, D. L.; Lodewijks, G.
2013-06-01
Dust emissions can have significant effects on the human health, environment and industry equipment. Understanding the dust generation process helps to select a suitable dust preventing approach and also is useful to evaluate the environmental impact of dust emission. To describe these processes, numerical methods such as Computational Fluid Dynamics (CFD) are widely used, however nowadays particle based methods like Discrete Element Method (DEM) allow researchers to model interaction between particles and fluid flow. In this study, air flow over a stockpile, dust emission, erosion and surface deformation of granular material in the form of stockpile are studied by using DEM and CFD as a coupled method. Two and three dimensional simulations are respectively developed for CFD and DEM methods to minimize CPU time. The standard κ-ɛ turbulence model is used in a fully developed turbulent flow. The continuous gas phase and the discrete particle phase link to each other through gas-particle void fractions and momentum transfer. In addition to stockpile deformation, dust dispersion is studied and finally the accuracy of stockpile deformation results obtained by CFD-DEM modelling will be validated by the agreement with the existing experimental data.
Discrete element method model and damping performance of bean bag dampers
NASA Astrophysics Data System (ADS)
Zhang, Chao; Chen, Tianning; Wang, Xiaopeng; Li, Yinggang
2014-11-01
Bean bag dampers (BBDs) have been widely applied in engineering to attenuate the vibration of a structural system, but the theoretical analysis on BBDs has been scarcely reported because of their nonlinear damping performance and complex mechanism. In this work, a three-dimensional model of a BBD was established based on the discrete element method (DEM); its flexible boundary was discretized. The model was verified by comparing simulation with test data. Based on the model, the selection of proper particle diameter on the flexible boundary of the BBD was discussed first, and then the effects of internal particle size of the BBD, the BBD's tightness and the gap between BBD and the inner wall of its enclosure on the energy dissipation capacity were studied. Moreover, the filling ratio of BBD (total internal particles' volume/the flexible boundary's capacity) was defined to quantitatively describe the tightness of BBD, and the effects of the internal particle size, the natural frequency of primary system and the enclosure size on the optimum tightness of the BBD were also considered. The results can be used as a guide in the design of BBDs.
A method of coupling discrete dislocation plasticity to the crystal plasticity finite element method
NASA Astrophysics Data System (ADS)
Xu, Y.; Balint, D. S.; Dini, D.
2016-05-01
A method of concurrent coupling of planar discrete dislocation plasticity (DDP) and a crystal plasticity finite element (CPFE) method was devised for simulating plastic deformation in large polycrystals with discrete dislocation resolution in a single grain or cluster of grains for computational efficiency; computation time using the coupling method can be reduced by an order of magnitude compared to DDP. The method is based on an iterative scheme initiated by a sub-model calculation, which ensures displacement and traction compatibility at all nodes at the interface between the DDP and CPFE domains. The proposed coupling approach is demonstrated using two plane strain problems: (i) uniaxial tension of a bi-crystal film and (ii) indentation of a thin film on a substrate. The latter was also used to demonstrate that the rigid substrate assumption used in earlier DDP studies is inadequate for indentation depths that are large compared to the film thickness, i.e. the effect of the plastic substrate modelled using CPFE becomes important. The coupling method can be used to study a wider range of indentation depths than previously possible using DDP alone, without sacrificing the indentation size effect regime captured by DDP. The method is general and can be applied to any problem where finer resolution of dislocation mediated plasticity is required to study the mechanical response of polycrystalline materials, e.g. to capture size effects locally within a larger elastic/plastic boundary value problem.
NASA Technical Reports Server (NTRS)
Thompson, David S.; Soni, Bharat K.
2000-01-01
An integrated software package, ICEG2D, was developed to automate computational fluid dynamics (CFD) simulations for single-element airfoils with ice accretion. ICEG2D is designed to automatically perform three primary functions: (1) generating a grid-ready, surface definition based on the geometrical characteristics of the iced airfoil surface, (2) generating a high-quality grid using the generated surface point distribution, and (3) generating the input and restart files needed to run the general purpose CFD solver NPARC. ICEG2D can be executed in batch mode using a script file or in an interactive mode by entering directives from a command line. This report summarizes activities completed in the first year of a three-year research and development program to address issues related to CFD simulations for aircraft components with ice accretion. Specifically, this document describes the technology employed in the software, the installation procedure, and a description of the operation of the software package. Validation of the geometry and grid generation modules of ICEG2D is also discussed.
Fracture and impulse based finite-discrete element modeling of fragmentation
NASA Astrophysics Data System (ADS)
Paluszny, A.; Tang, X. H.; Zimmerman, R. W.
2013-11-01
A numerical method for fragmentation is presented that combines the finite element method with the impulse-based discrete element method (impulse-based FDEM). In contrast to existing methods, fragments are not represented as a conglomeration of spheres; instead, their shapes are represented using solid modeling techniques, and are the result of multiple fracture growth. Fracture growth within each three-dimensional fragment is controlled by stress intensity factors computed using the finite element method and the reduced virtual integration technique. Non-convex fragment interaction and movement is modeled using impulse dynamics, rather than a penalty-based method. Collisions leading to fracture are handled individually by propagating pre-existing internal flaws and cracks. The method utilizes decoupled geometry and mesh representation, and local failure and propagation criteria. Fractures that reach volume boundaries lead to further fragmentation. The approach is demonstrated by the fragmentation of a sphere, which exhibits a velocity-dependent fragment size distribution. The distribution is characterized by a two-parameter Weibull distribution, an emergent property of the simulation. Results are in good agreement with experimental data.
Hasanzadeh, Mohammad; Shadjou, Nasrin; Mokhtarzadeh, Ahad; Ramezani, Mohammad
2016-11-01
Graphene is a 2-D carbon nanomaterial with many distinctive properties that are electrochemically beneficial, such as large surface-to-volume ratio, lowered power usage, high conductivity and electron mobility. Graphene-based electrochemical immune-devices have recently gained much importance for detecting antigens and biomarkers responsible for cancer diagnosis. This review describes fabrication and chemical modification of the surfaces of graphene for immunesensing applications. We also present a comprehensive overview of current developments and key issues in the determination of some biological molecules with particular emphasis on evaluating the models. This review focuses mostly on new developments in the last 5years in development of chip architecture and integration, different sensing modes that can be used in conjunction with microfluidics, and new applications that have emerged or have been demonstrated; it also aims to point out where future research can be directed to in these areas. PMID:27524045
NASA Astrophysics Data System (ADS)
Balzer, Karsten; Bauch, Sebastian; Bonitz, Michael
2010-04-01
In this contribution, we discuss the finite-element discrete variable representation (FE-DVR) of the nonequilibrium Green's function and its implications on the description of strongly inhomogeneous quantum systems. In detail, we show that the complementary features of FEs and the DVR allow for a notably more efficient solution of the two-time Schwinger/Keldysh/Kadanoff-Baym equations compared to a general basis approach. Particularly, the use of the FE-DVR leads to an essential speedup in computing the self-energies. As atomic and molecular examples we consider the He atom and the linear version of H+3 in one spatial dimension. For these closed-shell models we, in Hartree-Fock and second Born approximation, compute the ground-state properties and compare with the exact findings obtained from the solution of the few-particle time-dependent Schrödinger equation.
NASA Technical Reports Server (NTRS)
Koval, L. R.
1980-01-01
In the context of the transmission of airborne noise into an aircraft fuselage, a mathematical model is presented for the transmission of an oblique plane sound wave into a finite cylindrical shell stiffened by stringers and ring frames. The rings and stringers are modeled as discrete structural elements. The numerical case studied was typical of a narrow-bodied jet transport fuselage. The numerical results show that the ring-frequency dip in the transmission loss curve that is present for a monocoque shell is still present in the case of a stiffened shell. The ring frequency effect is a result of the cylindrical geometry of the shell. Below the ring frequency, stiffening does not appear to have any significant effect on transmission loss, but above the ring frequency, stiffeners can enhance the transmission loss of a cylindrical shell.
Discrete Element Method simulations of the saturation of aeolian sand transport
NASA Astrophysics Data System (ADS)
Pähtz, Thomas; Omeradžić, Amir; Carneiro, Marcus V.; Araújo, Nuno A. M.; Herrmann, Hans J.
2015-03-01
The saturation length of aeolian sand transport (Ls), characterizing the distance needed by wind-blown sand to adapt to changes in the wind shear, is essential for accurate modeling of the morphodynamics of Earth's sandy landscapes and for explaining the formation and shape of sand dunes. In the last decade, it has become a widely accepted hypothesis that Ls is proportional to the characteristic distance needed by transported particles to reach the wind speed (the "drag length"). Here we challenge this hypothesis. From extensive numerical Discrete Element Method simulations, we find that, for medium and strong winds, Ls∝Vs2/g, where Vs is the saturated value of the average speed of sand particles traveling above the surface and g is the gravitational constant. We show that this proportionality is consistent with a recent analytical model, in which the drag length is just one of four similarly important length scales relevant for sand transport saturation.
Discrete element method study of fuel relocation and dispersal during loss-of-coolant accidents
NASA Astrophysics Data System (ADS)
Govers, K.; Verwerft, M.
2016-09-01
The fuel fragmentation, relocation and dispersal (FFRD) during LOCA transients today retain the attention of the nuclear safety community. The fine fragmentation observed at high burnup may, indeed, affect the Emergency Core Cooling System performance: accumulation of fuel debris in the cladding ballooned zone leads to a redistribution of the temperature profile, while dispersal of debris might lead to coolant blockage or to debris circulation through the primary circuit. This work presents a contribution, by discrete element method, towards a mechanistic description of the various stages of FFRD. The fuel fragments are described as a set of interacting particles, behaving as a granular medium. The model shows qualitative and quantitative agreement with experimental observations, such as the packing efficiency in the balloon, which is shown to stabilize at about 55%. The model is then applied to study fuel dispersal, for which experimental parametric studies are both difficult and expensive.
NASA Astrophysics Data System (ADS)
Wendling, A.; Daniel, J. L.; Hivet, G.; Vidal-Sallé, E.; Boisse, P.
2015-12-01
Numerical simulation is a powerful tool to predict the mechanical behavior and the feasibility of composite parts. Among the available numerical approaches, as far as woven reinforced composites are concerned, 3D finite element simulation at the mesoscopic scale leads to a good compromise between realism and complexity. At this scale, the fibrous reinforcement is modeled by an interlacement of yarns assumed to be homogeneous that have to be accurately represented. Among the numerous issues induced by these simulations, the first one consists in providing a representative meshed geometrical model of the unit cell at the mesoscopic scale. The second one consists in enabling a fast data input in the finite element software (contacts definition, boundary conditions, elements reorientation, etc.) so as to obtain results within reasonable time. Based on parameterized 3D CAD modeling tool of unit-cells of dry fabrics already developed, this paper presents an efficient strategy which permits an automated meshing of the models with 3D hexahedral elements and to accelerate of several orders of magnitude the simulation data input. Finally, the overall modeling strategy is illustrated by examples of finite element simulation of the mechanical behavior of fabrics.
NASA Astrophysics Data System (ADS)
Song, Hengxu; Papanikolaou, Stefanos; van der Giessen, Erik
2015-03-01
It is well known for almost three decades that crystal plasticity in metals, such as Cu, is strongly rate dependent at strain rates higher than 10⌃3/s. This rate sensitivity is typically attributed to dislocation drag effects, but there appears to be a large range of possible high-rate-sensitivity exponents, depending on the sample and the experimental group. Thus, one may hypothesize that the dislocation structure has a strong influence on these effects. We elucidate the origins of rate effects in crystal plasticity and their connection with relaxed, before applying stress, dislocation structures by investigating simple bending in a model of discrete dislocation plasticity in two dimensions. We find that the high-strain-rate sensitivity changes significantly as a function of strain, different material treatment (annealed or not) and properties of dislocation sources (surface vs. bulk nucleation). We characterize in detail the emerging patterning in the dislocation structure and we provide predictions for future experiments on the dependence of the rate sensitivity on dislocation-related characteristics.
NASA Technical Reports Server (NTRS)
Morrison, Joseph H.
1998-01-01
This report details calculations for the McDonnell-Douglas 30P/30N and the NHLP-2D three-element highlift configurations. Calculations were performed with the Reynolds averaged Navier-Stokes code ISAAC to study the effects of various numerical issues on high lift predictions. These issues include the effect of numerical accuracy on the advection terms of the turbulence equations, Navier-Stokes versus the thin-layer Navier-Stokes approximation, an alternative formulation of the production term, and the performance of several turbulence models. The effect of the transition location on the NHLP-2D flow solution was investigated. Two empirical transition models were used to estimate the transition location.
Calio, I.; Cannizzaro, F.; Marletta, M.; Panto, B.; D'Amore, E.
2008-07-08
In the present study a new discrete-element approach for the evaluation of the seismic resistance of composite reinforced concrete-masonry structures is presented. In the proposed model, unreinforced masonry panels are modelled by means of two-dimensional discrete-elements, conceived by the authors for modelling masonry structures, whereas the reinforced concrete elements are modelled by lumped plasticity elements interacting with the masonry panels through nonlinear interface elements. The proposed procedure was adopted for the assessment of the seismic response of a case study confined-masonry building which was conceived to be a typical representative of a wide class of residential buildings designed to the requirements of the 1909 issue of the Italian seismic code and widely adopted in the aftermath of the 1908 earthquake for the reconstruction of the cities of Messina and Reggio Calabria.
NASA Astrophysics Data System (ADS)
Li, Jianbao; Wang, Yue-Sheng; Zhang, Chuanzeng
2010-05-01
In this paper, a finite element method based on the ABAQUS code and user subroutine is presented to evaluate the propagation of acoustic waves in the two-dimensional phononic crystals with Archimedean-like tilings. Two systems composed of cylinder scatters embedded in a host in Ladybug and Bathroom lattices are considered. Complete and accurate band structures and transmission spectra are obtained to identify the band gaps and eigenmodes. We found that Archimedean-like structures can have some advantages over the traditional square lattice regarding the completeness of the gap and its position and width. Also, due to the same square primitive unit cell and the first Brillouin zone, the two square-like lattices have similar acoustic response in lower bands. The results indicate that the finite element method is precise for the band structure computation of the complex phononic crystals with Archimedean tilings.
NASA Technical Reports Server (NTRS)
Krueger, Ronald; Paris, Isbelle L.; OBrien, T. Kevin; Minguet, Pierre J.
2004-01-01
The influence of two-dimensional finite element modeling assumptions on the debonding prediction for skin-stiffener specimens was investigated. Geometrically nonlinear finite element analyses using two-dimensional plane-stress and plane-strain elements as well as three different generalized plane strain type approaches were performed. The computed skin and flange strains, transverse tensile stresses and energy release rates were compared to results obtained from three-dimensional simulations. The study showed that for strains and energy release rate computations the generalized plane strain assumptions yielded results closest to the full three-dimensional analysis. For computed transverse tensile stresses the plane stress assumption gave the best agreement. Based on this study it is recommended that results from plane stress and plane strain models be used as upper and lower bounds. The results from generalized plane strain models fall between the results obtained from plane stress and plane strain models. Two-dimensional models may also be used to qualitatively evaluate the stress distribution in a ply and the variation of energy release rates and mixed mode ratios with delamination length. For more accurate predictions, however, a three-dimensional analysis is required.
The semi-discrete Galerkin finite element modelling of compressible viscous flow past an airfoil
NASA Technical Reports Server (NTRS)
Meade, Andrew J., Jr.
1992-01-01
A method is developed to solve the two-dimensional, steady, compressible, turbulent boundary-layer equations and is coupled to an existing Euler solver for attached transonic airfoil analysis problems. The boundary-layer formulation utilizes the semi-discrete Galerkin (SDG) method to model the spatial variable normal to the surface with linear finite elements and the time-like variable with finite differences. A Dorodnitsyn transformed system of equations is used to bound the infinite spatial domain thereby permitting the use of a uniform finite element grid which provides high resolution near the wall and automatically follows boundary-layer growth. The second-order accurate Crank-Nicholson scheme is applied along with a linearization method to take advantage of the parabolic nature of the boundary-layer equations and generate a non-iterative marching routine. The SDG code can be applied to any smoothly-connected airfoil shape without modification and can be coupled to any inviscid flow solver. In this analysis, a direct viscous-inviscid interaction is accomplished between the Euler and boundary-layer codes, through the application of a transpiration velocity boundary condition. Results are presented for compressible turbulent flow past NACA 0012 and RAE 2822 airfoils at various freestream Mach numbers, Reynolds numbers, and angles of attack. All results show good agreement with experiment, and the coupled code proved to be a computationally-efficient and accurate airfoil analysis tool.
Herbold, E. B.; Walton, O.; Homel, M. A.
2015-10-26
This document serves as a final report to a small effort where several improvements were added to a LLNL code GEODYN-L to develop Discrete Element Method (DEM) algorithms coupled to Lagrangian Finite Element (FE) solvers to investigate powder-bed formation problems for additive manufacturing. The results from these simulations will be assessed for inclusion as the initial conditions for Direct Metal Laser Sintering (DMLS) simulations performed with ALE3D. The algorithms were written and performed on parallel computing platforms at LLNL. The total funding level was 3-4 weeks of an FTE split amongst two staff scientists and one post-doc. The DEM simulations emulated, as much as was feasible, the physical process of depositing a new layer of powder over a bed of existing powder. The DEM simulations utilized truncated size distributions spanning realistic size ranges with a size distribution profile consistent with realistic sample set. A minimum simulation sample size on the order of 40-particles square by 10-particles deep was utilized in these scoping studies in order to evaluate the potential effects of size segregation variation with distance displaced in front of a screed blade. A reasonable method for evaluating the problem was developed and validated. Several simulations were performed to show the viability of the approach. Future investigations will focus on running various simulations investigating powder particle sizing and screen geometries.
NASA Astrophysics Data System (ADS)
Vašinová Galiová, Michaela; Čopjaková, Renata; Škoda, Radek; Štěpánková, Kateřina; Vaňková, Michaela; Kuta, Jan; Prokeš, Lubomír; Kynický, Jindřich; Kanický, Viktor
2014-10-01
A 213 nm Nd:YAG-based laser ablation (LA) system coupled to quadrupole-based inductively coupled plasma-mass spectrometer and an ArF* excimer-based LA-system coupled to a double-focusing sector field inductively coupled plasma-mass spectrometer were employed to study the spatial distribution of various elements in kidney stones (uroliths). Sections of the surfaces of uroliths were ablated according to line patterns to investigate the elemental profiles for the different urolith growth zones. This exploratory study was mainly focused on the distinguishing of the main constituents of urinary calculus fragments by means of LA-ICP-mass spectrometry. Changes in the ablation rate for oxalate and phosphate phases related to matrix density and hardness are discussed. Elemental association was investigated on the basis of 2D mapping. The possibility of using NIST SRM 1486 Bone Meal as an external standard for calibration was tested. It is shown that LA-ICP-MS is helpful for determination of the mineralogical composition and size of all phases within the analyzed surface area, for tracing down elemental associations and for documenting the elemental content of urinary stones. LA-ICP-MS results (elemental contents and maps) are compared to those obtained with electron microprobe analysis and solution analysis ICP-MS.
NASA Astrophysics Data System (ADS)
Le Hardy, D.; Favennec, Y.; Rousseau, B.
2016-08-01
The 2D radiative transfer equation coupled with specular reflection boundary conditions is solved using finite element schemes. Both Discontinuous Galerkin and Streamline-Upwind Petrov-Galerkin variational formulations are fully developed. These two schemes are validated step-by-step for all involved operators (transport, scattering, reflection) using analytical formulations. Numerical comparisons of the two schemes, in terms of convergence rate, reveal that the quadratic SUPG scheme proves efficient for solving such problems. This comparison constitutes the main issue of the paper. Moreover, the solution process is accelerated using block SOR-type iterative methods, for which the determination of the optimal parameter is found in a very cheap way.
NASA Technical Reports Server (NTRS)
Thompson David S.; Soni, Bharat K.
2001-01-01
An integrated geometry/grid/simulation software package, ICEG2D, is being developed to automate computational fluid dynamics (CFD) simulations for single- and multi-element airfoils with ice accretions. The current version, ICEG213 (v2.0), was designed to automatically perform four primary functions: (1) generate a grid-ready surface definition based on the geometrical characteristics of the iced airfoil surface, (2) generate high-quality structured and generalized grids starting from a defined surface definition, (3) generate the input and restart files needed to run the structured grid CFD solver NPARC or the generalized grid CFD solver HYBFL2D, and (4) using the flow solutions, generate solution-adaptive grids. ICEG2D (v2.0) can be operated in either a batch mode using a script file or in an interactive mode by entering directives from a command line within a Unix shell. This report summarizes activities completed in the first two years of a three-year research and development program to address automation issues related to CFD simulations for airfoils with ice accretions. As well as describing the technology employed in the software, this document serves as a users manual providing installation and operating instructions. An evaluation of the software is also presented.
NASA Astrophysics Data System (ADS)
Ren, Xiaotao; Corcolle, Romain; Daniel, Laurent
2016-02-01
The use of soft magnetic composites (SMCs) in electrical engineering applications is growing. SMCs provide an effective alternative to laminated steels because they exhibit a high permeability with low eddy current losses. Losses are a critical feature in the design of electrical machines, and it is necessary to evaluate the role of microstructure and constitutive properties of SMCs during the predesign stage. In this paper we propose a simplified finite element approach to compute eddy current losses in these materials. The computations allow to quantify the role of exciting source and material properties on eddy current losses. This analysis can later be used in the development of homogenization models for SMC. Contribution to the topical issue "Numelec 2015 - Elected submissions", edited by Adel Razek
Normal fault growth above pre-existing structures: insights from discrete element modelling
NASA Astrophysics Data System (ADS)
Wrona, Thilo; Finch, Emma; Bell, Rebecca; Jackson, Christopher; Gawthorpe, Robert; Phillips, Thomas
2016-04-01
In extensional systems, pre-existing structures such as shear zones may affect the growth, geometry and location of normal faults. Recent seismic reflection-based observations from the North Sea suggest that shear zones not only localise deformation in the host rock, but also in the overlying sedimentary succession. While pre-existing weaknesses are known to localise deformation in the host rock, their effect on deformation in the overlying succession is less well understood. Here, we use 3-D discrete element modelling to determine if and how kilometre-scale shear zones affect normal fault growth in the overlying succession. Discrete element models use a large number of interacting particles to describe the dynamic evolution of complex systems. The technique has therefore been applied to describe fault and fracture growth in a variety of geological settings. We model normal faulting by extending a 60×60×30 km crustal rift-basin model including brittle and ductile interactions and gravitation and isostatic forces by 30%. An inclined plane of weakness which represents a pre-existing shear zone is introduced in the lower section of the upper brittle layer at the start of the experiment. The length, width, orientation and dip of the weak zone are systematically varied between experiments to test how these parameters control the geometric and kinematic development of overlying normal fault systems. Consistent with our seismic reflection-based observations, our results show that strain is indeed localised in and above these weak zones. In the lower brittle layer, normal faults nucleate, as expected, within the zone of weakness and control the initiation and propagation of neighbouring faults. Above this, normal faults nucleate throughout the overlying strata where their orientations are strongly influenced by the underlying zone of weakness. These results challenge the notion that overburden normal faults simply form due to reactivation and upwards propagation of pre
NASA Astrophysics Data System (ADS)
Sakaris, C. S.; Sakellariou, J. S.; Fassois, S. D.
2016-06-01
This study focuses on the problem of vibration-based damage precise localization via data-based, time series type, methods for structures consisting of 1D, 2D, or 3D elements. A Generalized Functional Model Based method is postulated based on an expanded Vector-dependent Functionally Pooled ARX (VFP-ARX) model form, capable of accounting for an arbitrary structural topology. The FP model's operating parameter vector elements are properly constrained to reflect any given topology. Damage localization is based on operating parameter vector estimation within the specified topology, so that the location estimate and its uncertainty bounds are statistically optimal. The method's effectiveness is experimentally demonstrated through damage precise localization on a laboratory spatial truss structure using various damage scenarios and a single pair of random excitation - vibration response signals in a low and limited frequency bandwidth.
NASA Astrophysics Data System (ADS)
Sauer, Roger A.
2013-08-01
Recently an enriched contact finite element formulation has been developed that substantially increases the accuracy of contact computations while keeping the additional numerical effort at a minimum reported by Sauer (Int J Numer Meth Eng, 87: 593-616, 2011). Two enrich-ment strategies were proposed, one based on local p-refinement using Lagrange interpolation and one based on Hermite interpolation that produces C 1-smoothness on the contact surface. Both classes, which were initially considered for the frictionless Signorini problem, are extended here to friction and contact between deformable bodies. For this, a symmetric contact formulation is used that allows the unbiased treatment of both contact partners. This paper also proposes a post-processing scheme for contact quantities like the contact pressure. The scheme, which provides a more accurate representation than the raw data, is based on an averaging procedure that is inspired by mortar formulations. The properties of the enrichment strategies and the corresponding post-processing scheme are illustrated by several numerical examples considering sliding and peeling contact in the presence of large deformations.
A New Discrete Element Analysis Method for Predicting Hip Joint Contact Stresses
Abraham, Christine L.; Maas, Steve A.; Weiss, Jeffrey A.; Ellis, Benjamin J.; Peters, Christopher L.; Anderson, Andrew E.
2013-01-01
Quantifying cartilage contact stress is paramount to understanding hip osteoarthritis. Discrete element analysis (DEA) is a computationally efficient method to estimate cartilage contact stresses. Previous applications of DEA have underestimated cartilage stresses and yielded unrealistic contact patterns because they assumed constant cartilage thickness and/or concentric joint geometry. The study objectives were to: 1) develop a DEA model of the hip joint with subject-specific bone and cartilage geometry, 2) validate the DEA model by comparing DEA predictions to those of a validated finite element analysis (FEA) model, and 3) verify both the DEA and FEA models with a linear-elastic boundary value problem. Springs representing cartilage in the DEA model were given lengths equivalent to the sum of acetabular and femoral cartilage thickness and joint space in the FEA model. Material properties and boundary/loading conditions were equivalent. Walking, descending, and ascending stairs were simulated. Solution times for DEA and FEA models were ~7 seconds and ~65 minutes, respectively. Irregular, complex contact patterns predicted by DEA were in excellent agreement with FEA. DEA contact areas were 7.5%, 9.7% and 3.7% less than FEA for walking, descending stairs, and ascending stairs, respectively. DEA models predicted higher peak contact stresses (9.8–13.6 MPa) and average contact stresses (3.0–3.7 MPa) than FEA (6.2–9.8 and 2.0–2.5 MPa, respectively). DEA overestimated stresses due to the absence of the Poisson’s effect and a direct contact interface between cartilage layers. Nevertheless, DEA predicted realistic contact patterns when subject-specific bone geometry and cartilage thickness were used. This DEA method may have application as an alternative to FEA for pre-operative planning of joint-preserving surgery such as acetabular reorientation during peri-acetabular osteotomy. PMID:23453394
On the effects of geometry in discrete element numerical earthquake simulations
NASA Astrophysics Data System (ADS)
McGinnis, Seth Aaron
2001-07-01
Computer simulation is a widely-used component of earthquake research, but while many computer models of earthquakes exist, there are none that simulate both sub-fault activity and three-dimensional geometry. I develop a computer model of earthquakes that simulates activity on fault systems with three-dimensional geometry by calculating stress transfer between fault elements as a three-dimensional tensor quantity. This model is a discrete, quasi-static, cellular-automaton type model that generates failure cascade sequences of all sizes. The fault is represented as a collection of rectangular sub-faults or "fault patches" that are not constrained to a two-dimensional plane. Stress transfer is calculated as a tensor field originating from point sources in a linear elastic whole-space, though the effects of normal stress on the friction holding the surfaces of a fault element in place are neglected. I then develop a procedure for studying the effects of geometry on the evolution of synthetic event histories in a computer model by systematically varying the configuration of a z-shaped or "zig-zag" fault and studying the results using scaling, clustering, correlation, and phase dynamic probability change (PDPC) analysis. I also study the effects of roughness and coupling parameters. I find that, in the absence of normal stress effects, geometry does not act as a barrier to the development and propagation of events, but that differences in the rate of stress accumulation due to tectonic loading forces do; that geometric roughness does not change the dynamics of the system in a qualitative way; and that the PDPC analysis methodology cannot be effectively applied to simulation data of the quality that can be currently generated.
The ESyS_Particle: A New 3-D Discrete Element Model with Single Particle Rotation
NASA Astrophysics Data System (ADS)
Wang, Yucang; Mora, Peter
In this paper, the Discrete Element Model (DEM) is reviewed, and the ESyS_Particle, our new version of DEM, is introduced. We particularly highlight some of the major physical concerns about DEMs and major differences between our model and most current DEMs. In the new model, single particle rotation is introduced and represented by a unit quaternion. For each 3-D particle, six degrees of freedom are employed: three for translational motion, and three for orientation. Six kinds of relative motions are permitted between two neighboring particles, and six interactions are transferred, i.e., radial, two shearing forces, twisting and two bending torques. The relative rotation between two particles is decomposed into two sequence-independent rotations such that all interactions due to the relative motions between interactive rigid bodies can be uniquely determined. This algorithm can give more accurate results because physical principles are obeyed. A theoretical analysis about how to choose the model parameters is presented. Several numerical tests have been carried out, the results indicate that most laboratory tests can be well reproduced using our model.
Numerical Simulation of Dry Granular Flow Impacting a Rigid Wall Using the Discrete Element Method
Wu, Fengyuan; Fan, Yunyun; Liang, Li; Wang, Chao
2016-01-01
This paper presents a clump model based on Discrete Element Method. The clump model was more close to the real particle than a spherical particle. Numerical simulations of several tests of dry granular flow impacting a rigid wall flowing in an inclined chute have been achieved. Five clump models with different sphericity have been used in the simulations. By comparing the simulation results with the experimental results of normal force on the rigid wall, a clump model with better sphericity was selected to complete the following numerical simulation analysis and discussion. The calculation results of normal force showed good agreement with the experimental results, which verify the effectiveness of the clump model. Then, total normal force and bending moment of the rigid wall and motion process of the granular flow were further analyzed. Finally, comparison analysis of the numerical simulations using the clump model with different grain composition was obtained. By observing normal force on the rigid wall and distribution of particle size at the front of the rigid wall at the final state, the effect of grain composition on the force of the rigid wall has been revealed. It mainly showed that, with the increase of the particle size, the peak force at the retaining wall also increase. The result can provide a basis for the research of relevant disaster and the design of protective structures. PMID:27513661
The evolutional model of oblique-rifting basin : Insights from discrete element method
NASA Astrophysics Data System (ADS)
Cheng, I.-Wen; Yang, Kenn-Ming; Wu, Jong-Chang
2016-04-01
The geometry of oblique-rifting basin is strongly related with the angle (α) between the trend of rift and that of regional major extensional stress. The main purpose of this study is to investigate characteristics of geometry and kinematics of structure and tectono-stratigraphy during the evolution of oblique-rifting basin. In this study, we simulated the oblique-rifting basin model of various α with Particle Flow Code 3-Dimensions-(PFC 3D). The main theory of PFC 3D is based on the Discrete Element Method (DEM), in which parameters are applied to every particle in the models. We applied forces acting on both sides of rift axis, whichα are 45°, 60°, 75° and 90° respectively, to simulatebasin formation under oblique-rifting process. The study results of simulation models indicated that:1. the en echelon faults in the rifting basins are sub-orthogonal to the trend of major extensional stress; 2. the density of en echelon faults in rift basins decreasesgradually when α is close to 45°; 3. in these models, the α angles, which are 45°, 60°, 75° and 90°, correspond tothe angles of 0°, 15°-20°, 25°-30° and 50°-60° between the rift trend and en echelon faults trend. According tothe simulation results, the possible dircetions of major extensional stresses during the formation of oblique-rifting basin can be speculated.
A discrete element based simulation framework to investigate particulate spray deposition processes
Mukherjee, Debanjan Zohdi, Tarek I.
2015-06-01
This work presents a computer simulation framework based on discrete element method to analyze manufacturing processes that comprise a loosely flowing stream of particles in a carrier fluid being deposited on a target surface. The individual particulate dynamics under the combined action of particle collisions, fluid–particle interactions, particle–surface contact and adhesive interactions is simulated, and aggregated to obtain global system behavior. A model for deposition which incorporates the effect of surface energy, impact velocity and particle size, is developed. The fluid–particle interaction is modeled using appropriate spray nozzle gas velocity distributions and a one-way coupling between the phases. It is found that the particle response times and the release velocity distribution of particles have a combined effect on inter-particle collisions during the flow along the spray. It is also found that resolution of the particulate collisions close to the target surface plays an important role in characterizing the trends in the deposit pattern. Analysis of the deposit pattern using metrics defined from the particle distribution on the target surface is provided to characterize the deposition efficiency, deposit size, and scatter due to collisions.
Optimizing the Pipe Diameter of the Pipe Belt Conveyor Based on Discrete Element Method
NASA Astrophysics Data System (ADS)
Guo, Yong-cun; Wang, Shuang; Hu, Kun; Li, De-yong
2016-03-01
In order to increase the transport volume of the pipe belt conveyor and reduce lateral pressure of the supporting roller set, this study aims to optimize the pipe diameter of the pipe belt conveyor. A mechanical model of the pipe belt conveyor with six supporting roller sets in the belt bearing section was built based on the infinitesimal method, and the formula for calculating the lateral pressure of each supporting roller was deduced on the basis of reasonable assumption. Simulated analysis was carried out on the operation process of the pipe belt conveyor by using the discrete element method. The result showed that, when the other conditions were certain, as the pipe diameter increased, the average lateral pressure of the supporting roller set increased, with a gradually decreasing increment, which was consistent with the calculated result of the theoretical formula. An optimized pipe diameter under the current conditions was obtained by fitting the curve of the formula for calculating the transport volume of the pipe belt conveyor and its simulation curve. It provided a certain reference value for improving the transport efficiency and prolonging the service life of the pipe belt conveyor.
Microstructure-based modeling of snow mechanics: a discrete element approach
NASA Astrophysics Data System (ADS)
Hagenmuller, P.; Chambon, G.; Naaim, M.
2015-10-01
Rapid and large deformations of snow are mainly controlled by grain rearrangements, which occur through the failure of cohesive bonds and the creation of new contacts. We exploit a granular description of snow to develop a discrete element model based on the full 3-D microstructure captured by microtomography. The model assumes that snow is composed of rigid grains interacting through localized contacts accounting for cohesion and friction. The geometry of the grains and of the intergranular bonding system are explicitly defined from microtomographic data using geometrical criteria based on curvature and contiguity. Single grains are represented as rigid clumps of spheres. The model is applied to different snow samples subjected to confined compression tests. A detailed sensitivity analysis shows that artifacts introduced by the modeling approach and the influence of numerical parameters are limited compared to variations due to the geometry of the microstructure. The model shows that the compression behavior of snow is mainly controlled by the density of the samples, but that deviations from a pure density parameterization are not insignificant during the first phase of deformation. In particular, the model correctly predicts that, for a given density, faceted crystals are less resistant to compression than rounded grains or decomposed snow. For larger compression strains, no clear differences between snow types are observed.
Discrete element simulation of charging and mixed layer formation in the ironmaking blast furnace
NASA Astrophysics Data System (ADS)
Mitra, Tamoghna; Saxén, Henrik
2015-11-01
The burden distribution in the ironmaking blast furnace plays an important role for the operation as it affects the gas flow distribution, heat and mass transfer, and chemical reactions in the shaft. This work studies certain aspects of burden distribution by small-scale experiments and numerical simulation by the discrete element method (DEM). Particular attention is focused on the complex layer-formation process and the problems associated with estimating the burden layer distribution by burden profile measurements. The formation of mixed layers is studied, and a computational method for estimating the extent of the mixed layer, as well as its voidage, is proposed and applied on the results of the DEM simulations. In studying a charging program and its resulting burden distribution, the mixed layers of coke and pellets were found to show lower voidage than the individual burden layers. The dynamic evolution of the mixed layer during the charging process is also analyzed. The results of the study can be used to gain deeper insight into the complex charging process of the blast furnace, which is useful in the design of new charging programs and for mathematical models that do not consider the full behavior of the particles in the burden layers.
Shale Fracture Analysis using the Combined Finite-Discrete Element Method
NASA Astrophysics Data System (ADS)
Carey, J. W.; Lei, Z.; Rougier, E.; Knight, E. E.; Viswanathan, H.
2014-12-01
Hydraulic fracturing (hydrofrac) is a successful method used to extract oil and gas from highly carbonate rocks like shale. However, challenges exist for industry experts estimate that for a single $10 million dollar lateral wellbore fracking operation, only 10% of the hydrocarbons contained in the rock are extracted. To better understand how to improve hydrofrac recovery efficiencies and to lower its costs, LANL recently funded the Laboratory Directed Research and Development (LDRD) project: "Discovery Science of Hydraulic Fracturing: Innovative Working Fluids and Their Interactions with Rocks, Fractures, and Hydrocarbons". Under the support of this project, the LDRD modeling team is working with the experimental team to understand fracture initiation and propagation in shale rocks. LANL's hybrid hydro-mechanical (HM) tool, the Hybrid Optimization Software Suite (HOSS), is being used to simulate the complex fracture and fragment processes under a variety of different boundary conditions. HOSS is based on the combined finite-discrete element method (FDEM) and has been proven to be a superior computational tool for multi-fracturing problems. In this work, the comparison of HOSS simulation results to triaxial core flooding experiments will be presented.
Numerical sedimentation particle-size analysis using the Discrete Element Method
NASA Astrophysics Data System (ADS)
Bravo, R.; Pérez-Aparicio, J. L.; Gómez-Hernández, J. J.
2015-12-01
Sedimentation tests are widely used to determine the particle size distribution of a granular sample. In this work, the Discrete Element Method interacts with the simulation of flow using the well known one-way-coupling method, a computationally affordable approach for the time-consuming numerical simulation of the hydrometer, buoyancy and pipette sedimentation tests. These tests are used in the laboratory to determine the particle-size distribution of fine-grained aggregates. Five samples with different particle-size distributions are modeled by about six million rigid spheres projected on two-dimensions, with diameters ranging from 2.5 ×10-6 m to 70 ×10-6 m, forming a water suspension in a sedimentation cylinder. DEM simulates the particle's movement considering laminar flow interactions of buoyant, drag and lubrication forces. The simulation provides the temporal/spatial distributions of densities and concentrations of the suspension. The numerical simulations cannot replace the laboratory tests since they need the final granulometry as initial data, but, as the results show, these simulations can identify the strong and weak points of each method and eventually recommend useful variations and draw conclusions on their validity, aspects very difficult to achieve in the laboratory.
Study on small-strain behaviours of methane hydrate sandy sediments using discrete element method
Yu Yanxin; Cheng Yipik; Xu Xiaomin; Soga, Kenichi
2013-06-18
Methane hydrate bearing soil has attracted increasing interest as a potential energy resource where methane gas can be extracted from dissociating hydrate-bearing sediments. Seismic testing techniques have been applied extensively and in various ways, to detect the presence of hydrates, due to the fact that hydrates increase the stiffness of hydrate-bearing sediments. With the recognition of the limitations of laboratory and field tests, wave propagation modelling using Discrete Element Method (DEM) was conducted in this study in order to provide some particle-scale insights on the hydrate-bearing sandy sediment models with pore-filling and cementation hydrate distributions. The relationship between shear wave velocity and hydrate saturation was established by both DEM simulations and analytical solutions. Obvious differences were observed in the dependence of wave velocity on hydrate saturation for these two cases. From the shear wave velocity measurement and particle-scale analysis, it was found that the small-strain mechanical properties of hydrate-bearing sandy sediments are governed by both the hydrate distribution patterns and hydrate saturation.
Maginot, P. G.; Ragusa, J. C.; Morel, J. E.
2013-07-01
We examine several possible methods of mass matrix lumping for discontinuous finite element discrete ordinates transport using a Lagrange interpolatory polynomial trial space. Though positive outflow angular flux is guaranteed with traditional mass matrix lumping in a purely absorbing 1-D slab cell for the linear discontinuous approximation, we show that when used with higher degree interpolatory polynomial trial spaces, traditional lumping does yield strictly positive outflows and does not increase in accuracy with an increase in trial space polynomial degree. As an alternative, we examine methods which are 'self-lumping'. Self-lumping methods yield diagonal mass matrices by using numerical quadrature restricted to the Lagrange interpolatory points. Using equally-spaced interpolatory points, self-lumping is achieved through the use of closed Newton-Cotes formulas, resulting in strictly positive outflows in pure absorbers for odd power polynomials in 1-D slab geometry. By changing interpolatory points from the traditional equally-spaced points to the quadrature points of the Gauss-Legendre or Lobatto-Gauss-Legendre quadratures, it is possible to generate solution representations with a diagonal mass matrix and a strictly positive outflow for any degree polynomial solution representation in a pure absorber medium in 1-D slab geometry. Further, there is no inherent limit to local truncation error order of accuracy when using interpolatory points that correspond to the quadrature points of high order accuracy numerical quadrature schemes. (authors)
Zhou, Jing; Huang, Hai; Deo, Milind
2015-10-01
The interaction between hydraulic fractures (HF) and natural fractures (NF) will lead to complex fracture networks due to the branching and merging of natural and hydraulic fractures in unconventional reservoirs. In this paper, a newly developed hydraulic fracturing simulator based on discrete element method is used to predict the generation of complex fracture network in the presence of pre-existing natural fractures. By coupling geomechanics and reservoir flow within a dual lattice system, this simulator can effectively capture the poro-elastic effects and fluid leakoff into the formation. When HFs are intercepting single or multiple NFs, complex mechanisms such as direct crossing, arresting, dilating and branching can be simulated. Based on the model, the effects of injected fluid rate and viscosity, the orientation and permeability of NFs and stress anisotropy on the HF-NF interaction process are investigated. Combined impacts from multiple parameters are also examined in the paper. The numerical results show that large values of stress anisotropy, intercepting angle, injection rate and viscosity will impede the opening of NFs.
A discrete element based simulation framework to investigate particulate spray deposition processes
NASA Astrophysics Data System (ADS)
Mukherjee, Debanjan; Zohdi, Tarek I.
2015-06-01
This work presents a computer simulation framework based on discrete element method to analyze manufacturing processes that comprise a loosely flowing stream of particles in a carrier fluid being deposited on a target surface. The individual particulate dynamics under the combined action of particle collisions, fluid-particle interactions, particle-surface contact and adhesive interactions is simulated, and aggregated to obtain global system behavior. A model for deposition which incorporates the effect of surface energy, impact velocity and particle size, is developed. The fluid-particle interaction is modeled using appropriate spray nozzle gas velocity distributions and a one-way coupling between the phases. It is found that the particle response times and the release velocity distribution of particles have a combined effect on inter-particle collisions during the flow along the spray. It is also found that resolution of the particulate collisions close to the target surface plays an important role in characterizing the trends in the deposit pattern. Analysis of the deposit pattern using metrics defined from the particle distribution on the target surface is provided to characterize the deposition efficiency, deposit size, and scatter due to collisions.
Discrete Element Simulations of Granular Flow in a Pebble Bed Nuclear Reactor
NASA Astrophysics Data System (ADS)
Grest, Gary S.; Rycroft, Chris H.; Landry, James W.
2005-03-01
Pebble-bed reactor technology, which is currently being revived around the world, raises fundamental questions about granular flow in silos. The reactor core is composed of spherical billiard-ball sized (6cm diameter) graphite fuel pebbles containing sand-sized uranium fuel particles. The fuel pebbles drain very slowly through the core as a continuous refueling process. In some designs, a dynamical central column is formed from graphite moderator pebbles, physically identical to the fuel pebbles without any fuel. The total number of pebbles is of order 440,000 in a cell approximately 3.5m in diameter and 8.5m tall. Using discrete element (molecular dynamics) simulations we have studied a full scale model of the system. We find that the interface between the fuel and moderator particles remains sharp, as there is very little horizontal motion of the pebbles as they flow through the reactor. We measure mean velocity profiles and compare to various continuum models. We also investigated the feasibility of a bi-disperse core, containing smaller moderator pebbles, with the same size fuel pebbles, which could improve performance by focusing helium gas flow on the hotter fuel region. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04- 94AL85000.
Just, Sarah; Toschkoff, Gregor; Funke, Adrian; Djuric, Dejan; Scharrer, Georg; Khinast, Johannes; Knop, Klaus; Kleinebudde, Peter
2013-03-01
Coating of solid dosage forms is an important unit operation in the pharmaceutical industry. In recent years, numerical simulations of drug manufacturing processes have been gaining interest as process analytical technology tools. The discrete element method (DEM) in particular is suitable to model tablet-coating processes. For the development of accurate simulations, information on the material properties of the tablets is required. In this study, the mechanical parameters Young's modulus, coefficient of restitution (CoR), and coefficients of friction (CoF) of gastrointestinal therapeutic systems (GITS) and of active-coated GITS were measured experimentally. The dynamic angle of repose of these tablets in a drum coater was investigated to revise the CoF. The resulting values were used as input data in DEM simulations to compare simulation and experiment. A mean value of Young's modulus of 31.9 MPa was determined by the uniaxial compression test. The CoR was found to be 0.78. For both tablet-steel and tablet-tablet friction, active-coated GITS showed a higher CoF compared with GITS. According to the values of the dynamic angle of repose, the CoF was adjusted to obtain consistent tablet motion in the simulation and in the experiment. On the basis of this experimental characterization, mechanical parameters are integrated into DEM simulation programs to perform numerical analysis of coating processes. PMID:23354469
DNS with Discrete Element Modeling of Suspended Sediment Particles in an Open Channel Flow
NASA Astrophysics Data System (ADS)
Paksereht, Pedram; Apte, Sourabh; Finn, Justin
2015-11-01
Interactions of glass particles in water in a turbulent open channel flow over a smooth bed with gravity perpendicular to the mean flow is examined using direct numerical simulation (DNS) together with Lagrangian Discrete-Element-Model (DEM) for particles. The turbulent Reynolds number (Reτ) is 710 corresponding to the experimental observations of Righetti & Romano (JFM, 2004). Particles of size 200 microns with volume loading on the order of 10-3 are simulated using four-way coupling with standard models for drag, added mass, lift, pressure, and inter-particle collision forces. The presence of particles affect the outer as well as inner region of the wall layer where particle inertia and concentration are higher. The DNS-DEM is able to capture the fluid-particle interactions in the outer layer accurately. However, in the inner layer, an increase in mean as well as rms fluid velocity, as observed in the experiments, is not predicted by the DNS-DEM model. It is conjectured that particles slide and roll on the bottom wall, creating slip-like condition. Predictions using different models for drag and lift forces, as well as strong torque coupling are explored and compared with experimental data. Funding: NSF project #1133363, Sediment-Bed-Turbulence Coupling in Oscillatory Flows.
Borehole Breakouts Induced in Arkosic Sandstones and a Discrete Element Analysis
NASA Astrophysics Data System (ADS)
Lee, H.; Moon, T.; Haimson, B. C.
2016-04-01
A series of laboratory drilling experiments were conducted on two arkosic sandstones (Tenino and Tablerock) under polyaxial far-field stress conditions (σ h ≠ σ H ≠ σ v ). V-shaped breakouts, aligned with the σ h direction and revealing stress-dependent dimensions (width and length), were observed in the sandstones. The microscale damage pattern leading to the breakouts, however, is different between the two, which is attributed to the difference in their cementation. The dominant micromechanism in Tenino sandstone is intergranular microcracking occurring in clay minerals filling the spaces between clastic grains. On the other hand, intra- and transgranular microcracking taking place in the grain itself prevails in Tablerock sandstone. To capture the grain-scale damage and reproduce the failure localization observed around the borehole in the laboratory, we used a discrete element (DE) model in which a grain breakage algorithm was implemented. The microparameters needed in the numerical model were calibrated by running material tests and comparing the macroscopic responses of the model to the ones measured in the laboratory. It is shown that DE modeling is capable of simulating the microscale damage of the rock and replicating the localized damage zone observed in the laboratory. In addition, the numerically induced breakout width is determined at a very early stage of the damage localization and is not altered for the rest of the failure process.
Numerical Simulation of Dry Granular Flow Impacting a Rigid Wall Using the Discrete Element Method.
Wu, Fengyuan; Fan, Yunyun; Liang, Li; Wang, Chao
2016-01-01
This paper presents a clump model based on Discrete Element Method. The clump model was more close to the real particle than a spherical particle. Numerical simulations of several tests of dry granular flow impacting a rigid wall flowing in an inclined chute have been achieved. Five clump models with different sphericity have been used in the simulations. By comparing the simulation results with the experimental results of normal force on the rigid wall, a clump model with better sphericity was selected to complete the following numerical simulation analysis and discussion. The calculation results of normal force showed good agreement with the experimental results, which verify the effectiveness of the clump model. Then, total normal force and bending moment of the rigid wall and motion process of the granular flow were further analyzed. Finally, comparison analysis of the numerical simulations using the clump model with different grain composition was obtained. By observing normal force on the rigid wall and distribution of particle size at the front of the rigid wall at the final state, the effect of grain composition on the force of the rigid wall has been revealed. It mainly showed that, with the increase of the particle size, the peak force at the retaining wall also increase. The result can provide a basis for the research of relevant disaster and the design of protective structures. PMID:27513661
Nye, Ben; Kulchitsky, Anton V; Johnson, Jerome B
2014-01-01
This paper describes a new method for representing concave polyhedral particles in a discrete element method as unions of convex dilated polyhedra. This method offers an efficient way to simulate systems with a large number of (generally concave) polyhedral particles. The method also allows spheres, capsules, and dilated triangles to be combined with polyhedra using the same approach. The computational efficiency of the method is tested in two different simulation setups using different efficiency metrics for seven particle types: spheres, clusters of three spheres, clusters of four spheres, tetrahedra, cubes, unions of two octahedra (concave), and a model of a computer tomography scan of a lunar simulant GRC-3 particle. It is shown that the computational efficiency of the simulations degrades much slower than the increase in complexity of the particles in the system. The efficiency of the method is based on the time coherence of the system, and an efficient and robust distance computation method between polyhedra as particles never intersect for dilated particles. PMID:26300584
Numerical modelling of granular subglacial deformation using the discrete element method
NASA Astrophysics Data System (ADS)
Damsgaard, Anders; Egholm, David L.; Piotrowski, Jan A.; Tulaczyk, Slawek; Larsen, Nicolaj K.
2013-04-01
We apply the Discrete Element Method (DEM) to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to warm-based subglacial environments. As a method of validation of the simulated macromechanical behavior, and as a calibration benchmark, we compare results from successive laboratory ring-shear experiments on simple granular materials to results from similar numerical experiments. Overall, there is good agreement between the geotechnical behavior of the materials in the analogue and numerical experiments, and materials deform by an elasto-plastic rheology under the applied effective normal stress and shear velocity. By using the numerical approach it is possible to make a detailed analysis of the material dynamics and shear zone development during progressive shear strain. By visualizing the particles according to the sum of contact forces exerted onto them, the geometry of the heterogeneous stress network is visible in the form of force-carrying grain bridges and adjacent, volumetrically dominant, inactive zones. We demonstrate how the shear zone thickness and dilation is dependent on the effective deviatoric normal stress, where higher stresses mobilize material to greater depths. The data-parallel nature of the basic DEM formulation makes the problem ideal for utilizing the high arithmetic potential of modern general-purpose GPU's. Using the Nvidia Cuda C toolkit, the algorithm is formulated for spherical particles in three dimensions with a linear-elastic soft-body contact model.
NASA Astrophysics Data System (ADS)
Yan, Z.; Wilkinson, S. K.; Stitt, E. H.; Marigo, M.
2015-09-01
Selection or calibration of particle property input parameters is one of the key problematic aspects for the implementation of the discrete element method (DEM). In the current study, a parametric multi-level sensitivity method is employed to understand the impact of the DEM input particle properties on the bulk responses for a given simple system: discharge of particles from a flat bottom cylindrical container onto a plate. In this case study, particle properties, such as Young's modulus, friction parameters and coefficient of restitution were systematically changed in order to assess their effect on material repose angles and particle flow rate (FR). It was shown that inter-particle static friction plays a primary role in determining both final angle of repose and FR, followed by the role of inter-particle rolling friction coefficient. The particle restitution coefficient and Young's modulus were found to have insignificant impacts and were strongly cross correlated. The proposed approach provides a systematic method that can be used to show the importance of specific DEM input parameters for a given system and then potentially facilitates their selection or calibration. It is concluded that shortening the process for input parameters selection and calibration can help in the implementation of DEM.
NASA Technical Reports Server (NTRS)
Li, Fei; Choudhari, Meelan M.; Chang, Chau-Lyan; Streett, Craig L.; Carpenter, Mark H.
2011-01-01
A combination of parabolized stability equations and secondary instability theory has been applied to a low-speed swept airfoil model with a chord Reynolds number of 7.15 million, with the goals of (i) evaluating this methodology in the context of transition prediction for a known configuration for which roughness based crossflow transition control has been demonstrated under flight conditions and (ii) of analyzing the mechanism of transition delay via the introduction of discrete roughness elements (DRE). Roughness based transition control involves controlled seeding of suitable, subdominant crossflow modes, so as to weaken the growth of naturally occurring, linearly more unstable crossflow modes. Therefore, a synthesis of receptivity, linear and nonlinear growth of stationary crossflow disturbances, and the ensuing development of high frequency secondary instabilities is desirable to understand the experimentally observed transition behavior. With further validation, such higher fidelity prediction methodology could be utilized to assess the potential for crossflow transition control at even higher Reynolds numbers, where experimental data is currently unavailable.
NASA Astrophysics Data System (ADS)
Gallego, A.; Moreno-García, P.; Casanova, Cesar F.
2013-06-01
Structural studies to find defects (in particular delaminations) in composite plates have been very prevalent in the Structural Health Monitoring field. The present work develops a new method to detect delaminations in CFRP (Carbon Fiber Reinforced Polymer) plates. In this paper the method is validated with numerical simulations, which come to support its adequacy for use with real acquisition data. This is done firstly through the implementation of a delaminated plate finite element. Using the classical lamination plate theory, delamination is considered in the kinematic equations through jump functions and additional degrees of freedom. The element allows the introduction of nd delaminations through its thickness. Classical QMITC (Quadrilateral Mixed Interpolation Tensorial Components) and DKQ (Discrete Kirchhoff Quadrilateral) elements are used for the membrane and bending FEM (Finite Element Method) formulation. Second, using the vibration modes obtained with the FEM, a damage location technique based on the variational Ritz method and Wavelet Analysis is proposed. The approach has the advantage of requiring only damaged modes and not the healthy ones. Both FEM simulations and Ritz/Wavelet damage detection schemes are applied in an orthotropic CFRP plate with the stacking sequence [0/90]3S. In addition, the influence of delamination thickness position, boundary conditions and added noise (in order to simulate experimental measures) was studied.
Sanchez, R.
2012-07-01
Nonlinear acceleration of a continuous finite element (CFE) discretization of the transport equation requires a modification of the transport solution in order to achieve local conservation, a condition used in nonlinear acceleration to define the stopping criterion. In this work we implement a coarse-mesh finite difference acceleration for a CFE discretization of the second-order self adjoint angular flux (SAAF) form of the transport equation and use a post processing to enforce local conservation. Numerical results are given for one-group source calculations of one-dimensional slabs. We also give a formal derivation of the boundary conditions for the SAAF. (authors)
Richard Sanchez; Cristian Rabiti; Yaqi Wang
2013-11-01
Nonlinear acceleration of a continuous finite element (CFE) discretization of the transport equation requires a modification of the transport solution in order to achieve local conservation, a condition used in nonlinear acceleration to define the stopping criterion. In this work we implement a coarse-mesh finite difference acceleration for a CFE discretization of the second-order self-adjoint angular flux (SAAF) form of the transport equation and use a postprocessing to enforce local conservation. Numerical results are given for one-group source calculations of one-dimensional slabs. We also give a novel formal derivation of the boundary conditions for the SAAF.
MAGNUM-2D computer code: user's guide
England, R.L.; Kline, N.W.; Ekblad, K.J.; Baca, R.G.
1985-01-01
Information relevant to the general use of the MAGNUM-2D computer code is presented. This computer code was developed for the purpose of modeling (i.e., simulating) the thermal and hydraulic conditions in the vicinity of a waste package emplaced in a deep geologic repository. The MAGNUM-2D computer computes (1) the temperature field surrounding the waste package as a function of the heat generation rate of the nuclear waste and thermal properties of the basalt and (2) the hydraulic head distribution and associated groundwater flow fields as a function of the temperature gradients and hydraulic properties of the basalt. MAGNUM-2D is a two-dimensional numerical model for transient or steady-state analysis of coupled heat transfer and groundwater flow in a fractured porous medium. The governing equations consist of a set of coupled, quasi-linear partial differential equations that are solved using a Galerkin finite-element technique. A Newton-Raphson algorithm is embedded in the Galerkin functional to formulate the problem in terms of the incremental changes in the dependent variables. Both triangular and quadrilateral finite elements are used to represent the continuum portions of the spatial domain. Line elements may be used to represent discrete conduits. 18 refs., 4 figs., 1 tab.
NASA Astrophysics Data System (ADS)
Yoon, Jeoung Seok; Zang, Arno; Zimmermann, Günter; Stephansson, Ove
2016-04-01
Operation of fluid injection into and withdrawal from the subsurface for various purposes has been known to induce earthquakes. Such operations include hydraulic fracturing for shale gas extraction, hydraulic stimulation for Enhanced Geothermal System development and waste water disposal. Among these, several damaging earthquakes have been reported in the USA in particular in the areas of high-rate massive amount of wastewater injection [1] mostly with natural fault systems. Oil and gas production have been known to induce earthquake where pore fluid pressure decreases in some cases by several tens of Mega Pascal. One recent seismic event occurred in November 2013 near Azle, Texas where a series of earthquakes began along a mapped ancient fault system [2]. It was studied that a combination of brine production and waste water injection near the fault generated subsurface pressures sufficient to induced earthquakes on near-critically stressed faults. This numerical study aims at investigating the occurrence mechanisms of such earthquakes induced by fluid injection [3] and withdrawal by using hydro-geomechanical coupled dynamic simulator (Itasca's Particle Flow Code 2D). Generic models are setup to investigate the sensitivity of several parameters which include fault orientation, frictional properties, distance from the injection well to the fault, amount of fluid withdrawal around the injection well, to the response of the fault systems and the activation magnitude. Fault slip movement over time in relation to the diffusion of pore pressure is analyzed in detail. Moreover, correlations between the spatial distribution of pore pressure change and the locations of induced seismic events and fault slip rate are investigated. References [1] Keranen KM, Weingarten M, Albers GA, Bekins BA, Ge S, 2014. Sharp increase in central Oklahoma seismicity since 2008 induced by massive wastewater injection, Science 345, 448, DOI: 10.1126/science.1255802. [2] Hornbach MJ, DeShon HR
Lerche, Ernesto; Van Eester, Dirk
2011-12-23
Fourier analysis in the poloidal direction is a standard ingredient in present-day 2D wave equation solvers describing radio frequency waves in hot tokamak plasmas. Although a powerful and elegant technique, Fourier analysis has the disadvantage that a large number of modes is needed to describe the field pattern on a magnetic surface if a short wavelength mode exists on any - even very small - subpart of the particle trajectory. The present paper examines the potential of a method that does not suffer from this drawback: a finite element technique relying on simple linear or cubic area base functions that are defined on irregular elementary surfaces of triangular shape. The wave equation is solved in its weak Galerkin variational form and for realistic 2D tokamak geometry, accounting for the toroidal curvature but assuming the toroidal angle is ignorable, allowing to study the wave pattern for each of the independent toroidal modes excited by the antenna individually.The locally uniform full hot plasma dielectric tensor to all orders in finite Larmor radius was adopted. As the main intended application is the study of fast wave behavior (heating and current drive) at arbitrary harmonics, the wave vector complex amplitude appearing in the dielectric tensor is determined through a local dispersion root evaluation. High frequency fast wave propagation and damping is provided as an illustration in view of possible application of this type of current drive in future high density reactor-like tokamaks.
NASA Astrophysics Data System (ADS)
Key, K.
2013-12-01
This work announces the public release of an open-source inversion code named MARE2DEM (Modeling with Adaptively Refined Elements for 2D Electromagnetics). Although initially designed for the rapid inversion of marine electromagnetic data, MARE2DEM now supports a wide variety of acquisition configurations for both offshore and onshore surveys that utilize electric and magnetic dipole transmitters or magnetotelluric plane waves. The model domain is flexibly parameterized using a grid of arbitrarily shaped polygonal regions, allowing for complicated structures such as topography or seismically imaged horizons to be easily assimilated. MARE2DEM efficiently solves the forward problem in parallel by dividing the input data parameters into smaller subsets using a parallel data decomposition algorithm. The data subsets are then solved in parallel using an automatic adaptive finite element method that iterative solves the forward problem on successively refined finite element meshes until a specified accuracy tolerance is met, thus freeing the end user from the burden of designing an accurate numerical modeling grid. Regularized non-linear inversion for isotropic or anisotropic conductivity is accomplished with a new implementation of Occam's method referred to as fast-Occam, which is able to minimize the objective function in much fewer forward evaluations than the required by the original method. This presentation will review the theoretical considerations behind MARE2DEM and use a few recent offshore EM data sets to demonstrate its capabilities and to showcase the software interface tools that streamline model building and data inversion.
Discrete Element Method (DEM) Application to The Cone Penetration Test Using COUPi Model
NASA Astrophysics Data System (ADS)
Kulchitsky, A. V.; Johnson, J.; Wilkinson, A.; DeGennaro, A. J.; Duvoy, P.
2011-12-01
The cone penetration test (CPT) is a soil strength measurement method to determine the tip resistance and sleeve friction versus depth while pushing a cone into regolith with controlled slow quasi-static speed. This test can also be used as an excellent tool to validate the discrete element method (DEM) model by comparing tip resistance and sleeve friction from experiments to model results. DEM by nature requires significant computational resources even for a limited number of particles. Thus, it is important to find particle and ensemble parameters that produce valuable results within reasonable computation times. The Controllable Objects Unbounded Particles Interaction (COUPi) model is a general physical DEM code being developed to model machine/regolith interactions as part of a NASA Lunar Science Institute sponsored project on excavation and mobility modeling. In this work, we consider how different particle shape and size distributions defined in the DEM influence the cone tip and friction sleeve resistance in a CPT DEM simulation. The results are compared to experiments with cone penetration in JSC-1A lunar regolith simulant. The particle shapes include spherical particles, particles composed from the union of three spheres, and some simple polyhedra. This focus is driven by the soil mechanics rule of thumb that particle size and shape distributions are the two most significant factors affecting soil strength. In addition to the particle properties, the packing configuration of an ensemble strongly affects soil strength. Bulk density of the regolith is an important characteristic that significantly influences the tip resistance and sleeve friction (Figure 1). We discuss different approaches used to control granular density in the DEM, including how to obtain higher bulk densities, using numerical "shaking" techniques and varying the friction coefficient during computations.
Fish Passage though Hydropower Turbines: Simulating Blade Strike using the Discrete Element Method
Richmond, Marshall C.; Romero Gomez, Pedro DJ
2014-12-08
mong the hazardous hydraulic conditions affecting anadromous and resident fish during their passage though turbine flows, two are believed to cause considerable injury and mortality: collision on moving blades and decompression. Several methods are currently available to evaluate these stressors in installed turbines, i.e. using live fish or autonomous sensor devices, and in reduced-scale physical models, i.e. registering collisions from plastic beads. However, a priori estimates with computational modeling approaches applied early in the process of turbine design can facilitate the development of fish-friendly turbines. In the present study, we evaluated the frequency of blade strike and nadir pressure environment by modeling potential fish trajectories with the Discrete Element Method (DEM) applied to fish-like composite particles. In the DEM approach, particles are subjected to realistic hydraulic conditions simulated with computational fluid dynamics (CFD), and particle-structure interactions—representing fish collisions with turbine blades—are explicitly recorded and accounted for in the calculation of particle trajectories. We conducted transient CFD simulations by setting the runner in motion and allowing for better turbulence resolution, a modeling improvement over the conventional practice of simulating the system in steady state which was also done here. While both schemes yielded comparable bulk hydraulic performance, transient conditions exhibited a visual improvement in describing flow variability. We released streamtraces (steady flow solution) and DEM particles (transient solution) at the same location from where sensor fish (SF) have been released in field studies of the modeled turbine unit. The streamtrace-based results showed a better agreement with SF data than the DEM-based nadir pressures did because the former accounted for the turbulent dispersion at the intake but the latter did not. However, the DEM-based strike frequency is more
NASA Astrophysics Data System (ADS)
Duan, K.; Kwok, C. Y.
2016-04-01
The aim of this study is to better understand the mechanisms controlling the initiation, propagation, and ultimate pattern of borehole breakouts in shale formation when drilled parallel with and perpendicular to beddings. A two-dimensional discrete element model is constructed to explicitly represent the microstructure of inherently anisotropic rocks by inserting a series of individual smooth joints into an assembly of bonded rigid discs. Both isotropic and anisotropic hollow square-shaped samples are generated to represent the wellbores drilled perpendicular to and parallel with beddings at reduced scale. The isotropic model is validated by comparing the stress distribution around borehole wall and along X axis direction with analytical solutions. Effects of different factors including the particle size distribution, borehole diameter, far-field stress anisotropy, and rock anisotropy are systematically evaluated on the stress distribution and borehole breakout propagation. Simulation results reveal that wider particle size distribution results in the local stress perturbations which cause localization of cracks. Reduction of borehole diameter significantly alters the crack failure from tensile to shear and raises the critical pressure. Rock anisotropy plays an important role on the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress. Far-field stress anisotropy plays a dominant role in the shape of borehole breakout when drilled perpendicular to beddings while a secondary role when drilled parallel with beddings. Results from this study can provide fundamental insights on the underlying particle-scale mechanisms for previous findings in laboratory and field on borehole stability in anisotropic rock.
Discrete Element Modeling of Stick-Slip Instability and Induced Microseismicity
NASA Astrophysics Data System (ADS)
Khazaei, Cyrus; Hazzard, Jim; Chalaturnyk, Rick
2016-03-01
Using Particle Flow Code, a discrete element model is presented in this paper that allows direct modeling of stick-slip behavior in pre-existing weak planes such as joints, beddings, and faults. The model is used to simulate a biaxial sliding experiment from literature on a saw-cut specimen of Sierra granite with a single fault. The fault is represented by the smooth-joint contact model. Also, an algorithm is developed to record the stick-slip induced microseismic events along the fault. Once the results compared well with laboratory data, a parametric study was conducted to investigate the evolution of the model's behavior due to varying factors such as resolution of the model, particle elasticity, fault coefficient of friction, fault stiffness, and normal stress. The results show a decrease in shear strength of the fault in the models with smaller particles, smaller coefficient of friction of the fault, harder fault surroundings, softer faults, and smaller normal stress on the fault. Also, a higher rate of displacement was observed for conditions resulting in smaller shear strength. An increase in b-values was observed by increasing the resolution or decreasing the normal stress on the fault, while b-values were not sensitive to changes in elasticity of the fault or its surrounding region. A larger number of recorded events were observed for the models with finer particles, smaller coefficient of friction of the fault, harder fault surroundings, harder fault, and smaller normal stress on the fault. The results suggest that it is possible for the two ends of a fault to be still while there are patches along the fault undergoing stick-slips. Such local stick-slips seem to provide a softer surrounding for their neighbor patches facilitating their subsequent stick-slips.
Investigation of Crack Propagation in Rock using Discrete Sphero-Polyhedral Element Method
NASA Astrophysics Data System (ADS)
Behraftar, S.; Galindo-torres, S. A.; Scheuermann, A.; Li, L.; Williams, D.
2014-12-01
In this study a micro-mechanical model is developed to study the fracture propagation process in rocks. The model is represented by an array of bonded particles simulated by the Discrete Sphero-Polyhedral Element Model (DSEM), which was introduced by the authors previously and has been shown to be a suitable technique to model rock [1]. It allows the modelling of particles of general shape, with no internal porosity. The motivation behind using this technique is the desire to microscopically investigate the fracture propagation process and study the relationship between the microscopic and macroscopic behaviour of rock. The DSEM method is used to model the Crack Chevron Notch Brazilian Disc (CCNBD) test suggested by the International Society of Rock Mechanics (ISRM) for determining the fracture toughness of rock specimens. CCNBD samples with different crack inclination angles, are modelled to investigate their fracture mode. The Crack Mouth Opening Displacement (CMOD) is simulated and the results are validated using experimental results obtained from a previous study [2]. Fig. 1 shows the simulated and experimental results of crack propagation for different inclination angles of CCNBD specimens. The DSEM method can be used to predict crack trajectory and quantify crack propagation during loading. References: 1. Galindo-Torres, S. A., et al. "Breaking processes in three-dimensional bonded granular materials with general shapes." Computer Physics Communications 183.2 (2012): 266-277. 2. Erarslan, N., and D. J. Williams. "Mixed-mode fracturing of rocks under static and cyclic loading." Rock mechanics and rock engineering 46.5 (2013): 1035-1052.
Utilizing the Discrete Element Method for the Modeling of Viscosity in Concentrated Suspensions.
Kroupa, Martin; Vonka, Michal; Soos, Miroslav; Kosek, Juraj
2016-08-23
The rheological behavior of concentrated suspensions is a complicated problem because it originates in the collective motion of particles and their interaction with the surrounding fluid. For this reason, it is difficult to accurately model the effect of various system parameters on the viscosity even for highly simplified systems. We model the viscosity of a hard-sphere suspension subjected to high shear rates using the dynamic discrete element method (DEM) in three spatial dimensions. The contact interaction between particles was described by the Hertz model of elastic spheres (soft-sphere model), and the interaction of particles with flow was accounted for by the two-way coupling approach. The hydrodynamic interaction between particles was described by the lubrication theory accounting for the slip on particle surfaces. The viscosity in a simple-shear model was evaluated from the force balance on the wall. The obtained results are in close agreement with literature data for systems with hard spheres. Namely, the viscosity is shown to be independent of shear rate and primary particle size for monodisperse suspensions. In accordance with theory and experimental data, the viscosity grows rapidly with particle volume fraction. We show that this rheological behavior is predominantly caused by the lubrication forces. A novel approach based on the slip of water on a particle surface was developed to overcome the divergent behavior of lubrication forces. This approach was qualitatively validated with literature data from AFM measurements using a colloidal probe. The model presented in this work represents a new, robust, and versatile approach to the modeling of viscosity in suspensions with the possibility to include various interaction models and study their effect on viscosity. PMID:27479150
Modeling of crack propagation in weak snowpack layers using the discrete element method
NASA Astrophysics Data System (ADS)
Gaume, J.; van Herwijnen, A.; Chambon, G.; Schweizer, J.; Birkeland, K. W.
2015-01-01
Dry-snow slab avalanches are generally caused by a sequence of fracture processes including (1) failure initiation in a weak snow layer underlying a cohesive slab, (2) crack propagation within the weak layer and (3) tensile fracture through the slab which leads to its detachment. During the past decades, theoretical and experimental work has gradually led to a better understanding of the fracture process in snow involving the collapse of the structure in the weak layer during fracture. This now allows us to better model failure initiation and the onset of crack propagation, i.e. to estimate the critical length required for crack propagation. On the other hand, our understanding of dynamic crack propagation and fracture arrest propensity is still very limited. For instance, it is not uncommon to perform field measurements with widespread crack propagation on one day, while a few days later, with very little changes to the snowpack, crack propagation does not occur anymore. Thus far, there is no clear theoretical framework to interpret such observations, and it is not clear how and which snowpack properties affect dynamic crack propagation. To shed more light on this issue, we performed numerical propagation saw test (PST) experiments applying the discrete element (DE) method and compared the numerical results with field measurements based on particle tracking. The goal is to investigate the influence of weak layer failure and the mechanical properties of the slab on crack propagation and fracture arrest propensity. Crack propagation speeds and distances before fracture arrest were derived from the DE simulations for different snowpack configurations and mechanical properties. Then, the relation between mechanical parameters of the snowpack was taken into account so as to compare numerical and experimental results, which were in good agreement, suggesting that the simulations can reproduce crack propagation in PSTs. Finally, an in-depth analysis of the mechanical
Modeling of crack propagation in weak snowpack layers using the discrete element method
NASA Astrophysics Data System (ADS)
Gaume, J.; van Herwijnen, A.; Chambon, G.; Birkeland, K. W.; Schweizer, J.
2015-10-01
Dry-snow slab avalanches are generally caused by a sequence of fracture processes including (1) failure initiation in a weak snow layer underlying a cohesive slab, (2) crack propagation within the weak layer and (3) tensile fracture through the slab which leads to its detachment. During the past decades, theoretical and experimental work has gradually led to a better understanding of the fracture process in snow involving the collapse of the structure in the weak layer during fracture. This now allows us to better model failure initiation and the onset of crack propagation, i.e., to estimate the critical length required for crack propagation. On the other hand, our understanding of dynamic crack propagation and fracture arrest propensity is still very limited. To shed more light on this issue, we performed numerical propagation saw test (PST) experiments applying the discrete element (DE) method and compared the numerical results with field measurements based on particle tracking. The goal is to investigate the influence of weak layer failure and the mechanical properties of the slab on crack propagation and fracture arrest propensity. Crack propagation speeds and distances before fracture arrest were derived from the DE simulations for different snowpack configurations and mechanical properties. Then, in order to compare the numerical and experimental results, the slab mechanical properties (Young's modulus and strength) which are not measured in the field were derived from density. The simulations nicely reproduced the process of crack propagation observed in field PSTs. Finally, the mechanical processes at play were analyzed in depth which led to suggestions for minimum column length in field PSTs.
NASA Astrophysics Data System (ADS)
Wirtz, T.; Philipp, P.; Audinot, J.-N.; Dowsett, D.; Eswara, S.
2015-10-01
Secondary ion mass spectrometry (SIMS) constitutes an extremely sensitive technique for imaging surfaces in 2D and 3D. Apart from its excellent sensitivity and high lateral resolution (50 nm on state-of-the-art SIMS instruments), advantages of SIMS include high dynamic range and the ability to differentiate between isotopes. This paper first reviews the underlying principles of SIMS as well as the performance and applications of 2D and 3D SIMS elemental imaging. The prospects for further improving the capabilities of SIMS imaging are discussed. The lateral resolution in SIMS imaging when using the microprobe mode is limited by (i) the ion probe size, which is dependent on the brightness of the primary ion source, the quality of the optics of the primary ion column and the electric fields in the near sample region used to extract secondary ions; (ii) the sensitivity of the analysis as a reasonable secondary ion signal, which must be detected from very tiny voxel sizes and thus from a very limited number of sputtered atoms; and (iii) the physical dimensions of the collision cascade determining the origin of the sputtered ions with respect to the impact site of the incident primary ion probe. One interesting prospect is the use of SIMS-based correlative microscopy. In this approach SIMS is combined with various high-resolution microscopy techniques, so that elemental/chemical information at the highest sensitivity can be obtained with SIMS, while excellent spatial resolution is provided by overlaying the SIMS images with high-resolution images obtained by these microscopy techniques. Examples of this approach are given by presenting in situ combinations of SIMS with transmission electron microscopy (TEM), helium ion microscopy (HIM) and scanning probe microscopy (SPM).
Wirtz, T; Philipp, P; Audinot, J-N; Dowsett, D; Eswara, S
2015-10-30
Secondary ion mass spectrometry (SIMS) constitutes an extremely sensitive technique for imaging surfaces in 2D and 3D. Apart from its excellent sensitivity and high lateral resolution (50 nm on state-of-the-art SIMS instruments), advantages of SIMS include high dynamic range and the ability to differentiate between isotopes. This paper first reviews the underlying principles of SIMS as well as the performance and applications of 2D and 3D SIMS elemental imaging. The prospects for further improving the capabilities of SIMS imaging are discussed. The lateral resolution in SIMS imaging when using the microprobe mode is limited by (i) the ion probe size, which is dependent on the brightness of the primary ion source, the quality of the optics of the primary ion column and the electric fields in the near sample region used to extract secondary ions; (ii) the sensitivity of the analysis as a reasonable secondary ion signal, which must be detected from very tiny voxel sizes and thus from a very limited number of sputtered atoms; and (iii) the physical dimensions of the collision cascade determining the origin of the sputtered ions with respect to the impact site of the incident primary ion probe. One interesting prospect is the use of SIMS-based correlative microscopy. In this approach SIMS is combined with various high-resolution microscopy techniques, so that elemental/chemical information at the highest sensitivity can be obtained with SIMS, while excellent spatial resolution is provided by overlaying the SIMS images with high-resolution images obtained by these microscopy techniques. Examples of this approach are given by presenting in situ combinations of SIMS with transmission electron microscopy (TEM), helium ion microscopy (HIM) and scanning probe microscopy (SPM). PMID:26436905
Approach to failure in a discrete element model of the compressive failure of porous rocks (Invited)
NASA Astrophysics Data System (ADS)
Kun, F.; Varga, I.; Lennartz-Sassinek, S.; Main, I. G.
2013-12-01
We investigate how a porous rock sample approaches failure under uniaxial compression. Computer simulations are carried out in the framework of a discrete element model (DEM) which takes into account both the microstructure of the material and the dynamics of local fracturing, revealing much more detail and observation bandwidth in then granular mechanics than possible during standard laboratory tests. The synthetic sample is generated by sedimentation of randomly-sized spherical particles with a log-normal size distribution inside a cylindrical container. The cohesive interaction of particles is represented by beam elements that break when overstressed. The breaking rule takes into account both stretching and shear of particle contacts. When particles not connected by a beam come into contact their interaction is described by the Hertz contact law. The time evolution of the system is generated by molecular dynamics simulations in three dimensions. Computer simulations showed that under strain controlled uniaxial loading of the system micro-cracks initially nucleate in an uncorrelated way all over the sample. As loading proceeds localization occurs, i.e. the damage concentrates into a narrow damage band. Inside the damage band the material is crushed, into a poorly sorted mixture of fine powder and larger fragments with a power-law mass distribution, as observed in fault wear products (gouge) in natural and laboratory faults. Dynamic bursts of radiated energy, analogous to acoustic emissions observed in laboratory experiments, are identified as correlated trails of local fracture emerging as the consequence of stress redistribution. Characteristic quantities of burst such as size/rupture area, released elastic energy, and duration proved to have power law probability-size distributions over a broad range. The energy and duration of bursts have power law dependence on the rupture area created. As the system approaches macroscopic failure consecutive bursts become
Hoffman, E.L.; Ammerman, D.J.
1995-04-01
A series of tests investigating dynamic pulse buckling of a cylindrical shell under axial impact is compared to several 2D and 3D finite element simulations of the event. The purpose of the work is to investigate the performance of various analysis codes and element types on a problem which is applicable to radioactive material transport packages, and ultimately to develop a benchmark problem to qualify finite element analysis codes for the transport package design industry. Four axial impact tests were performed on 4 in-diameter, 8 in-long, 304 L stainless steel cylinders with a 3/16 in wall thickness. The cylinders were struck by a 597 lb mass with an impact velocity ranging from 42.2 to 45.1 ft/sec. During the impact event, a buckle formed at each end of the cylinder, and one of the two buckles became unstable and collapsed. The instability occurred at the top of the cylinder in three tests and at the bottom in one test. Numerical simulations of the test were performed using the following codes and element types: PRONTO2D with axisymmetric four-node quadrilaterals; PRONTO3D with both four-node shells and eight-node hexahedrons; and ABAQUS/Explicit with axisymmetric two-node shells and four-node quadrilaterals, and 3D four-node shells and eight-node hexahedrons. All of the calculations are compared to the tests with respect to deformed shape and impact load history. As in the tests, the location of the instability is not consistent in all of the calculations. However, the calculations show good agreement with impact load measurements with the exception of an initial load spike which is proven to be the dynamic response of the load cell to the impact. Finally, the PRONIT02D calculation is compared to the tests with respect to strain and acceleration histories. Accelerometer data exhibited good qualitative agreement with the calculations. The strain comparisons show that measurements are very sensitive to gage placement.
Influence of mobile shale on thrust faults: Insights from discrete element simulations
NASA Astrophysics Data System (ADS)
Dean, S. L.; Morgan, J. K.
2013-12-01
We use two-dimensional discrete element method (DEM) simulations to study the effects of a two-layer mechanical stratigraphy on a gravitationally collapsing passive margin. The system consists of an upslope sedimentary wedge, overlying an extensional zone that is linked at depth with a downslope fold and thrust belt. The behavior of the system is dependent on the material properties and thickness of the competent units. The models are initially composed of a mobile shale unit overlain by a pre-delta unit. In DEM materials, the bulk rheology of the granular material is a product of the particle interactions, depending on a range of parameters, including friction and elastic moduli. Natural mobile shales underlying deltas are presumed to be viscous, and are therefore represented in DEM as very weak non-cohesive particles. The unbonded particles respond to loading by moving to areas of lower stress, i.e. out from beneath a growing sediment wedge. The bulk motion of the particles therefore flows away from the upslope extensional zone. Apparent viscosity is introduced in DEM materials due to time dependent numerical parameters such as viscous damping of particle motions. We characterized this apparent viscosity of this mobile shale unit with a series of shear box tests, with varying shear strain rates. The mobile shale particles have a viscosity of about 108 Pa*s, which is low for mobile shale. The low viscosity of our numerical materials can be compensated for by scaling time in our models, because the simulations are driven by sedimentary loading. By increasing the sedimentation rate by many orders of magnitude, we can approximate the natural values of shear stress in our simulations. Results are compared with the Niger Delta type locale for shale tectonics. The simulations succeed in creating an overall linked extensional-contractional system, as well as creating individual structures such as popups and intersecting forethrusts and backthrusts. In addition, toe
Blacker, Teddy D.
1994-01-01
An automatic quadrilateral surface discretization method and apparatus is provided for automatically discretizing a geometric region without decomposing the region. The automated quadrilateral surface discretization method and apparatus automatically generates a mesh of all quadrilateral elements which is particularly useful in finite element analysis. The generated mesh of all quadrilateral elements is boundary sensitive, orientation insensitive and has few irregular nodes on the boundary. A permanent boundary of the geometric region is input and rows are iteratively layered toward the interior of the geometric region. Also, an exterior permanent boundary and an interior permanent boundary for a geometric region may be input and the rows are iteratively layered inward from the exterior boundary in a first counter clockwise direction while the rows are iteratively layered from the interior permanent boundary toward the exterior of the region in a second clockwise direction. As a result, a high quality mesh for an arbitrary geometry may be generated with a technique that is robust and fast for complex geometric regions and extreme mesh gradations.
A finite element technique for a system of fully-discrete time-dependent Joule heating equations
NASA Astrophysics Data System (ADS)
Chin, Pius W. M.
2016-06-01
A system of decoupled nonlinear fully-discrete time-dependent Joule heating equation is studied. Instead of the traditional technique of combining the Euler and the finite element methods, we design a reliable scheme consisting of coupling the Non-standard finite difference in the time space and finite element method in the space variables. We prove for the optimal rate of convergence of the solution of the said scheme in both the H1 as well as the L2-norms. Furthermore, we show that the scheme under study preserves the properties of the exact solution. Numerical experiments are provided to confirm our theoretical analysis.
NASA Astrophysics Data System (ADS)
Tsamados, Michel; Heorton, Harry; Feltham, Daniel; Muir, Alan; Baker, Steven
2016-04-01
The new elastic-plastic anisotropic (EAP) rheology that explicitly accounts for the sub-continuum anisotropy of the sea ice cover has been implemented into the latest version of the Los Alamos sea ice model CICE. The EAP rheology is widely used in the climate modeling scientific community (i.e. CPOM stand alone, RASM high resolution regional ice-ocean model, MetOffice fully coupled model). Early results from sensitivity studies (Tsamados et al, 2013) have shown the potential for an improved representation of the observed main sea ice characteristics with a substantial change of the spatial distribution of ice thickness and ice drift relative to model runs with the reference visco-plastic (VP) rheology. The model contains one new prognostic variable, the local structure tensor, which quantifies the degree of anisotropy of the sea ice, and two parameters that set the time scale of the evolution of this tensor. Observations from high resolution satellite SAR imagery as well as numerical simulation results from a discrete element model (DEM, see Wilchinsky, 2010) have shown that these individual floes can organize under external wind and thermal forcing to form an emergent isotropic sea ice state (via thermodynamic healing, thermal cracking) or an anisotropic sea ice state (via Coulombic failure lines due to shear rupture). In this work we use for the first time in the context of sea ice research a mathematical metric, the Tensorial Minkowski functionals (Schroeder-Turk, 2010), to measure quantitatively the degree of anisotropy and alignment of the sea ice at different scales. We apply the methodology on the GlobICE Envisat satellite deformation product (www.globice.info), on a prototype modified version of GlobICE applied on Sentinel-1 Synthetic Aperture Radar (SAR) imagery and on the DEM ice floe aggregates. By comparing these independent measurements of the sea ice anisotropy as well as its temporal evolution against the EAP model we are able to constrain the
A new meso-scale discrete element model to study deposit differences in tsunamis and storms
NASA Astrophysics Data System (ADS)
Cheng, W.; Weiss, R.
2014-12-01
A fundamental question in tsunami and storm studies is how to differentiate their deposits, which is key to the understanding of past events. Currently, there is no consistent differences due to wide variability of causative forces, topography, sediment source and post-depositional changes. One avenue to resolve these issues can potentially be numerical modeling. Conventional depth-averaged models help us learn general interactions between flow and sediments, but fail to reproduce small-scale depositional structures. We present a new meso-scale sediment transport model. The goal is to advance our knowledge of characteristic differences between storm and tsunami deposits and their relationship with the hydrodynamic processes in tsunamis and storms. Our transport model is based on the Discrete Element Method (DEM). While it is ideal to model every single sediment grains, contemporary computational power will be quickly exhausted due to the scale of interest. Therefore we employ the meso-scale method where a particle represents a group of grains. The volume of each particle is determined dynamically based on pickup rate from the bed and transport rate at the boundaries. During transport, it is assumed that the particle does not change. The motion of particles is governed by Newton's Second Law, with wave motion superimposed on its settling velocities. Hindered settling is implemented to allow interactions between particles through changes of local sediment concentration. Particles are deposited when they reach the bed, and merged into the top layer. Deposits consist of layers that are of the same constant thickness. Bed avalanching could occur where slope exceeds a certain threshold. The Nonlinear Shallow Water Equation (NSWE) is employed to model hydrodynamics. The system of NSWE is solved with a second-order upwind FVM numerical scheme. Wetting and drying is also implemented to handle inundation. In order to couple the depth integrated NSWE with DEM, a velocity
Fish passage through hydropower turbines: Simulating blade strike using the discrete element method
NASA Astrophysics Data System (ADS)
Richmond, M. C.; Romero-Gomez, P.
2014-03-01
Among the hazardous hydraulic conditions affecting anadromous and resident fish during their passage though hydro-turbines two common physical processes can lead to injury and mortality: collisions/blade-strike and rapid decompression. Several methods are currently available to evaluate these stressors in installed turbines, e.g. using live fish or autonomous sensor devices, and in reduced-scale physical models, e.g. registering collisions from plastic beads. However, a priori estimates with computational modeling approaches applied early in the process of turbine design can facilitate the development of fish-friendly turbines. In the present study, we evaluated the frequency of blade strike and rapid pressure change by modeling potential fish trajectories with the Discrete Element Method (DEM) applied to fish-like composite particles. In the DEM approach, particles are subjected to realistic hydraulic conditions simulated with computational fluid dynamics (CFD), and particle-structure interactions-representing fish collisions with turbine components such as blades-are explicitly recorded and accounted for in the calculation of particle trajectories. We conducted transient CFD simulations by setting the runner in motion and allowing for unsteady turbulence using detached eddy simulation (DES), as compared to the conventional practice of simulating the system in steady state (which was also done here for comparison). While both schemes yielded comparable bulk hydraulic performance values, transient conditions exhibited an improvement in describing flow temporal and spatial variability. We released streamtraces (in the steady flow solution) and DEM particles (transient solution) at the same locations where sensor fish (SF) were released in previous field studies of the advanced turbine unit. The streamtrace- based results showed a better agreement with SF data than the DEM-based nadir pressures did because the former accounted for the turbulent dispersion at the