Linking continuum mechanics and 3D discrete dislocation simulations
El-Azab, A. A.; Fivel, M.
1998-10-18
A technique is developed for linking the methods of discrete dislocation dynamics simulation and finite element to treat elasto-plasticity problems. The overall formulation views the plastically deforming crystal as an elastic crystal with continuously changing dislocation microstructure which is tracked by the numerical dynamics simulation. The FEM code needed in this regard is based on linear elasticity only. This formulation presented here is focused on a continuous updating of the outer shape of the crystal, for possible regeneration of the FEM mesh, and adjustment of the surface geometry, in particular the surface normal. The method is expected to be potentially applicable to the nano- indentation experiments, where the zone around the indenter-crystal contact undergoes significant permanent deformation, the rigorous determination of which is very important to the calculation of the indentation print area and in turn, the surface hardness. Furthermore, the technique is expected to account for the plastic history of the surface displacement under the indenter. Other potential applications are mentioned in the text.
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.
Jones, Reese E.; Zimmerman, Jonathan A.; Po, Giacomo; ...
2016-02-01
Accurate simulation of the plastic deformation of ductile metals is important to the design of structures and components to performance and failure criteria. Many techniques exist that address the length scales relevant to deformation processes, including dislocation dynamics (DD), which models the interaction and evolution of discrete dislocation line segments, and crystal plasticity (CP), which incorporates the crystalline nature and restricted motion of dislocations into a higher scale continuous field framework. While these two methods are conceptually related, there have been only nominal efforts focused at the global material response that use DD-generated information to enhance the fidelity of CPmore » models. To ascertain to what degree the predictions of CP are consistent with those of DD, we compare their global and microstructural response in a number of deformation modes. After using nominally homogeneous compression and shear deformation dislocation dynamics simulations to calibrate crystal plasticity ow rule parameters, we compare not only the system-level stress-strain response of prismatic wires in torsion but also the resulting geometrically necessary dislocation density fields. To establish a connection between explicit description of dislocations and the continuum assumed with crystal plasticity simulations we ascertain the minimum length-scale at which meaningful dislocation density fields appear. Furthermore, our results show that, for the case of torsion, that the two material models can produce comparable spatial dislocation density distributions.« less
Jones, Reese E.; Zimmerman, Jonathan A.; Po, Giacomo; Mandadapu, Kranthi
2016-02-01
Accurate simulation of the plastic deformation of ductile metals is important to the design of structures and components to performance and failure criteria. Many techniques exist that address the length scales relevant to deformation processes, including dislocation dynamics (DD), which models the interaction and evolution of discrete dislocation line segments, and crystal plasticity (CP), which incorporates the crystalline nature and restricted motion of dislocations into a higher scale continuous field framework. While these two methods are conceptually related, there have been only nominal efforts focused at the global material response that use DD-generated information to enhance the fidelity of CP models. To ascertain to what degree the predictions of CP are consistent with those of DD, we compare their global and microstructural response in a number of deformation modes. After using nominally homogeneous compression and shear deformation dislocation dynamics simulations to calibrate crystal plasticity ow rule parameters, we compare not only the system-level stress-strain response of prismatic wires in torsion but also the resulting geometrically necessary dislocation density fields. To establish a connection between explicit description of dislocations and the continuum assumed with crystal plasticity simulations we ascertain the minimum length-scale at which meaningful dislocation density fields appear. Furthermore, our results show that, for the case of torsion, that the two material models can produce comparable spatial dislocation density distributions.
Discrete dislocations in graphene
NASA Astrophysics Data System (ADS)
Ariza, M. P.; Ortiz, M.
2010-05-01
In this work, we present an application of the theory of discrete dislocations of Ariza and Ortiz (2005) to the analysis of dislocations in graphene. Specifically, we discuss the specialization of the theory to graphene and its further specialization to the force-constant model of Aizawa et al. (1990). The ability of the discrete-dislocation theory to predict dislocation core structures and energies is critically assessed for periodic arrangements of dislocation dipoles and quadrupoles. We show that, with the aid of the discrete Fourier transform, those problems are amenable to exact solution within the discrete-dislocation theory, which confers the theory a distinct advantage over conventional atomistic models. The discrete dislocations exhibit 5-7 ring core structures that are consistent with observation and result in dislocation energies that fall within the range of prediction of other models. The asymptotic behavior of dilute distributions of dislocations is characterized analytically in terms of a discrete prelogarithmic energy tensor. Explicit expressions for this discrete prelogarithmic energy tensor are provided up to quadratures.
NASA Astrophysics Data System (ADS)
Messner, Mark C.; Rhee, Moono; Arsenlis, Athanasios; Barton, Nathan R.
2017-06-01
This work develops a method for calibrating a crystal plasticity model to the results of discrete dislocation (DD) simulations. The crystal model explicitly represents junction formation and annihilation mechanisms and applies these mechanisms to describe hardening in hexagonal close packed metals. The model treats these dislocation mechanisms separately from elastic interactions among populations of dislocations, which the model represents through a conventional strength-interaction matrix. This split between elastic interactions and junction formation mechanisms more accurately reproduces the DD data and results in a multi-scale model that better represents the lower scale physics. The fitting procedure employs concepts of machine learning—feature selection by regularized regression and cross-validation—to develop a robust, physically accurate crystal model. The work also presents a method for ensuring the final, calibrated crystal model respects the physical symmetries of the crystal system. Calibrating the crystal model requires fitting two linear operators: one describing elastic dislocation interactions and another describing junction formation and annihilation dislocation reactions. The structure of these operators in the final, calibrated model reflect the crystal symmetry and slip system geometry of the DD simulations.
NASA Astrophysics Data System (ADS)
Khraishi, Tariq A.; Zbib, Hussein M.; de La Rubia, Tomas Diaz; Victoria, Max
2002-04-01
When irradiated, metals undergo significant internal damage accumulation and degradation of mechanical properties. Damage takes the form of a high number density of nanosize defect clusters (stacking-fault tetrahedrons (SFTs) or interstitial loops). The alteration of mechanical properties is manifested in a hardening behavior and localized plastic deformation in defect-free channels. This work uses discrete dislocation dynamics (DD) to capture these effects. It sets the framework for the elastic interaction between gliding dislocations and defect clusters and details a scheme for loop unfaulting and absorption into dislocations. Here, it is shown that SFTs represents weaker pinning points for dislocation motion than parent dislocation loops. It is also shown that appreciable yield drop can be attributed to high density of defects decorating the dislocations. Strong obstacles cause dislocations in Cu to continually double cross slip causing the formation of defect-free channels. Finally, the correlation between yield stress increase and defect number density is in excellent agreement with the experiment.
Electromechanical simulations of dislocations
NASA Astrophysics Data System (ADS)
Skiba, Oxana; Gracie, Robert; Potapenko, Stanislav
2013-04-01
Improving the reliability of micro-electronic devices depends in part on developing a more in-depth understanding of dislocations because dislocations are barriers to charge carriers. To this end, the quasi-static simulation of discrete dislocations dynamics in materials under mechanical and electrical loads is presented. The simulations are based on the extended finite element method, where dislocations are modelled as internal discontinuities. The strong and weak forms of the boundary value problem for the coupled system are presented. The computation of the Peach-Koehler force using the J-integral is discussed. Examples to illustrate the accuracy of the simulations are presented. The motion of the network of the dislocations under different electrical and mechanical loads is simulated. It was shown that even in weak piezoelectric materials the effect of the electric field on plastic behaviour is significant.
Discrete Dislocation Dynamics Simulations of Twin Size-Effects in Magnesium
2015-01-01
dynamics simulations to study the mechanical behavior of micro-twinned Mg. Strong strain hardening was captured by current simulations, which is...significant grain refinement and strong hardening . In addition, Knezevic et al. [3] showed that compression twins in the tension-twinned grains attribute to...the hardening behavior. Moreover, Barnett et al. [4] observed the formation of low angle boundaries arising from the dislocation-TB interaction, which
Fast Fourier transform discrete dislocation dynamics
NASA Astrophysics Data System (ADS)
Graham, J. T.; Rollett, A. D.; LeSar, R.
2016-12-01
Discrete dislocation dynamics simulations have been generally limited to modeling systems described by isotropic elasticity. Effects of anisotropy on dislocation interactions, which can be quite large, have generally been ignored because of the computational expense involved when including anisotropic elasticity. We present a different formalism of dislocation dynamics in which the dislocations are represented by the deformation tensor, which is a direct measure of the slip in the lattice caused by the dislocations and can be considered as an eigenstrain. The stresses arising from the dislocations are calculated with a fast Fourier transform (FFT) method, from which the forces are determined and the equations of motion are solved. Use of the FFTs means that the stress field is only available at the grid points, which requires some adjustments/regularizations to be made to the representation of the dislocations and the calculation of the force on individual segments, as is discussed hereinafter. A notable advantage of this approach is that there is no computational penalty for including anisotropic elasticity. We review the method and apply it in a simple dislocation dynamics calculation.
Fan, Haidong; Aubry, Sylvie; Arsenlis, Athanasios; El-Awady, Jaafar A.
2015-04-13
The mechanical response of micro-twinned polycrystalline magnesium was studied through three-dimensional discrete dislocation dynamics (DDD). A systematic interaction model between dislocations and (1012) tension twin boundaries (TBs) was proposed and introduced into the DDD framework. In addition, a nominal grain boundary (GB) model agreeing with experimental results was also introduced to mimic the GB’s barrier effect. The current simulation results show that TBs act as a strong obstacle to gliding dislocations, which contributes significantly to the hardening behavior of magnesium. On the other hand, the deformation accommodated by twinning plays a softening role. Therefore, the concave shape of the Mg stress-strain curve results from the competition between dislocation-TB induced hardening and twinning deformation induced softening. At low strain levels, twinning deformation induced softening dominates and a decreasing hardening rate is observed in Stage-I. In Stage-II, both the hardening and softening effects decline, but twinning deformation induced softening declines faster, which leads to an increasing hardening rate.
Fan, Haidong; Aubry, Sylvie; Arsenlis, Athanasios; ...
2015-04-13
The mechanical response of micro-twinned polycrystalline magnesium was studied through three-dimensional discrete dislocation dynamics (DDD). A systematic interaction model between dislocations and (1012) tension twin boundaries (TBs) was proposed and introduced into the DDD framework. In addition, a nominal grain boundary (GB) model agreeing with experimental results was also introduced to mimic the GB’s barrier effect. The current simulation results show that TBs act as a strong obstacle to gliding dislocations, which contributes significantly to the hardening behavior of magnesium. On the other hand, the deformation accommodated by twinning plays a softening role. Therefore, the concave shape of the Mgmore » stress-strain curve results from the competition between dislocation-TB induced hardening and twinning deformation induced softening. At low strain levels, twinning deformation induced softening dominates and a decreasing hardening rate is observed in Stage-I. In Stage-II, both the hardening and softening effects decline, but twinning deformation induced softening declines faster, which leads to an increasing hardening rate.« less
Lehtinen, Arttu; Granberg, Fredric; Laurson, Lasse; Nordlund, Kai; Alava, Mikko J
2016-01-01
The stress-driven motion of dislocations in crystalline solids, and thus the ensuing plastic deformation process, is greatly influenced by the presence or absence of various pointlike defects such as precipitates or solute atoms. These defects act as obstacles for dislocation motion and hence affect the mechanical properties of the material. Here we combine molecular dynamics studies with three-dimensional discrete dislocation dynamics simulations in order to model the interaction between different kinds of precipitates and a 1/2〈111〉{110} edge dislocation in BCC iron. We have implemented immobile spherical precipitates into the ParaDis discrete dislocation dynamics code, with the dislocations interacting with the precipitates via a Gaussian potential, generating a normal force acting on the dislocation segments. The parameters used in the discrete dislocation dynamics simulations for the precipitate potential, the dislocation mobility, shear modulus, and dislocation core energy are obtained from molecular dynamics simulations. We compare the critical stresses needed to unpin the dislocation from the precipitate in molecular dynamics and discrete dislocation dynamics simulations in order to fit the two methods together and discuss the variety of the relevant pinning and depinning mechanisms.
High-temperature discrete dislocation plasticity
NASA Astrophysics Data System (ADS)
Keralavarma, S. M.; Benzerga, A. A.
2015-09-01
A framework for solving problems of dislocation-mediated plasticity coupled with point-defect diffusion is presented. The dislocations are modeled as line singularities embedded in a linear elastic medium while the point defects are represented by a concentration field as in continuum diffusion theory. Plastic flow arises due to the collective motion of a large number of dislocations. Both conservative (glide) and nonconservative (diffusion-mediated climb) motions are accounted for. Time scale separation is contingent upon the existence of quasi-equilibrium dislocation configurations. A variational principle is used to derive the coupled governing equations for point-defect diffusion and dislocation climb. Superposition is used to obtain the mechanical fields in terms of the infinite-medium discrete dislocation fields and an image field that enforces the boundary conditions while the point-defect concentration is obtained by solving the stress-dependent diffusion equations on the same finite-element grid. Core-level boundary conditions for the concentration field are avoided by invoking an approximate, yet robust kinetic law. Aspects of the formulation are general but its implementation in a simple plane strain model enables the modeling of high-temperature phenomena such as creep, recovery and relaxation in crystalline materials. With emphasis laid on lattice vacancies, the creep response of planar single crystals in simple tension emerges as a natural outcome in the simulations. A large number of boundary-value problem solutions are obtained which depict transitions from diffusional to power-law creep, in keeping with long-standing phenomenological theories of creep. In addition, some unique experimental aspects of creep in small scale specimens are also reproduced in the simulations.
Parallel Dislocation Simulator
2006-10-30
ParaDiS is software capable of simulating the motion, evolution, and interaction of dislocation networks in single crystals using massively parallel computer architectures. The software is capable of outputting the stress-strain response of a single crystal whose plastic deformation is controlled by the dislocation processes.
Enabling Strain Hardening Simulations with Dislocation Dynamics
Arsenlis, A; Cai, W
2006-12-20
Numerical algorithms for discrete dislocation dynamics simulations are investigated for the purpose of enabling strain hardening simulations of single crystals on massively parallel computers. The algorithms investigated include the /(N) calculation of forces, the equations of motion, time integration, adaptive mesh refinement, the treatment of dislocation core reactions, and the dynamic distribution of work on parallel computers. A simulation integrating all of these algorithmic elements using the Parallel Dislocation Simulator (ParaDiS) code is performed to understand their behavior in concert, and evaluate the overall numerical performance of dislocation dynamics simulations and their ability to accumulate percents of plastic strain.
3D discrete dislocation models of thin-film plasticity
Hartmaier, A.; Gumbsch, P.; Fivel, M.C.; Canova, G.R.
1998-12-31
Three-dimensional simulation schemes for discrete dislocation dynamics (DDD) have been used successfully to investigate plasticity of bulk materials. The adaptation of these DDD schemes to a description of thin-film plasticity requires detailed modeling of the interfaces and surfaces of the film. One possible method is to compensate for the normal stresses that a dislocation distribution exerts on a surface by appropriate point loads. This traction-compensation method is extended to a free standing film (two opposing surfaces). The extension to a thin film on a substrate is possible.
Microstructural comparison of the kinematics of discrete and continuum dislocations models
NASA Astrophysics Data System (ADS)
Sandfeld, Stefan; Po, Giacomo
2015-12-01
The Continuum Dislocation Dynamics (CDD) theory and the Discrete Dislocation Dynamics (DDD) method are compared based on concise mathematical formulations of the coarse graining of discrete data. A numerical tool for converting from a discrete to a continuum representation of a given dislocation configuration is developed, which allows to directly compare both simulation approaches based on continuum quantities (e.g. scalar density, geometrically necessary densities, mean curvature). Investigating the evolution of selected dislocation configurations within analytically given velocity fields for both DDD and CDD reveals that CDD contains a surprising number of important microstructural details.
Green’s function molecular dynamics meets discrete dislocation plasticity
NASA Astrophysics Data System (ADS)
Venugopalan, Syam P.; Müser, Martin H.; Nicola, Lucia
2017-09-01
Metals deform plastically at the asperity level when brought in contact with a counter body even when the nominal contact pressure is small. Modeling the plasticity of solids with rough surfaces is challenging due to the multi-scale nature of surface roughness and the length-scale dependence of plasticity. While discrete-dislocation plasticity (DDP) simulations capture size-dependent plasticity by keeping track of the motion of individual dislocations, only simple two-dimensional surface geometries have so far been studied with DDP. The main computational bottleneck in contact problems modeled by DDP is the calculation of the dislocation image fields. We address this issue by combining two-dimensional DDP with Green’s function molecular dynamics. The resulting method allows for an efficient boundary-value-method based treatment of elasticity in the presence of dislocations. We demonstrate that our method captures plasticity quantitatively from single to many dislocations and that it scales more favorably with system size than conventional methods. We also derive the relevant Green’s functions for elastic slabs of finite width allowing arbitrary boundary conditions on top and bottom surface to be simulated.
Reprint of: Dynamics of discrete screw dislocations on glide directions
NASA Astrophysics Data System (ADS)
Alicandro, R.; De Luca, L.; Garroni, A.; Ponsiglione, M.
2016-12-01
We consider a simple discrete model for screw dislocations in crystals. Using a variational discrete scheme we study the motion of a configuration of dislocations toward low energy configurations. We deduce an effective fully overdamped dynamics that follows the maximal dissipation criterion introduced in Cermelli and Gurtin (1999) and predicts motion along the glide directions of the crystal.
Dynamics of discrete screw dislocations on glide directions
NASA Astrophysics Data System (ADS)
Alicandro, R.; De Luca, L.; Garroni, A.; Ponsiglione, M.
2016-07-01
We consider a simple discrete model for screw dislocations in crystals. Using a variational discrete scheme we study the motion of a configuration of dislocations toward low energy configurations. We deduce an effective fully overdamped dynamics that follows the maximal dissipation criterion introduced in Cermelli and Gurtin (1999) and predicts motion along the glide directions of the crystal.
2015-01-01
to the basal plane leading to predominant basal slip, as shown in Fig. 4(a), since the Schmid factor on these planes is maxi - mum. The low yield...slip is the hardest slip mode. It is also observed from Fig. 3 that regardless of the loading directions, all single crystals results exhibit weak ...In Section 3.1, the dislocation forest hardening effect for single crystals was observed to be weak (see Fig. 3). Furthermore, GBs do produce
NASA Astrophysics Data System (ADS)
Bonny, G.; Terentyev, D.; Elena, J.; Zinovev, A.; Minov, B.; Zhurkin, E. E.
2016-05-01
Upon irradiation, iron based steels used for nuclear applications contain dislocation loops of both < 100 > and ½ < 111 > type. Both types of loops are known to contribute to the radiation hardening and embrittlement of steels. In the literature many molecular dynamics works studying the interaction of dislocations with dislocation loops are available. Recently, based on such studies, a thermo-mechanical model to threat the dislocation - dislocation loop (DL) interaction within a discrete dislocation dynamics framework was developed for ½ < 111 > loops. In this work, we make a literature review of the dislocation - DL interaction in bcc iron. We also perform molecular dynamics simulations to derive the stress-energy function for < 100 > loops. As a result we deliver the function of the activation energy versus activation stress for < 100 > loops that can be applied in a discrete dislocation dynamics framework.
NASA Astrophysics Data System (ADS)
Gurrutxaga-Lerma, Benat; Sutton, Adrian; Eakins, Daniel; Balint, Daniel; Dini, Daniele
2013-06-01
This talk intends to offer some insight as to how Discrete Dislocation Plasticity (DDP) can be adapted to simulate plastic relaxation processes under weak shock loading and high strain rates. In those circumstances, dislocations are believed to be the main cause of plastic relaxation in crystalline solids. Direct simulation of dislocations as the dynamic agents of plastic relaxation in those cases remains a challenge. DDP, where dislocations are modelled as discrete discontinuities in elastic continuum media, is often unable to adequately simulate plastic relaxation because it treats dislocation motion quasi-statically, thus neglecting the time-dependent nature of the elastic fields and assuming that they instantaneously acquire the shape and magnitude predicted by elastostatics. Under shock loading, this assumption leads to several artefacts that can only be overcome with a fully time-dependent formulation of the elastic fields. In this talk one of such formulations for the creation, annihilation and arbitrary motion of straight edge dislocations will be presented. These solutions are applied in a two-dimensional model of time-dependent plastic relaxation under shock loading, and some relevant results will be presented. EPSRC CDT in Theory and Simulation of Materials
NASA Astrophysics Data System (ADS)
Gao, Yuan; Zhuang, Zhuo; You, XiaoChuan
2011-04-01
We develop a new hierarchical dislocation-grain boundary (GB) interaction model to predict the mechanical behavior of polycrystalline metals at micro and submicro scales by coupling 3D Discrete Dislocation Dynamics (DDD) simulation with the Molecular Dynamics (MD) simulation. At the microscales, the DDD simulations are responsible for capturing the evolution of dislocation structures; at the nanoscales, the MD simulations are responsible for obtaining the GB energy and ISF energy which are then transferred hierarchically to the DDD level. In the present model, four kinds of dislocation-GB interactions, i.e. transmission, absorption, re-emission and reflection, are all considered. By this methodology, the compression of a Cu micro-sized bi-crystal pillar is studied. We investigate the characteristic mechanical behavior of the bi-crystal compared with that of the single-crystal. Moreover, the comparison between the present penetrable model of GB and the conventional impenetrable model also shows the accuracy and efficiency of the present model.
ParaDiS-FEM dislocation dynamics simulation code primer
Tang, M; Hommes, G; Aubry, S; Arsenlis, A
2011-09-27
The ParaDiS code is developed to study bulk systems with periodic boundary conditions. When we try to perform discrete dislocation dynamics simulations for finite systems such as thin films or cylinders, the ParaDiS code must be extended. First, dislocations need to be contained inside the finite simulation box; Second, dislocations inside the finite box experience image stresses due to the free surfaces. We have developed in-house FEM subroutines to couple with the ParaDiS code to deal with free surface related issues in the dislocation dynamics simulations. This primer explains how the coupled code was developed, the main changes from the ParaDiS code, and the functions of the new FEM subroutines.
NASA Astrophysics Data System (ADS)
Gao, Siwen; Fivel, Marc; Ma, Anxin; Hartmaier, Alexander
2015-03-01
In the characteristic γ / γ ‧ microstructure of single crystal superalloys, misfit stresses occur due to a significant lattice mismatch of those two phases. The magnitude of this lattice mismatch depends on the chemical composition of both phases as well as on temperature. Furthermore, the lattice mismatch of γ and γ ‧ phases can be either positive or negative in sign. The internal stresses caused by such lattice mismatch play a decisive role for the micromechanical processes that lead to the observed macroscopic athermal deformation behavior of these high-temperature alloys. Three-dimensional discrete dislocation dynamics (DDD) simulations are applied to investigate dislocation glide in γ matrix channels and shearing of γ ‧ precipitates by superdislocations under externally applied uniaxial stresses, by fully taking into account internal misfit stresses. Misfit stress fields are calculated by the fast Fourier transformation (FFT) method and hybridized with DDD simulations. For external loading along the crystallographic [001] direction of the single crystal, it was found that the different internal stress states for negative and positive lattice mismatch result in non-uniform dislocation movement and different dislocation patterns in horizontal and vertical γ matrix channels. Furthermore, positive lattice mismatch produces a lower deformation rate than negative lattice mismatch under the same tensile loading, but for an increasing magnitude of lattice mismatch, the deformation resistance always diminishes. Hence, the best deformation performance is expected to result from alloys with either small positive, or even better, vanishing lattice mismatch between γ and γ ‧ phase.
NASA Astrophysics Data System (ADS)
Gao, Siwen; Fivel, Marc; Ma, Anxin; Hartmaier, Alexander
2017-05-01
A three-dimensional (3D) discrete dislocation dynamics (DDD) creep model is developed to investigate creep behavior under uniaxial tensile stress along the crystallographic [001] direction in Ni-base single crystal superalloys, which takes explicitly account of dislocation glide, climb and vacancy diffusion, but neglects phase transformation like rafting of γ‧ precipitates. The vacancy diffusion model takes internal stresses by dislocations and mismatch strains into account and it is coupled to the dislocation dynamics model in a numerically efficient way. This model is helpful for understanding the fundamental creep mechanisms in superalloys and clarifying the effects of dislocation glide and climb on creep deformation. In cases where the precipitate cutting rarely occurs, e.g. due to the high anti-phase boundary energy and the lack of superdislocations, the dislocation glide in the γ matrix and the dislocation climb along the γ/γ‧ interface dominate plastic deformation. The simulation results show that a high temperature or a high stress both promote dislocation motion and multiplication, so as to cause a large creep strain. Dislocation climb accelerated by high temperature only produces a small plastic strain, but relaxes the hardening caused by the filling γ channels and lets dislocations further glide and multiply. The strongest variation of vacancy concentration occurs in the horizontal channels, where more mixed dislocations exit and tend to climb. The increasing internal stresses due to the increasing dislocation density are easily overcome by dislocations under a high external stress that leads to a long-term dislocation glide accompanied by multiplication.
Discrete dislocation plasticity analysis of the wedge indentation of films
NASA Astrophysics Data System (ADS)
Balint, D. S.; Deshpande, V. S.; Needleman, A.; Van der Giessen, E.
2006-11-01
The plane strain indentation of single crystal films on a rigid substrate by a rigid wedge indenter is analyzed using discrete dislocation plasticity. The crystals have three slip systems at ±35.3∘ and 90∘ with respect to the indentation direction. The analyses are carried out for three values of the film thickness, 2, 10 and 50 μm, and with the dislocations all of edge character modeled as line singularities in a linear elastic material. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation are incorporated through a set of constitutive rules. Over the range of indentation depths considered, the indentation pressure for the 10 and 50 μm thick films decreases with increasing contact size and attains a contact size-independent value for contact lengths A>4 μm. On the other hand, for the 2 μm films, the indentation pressure first decreases with increasing contact size and subsequently increases as the plastic zone reaches the rigid substrate. For the 10 and 50 μm thick films sink-in occurs around the indenter, while pile-up occurs in the 2 μm film when the plastic zone reaches the substrate. Comparisons are made with predictions obtained from other formulations: (i) the contact size-independent indentation pressure is compared with that given by continuum crystal plasticity; (ii) the scaling of the indentation pressure with indentation depth is compared with the relation proposed by Nix and Gao [1998. Indentation size effects in crystalline materials: a law for strain gradient plasticity. J. Mech. Phys. Solids 43, 411-423]; and (iii) the computed contact area is compared with that obtained from the estimation procedure of Oliver and Pharr [1992. An improved technique for determining hardness and elastic-modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7, 1564-1583].
Massively-Parallel Dislocation Dynamics Simulations
Cai, W; Bulatov, V V; Pierce, T G; Hiratani, M; Rhee, M; Bartelt, M; Tang, M
2003-06-18
Prediction of the plastic strength of single crystals based on the collective dynamics of dislocations has been a challenge for computational materials science for a number of years. The difficulty lies in the inability of the existing dislocation dynamics (DD) codes to handle a sufficiently large number of dislocation lines, in order to be statistically representative and to reproduce experimentally observed microstructures. A new massively-parallel DD code is developed that is capable of modeling million-dislocation systems by employing thousands of processors. We discuss the general aspects of this code that make such large scale simulations possible, as well as a few initial simulation results.
The discrete-continuum connection in dislocation dynamics: I. Time coarse graining of cross slip
NASA Astrophysics Data System (ADS)
Xia, Shengxu; Belak, James; El-Azab, Anter
2016-10-01
A recent continuum dislocation dynamics formalism (Xia and El-Azab 2015 Model. Simul. Mater. Sci. Eng. 23 055009) has been enriched by incorporating an improved cross slip model. 3D discrete dislocation dynamics simulations were used to collect cross slip rate data in the form of time series that were analysed to estimate the correlation time for cross slip, which was subsequently used as a time scale for local window averaging of the collected cross slip rate data. This time averaging filters out the cross slip rate fluctuations over time intervals less than the correlation time, thus resulting in relatively smoother time series for the cross slip rates. The coarse grained series were further cast in the form of smooth trends with superposed fluctuations and implemented in continuum dislocation dynamics simulations using a Monte Carlo scheme. This approach resulted in a significant improvement of the predicted stress-strain response and a more realistic dislocation cell structure evolution. The similitude law for the average cell size evolution with inverse of stress, however, remains unaffected by the cross slip rates used in continuum dislocation dynamics.
Mobility Laws in Dislocation Dynamics Simulations
Cai, W; Bulatov, V V
2003-10-21
Prediction of the plastic deformation behavior of single crystals based on the collective dynamics of dislocations has been a challenge for computational materials science for a number of years. The difficulty lies in the inability of existing dislocation dynamics (DD) codes to handle a sufficiently large number of dislocation lines, to establish a statistically representative model of crystal plasticity. A new massively-parallel DD code is developed that is capable of modeling million-dislocation systems by employing thousands of processors. We discuss an important ingredient of this code--the mobility laws dictating the behavior of individual dislocations. They are materials input for DD simulations and are constructed based on the understanding of dislocation motion at the atomistic level.
NASA Astrophysics Data System (ADS)
Bertin, N.; Upadhyay, M. V.; Pradalier, C.; Capolungo, L.
2015-09-01
In this paper, we propose a novel full-field approach based on the fast Fourier transform (FFT) technique to compute mechanical fields in periodic discrete dislocation dynamics (DDD) simulations for anisotropic materials: the DDD-FFT approach. By coupling the FFT-based approach to the discrete continuous model, the present approach benefits from the high computational efficiency of the FFT algorithm, while allowing for a discrete representation of dislocation lines. It is demonstrated that the computational time associated with the new DDD-FFT approach is significantly lower than that of current DDD approaches when large number of dislocation segments are involved for isotropic and anisotropic elasticity, respectively. Furthermore, for fine Fourier grids, the treatment of anisotropic elasticity comes at a similar computational cost to that of isotropic simulation. Thus, the proposed approach paves the way towards achieving scale transition from DDD to mesoscale plasticity, especially due to the method’s ability to incorporate inhomogeneous elasticity.
NASA Astrophysics Data System (ADS)
Liao, Yiliang; Ye, Chang; Gao, Huang; Kim, Bong-Joong; Suslov, Sergey; Stach, Eric A.; Cheng, Gary J.
2011-07-01
Warm laser shock peening (WLSP) is a new high strain rate surface strengthening process that has been demonstrated to significantly improve the fatigue performance of metallic components. This improvement is mainly due to the interaction of dislocations with highly dense nanoscale precipitates, which are generated by dynamic precipitation during the WLSP process. In this paper, the dislocation pinning effects induced by the nanoscale precipitates during WLSP are systematically studied. Aluminum alloy 6061 and AISI 4140 steel are selected as the materials with which to conduct WLSP experiments. Multiscale discrete dislocation dynamics (MDDD) simulation is conducted in order to investigate the interaction of dislocations and precipitates during the shock wave propagation. The evolution of dislocation structures during the shock wave propagation is studied. The dislocation structures after WLSP are characterized via transmission electron microscopy and are compared with the results of the MDDD simulation. The results show that nano-precipitates facilitate the generation of highly dense and uniformly distributed dislocation structures. The dislocation pinning effect is strongly affected by the density, size, and space distribution of nano-precipitates.
A Dynamic Discrete Dislocation Plasticity study of elastodynamic shielding of stationary cracks
NASA Astrophysics Data System (ADS)
Gurrutxaga-Lerma, B.; Balint, D. S.; Dini, D.; Sutton, A. P.
2017-01-01
Employing Dynamic Discrete Dislocation Plasticity (D3P), an elastodynamic analysis of the shielding of a stationary crack tip by dislocations is studied. Dislocations are generated via Frank-Read sources, and make a negligible contribution to the shielding of the crack tip, whereas dislocations generated at the crack tip via homogeneous nucleation dominate the shielding. Their effect is found to be highly localised around the crack, leading to a magnification of the shielding when compared to time-independent, elastostatic predictions. The resulting attenuation of KI(t) is computed, and is found to be directly proportional to the applied load and to √{ t }.
Liu, Guisen; Cheng, Xi; Wang, Jian; Chen, Kaiguo; Shen, Yao
2017-01-01
Prediction of Peierls stress associated with dislocation glide is of fundamental concern in understanding and designing the plasticity and mechanical properties of crystalline materials. Here, we develop a nonlocal semi-discrete variational Peierls-Nabarro (SVPN) model by incorporating the nonlocal atomic interactions into the semi-discrete variational Peierls framework. The nonlocal kernel is simplified by limiting the nonlocal atomic interaction in the nearest neighbor region, and the nonlocal coefficient is directly computed from the dislocation core structure. Our model is capable of accurately predicting the displacement profile, and the Peierls stress, of planar-extended core dislocations in face-centered cubic structures. Our model could be extended to study more complicated planar-extended core dislocations, such as <110> {111} dislocations in Al-based and Ti-based intermetallic compounds. PMID:28252102
Liu, Guisen; Cheng, Xi; Wang, Jian; Chen, Kaiguo; Shen, Yao
2017-03-02
Prediction of Peierls stress associated with dislocation glide is of fundamental concern in understanding and designing the plasticity and mechanical properties of crystalline materials. Here, we develop a nonlocal semi-discrete variational Peierls-Nabarro (SVPN) model by incorporating the nonlocal atomic interactions into the semi-discrete variational Peierls framework. The nonlocal kernel is simplified by limiting the nonlocal atomic interaction in the nearest neighbor region, and the nonlocal coefficient is directly computed from the dislocation core structure. Our model is capable of accurately predicting the displacement profile, and the Peierls stress, of planar-extended core dislocations in face-centered cubic structures. Our model could be extended to study more complicated planar-extended core dislocations, such as <110> {111} dislocations in Al-based and Ti-based intermetallic compounds.
NASA Astrophysics Data System (ADS)
Liu, Guisen; Cheng, Xi; Wang, Jian; Chen, Kaiguo; Shen, Yao
2017-03-01
Prediction of Peierls stress associated with dislocation glide is of fundamental concern in understanding and designing the plasticity and mechanical properties of crystalline materials. Here, we develop a nonlocal semi-discrete variational Peierls-Nabarro (SVPN) model by incorporating the nonlocal atomic interactions into the semi-discrete variational Peierls framework. The nonlocal kernel is simplified by limiting the nonlocal atomic interaction in the nearest neighbor region, and the nonlocal coefficient is directly computed from the dislocation core structure. Our model is capable of accurately predicting the displacement profile, and the Peierls stress, of planar-extended core dislocations in face-centered cubic structures. Our model could be extended to study more complicated planar-extended core dislocations, such as <110> {111} dislocations in Al-based and Ti-based intermetallic compounds.
Size dependence of yield strength simulated by a dislocation-density function dynamics approach
NASA Astrophysics Data System (ADS)
Leung, P. S. S.; Leung, H. S.; Cheng, B.; Ngan, A. H. W.
2015-04-01
The size dependence of the strength of nano- and micron-sized crystals is studied using a new simulation approach in which the dynamics of the density functions of dislocations are modeled. Since any quantity of dislocations can be represented by a density, this approach can handle large systems containing large quantities of dislocations, which may handicap discrete dislocation dynamics schemes due to the excessive computation time involved. For this reason, pillar sizes spanning a large range, from the sub-micron to micron regimes, can be simulated. The simulation results reveal the power-law relationship between strength and specimen size up to a certain size, beyond which the strength varies much more slowly with size. For specimens smaller than ∼4000b, their strength is found to be controlled by the dislocation depletion condition, in which the total dislocation density remains almost constant throughout the loading process. In specimens larger than ∼4000b, the initial dislocation distribution is of critical importance since the presence of dislocation entanglements is found to obstruct deformation in the neighboring regions within a distance of ∼2000b. This length scale suggests that the effects of dense dislocation clusters are greater in intermediate-sized specimens (e.g. 4000b and 8000b) than in larger specimens (e.g. 16 000b), according to the weakest-link concept.
NASA Astrophysics Data System (ADS)
Shishvan, Siamak S.; Pollock, Tresa M.; McMeeking, Robert M.; Deshpande, Vikram S.
2017-01-01
We present a discrete dislocation plasticity (DDP) framework to analyse the high temperature deformation of multi-phase materials (composites) comprising a matrix and inclusions. Deformation of the phases is by climb-assisted glide of the dislocations while the particles can also deform due to stress-driven interfacial diffusion. The general framework is used to analyse the uniaxial tensile deformation of a composite comprising elastic particles with dislocation plasticity only present in the matrix phase. When dislocation motion is restricted to only glide within the matrix a strong size effect of the composite strength is predicted with the strength increasing with decreasing unit cell size due to dislocations forming pile-ups against the matrix/particle interface. Interfacial diffusion decreases the composite strength as it enhances the elongation of the elastic particles along the loading direction. When dislocation motion occurs by climb-assisted glide within the matrix the size effect of the strength is reduced as dislocations no longer arrange high energy pile-up structures but rather form lower energy dislocation cell networks. While interfacial diffusion again reduces the composite strength, in contrast to continuum plasticity predictions, the elongation of the particles is almost independent of the interfacial diffusion constant. Rather, in DDP the reduction in composite strength due to interfacial diffusion is a result of changes in the dislocation structures within the matrix and the associated enhanced dislocation climb rates in the matrix.
Provatas, Nikolas; Leonard, Francois Leonard; Mahon, Jennifer; Haataja, Mikko
2005-06-01
In this letter, we examine the effects of discrete mobile dislocations on spinodal decomposition kinetics in lattice mismatched binary alloys. By employing a novel continuum model, we demonstrate that the effects of dislocation mobility on domain coarsening kinetics can be expressed in a unified manner through a scaling function, describing a crossover from t{sup 1/2} to t{sup 1/3} behavior.
Empirical potential simulations of interstitial dislocation loops in uranium dioxide
NASA Astrophysics Data System (ADS)
Le Prioux, Arno; Fossati, Paul; Maillard, Serge; Jourdan, Thomas; Maugis, Philippe
2016-10-01
Stoichiometric circular shaped interstitial dislocation loop energies are calculated in stoichiometric UO2 by empirical potential simulation. The Burgers vector directions studied are <110> and <111>. The main structural properties of each type of interstitial dislocation loop are determined, including stacking fault energy. Defect energies are compared and a maximum size for stable <111> dislocation loops before transition to <110> dislocation loops is given. A model of dislocation loop energy based on elasticity theory is then fitted on the basis of these simulation results.
Recent Progress in Discrete Dislocation Dynamics and Its Applications to Micro Plasticity
NASA Astrophysics Data System (ADS)
Po, Giacomo; Mohamed, Mamdouh S.; Crosby, Tamer; Erel, Can; El-Azab, Anter; Ghoniem, Nasr
2014-10-01
We present a self-contained review of the discrete dislocation dynamics (DDD) method for the numerical investigation of plasticity in crystals, focusing on recent development and implementation progress. The review covers the theoretical foundations of DDD within the framework of incompatible elasticity, its numerical implementation via the nodal method, the extension of the method to finite domains and several implementation details. Applications of the method to current topics in micro-plasticity are presented, including the size effects in nano-indentation, the evolution of the dislocation microstructure in persistent slip bands, and the phenomenon of dislocation avalanches in micro-pillar compression.
Atomistic simulations of dislocation pileup: Grain boundaries interaction
Wang, Jian
2015-05-27
Here, using molecular dynamics (MD) simulations, we studied the dislocation pileup–grain boundary (GB) interactions. Two Σ11 asymmetrical tilt grain boundaries in Al are studied to explore the influence of orientation relationship and interface structure on dislocation activities at grain boundaries. To mimic the reality of a dislocation pileup in a coarse-grained polycrystalline, we optimized the dislocation population in MD simulations and developed a predict-correct method to create a dislocation pileup in MD simulations. MD simulations explored several kinetic processes of dislocations–GB reactions: grain boundary sliding, grain boundary migration, slip transmission, dislocation reflection, reconstruction of grain boundary, and the correlation ofmore » these kinetic processes with the available slip systems across the GB and atomic structures of the GB.« less
Atomistic simulations of dislocation pileup: Grain boundaries interaction
Wang, Jian
2015-05-27
Here, using molecular dynamics (MD) simulations, we studied the dislocation pileup–grain boundary (GB) interactions. Two Σ11 asymmetrical tilt grain boundaries in Al are studied to explore the influence of orientation relationship and interface structure on dislocation activities at grain boundaries. To mimic the reality of a dislocation pileup in a coarse-grained polycrystalline, we optimized the dislocation population in MD simulations and developed a predict-correct method to create a dislocation pileup in MD simulations. MD simulations explored several kinetic processes of dislocations–GB reactions: grain boundary sliding, grain boundary migration, slip transmission, dislocation reflection, reconstruction of grain boundary, and the correlation of these kinetic processes with the available slip systems across the GB and atomic structures of the GB.
Joint dislocation ... It may be hard to tell a dislocated joint from a broken bone . Both are emergencies that ... to repair a ligament that tears when the joint is dislocated is needed. Injuries to nerves and ...
NASA Astrophysics Data System (ADS)
Drouet, Julie; Dupuy, Laurent; Onimus, Fabien; Mompiou, Frédéric; Perusin, Simon; Ambard, Antoine
2014-06-01
The mechanical behavior of Pressurized Water Reactor fuel cladding tubes made of zirconium alloys is strongly affected by neutron irradiation due to the high density of radiation induced dislocation loops. In order to investigate the interaction mechanisms between gliding dislocations and loops in zirconium, a new nodal dislocation dynamics code, adapted to Hexagonal Close Packed metals, has been used. Various configurations have been systematically computed considering different glide planes, basal or prismatic, and different characters, edge or screw, for gliding dislocations with -type Burgers vectors. Simulations show various interaction mechanisms such as (i) absorption of a loop on an edge dislocation leading to the formation of a double super-jog, (ii) creation of a helical turn, on a screw dislocation, that acts as a strong pinning point or (iii) sweeping of a loop by a gliding dislocation. It is shown that the clearing of loops is more favorable when the dislocation glides in the basal plane than in the prismatic plane explaining the easy dislocation channeling in the basal plane observed after neutron irradiation by transmission electron microscopy.
Numerical Simulation of Dislocation Interactions in Strained Layers
NASA Astrophysics Data System (ADS)
Schwarz, Klaus W.
1998-03-01
Because both the dynamics and the interactions of dislocations are complicated, little is known about the effects that individual dislocations have on each other when they come into close proximity, and, more generally, about the evolution of collections of strongly interacting dislocations. The power of modern supercomputers allows one to address these issues by direct numerical simulation. In our program the stress tensor which moves the dislocations is calculated at every point by evaluating the full Peach-Koehler expression over all of the dislocations present. The self-interaction of the dislocations is regularized by the Brown method of splitting the dislocation in half, moving the two halves outward by some core parameter, and averaging the result. The code can be used to study the interactions between arbitrarily configured dislocations, located on any allowed glide plane, passing from one glide plane to another, and having any allowed Burgers vector, and has been applied to model the behavior of dislocations in semiconducting thin films. The behavior of a threading dislocation as it tries to pass over a misfit dislocation in the layer interface has been investigated, and the blocking effect of the misfit is found to be considerably less than previously estimated. Subsequent studies have calculated the behavior of Frank-Read sources, spiral sources, and corner sources in a strained layer, as well as that of multiple sources operating on various glide planes. For a model in which the layer is assumed to relax through the excitation of a low density of Frank-Read sources, it is found that the interactions between the emitted dislocations result in elaborate dislocation networks. The predicted patterns are strikingly similar to those observed in actual relaxed layers.
... Dislocations can occur in contact sports, such as football and hockey, and in sports in which falls ... downhill skiing, gymnastics and volleyball. Basketball players and football players also commonly dislocate joints in their fingers ...
Discrete dislocation plasticity analysis of loading rate-dependent static friction
NASA Astrophysics Data System (ADS)
Song, H.; Deshpande, V. S.; Van der Giessen, E.
2016-08-01
From a microscopic point of view, the frictional force associated with the relative sliding of rough surfaces originates from deformation of the material in contact, by adhesion in the contact interface or both. We know that plastic deformation at the size scale of micrometres is not only dependent on the size of the contact, but also on the rate of deformation. Moreover, depending on its physical origin, adhesion can also be size and rate dependent, albeit different from plasticity. We present a two-dimensional model that incorporates both discrete dislocation plasticity inside a face-centred cubic crystal and adhesion in the interface to understand the rate dependence of friction caused by micrometre-size asperities. The friction strength is the outcome of the competition between adhesion and discrete dislocation plasticity. As a function of contact size, the friction strength contains two plateaus: at small contact length (≲0.6 μ m), the onset of sliding is fully controlled by adhesion while for large contact length (≳10 μ m), the friction strength approaches the size-independent plastic shear yield strength. The transition regime at intermediate contact size is a result of partial de-cohesion and size-dependent dislocation plasticity, and is determined by dislocation properties, interfacial properties as well as by the loading rate.
A discrete dislocation analysis of mixed mode fracture at bimaterial interfaces
NASA Astrophysics Data System (ADS)
O'Day, Michael; Curtin, William
2004-03-01
The influence of mode mixity on crack growth and failure at a metal/ceramic bimaterial interface is examined within the discrete dislocation (DD) plasticity framework. In this method, plasticity occurs via the motion of a large number of dislocations embedded in a linearly elastic medium. No plastic constitutive law is required, however a set of rules governing dislocation nucleation, motion and annihilation is necessary. The numerical procedure uses a superposition technique, developed specifically to allow the efficient solution of DD problems with elastic inhomogeneities. An interface crack exists in the unloaded configuration, and a mode independent cohesive zone law characterizes the interface ahead of the crack tip. The influence of mode mixity on crack growth resistance curve (R-curve) behavior is qualitatively similar to continuum plasticity calculations, where increasing mode mixity leads to increasing toughness. However, deviations can arise due to (i) statistical effects, and (ii) the discrete nature of plasticity. Crack blunting, dislocation patterning and the existence of preferential slip planes all emerge naturally from the boundary value problem solution and give insight into observed R-curve trends.
Finite versus small strain discrete dislocation analysis of cantilever bending of single crystals
NASA Astrophysics Data System (ADS)
Irani, Nilgoon; Remmers, Joris J. C.; Deshpande, Vikram S.
2017-08-01
Plastic size effects in single crystals are investigated by using finite strain and small strain discrete dislocation plasticity to analyse the response of cantilever beam specimens. Crystals with both one and two active slip systems are analysed, as well as specimens with different beam aspect ratios. Over the range of specimen sizes analysed here, the bending stress versus applied tip displacement response has a strong hardening plastic component. This hardening rate increases with decreasing specimen size. The hardening rates are slightly lower when the finite strain discrete dislocation plasticity (DDP) formulation is employed as curving of the slip planes is accounted for in the finite strain formulation. This relaxes the back-stresses in the dislocation pile-ups and thereby reduces the hardening rate. Our calculations show that in line with the pure bending case, the bending stress in cantilever bending displays a plastic size dependence. However, unlike pure bending, the bending flow strength of the larger aspect ratio cantilever beams is appreciably smaller. This is attributed to the fact that for the same applied bending stress, longer beams have lower shear forces acting upon them and this results in a lower density of statistically stored dislocations.
NASA Astrophysics Data System (ADS)
Waheed, S.; Hao, R.; Bhowmik, A.; Balint, D. S.; Giuliani, F.
2017-07-01
In this study, sequential sputter deposition, diffusion bonding and focused ion beam milling are used to fabricate sapphire micropillars encapsulating a thin single crystal niobium film. A distinct Bauschinger effect is observed during the cyclic axial compression of the samples. Plain strain discrete dislocation plasticity is used to interpret the experimental results obtained for the encapsulated film-micropillar geometry. The simulations show that the experimental samples correspond to a saturated source density regime, producing the maximum Bauschinger effect for the chosen mean nucleation strength. Next, the source density and mean nucleation strength are shown to have a coupled effect on the size of the Bauschinger effect, understood in terms of the differing number of pile-ups occurring per source in the film. The coupled effect is found to be represented by the density of dislocations annihilated upon unloading: a consistent linear relationship is observed between the size of the Bauschinger effect and the annihilated dislocation density over the entire source density and nucleation strength parameter space investigated. It is found that different film orientations fulfil the same linear relationship, whereas changing the film thickness causes the slope of the linear trend to vary suggesting a length-scale dependence on reverse plasticity. Finally, all results are found to be unified by a power-law relationship quantifying the Bauschinger effect of the form {{{Γ }}}{{B}}\\propto {{Δ }}{ρ }{{ann}}{l}n where it is argued that the number of dislocations undergoing reverse glide in the confined film is represented by {{Δ }}{ρ }{{ann}}, the mean free path of dislocations by l and the effect of hardening processes by the exponent n. The net reverse glide is thus represented by {{Δ }}{ρ }{{ann}}{l}n which can be used as a measure of the Bauschinger effect.
NASA Technical Reports Server (NTRS)
Saether, Erik; Hochhalter, Jacob D.; Glaessgen, Edward H.
2012-01-01
A multiscale modeling methodology that combines the predictive capability of discrete dislocation plasticity and the computational efficiency of continuum crystal plasticity is developed. Single crystal configurations of different grain sizes modeled with periodic boundary conditions are analyzed using discrete dislocation plasticity (DD) to obtain grain size-dependent stress-strain predictions. These relationships are mapped into crystal plasticity parameters to develop a multiscale DD/CP model for continuum level simulations. A polycrystal model of a structurally-graded microstructure is developed, analyzed and used as a benchmark for comparison between the multiscale DD/CP model and the DD predictions. The multiscale DD/CP model follows the DD predictions closely up to an initial peak stress and then follows a strain hardening path that is parallel but somewhat offset from the DD predictions. The difference is believed to be from a combination of the strain rate in the DD simulation and the inability of the DD/CP model to represent non-monotonic material response.
NASA Astrophysics Data System (ADS)
Huang, Minsheng; Li, Zhenhuan
2013-12-01
To model the deformation of single crystal nickel based superalloys (SCNBS) with low stacking fault energy (SFE), three-dimensional discrete dislocation dynamics (3D-DDD) is extended by incorporating dislocation dissociation mechanism. The present 3D-DDD simulations show that, consistent with the existing TEM observation, the leading partial can enter the matrix channel efficiently while the trailing partial can hardly glide into it when the dislocation dissociation is taken into account. To determine whether the dislocation dissociation can occur or not, a critical percolation stress (CPS) based criterion is suggested. According to this CPS criterion, for SCNBS there exists a critical matrix channel width. When the channel width is lower than this critical value, the dislocation tends to dissociate into an extended configuration and vice versa. To clarify the influence of dislocation dissociation on CPS, the classical Orowan formula is improved by incorporating the SFE. Moreover, the present 3D-DDD simulations also show that the yielding stress of SCNBSs with low SFE may be overestimated up to 30% if the dislocation dissociation is ignored. With dislocation dissociation being considered, the size effect due to the width of γ matrix channel and the length of γ‧ precipitates on the stress-strain responses of SCNBS can be enhanced remarkably. In addition, due to the strong constraint effect by the two-phase microstructure in SCNBS, the configuration of formed junctions is quite different from that in single phase crystals such as Cu. The present results not only provide clear understanding of the two-phase microstructure levelled microplastic mechanisms in SCNBSs with low SFE, but also help to develop new continuum-levelled constitutive laws for SCNBSs.
NASA Technical Reports Server (NTRS)
Saether, Erik; Hochhalter, Jacob D.; Glaessgen, Edward H.; Mishin, Yuri
2014-01-01
A multiscale modeling methodology is developed for structurally-graded material microstructures. Molecular dynamic (MD) simulations are performed at the nanoscale to determine fundamental failure mechanisms and quantify material constitutive parameters. These parameters are used to calibrate material processes at the mesoscale using discrete dislocation dynamics (DD). Different grain boundary interactions with dislocations are analyzed using DD to predict grain-size dependent stress-strain behavior. These relationships are mapped into crystal plasticity (CP) parameters to develop a computationally efficient finite element-based DD/CP model for continuum-level simulations and complete the multiscale analysis by predicting the behavior of macroscopic physical specimens. The present analysis is focused on simulating the behavior of a graded microstructure in which grain sizes are on the order of nanometers in the exterior region and transition to larger, multi-micron size in the interior domain. This microstructural configuration has been shown to offer improved mechanical properties over homogeneous coarse-grained materials by increasing yield stress while maintaining ductility. Various mesoscopic polycrystal models of structurally-graded microstructures are generated, analyzed and used as a benchmark for comparison between multiscale DD/CP model and DD predictions. A final series of simulations utilize the DD/CP analysis method exclusively to study macroscopic models that cannot be analyzed by MD or DD methods alone due to the model size.
How to identify dislocations in molecular dynamics simulations?
NASA Astrophysics Data System (ADS)
Li, Duo; Wang, FengChao; Yang, ZhenYu; Zhao, YaPu
2014-12-01
Dislocations are of great importance in revealing the underlying mechanisms of deformed solid crystals. With the development of computational facilities and technologies, the observations of dislocations at atomic level through numerical simulations are permitted. Molecular dynamics (MD) simulation suggests itself as a powerful tool for understanding and visualizing the creation of dislocations as well as the evolution of crystal defects. However, the numerical results from the large-scale MD simulations are not very illuminating by themselves and there exist various techniques for analyzing dislocations and the deformed crystal structures. Thus, it is a big challenge for the beginners in this community to choose a proper method to start their investigations. In this review, we summarized and discussed up to twelve existing structure characterization methods in MD simulations of deformed crystal solids. A comprehensive comparison was made between the advantages and disadvantages of these typical techniques. We also examined some of the recent advances in the dynamics of dislocations related to the hydraulic fracturing. It was found that the dislocation emission has a significant effect on the propagation and bifurcation of the crack tip in the hydraulic fracturing.
Atomistic Simulation of Dislocation-Defect Interactions in Cu
Wirth, B D; Bulatov, V V; Diaz de la Rubia, T
2001-01-01
The mechanisms of dislocation-defect interactions are of practical importance for developing quantitative structure-property relationships, mechanistic understanding of plastic flow localization and predictive models of mechanical behavior in metals under irradiation. In copper and other face centered cubic metals, high-energy particle irradiation produces hardening and shear localization. Post-irradiation microstructural examination in Cu reveals that irradiation has produced a high number density of nanometer sized stacking fault tetrahedra. Thus, the resultant irradiation hardening and shear localization is commonly attributed to the interaction between stacking fault tetrahedra and mobile dislocations, although the mechanism of this interaction is unknown. In this work, we present a comprehensive molecular dynamics simulation study that characterizes the interaction and fate of moving dislocations with stacking fault tetrahedra in Cu using an EAM interatomic potential. This work is intended to produce atomistic input into dislocation dynamics simulations of plastic flow localization in irradiated materials.
Dislocation dynamics simulations of plasticity at small scales
Zhou, Caizhi
2010-01-01
As metallic structures and devices are being created on a dimension comparable to the length scales of the underlying dislocation microstructures, the mechanical properties of them change drastically. Since such small structures are increasingly common in modern technologies, there is an emergent need to understand the critical roles of elasticity, plasticity, and fracture in small structures. Dislocation dynamics (DD) simulations, in which the dislocations are the simulated entities, offer a way to extend length scales beyond those of atomistic simulations and the results from DD simulations can be directly compared with the micromechanical tests. The primary objective of this research is to use 3-D DD simulations to study the plastic deformation of nano- and micro-scale materials and understand the correlation between dislocation motion, interactions and the mechanical response. Specifically, to identify what critical events (i.e., dislocation multiplication, cross-slip, storage, nucleation, junction and dipole formation, pinning etc.) determine the deformation response and how these change from bulk behavior as the system decreases in size and correlate and improve our current knowledge of bulk plasticity with the knowledge gained from the direct observations of small-scale plasticity. Our simulation results on single crystal micropillars and polycrystalline thin films can march the experiment results well and capture the essential features in small-scale plasticity. Furthermore, several simple and accurate models have been developed following our simulation results and can reasonably predict the plastic behavior of small scale materials.
The inverse hall-petch relation in nanocrystalline metals: A discrete dislocation dynamics analysis
NASA Astrophysics Data System (ADS)
Quek, Siu Sin; Chooi, Zheng Hoe; Wu, Zhaoxuan; Zhang, Yong Wei; Srolovitz, David J.
2016-03-01
When the grain size in polycrystalline materials is reduced to the nanometer length scale (nanocrystallinity), observations from experiments and atomistic simulations suggest that the yield strength decreases (softening) as the grain size is decreased. This is in contrast to the Hall-Petch relation observed in larger sized grains. We incorporated grain boundary (GB) sliding and dislocation emission from GB junctions into the classical DDD framework, and recovered the smaller is weaker relationship observed in nanocrystalline materials. This current model shows that the inverse Hall-Petch behavior can be obtained through a relief of stress buildup at GB junctions from GB sliding by emitting dislocations from the junctions. The yield stress is shown to vary with grain size, d, by a d 1 / 2 relationship when grain sizes are very small. However, pure GB sliding alone without further plastic accomodation by dislocation emission is grain size independent.
Quantifying Discretization Effects on Brain Trauma Simulations
2016-01-01
analyzed in each case were the variations in stress magnitude, spatial distribution, and wave patterns that arise inside the brain. The effects of...ARL-CR-0792 ● JAN 2016 US Army Research Laboratory Quantifying Discretization Effects on Brain Trauma Simulations prepared by...originator. ARL-CR-0792● JAN 2016 US Army Research Laboratory Quantifying Discretization Effects on Brain Trauma Simulations
Synchronization Of Parallel Discrete Event Simulations
NASA Technical Reports Server (NTRS)
Steinman, Jeffrey S.
1992-01-01
Adaptive, parallel, discrete-event-simulation-synchronization algorithm, Breathing Time Buckets, developed in Synchronous Parallel Environment for Emulation and Discrete Event Simulation (SPEEDES) operating system. Algorithm allows parallel simulations to process events optimistically in fluctuating time cycles that naturally adapt while simulation in progress. Combines best of optimistic and conservative synchronization strategies while avoiding major disadvantages. Algorithm processes events optimistically in time cycles adapting while simulation in progress. Well suited for modeling communication networks, for large-scale war games, for simulated flights of aircraft, for simulations of computer equipment, for mathematical modeling, for interactive engineering simulations, and for depictions of flows of information.
Periodic boundary conditions for dislocation dynamics simulations in three dimensions
Bulatov, V V; Rhee, M; Cai, W
2000-11-20
This article presents an implementation of periodic boundary conditions (PBC) for Dislocation Dynamics (DD) simulations in three dimensions (3D). We discuss fundamental aspects of PBC development, including preservation of translational invariance and line connectivity, the choice of initial configurations compatible with PBC and a consistent treatment of image stress. On the practical side, our approach reduces to manageable proportions the computational burden of updating the long-range elastic interactions among dislocation segments. The timing data confirms feasibility and practicality of PBC for large-scale DD simulations in 3D.
Dwell fatigue in two Ti alloys: An integrated crystal plasticity and discrete dislocation study
NASA Astrophysics Data System (ADS)
Zheng, Zebang; Balint, Daniel S.; Dunne, Fionn P. E.
2016-11-01
It is a well known and important problem in the aircraft engine industry that alloy Ti-6242 shows a significant reduction in fatigue life, termed dwell debit, if a stress dwell is included in the fatigue cycle, whereas Ti-6246 does not; the mechanistic explanation for the differing dwell debit of these alloys has remained elusive for decades. In this work, crystal plasticity modelling has been utilised to extract the thermal activation energies for pinned dislocation escape for both Ti alloys based on independent experimental data. This then allows the markedly different cold creep responses of the two alloys to be captured accurately and demonstrates why the observed near-identical rate sensitivity under non-dwell loading is entirely consistent with the dwell behaviour. The activation energies determined are then utilised within a recently developed thermally-activated discrete dislocation plasticity model to predict the strain rate sensitivities of the two alloys associated with nano-indentation into basal and prism planes. It is shown that Ti-6242 experiences a strong crystallographic orientation-dependent rate sensitivity while Ti-6246 does not which is shown to agree with recently published independent measurements; the dependence of rate sensitivity on indentation slip plane is also well captured. The thermally-activated discrete dislocation plasticity model shows that the incorporation of a stress dwell in fatigue loading leads to remarkable stress redistribution from soft to hard grains in the classical cold dwell fatigue rogue grain combination in alloy Ti-6242, but that no such load shedding occurs in alloy Ti-6246. The key property controlling the behaviour is the time constant of the thermal activation process relative to that of the loading. This work provides the first mechanistic basis to explain why alloy Ti-6242 shows a dwell debit but Ti-6246 does not.
A discrete event method for wave simulation
Nutaro, James J
2006-01-01
This article describes a discrete event interpretation of the finite difference time domain (FDTD) and digital wave guide network (DWN) wave simulation schemes. The discrete event method is formalized using the discrete event system specification (DEVS). The scheme is shown to have errors that are proportional to the resolution of the spatial grid. A numerical example demonstrates the relative efficiency of the scheme with respect to FDTD and DWN schemes. The potential for the discrete event scheme to reduce numerical dispersion and attenuation errors is discussed.
Continuum simulation of dislocation dynamics: Predictions for internal friction response
Greaney, P. Alex; Friedman, Lawrence H.; Chrzan, D.C.
2002-02-19
The amplitude dependent mechanical loss due to bosing of an idealized Frank-Read Source is studied using both simulation and analytical techniques. Dislocations are modeled within isotropic elasticity theory, and are assumed to be in the over-damped limit.
Running Parallel Discrete Event Simulators on Sierra
Barnes, P. D.; Jefferson, D. R.
2015-12-03
In this proposal we consider porting the ROSS/Charm++ simulator and the discrete event models that run under its control so that they run on the Sierra architecture and make efficient use of the Volta GPUs.
Program For Parallel Discrete-Event Simulation
NASA Technical Reports Server (NTRS)
Beckman, Brian C.; Blume, Leo R.; Geiselman, John S.; Presley, Matthew T.; Wedel, John J., Jr.; Bellenot, Steven F.; Diloreto, Michael; Hontalas, Philip J.; Reiher, Peter L.; Weiland, Frederick P.
1991-01-01
User does not have to add any special logic to aid in synchronization. Time Warp Operating System (TWOS) computer program is special-purpose operating system designed to support parallel discrete-event simulation. Complete implementation of Time Warp mechanism. Supports only simulations and other computations designed for virtual time. Time Warp Simulator (TWSIM) subdirectory contains sequential simulation engine interface-compatible with TWOS. TWOS and TWSIM written in, and support simulations in, C programming language.
NASA Technical Reports Server (NTRS)
Liu, H. W.; Gao, Q.
1991-01-01
The equivalence between continuous dislocation pileups and cracks is reviewed. The force on the leading dislocation is defined and a general method of calculation of the force is proposed. The equivalence relations are given. Based on the analysis by Eshelby, Frank, and Nabarro and the numerical calculations by Chou and Li and Armstrong et al, it will be shown that the force, F, on the locked leading dislocation of a discrete pileup is capable of characterizing uniquely the stress, strain, and displacement fields at the tip of the pileup, including the positions of the discrete mobile dislocations behind the leading dislocation. Conversely, the positions of the mobile dislocations can be used to measure F. If the propagation of micro-slips and the initiation of micro-fractures at the tip of a pileup are controlled by resolved shear stress and normal cleavage stress respectively, the resolved shear stress intensity coefficient, ReSIC, and the resolved cleavage stress intensity coefficient, ReCIC, must be constant.
Dislocation dynamics: simulation of plastic flow of bcc metals
Lassila, D H
2001-02-20
This is the final report for the LDRD strategic initiative entitled ''Dislocation Dynamic: Simulation of Plastic Flow of bcc Metals'' (tracking code: 00-SI-011). This report is comprised of 6 individual sections. The first is an executive summary of the project and describes the overall project goal, which is to establish an experimentally validated 3D dislocation dynamics simulation. This first section also gives some information of LLNL's multi-scale modeling efforts associated with the plasticity of bcc metals, and the role of this LDRD project in the multiscale modeling program. The last five sections of this report are journal articles that were produced during the course of the FY-2000 efforts.
Atomic and dislocation dynamics simulations of plastic deformation in reactor pressure vessel steel
NASA Astrophysics Data System (ADS)
Monnet, Ghiath; Domain, Christophe; Queyreau, Sylvain; Naamane, Sanae; Devincre, Benoit
2009-11-01
The collective behavior of dislocations in reactor pressure vessel (RPV) steel involves dislocation properties on different phenomenological scales. In the multiscale approach, adopted in this work, we use atomic simulations to provide input data for larger scale simulations. We show in this paper how first-principles calculations can be used to describe the Peierls potential of screw dislocations, allowing for the validation of the empirical interatomic potential used in molecular dynamics simulations. The latter are used to compute the velocity of dislocations as a function of the applied stress and the temperature. The mobility laws obtained in this way are employed in dislocation dynamics simulations in order to predict properties of plastic flow, namely dislocation-dislocation interactions and dislocation interactions with carbides at low and high temperature.
Atomistic simulations of elastic deformation and dislocation nucleation during nanoindentation
NASA Astrophysics Data System (ADS)
Lilleodden, E. T.; Zimmerman, J. A.; Foiles, S. M.; Nix, W. D.
2003-05-01
Nanoindentation experiments have shown that microstructural inhomogeneities across the surface of gold thin films lead to position-dependent nanoindentation behavior [Phys. Rev. B (2002), to be submitted]. The rationale for such behavior was based on the availability of dislocation sources at the grain boundary for initiating plasticity. In order to verify or refute this theory, a computational approach has been pursued. Here, a simulation study of the initial stages of indentation using the embedded atom method (EAM) is presented. First, the principles of the EAM are given, and a comparison is made between atomistic simulations and continuum models for elastic deformation. Then, the mechanism of dislocation nucleation in single crystalline gold is analyzed, and the effects of elastic anisotropy are considered. Finally, a systematic study of the indentation response in the proximity of a high angle, high sigma (low symmetry) grain boundary is presented; indentation behavior is simulated for varying indenter positions relative to the boundary. The results indicate that high angle grain boundaries are a ready source of dislocations in indentation-induced deformation.
NASA Astrophysics Data System (ADS)
Yang, Hui; Li, Zhenhuan; Huang, Minsheng
2014-12-01
Unlike common single crystals, the nickel-based single crystal superalloy shows surprisingly anomalous flow strength (i.e. with the increase of temperature, the yield strength first increases to a peak value and then decreases) and tension-compression (TC) asymmetry. A comprehensive three-dimensional discrete dislocation dynamics (3D-DDD) procedure was developed to model these abnormal mechanical properties. For this purpose, a series of complicated dynamic evolution details of Kear-Wilsdorf (KW) locks, which are closely related to the flow strength anomaly and TC asymmetry, were incorporated into this 3D-DDD framework. Moreover, the activation of the cubic slip system, which is the origin of the decrease in yield strength with increasing temperature at relatively high temperatures, was especially taken into account by introducing a competition criterion between the unlocking of the KW locks and the activation of the cubic slip system. To test our framework, a series of 3D-DDD simulations were performed on a representative volume cell model with a cuboidal Ni3Al precipitate phase embedded in a nickel matrix. Results show that the present 3D-DDD procedure can successfully capture the dynamic evolution of KW locks, the flow strength anomaly and TC asymmetry. Then, the underlying dislocation mechanisms leading to these abnormal mechanical responses were investigated and discussed in detail. Finally, a cyclic deformation of the nickel-based single crystal superalloy was modeled by using the present DDD model, with a special focus on the influence of KW locks on the Bauschinger effect and cyclic softening.
NASA Astrophysics Data System (ADS)
Gao, Siwen; Rajendran, Mohan Kumar; Fivel, Marc; Ma, Anxin; Shchyglo, Oleg; Hartmaier, Alexander; Steinbach, Ingo
2015-10-01
Three-dimensional discrete dislocation dynamics (DDD) simulations in combination with the phase-field method are performed to investigate the influence of different realistic Ni-base single crystal superalloy microstructures with the same volume fraction of {γ\\prime} precipitates on plastic deformation at room temperature. The phase-field method is used to generate realistic microstructures as the boundary conditions for DDD simulations in which a constant high uniaxial tensile load is applied along different crystallographic directions. In addition, the lattice mismatch between the γ and {γ\\prime} phases is taken into account as a source of internal stresses. Due to the high antiphase boundary energy and the rare formation of superdislocations, precipitate cutting is not observed in the present simulations. Therefore, the plastic deformation is mainly caused by dislocation motion in γ matrix channels. From a comparison of the macroscopic mechanical response and the dislocation evolution for different microstructures in each loading direction, we found that, for a given {γ\\prime} phase volume fraction, the optimal microstructure should possess narrow and homogeneous γ matrix channels.
Terentyev, Dmitry; Grammatikopoulos, P.; Bacon, D; Osetsky, Yu N
2008-01-01
Atomic-level simulations are used to investigate the interaction of an edge dislocation with h100i interstitial dislocation loops in airon at 300 K. Dislocation reactions are studied systematically for different loop positions and Burgers vector orientations, and results are compared for two different interatomic potentials. Reactions are wide-ranging and complex, but can be described in terms of conventional dislocation reactions in which Burgers vector is conserved. The fraction of interstitials left behind after dislocation breakaway varies from 25 to 100%. The nature of the reactions requiring high applied stress for breakaway is identified. The obstacle strengths of h100i loops, 1/2h111i loops and voids containing the same number (169) of point defects are compared. h100i loops with Burgers vector parallel to the dislocation glide plane are slightly stronger than h100i and 1/2h111i loops with inclined Burgers vector: voids are about 30% weaker than the stronger loops. However, small voids are stronger than small 1/2h111i loops. The complexity of some reactions and the variety of obstacle strengths poses a challenge for the development of continuum models of dislocation behaviour in irradiated iron. 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
An automatic and simple method for specifying dislocation features in atomistic simulations
NASA Astrophysics Data System (ADS)
Dai, Fu-Zhi; Zhang, Wen-Zheng
2015-03-01
An important aspect of atomistic simulations of imperfect crystalline materials is the characterization of dislocations. In this paper, we propose the use of singular values and singular vectors of the Nye tensor to specify the features of dislocations, including the cores, nodes, Burgers vectors and line directions. Atoms locate in dislocation cores and nodes are identified by non-trivial singular values of the Nye tensor, while Burgers vector direction and line direction are specified by those singular vectors corresponding to the biggest singular value. This method provides a powerful, intuitive and automatic tool for characterizing dislocations from simulation data. Its validity for characterizing dislocations is verified with three examples.
Parallel discrete event simulation using shared memory
NASA Technical Reports Server (NTRS)
Reed, Daniel A.; Malony, Allen D.; Mccredie, Bradley D.
1988-01-01
With traditional event-list techniques, evaluating a detailed discrete-event simulation-model can often require hours or even days of computation time. By eliminating the event list and maintaining only sufficient synchronization to ensure causality, parallel simulation can potentially provide speedups that are linear in the numbers of processors. A set of shared-memory experiments, using the Chandy-Misra distributed-simulation algorithm, to simulate networks of queues is presented. Parameters of the study include queueing network topology and routing probabilities, number of processors, and assignment of network nodes to processors. These experiments show that Chandy-Misra distributed simulation is a questionable alternative to sequential-simulation of most queueing network models.
NASA Astrophysics Data System (ADS)
Rao, Satish
2015-03-01
Experimental studies show strong strengthening effects for micrometer-scale FCC as well as two-phase superalloy crystals, even at high initial dislocation densities. This talk shows results from large-scale 3-D discrete dislocation simulations (DDS) used to explicitly model the deformation behavior of FCC Ni (flow stress and strain-hardening) as well as superalloy microcrystals for diameters ranging from 1 - 20 microns. The work shows that two size-sensitive athermal hardening processes, beyond forest and precipitation hardening, are sufficient to develop the dimensional scaling of the flow stress, stochastic stress variation, flow intermittency and, high initial strain-hardening rates, similar to experimental observations for various materials. In addition, 3D dislocation dynamics simulations are used to investigate strain-hardening characteristics and dislocation microstructure evolution with strain in large 20 micron size Ni microcrystals (bulk-like) under three different loading axes: 111, 001 and 110. Three different multi-slip loading axes, < 111 > , < 001 > and < 110 > , are explored for shear strains of ~0.03 and final dislocation densities of ~1013/m2. The orientation dependence of initial strain hardening rates and dislocation microstructure evolution with strain are discussed. The simulated strain hardening results are compared with experimental data under similar loading conditions from bulk single-crystal Ni. Finally, atomistic simulation results on the operation of single arm sources in Ni bipillars with a large angle grain boundary is discussed. The atomistic simulation results are compared with experimental mechanical behavior data on Cu bipillars with a similar large angle grain boundary. This work was supported by AFOSR (Dr. David Stargel), and by a grant of computer time from the DOD High Performance Computing Modernization Program, at the Aeronautical Systems Center/Major Shared Resource Center.
Simulation of Screw Dislocation Motion in Iron by Molecular Dynamics Simulations
Domain, Christophe; Monnet, Ghiath
2005-11-18
Molecular dynamics (MD) simulations are used to investigate the response of a/2<111> screw dislocation in iron submitted to pure shear strain. The dislocation glides and remains in a (110) plane; the motion occurs exclusively through the nucleation and propagation of double kinks. The critical stress is calculated as a function of the temperature. A new method is developed and used to determine the activation energy of the double kink mechanism from MD simulations. It is shown that the differences between experimental and simulation conditions lead to a significant difference in activation energy. These differences are explained, and the method developed provides the link between MD and mesoscopic simulations.
Discrete Molecular Dynamics Simulation of Biomolecules
NASA Astrophysics Data System (ADS)
Ding, Feng
2011-10-01
Discrete molecular dynamics (DMD) simulation of hard spheres was the first implementation of molecular dynamics (MD) in history. DMD simulations are computationally more efficient than continuous MD simulations due to simplified interaction potentials. However, also due to these simplified potentials, DMD has often been associated with coarse-grained modeling, and hence continuous MD has become the dominant approach used to study the internal dynamics of biomolecules. With the recent advances in DMD methodology, including the development of high-resolution models for biomolecules and approaches to increase DMD efficiency, DMD simulations are emerging as an important tool in the field of molecular modeling, including the study of protein folding, protein misfolding and aggregation, and protein engineering. Recently, DMD methodology has been applied to modeling RNA folding and protein-ligand recognition. With these improvements to DMD methodology and the continuous increase in available computational power, we expect a growing role of DMD simulations in our understanding of biology.
Distributed discrete event simulation. Final report
De Vries, R.C.
1988-02-01
The presentation given here is restricted to discrete event simulation. The complexity of and time required for many present and potential discrete simulations exceeds the reasonable capacity of most present serial computers. The desire, then, is to implement the simulations on a parallel machine. However, certain problems arise in an effort to program the simulation on a parallel machine. In one category of methods deadlock care arise and some method is required to either detect deadlock and recover from it or to avoid deadlock through information passing. In the second category of methods, potentially incorrect simulations are allowed to proceed. If the situation is later determined to be incorrect, recovery from the error must be initiated. In either case, computation and information passing are required which would not be required in a serial implementation. The net effect is that the parallel simulation may not be much better than a serial simulation. In an effort to determine alternate approaches, important papers in the area were reviewed. As a part of that review process, each of the papers was summarized. The summary of each paper is presented in this report in the hopes that those doing future work in the area will be able to gain insight that might not otherwise be available, and to aid in deciding which papers would be most beneficial to pursue in more detail. The papers are broken down into categories and then by author. Conclusions reached after examining the papers and other material, such as direct talks with an author, are presented in the last section. Also presented there are some ideas that surfaced late in the research effort. These promise to be of some benefit in limiting information which must be passed between processes and in better understanding the structure of a distributed simulation. Pursuit of these ideas seems appropriate.
Zhu, Wujun; Deng, Mingxi; Xiang, Yanxun; Xuan, Fu-Zhen; Liu, Changjun; Wang, Yi-Ning
2016-05-01
A nonlinear constitutive relationship was established to investigate nonlinear behaviors of ultrasonic wave propagation in plastically damaged media based on analyses of mixed dislocation evolution. Finite element simulations of longitudinal wave propagation in plastically deformed martensite stainless steel were performed based on the proposed nonlinear constitutive relationship, in which the contribution of mixed dislocation to acoustic nonlinearity was considered. The simulated results were validated by experimental measurements of plastically deformed 30Cr2Ni4MoV martensite stainless steels. Simulated and experimental results both reveal a monotonically increasing tendency of the normalized acoustic nonlinearity parameter as a function of plastic strain. Microscopic studies revealed that the changes of the acoustic nonlinearity are mainly attributed to dislocation evolutions, such as dislocation density, dislocation length, and the type and fraction of dislocations during plastic loading.
NASA Astrophysics Data System (ADS)
Zisman, Alexander
2016-04-01
Starting from Nye's tensor, alternative characteristics of crystal curvature indicative of dislocation content are considered subject to very low thickness of investigated matter under the free surface and discreteness of orientation sampling. Analysis within the framework of continuum mechanics, undertaken to allow for such conditions peculiar to the electron backscatter diffraction (EBSD) technique, has shown the variable part of orientations expressed in a vector form to be most sensitive to lattice defects when projected to the free surface plane. Hence, as verified with EBSD data on a grain junction in a low deformed IF steel, magnitude of the projected field allows one to map plastic strains inhomogeneous within grains whereas divergence of this field distinctly images and quantifies low-angle dislocation boundaries formed at low strains.
On the Connection Between the Discrete Dislocation Slip Model and the Orowan Equation
BRAGINSKY, MICHAEL V.; GLAZOV, MICHAEL V.; RICHMOND, OWEN
1999-09-08
Within the framework of thermodynamic theory of plasticity and specific structural-variables (associated with individual dislocations), a transition has been made to an expression containing one internal variable of the averaging type--the density of glissile dislocations, N{sub g}. This expression should be considered a tensorial generalization of the well-known Orowan's equation and relates it directly to the simplest possible case of normal flow in metallic materials. Since most metals display deviations from normality in the flow rule{sup 7} it also clearly indicates that more rigorous assessment of the relation between plastic strain rate and dislocation populations is required especially for materials displaying plastic instabilities in the form of dislocation patterning, strain-softening and strain-rate softening phenomena. The obtained result could be a useful starting point in establishing such rigorous macroscopic relations from microscopic considerations associated with individual dislocations and to find useful applications in dislocation density-related constitutive modeling of plastic deformation.
Simulating Electrophoresis with Discrete Charge and Drag
NASA Astrophysics Data System (ADS)
Mowitz, Aaron J.; Witten, Thomas A.
A charged asymmetric rigid cluster of colloidal particles in saline solution can respond in exotic ways to an electric field: it may spin or move transversely. These distinctive motions arise from the drag force of the neutralizing countercharge surrounding the cluster. Because of this drag, calculating the motion of arbitrary asymmetric objects with nonuniform charge is impractical by conventional methods. Here we present a new method of simulating electrophoresis, in which we replace the continuous object and the surrounding countercharge with discrete point-draggers, called Stokeslets. The balance of forces imposes a linear, self-consistent relation among the drag and Coulomb forces on the Stokeslets, which allows us to easily determine the object's motion via matrix inversion. By explicitly enforcing charge+countercharge neutrality, the simulation recovers the distinctive features of electrophoretic motion to few-percent accuracy using as few as 1000 Stokeslets. In particular, for uniformly charged objects, we observe the characteristic Smoluchowski independence of mobility on object size and shape. We then discuss electrophoretic motion of asymmetric objects, where our simulation method is particularly advantageous. This work is supported by a Grant from the US-Israel Binational Science Foundation.
Discrete photon implementation for plasma simulations
NASA Astrophysics Data System (ADS)
Fierro, Andrew; Stephens, Jacob; Beeson, Sterling; Dickens, James; Neuber, Andreas
2016-01-01
The self-produced light emission from pulsed plasma discharges and its impact on plasma development are challenging to characterize through simulation and modeling, chiefly due to the large number of radiating species and limited computer memory. Often, photo-processes, such as photo-ionization or photo-emission of electrons, are implemented through over-simplifying approximations or neglected altogether. Here, a method applicable to plasma simulations is implemented in a Particle-in-Cell /Monte Carlo Collision model, which is capable of discretely tracking photons and their corresponding wavelengths. Combined with the appropriate cross sections or quantum yields, a wavelength dependent model for photo-ionization or photo-emission may be implemented. Additionally, by resolving the wavelengths of each photon, an emission spectrum for a region of interest may be generated. Simulations for a pure nitrogen environment reveal that the calculated emission profile of the second positive system agrees well with the experimental spectrum of a pulsed, nanosecond discharge in the same spectral region.
Discrete photon implementation for plasma simulations
Fierro, Andrew Stephens, Jacob; Beeson, Sterling; Dickens, James; Neuber, Andreas
2016-01-15
The self-produced light emission from pulsed plasma discharges and its impact on plasma development are challenging to characterize through simulation and modeling, chiefly due to the large number of radiating species and limited computer memory. Often, photo-processes, such as photo-ionization or photo-emission of electrons, are implemented through over-simplifying approximations or neglected altogether. Here, a method applicable to plasma simulations is implemented in a Particle-in-Cell /Monte Carlo Collision model, which is capable of discretely tracking photons and their corresponding wavelengths. Combined with the appropriate cross sections or quantum yields, a wavelength dependent model for photo-ionization or photo-emission may be implemented. Additionally, by resolving the wavelengths of each photon, an emission spectrum for a region of interest may be generated. Simulations for a pure nitrogen environment reveal that the calculated emission profile of the second positive system agrees well with the experimental spectrum of a pulsed, nanosecond discharge in the same spectral region.
Strength of metals under vibrations - dislocation-density-function dynamics simulations
NASA Astrophysics Data System (ADS)
Cheng, B.; Leung, H. S.; Ngan, A. H. W.
2015-06-01
It is well known that ultrasonic vibration can soften metals, and this phenomenon has been widely exploited in industrial applications concerning metal forming and bonding. Recent experiments show that the simultaneous application of oscillatory stresses from audible to ultrasonic frequency ranges can lead to not only softening but also significant dislocation annihilation and subgrain formation in metal samples from the nano- to macro-size range. These findings indicate that the existing understanding of ultrasound softening - that the vibrations either impose additional stress waves to augment the quasi-static applied load, or cause heating of the metal, whereas the metal's intrinsic deformation resistance or mechanism remains unaltered - is far from complete. To understand the softening and the associated enhanced subgrain formation and dislocation annihilation, a new simulator based on the dynamics of dislocation-density functions is employed. This new simulator considers the flux, production and annihilation, as well as the Taylor and elastic interactions between dislocation densities. Softening during vibrations as well as enhanced cell formation is predicted. The simulations reveal the main mechanism for subcell formation under oscillatory loadings to be the enhanced elimination of statistically stored dislocations (SSDs) by the oscillatory stress, leaving behind geometrically necessary dislocations with low Schmid factors which then form the subgrain walls. The oscillatory stress helps the depletion of the SSDs, because the chance for them to meet up and annihilate is increased with reversals of dislocation motions. This is the first simulation effort to successfully predict the cell formation phenomenon under vibratory loadings.
NASA Astrophysics Data System (ADS)
Zhang, Yanqiu; Jiang, Shuyong; Zhu, Xiaoming; Zhao, Yanan
2016-08-01
Molecular dynamics simulation was performed to investigate dislocation mechanism of void growth at twin boundary (TB) of nanotwinned nickel. Simulation results show that the deformation of nanotwinned nickel containing a void at TB is dominated by the slip involving both leading and trailing partials, where the trailing partials are the dissociation products of stair-rod dislocations formed by the leading partials. The growth of a void at TB is attributed to the successive emission of the leading partials followed by trailing partials as well as the escape of these partial dislocations from the void surface.
Three-dimensional formulation of dislocation climb
NASA Astrophysics Data System (ADS)
Gu, Yejun; Xiang, Yang; Quek, Siu Sin; Srolovitz, David J.
2015-10-01
We derive a Green's function formulation for the climb of curved dislocations and multiple dislocations in three-dimensions. In this new dislocation climb formulation, the dislocation climb velocity is determined from the Peach-Koehler force on dislocations through vacancy diffusion in a non-local manner. The long-range contribution to the dislocation climb velocity is associated with vacancy diffusion rather than from the climb component of the well-known, long-range elastic effects captured in the Peach-Koehler force. Both long-range effects are important in determining the climb velocity of dislocations. Analytical and numerical examples show that the widely used local climb formula, based on straight infinite dislocations, is not generally applicable, except for a small set of special cases. We also present a numerical discretization method of this Green's function formulation appropriate for implementation in discrete dislocation dynamics (DDD) simulations. In DDD implementations, the long-range Peach-Koehler force is calculated as is commonly done, then a linear system is solved for the climb velocity using these forces. This is also done within the same order of computational cost as existing discrete dislocation dynamics methods.
NASA Astrophysics Data System (ADS)
Zhu, Yichao; Niu, Xiaohua; Xiang, Yang
2016-11-01
In continuum models of dislocations, proper formulations of short-range elastic interactions of dislocations are crucial for capturing various types of dislocation patterns formed in crystalline materials. In this article, the continuum dynamics of straight dislocations distributed on two parallel slip planes is modelled through upscaling the underlying discrete dislocation dynamics. Two continuum velocity field quantities are introduced to facilitate the discrete-to-continuum transition. The first one is the local migration velocity of dislocation ensembles which is found fully independent of the short-range dislocation correlations. The second one is the decoupling velocity of dislocation pairs controlled by a threshold stress value, which is proposed to be the effective flow stress for single slip systems. Compared to the almost ubiquitously adopted Taylor relationship, the derived flow stress formula exhibits two features that are more consistent with the underlying discrete dislocation dynamics: (i) the flow stress increases with the in-plane component of the dislocation density only up to a certain value, hence the derived formula admits a minimum inter-dislocation distance within slip planes; (ii) the flow stress smoothly transits to zero when all dislocations become geometrically necessary dislocations. A regime under which inhomogeneities in dislocation density grow is identified, and is further validated through comparison with discrete dislocation dynamical simulation results. Based on the findings in this article and in our previous works, a general strategy for incorporating short-range dislocation correlations into continuum models of dislocations is proposed.
Multiscale dislocation dynamics simulations of shock-induced plasticity in small volumes
NASA Astrophysics Data System (ADS)
Shehadeh, Mutasem A.
2012-04-01
Multiscale dislocation dynamics plasticity (MDDP) was used to investigate shock-induced deformation in monocrystalline copper. In order to enhance the numerical simulations, a periodic boundary condition was implemented in the continuum finite element (FE) scale so that the uniaxial compression of shocks could be attained. Additionally, lattice rotation was accounted for by modifying the dislocation dynamics (DD) code to update the dislocations' slip systems. The dislocation microstructures were examined in detail and a mechanism of microband formation is proposed for single- and multiple-slip deformation. The simulation results show that lattice rotation enhances microband formation in single slip by locally reorienting the slip plane. It is also illustrated that both confined and periodic boundary conditions can be used to achieve uniaxial compression; however, a periodic boundary condition yields a disturbed wave profile due to edge effects. Moreover, the boundary conditions and the loading rise time show no significant effects on shock-dislocations interaction and the resulting microstructures. MDDP results of high strain rate calculations are also compared with the predictions of the Armstrong-Zerilli model of dislocation generation and movement. This work confirms that the effect of resident dislocations on the strain rate can be neglected when a homogeneous nucleation mechanism is included.
NASA Astrophysics Data System (ADS)
Godiksen, R. B. N.; Schmidt, S.; Jensen, D. Juul
2008-09-01
Molecular dynamics simulations of grain boundary migration, where the driving pressure P is the excess stored energy due to dislocation structures, have been performed. This represents recrystallization in metals. Two types of dislocation structures have been simulated: (a) tilt dislocation boundaries, where edge dislocations are arranged as parallel arrays, (b) twist dislocation boundaries, where screw dislocations are arranged in interconnected dislocation networks. The velocity v and mobility M of the migrating grain boundaries have been calculated from the simulations. v and M are higher in twist-type simulations than in tilt-type simulations, although the activation energies are similar in the two cases. v ~ P is observed for tilt simulations where the driving pressure is changed by varying the density of dislocation boundaries and for twist simulations where the driving pressure is changed by varying the misorientation across dislocation boundaries. When the misorientations across edge dislocation boundaries are varied, however, the simulations show v ~ P2. It is suggested that this deviation from the usual v ~ P-relationship is due to local interactions between the grain boundary and nearby individual dislocations. Misorientation variations across grain boundaries have also been simulated, but the mobilities show little dependence on this. The present simulations result in mobilities and activation energies that are, respectively, significantly higher and somewhat lower than experimental values. A direct mimic of experimental observations is, however not the purpose of this study. Rather the present simulations are based on idealized dislocation structures and suggest that variations in the dislocation structures may play a dominant role in recrystallization dynamics and that local effects are very important phenomena, essential for the interpretation of recrystallization mechanisms.
Discrete Event Simulation of Distributed Team Communication
2012-03-22
executable system architecture approach to discrete events system modeling using sysml in conjunction with colored petri net . In Systems Conference, 2008 2nd...operators. Mitchell found that IMPRINT predictions of communication times and frequencies correlated with recorded communications amongst a platoon of
Assessing the Effectiveness of Biosurveillance Via Discrete Event Simulation
2011-03-01
EFFECTIVENESS OF BIOSURVEILLANCE VIA DISCRETE EVENT SIMULATION by Jason H. Dao March 2011 Thesis Advisor: Ronald D. Fricker, Jr. Second Reader...TYPE AND DATES COVERED Master’s Thesis 4. TITLE AND SUBTITLE Assessing the Effectiveness of Biosurveillance Via Discrete Event Simulation 6...the potential for disastrous outcomes is greater than it has ever been. In order to confront this threat, biosurveillance systems are utilized to
Optimization of Operations Resources via Discrete Event Simulation Modeling
NASA Technical Reports Server (NTRS)
Joshi, B.; Morris, D.; White, N.; Unal, R.
1996-01-01
The resource levels required for operation and support of reusable launch vehicles are typically defined through discrete event simulation modeling. Minimizing these resources constitutes an optimization problem involving discrete variables and simulation. Conventional approaches to solve such optimization problems involving integer valued decision variables are the pattern search and statistical methods. However, in a simulation environment that is characterized by search spaces of unknown topology and stochastic measures, these optimization approaches often prove inadequate. In this paper, we have explored the applicability of genetic algorithms to the simulation domain. Genetic algorithms provide a robust search strategy that does not require continuity and differentiability of the problem domain. The genetic algorithm successfully minimized the operation and support activities for a space vehicle, through a discrete event simulation model. The practical issues associated with simulation optimization, such as stochastic variables and constraints, were also taken into consideration.
NASA Astrophysics Data System (ADS)
Bringa, Eduardo; Ruestes, Carlos; Rodriguez Nieva, Joaquin; Tramontina, Diego; Tang, Yizhe; Meyers, Marc
2015-06-01
Mimicking shock compression experiments, our molecular dynamics simulations explore the mechanical response and plasticity effects under uniaxial high strain rate compression (10**7/s to 10**9/s) for Au and Ta single crystals with a collection of spherical nanovoids, with a radius of 3-4 nm, resulting in an initial porosity of %-10%. Dislocation analysis was used to evaluate and quantify the evolution of plasticity. The evolution of dislocations configuration and densities were predicted and successfully compared to an analysis based on Ashby's concept of geometrically-necessary dislocations. The temperature excursion during plastic deformation was used to estimate the mobile dislocation density. The results obtained are compared with a variety of dislocation-based constitutive models. Plastic activity leads to a decrease in porosity until voids disappear completely. Based on the atomistic simulations, a densification regime was observed in all nanoporous samples studied. With these results, a new strain- based porosity model for metals is proposed for simulations at the continuum scale. EB, CR and DT thank support from PICT-0092 and a SeCTyP-UNCuyo grant.
Discrete-Event Simulation in Chemical Engineering.
ERIC Educational Resources Information Center
Schultheisz, Daniel; Sommerfeld, Jude T.
1988-01-01
Gives examples, descriptions, and uses for various types of simulation systems, including the Flowtran, Process, Aspen Plus, Design II, GPSS, Simula, and Simscript. Explains similarities in simulators, terminology, and a batch chemical process. Tables and diagrams are included. (RT)
Understanding of edge and screw dislocations in nanostructures by modeling and simulations
NASA Astrophysics Data System (ADS)
Dontsova, Evgeniya
The role of the extended dislocation defects in nanostructures only recently began to be explored. In bulk materials, dislocations are modeled only away from their cores within the framework of the continuum mechanics. It is known that applying continuum modeling in the core region leads to divergences. In nanostructures, the core region dominates and new investigation methods are needed. This work contributes to the fundamental understanding of the role of dislocations in important carbon and zinc oxide nanostructures, by using atomistic investigation methods. In quasi-zero-dimensional structures, thesis describes the first attempt to rationalize dislocation processes in carbon nano-onions. Experiments show that carbon nano-onions exhibit an unusual dislocation dynamics with unexpected attraction of outer edge dislocation towards the core. Atomistic calculations combined with rigorous energy analysis attribute this behavior to an unusual inward driving force on the outer edge dislocation associated with a reduction in the number of dangling bonds. Moving on to quasi-one-dimensional nanostructures, we study the stability of screw-dislocated zinc oxide structures in the wurtzite phase with a symmetry-adapted molecular dynamics methodology, which introduces a significant simplification in the simulation domain size by accounting for the helical symmetry explicitly. The goal is to provide the theoretical support for a universal screw-dislocation-driven growth mechanism suggested by recent experiments. Moreover, the effects of axial screw dislocations on the electronic properties in helical zinc oxide nanowires and nanotubes are explored. We demonstrate significant screw-dislocation-induced band gap modifications that originate in the highly distorted cores. Finally, using the same objective technique, we investigate the stability against torsional deformations of quasi-one-dimensional graphene nanoribbons with bare, F-, and OH-saturated armchair edges. The prevalence
Harnessing atomistic simulations to predict the rate at which dislocations overcome obstacles
NASA Astrophysics Data System (ADS)
Saroukhani, S.; Nguyen, L. D.; Leung, K. W. K.; Singh, C. V.; Warner, D. H.
2016-05-01
Predicting the rate at which dislocations overcome obstacles is key to understanding the microscopic features that govern the plastic flow of modern alloys. In this spirit, the current manuscript examines the rate at which an edge dislocation overcomes an obstacle in aluminum. Predictions were made using different popular variants of Harmonic Transition State Theory (HTST) and compared to those of direct Molecular Dynamics (MD) simulations. The HTST predictions were found to be grossly inaccurate due to the large entropy barrier associated with the dislocation-obstacle interaction. Considering the importance of finite temperature effects, the utility of the Finite Temperature String (FTS) method was then explored. While this approach was found capable of identifying a prominent reaction tube, it was not capable of computing the free energy profile along the tube. Lastly, the utility of the Transition Interface Sampling (TIS) approach was explored, which does not need a free energy profile and is known to be less reliant on the choice of reaction coordinate. The TIS approach was found capable of accurately predicting the rate, relative to direct MD simulations. This finding was utilized to examine the temperature and load dependence of the dislocation-obstacle interaction in a simple periodic cell configuration. An attractive rate prediction approach combining TST and simple continuum models is identified, and the strain rate sensitivity of individual dislocation obstacle interactions is predicted.
Discrete element simulations of crumpling of thin sheets
NASA Astrophysics Data System (ADS)
Tallinen, T.; Åström, J. A.; Timonen, J.
2009-04-01
Forced crumpling of stiff self-avoiding sheets is studied by discrete element simulations. Simulations display stress condensation and scaling of ridge energy in agreement with theoretical expectations for elastic and frictionless sheets, and extends such behavior to elasto-plastic sheets. Crumpling of ideally elastic and frictionless sheets is compared to that of elasto-plastic sheets and sheets with friction.
An adaptive synchronization protocol for parallel discrete event simulation
Bisset, K.R.
1998-12-01
Simulation, especially discrete event simulation (DES), is used in a variety of disciplines where numerical methods are difficult or impossible to apply. One problem with this method is that a sufficiently detailed simulation may take hours or days to execute, and multiple runs may be needed in order to generate the desired results. Parallel discrete event simulation (PDES) has been explored for many years as a method to decrease the time taken to execute a simulation. Many protocols have been developed which work well for particular types of simulations, but perform poorly when used for other types of simulations. Often it is difficult to know a priori whether a particular protocol is appropriate for a given problem. In this work, an adaptive synchronization method (ASM) is developed which works well on an entire spectrum of problems. The ASM determines, using an artificial neural network (ANN), the likelihood that a particular event is safe to process.
Agent Frameworks for Discrete Event Social Simulations
2010-03-01
of a general modeling approach to social simulation that embeds a multi - agent system within a DES framework, and propose several reusable agent... agent system to simulate changes in the beliefs, values, and interests (BVIs) of large social groups (Alt, Jackson, Hudak, & Steven Lieberman, 2010...to events from A. 2.3 Cultural Geography Model The Cultural Geography (CG) Model is an implementation of a DESS that uses an embedded multi
NASA Astrophysics Data System (ADS)
Balusu, K.; Huang, H.
2017-04-01
A combined dislocation fan-finite element (DF-FE) method is presented for efficient and accurate simulation of dislocation nodal forces in 3D elastically anisotropic crystals with dislocations intersecting the free surfaces. The finite domain problem is decomposed into half-spaces with singular traction stresses, an infinite domain, and a finite domain with non-singular traction stresses. As such, the singular and non-singular parts of the traction stresses are addressed separately; the dislocation fan (DF) method is introduced to balance the singular traction stresses in the half-spaces while the finite element method (FEM) is employed to enforce the non-singular boundary conditions. The accuracy and efficiency of the DF method is demonstrated using a simple isotropic test case, by comparing it with the analytical solution as well as the FEM solution. The DF-FE method is subsequently used for calculating the dislocation nodal forces in a finite elastically anisotropic crystal, which produces dislocation nodal forces that converge rapidly with increasing mesh resolutions. In comparison, the FEM solution fails to converge, especially for nodes closer to the surfaces.
Discrete-Event Simulation Applied to Apparel Manufacturing
1990-06-01
manufacturing , e.g., machine tools, vehicles, appliances, etc. Very few applications of simulation and, particularly, of discrete- event simulation in the...industry has shown renewed interest in applications of computer-based tools to manufacturing systems. Simulation, which has been a widely used tool in...other industries, has received considerable attention for its possible applications in apparel manufacturing . !2 ! To date, however, little application
NASA Astrophysics Data System (ADS)
Tsuru, Tomohito; Shibutani, Yoji
Recent advances in miniaturization and highly-accurate measurement techniques have allowed mechanical properties to be measured at the nanometer scale. Nanoindentation has been widely used because of its applicability in ambient conditions. Unstable displacement burst or the abrupt growth of indent displacement after homogeneous elastic deformation observed in crystalline materials is a unique plastic deformation characteristic (nanoplasticity). In the present paper, a series of atomistic simulations of nanoindentation in single crystalline aluminum and copper are performed in analyzing the critical state for dislocation nucleation and interaction between dislocations beneath the indenter. With reference to the Hertzian solution based on isotropic linear elastic theory, both the anisotropic effect and nonlinear behavior of nanoindentation are discussed in detail. The discovery was made that the incipient yield process is strongly related to the triaxial stress state created beneath the indenter, and that energetically unfavorable interactions accompanied with cross slip induce the formation of prismatic dislocations.
Methodology for characterizing modeling and discretization uncertainties in computational simulation
ALVIN,KENNETH F.; OBERKAMPF,WILLIAM L.; RUTHERFORD,BRIAN M.; DIEGERT,KATHLEEN V.
2000-03-01
This research effort focuses on methodology for quantifying the effects of model uncertainty and discretization error on computational modeling and simulation. The work is directed towards developing methodologies which treat model form assumptions within an overall framework for uncertainty quantification, for the purpose of developing estimates of total prediction uncertainty. The present effort consists of work in three areas: framework development for sources of uncertainty and error in the modeling and simulation process which impact model structure; model uncertainty assessment and propagation through Bayesian inference methods; and discretization error estimation within the context of non-deterministic analysis.
NASA Astrophysics Data System (ADS)
Javanbakht, Mahdi; Levitas, Valery I.
2015-09-01
The complete system of phase field equations for coupled martensitic phase transformations (PTs), dislocation evolution, and mechanics at large strains is presented. Finite element method (FEM) is utilized to solve this system for two important problems. The first one is related to the simulation of shear strain-induced PT at the evolving dislocation pile-ups in a nanosized bicrystal. Plasticity plays a dual part in the interaction with PT. Dislocation pile-ups produce strong stress tensor concentrators that lead to barrierless martensite (M) nucleation. On the other hand, plasticity in the transforming grain relaxes these stress concentrators suppressing PT. The final stationary M morphology is governed by the local thermodynamic equilibrium, either at the interfaces or in terms of stresses averaged over the martensitic region or the entire grain. This is very surprising because of strong heterogeneity of stress fields and is in contrast to previous statements that phase equilibrium conditions do not enter the description of strain-induced PTs. The second problem is devoted to martensitic plate propagation through a bicrystal during temperature-induced PT. For elastic growth (without dislocations) and a large thermal driving force, a complex transformation path with plate branching and direct and reverse PTs is observed, which still ends with the same stationary nanostructure as for a smaller driving force and a traditional transformation path. Sharp grain boundary arrests plate growth at a relatively small driving force, exhibiting an athermal friction. For elastoplastic growth, the generation of dislocations produces athermal friction and arrests the plate below some critical driving force, leading to a morphological transition from plate to lath M. The width of the martensitic plate increases in comparison with elastic growth due to internal stress relaxation. Plate growth is accompanied by the nucleation of dislocations within M and remaining in M, the
Lill, J V; Broughton, J Q
2000-06-19
The method of Parrinello and Rahman is generalized to include slip in addition to deformation of the simulation cell. Equations of motion are derived, and a microscopic expression for traction is introduced. Lagrangian constraints are imposed so that the combination of deformation and slip conform to the invariant plane shear characteristic of martensites. Simulation of a model transformation demonstrates the nucleation and propagation of a glissile dislocation interface.
Correction of Discretization Errors Simulated at Supply Wells.
MacMillan, Gordon J; Schumacher, Jens
2015-01-01
Many hydrogeology problems require predictions of hydraulic heads in a supply well. In most cases, the regional hydraulic response to groundwater withdrawal is best approximated using a numerical model; however, simulated hydraulic heads at supply wells are subject to errors associated with model discretization and well loss. An approach for correcting the simulated head at a pumping node is described here. The approach corrects for errors associated with model discretization and can incorporate the user's knowledge of well loss. The approach is model independent, can be applied to finite difference or finite element models, and allows the numerical model to remain somewhat coarsely discretized and therefore numerically efficient. Because the correction is implemented external to the numerical model, one important benefit of this approach is that a response matrix, reduced model approach can be supported even when nonlinear well loss is considered.
NASA Astrophysics Data System (ADS)
Ouyanga, Chaojun; Lia, Zhenhuan; Huanga, Minsheng; Hua, Lili; Houa, Chuantao
2009-11-01
2D discrete dislocation dynamic modeling of compressed micro-pillars attached on a huge base is executed to study the size-dependent microplastic behavior of micro-pillars and the corresponding size effect. In addition to the conventional dimensional parameters of the micro-pillar such as the micro-pillar size and the height-to-width ratio, the micro-pillar taper angle and the dislocation slip plane orientation angle in the micro-pillar are also considered to address the size effect and its rich underlying mechanism. Computational results show that there are at least two operating mechanisms responsible for the plastic behavior of micro-pillars. One is associated with the dislocation free slip-out from the micro-pillar sidewall; the other is related to the dislocation pile-up at the base and the top end of the pillar. The overall mechanism governing the size effect of the micro-pillar rests with multi-factors, including the micro-pillar size, the height-to-width ratio, the micro-pillar taper and the slip plane orientation angle; however, whether the "free slip band" exists or not is the most important denotation. The well-known Schmid law still validates in the slender micro-pillars due to existence of the free slip band, whereas it may fail in the podgier micro-pillars due to absence of the free slip band; as a result, a complicated even "reverse" size effect appears.
NASA Astrophysics Data System (ADS)
Fan, Haidong; Wang, Qingyuan; Ouyang, Chaojun
2015-10-01
The defect-free channels were frequently observed in irradiated materials, i.e. copper, as a result of the stacking fault tetrahedron (SFT) interactions with dislocations. However, the underlying mechanisms for this process are still unclear to date. To address them, a comprehensive study on the interactions between SFTs and mixed dislocations was performed using molecular dynamics simulations. In particular, eight interaction geometries were considered, in terms of the dislocation Burgers vector directions, dislocation gliding directions and intersection positions on SFT. Various interaction outcomes were revealed after dislocation detachment. (1) SFT is fully absorbed through the transformation into Lomer dislocations, and subsequently moves out of free surfaces along the dislocation. (2) SFT is partially absorbed with the absorbed SFT base moving out of free surfaces along the dislocation. (3) SFT is not absorbed but sheared with ledges left on the SFT faces. (4) SFT is unaffected by the mixed dislocation. The current simulations, especially the full SFT absorption, provide important insights into the forming mechanisms of defect-free channels in irradiated materials.
Exploring Discretization Error in Simulation-Based Aerodynamic Databases
NASA Technical Reports Server (NTRS)
Aftosmis, Michael J.; Nemec, Marian
2010-01-01
This work examines the level of discretization error in simulation-based aerodynamic databases and introduces strategies for error control. Simulations are performed using a parallel, multi-level Euler solver on embedded-boundary Cartesian meshes. Discretization errors in user-selected outputs are estimated using the method of adjoint-weighted residuals and we use adaptive mesh refinement to reduce these errors to specified tolerances. Using this framework, we examine the behavior of discretization error throughout a token database computed for a NACA 0012 airfoil consisting of 120 cases. We compare the cost and accuracy of two approaches for aerodynamic database generation. In the first approach, mesh adaptation is used to compute all cases in the database to a prescribed level of accuracy. The second approach conducts all simulations using the same computational mesh without adaptation. We quantitatively assess the error landscape and computational costs in both databases. This investigation highlights sensitivities of the database under a variety of conditions. The presence of transonic shocks or the stiffness in the governing equations near the incompressible limit are shown to dramatically increase discretization error requiring additional mesh resolution to control. Results show that such pathologies lead to error levels that vary by over factor of 40 when using a fixed mesh throughout the database. Alternatively, controlling this sensitivity through mesh adaptation leads to mesh sizes which span two orders of magnitude. We propose strategies to minimize simulation cost in sensitive regions and discuss the role of error-estimation in database quality.
a Discrete Simulation of Tumor Growth Concerning Nutrient Influence
NASA Astrophysics Data System (ADS)
Sun, L.; Chang, Y. F.; Cai, X.
We develop a 2-D discrete model to simulate malignant cells growing in healthy tissues using a thermodynamic method on the basis of Potts model. After introducing a malignant seed in a healthy tissue, we use a set of adjustment factors, including the interaction between cells and nutrient, to simulate the growth of malignant cells under different environments. This allows us to investigate the effects of environment on malignant cell growth and the formation of cancer.
Synchronization of autonomous objects in discrete event simulation
NASA Technical Reports Server (NTRS)
Rogers, Ralph V.
1990-01-01
Autonomous objects in event-driven discrete event simulation offer the potential to combine the freedom of unrestricted movement and positional accuracy through Euclidean space of time-driven models with the computational efficiency of event-driven simulation. The principal challenge to autonomous object implementation is object synchronization. The concept of a spatial blackboard is offered as a potential methodology for synchronization. The issues facing implementation of a spatial blackboard are outlined and discussed.
Optimal open multistep discretization formulas for real-time simulation
NASA Technical Reports Server (NTRS)
Moerder, Daniel D.; Calise, Anthony J.; Clemmons, Paul
1993-01-01
The performance of digital real-time simulations is considered. A figure of merit is derived that quantifies a simulation's fidelity in terms of the time-domain discrepancy between its output and that of the plant it simulates, assuming that the plant is linearizable and asymptotically stable. This performance index is then used in deriving an easily automated procedure for calculating optimal values for free parameters in plant discretizations based on a generalized form of open linear multistep integration formulas. The theory is demonstrated in simulating the rigid-body dynamics of a fully articulated helicopter rotor blade system.
Parallel Stochastic discrete event simulation of calcium dynamics in neuron.
Ishlam Patoary, Mohammad Nazrul; Tropper, Carl; McDougal, Robert A; Zhongwei, Lin; Lytton, William W
2017-09-26
The intra-cellular calcium signaling pathways of a neuron depends on both biochemical reactions and diffusions. Some quasi-isolated compartments (e.g. spines) are so small and calcium concentrations are so low that one extra molecule diffusing in by chance can make a nontrivial difference in its concentration (percentage-wise). These rare events can affect dynamics discretely in such way that they cannot be evaluated by a deterministic simulation. Stochastic models of such a system provide a more detailed understanding of these systems than existing deterministic models because they capture their behavior at a molecular level. Our research focuses on the development of a high performance parallel discrete event simulation environment, Neuron Time Warp (NTW), which is intended for use in the parallel simulation of stochastic reaction-diffusion systems such as intra-calcium signaling. NTW is integrated with NEURON, a simulator which is widely used within the neuroscience community. We simulate two models, a calcium buffer and a calcium wave model. The calcium buffer model is employed in order to verify the correctness and performance of NTW by comparing it to a serial deterministic simulation in NEURON. We also derived a discrete event calcium wave model from a deterministic model using the stochastic IP3R structure.
Parallel discrete event simulation: A shared memory approach
NASA Technical Reports Server (NTRS)
Reed, Daniel A.; Malony, Allen D.; Mccredie, Bradley D.
1987-01-01
With traditional event list techniques, evaluating a detailed discrete event simulation model can often require hours or even days of computation time. Parallel simulation mimics the interacting servers and queues of a real system by assigning each simulated entity to a processor. By eliminating the event list and maintaining only sufficient synchronization to insure causality, parallel simulation can potentially provide speedups that are linear in the number of processors. A set of shared memory experiments is presented using the Chandy-Misra distributed simulation algorithm to simulate networks of queues. Parameters include queueing network topology and routing probabilities, number of processors, and assignment of network nodes to processors. These experiments show that Chandy-Misra distributed simulation is a questionable alternative to sequential simulation of most queueing network models.
NASA Astrophysics Data System (ADS)
Yuan, FuPing
2017-03-01
Strengthening in metals is traditionally achieved through the controlled creation of various grain boundaries (GBs), such as low-angle GBs, high-angle GBs, and twin boundaries (TBs). In the present study, a series of large-scale molecular dynamics simulations with spherical nanoindentation and carefully designed model were conducted to investigate and compare the strengthening effects of various GBs with nano-spacing as barriers of dislocation motion. Simulation results showed that high-angle twist GBs and TBs are similar barriers and low-angle twist GBs are less effective in obstructing dislocation motion. Corresponding atomistic mechanisms were also given. At a certain indentation depth, dislocation transmission and dislocation nucleation from the other side of boundaries were observed for low-angle twist GBs, whereas dislocations were completely blocked by high-angle twist GBs and TBs at the same indentation depth. The current findings should provide insights for comprehensive understanding of the strengthening effects of various GBs at nanoscale.
Disaster Response Modeling Through Discrete-Event Simulation
NASA Technical Reports Server (NTRS)
Wang, Jeffrey; Gilmer, Graham
2012-01-01
Organizations today are required to plan against a rapidly changing, high-cost environment. This is especially true for first responders to disasters and other incidents, where critical decisions must be made in a timely manner to save lives and resources. Discrete-event simulations enable organizations to make better decisions by visualizing complex processes and the impact of proposed changes before they are implemented. A discrete-event simulation using Simio software has been developed to effectively analyze and quantify the imagery capabilities of domestic aviation resources conducting relief missions. This approach has helped synthesize large amounts of data to better visualize process flows, manage resources, and pinpoint capability gaps and shortfalls in disaster response scenarios. Simulation outputs and results have supported decision makers in the understanding of high risk locations, key resource placement, and the effectiveness of proposed improvements.
Gurrutxaga-Lerma, Beñat; Balint, Daniel S; Dini, Daniele; Eakins, Daniel E; Sutton, Adrian P
2015-05-01
When a metal is subjected to extremely rapid compression, a shock wave is launched that generates dislocations as it propagates. The shock wave evolves into a characteristic two-wave structure, with an elastic wave preceding a plastic front. It has been known for more than six decades that the amplitude of the elastic wave decays the farther it travels into the metal: this is known as "the decay of the elastic precursor." The amplitude of the elastic precursor is a dynamic yield point because it marks the transition from elastic to plastic behavior. In this Letter we provide a full explanation of this attenuation using the first method of dislocation dynamics to treat the time dependence of the elastic fields of dislocations explicitly. We show that the decay of the elastic precursor is a result of the interference of the elastic shock wave with elastic waves emanating from dislocations nucleated in the shock front. Our simulations reproduce quantitatively recent experiments on the decay of the elastic precursor in aluminum and its dependence on strain rate.
Advanced time integration algorithms for dislocation dynamics simulations of work hardening
Sills, Ryan B.; Aghaei, Amin; Cai, Wei
2016-04-25
Efficient time integration is a necessity for dislocation dynamics simulations of work hardening to achieve experimentally relevant strains. In this work, an efficient time integration scheme using a high order explicit method with time step subcycling and a newly-developed collision detection algorithm are evaluated. First, time integrator performance is examined for an annihilating Frank–Read source, showing the effects of dislocation line collision. The integrator with subcycling is found to significantly out-perform other integration schemes. The performance of the time integration and collision detection algorithms is then tested in a work hardening simulation. The new algorithms show a 100-fold speed-up relativemore » to traditional schemes. As a result, subcycling is shown to improve efficiency significantly while maintaining an accurate solution, and the new collision algorithm allows an arbitrarily large time step size without missing collisions.« less
Advanced time integration algorithms for dislocation dynamics simulations of work hardening
Sills, Ryan B.; Aghaei, Amin; Cai, Wei
2016-04-25
Efficient time integration is a necessity for dislocation dynamics simulations of work hardening to achieve experimentally relevant strains. In this work, an efficient time integration scheme using a high order explicit method with time step subcycling and a newly-developed collision detection algorithm are evaluated. First, time integrator performance is examined for an annihilating Frank–Read source, showing the effects of dislocation line collision. The integrator with subcycling is found to significantly out-perform other integration schemes. The performance of the time integration and collision detection algorithms is then tested in a work hardening simulation. The new algorithms show a 100-fold speed-up relative to traditional schemes. As a result, subcycling is shown to improve efficiency significantly while maintaining an accurate solution, and the new collision algorithm allows an arbitrarily large time step size without missing collisions.
Advanced time integration algorithms for dislocation dynamics simulations of work hardening
NASA Astrophysics Data System (ADS)
Sills, Ryan B.; Aghaei, Amin; Cai, Wei
2016-05-01
Efficient time integration is a necessity for dislocation dynamics simulations of work hardening to achieve experimentally relevant strains. In this work, an efficient time integration scheme using a high order explicit method with time step subcycling and a newly-developed collision detection algorithm are evaluated. First, time integrator performance is examined for an annihilating Frank-Read source, showing the effects of dislocation line collision. The integrator with subcycling is found to significantly out-perform other integration schemes. The performance of the time integration and collision detection algorithms is then tested in a work hardening simulation. The new algorithms show a 100-fold speed-up relative to traditional schemes. Subcycling is shown to improve efficiency significantly while maintaining an accurate solution, and the new collision algorithm allows an arbitrarily large time step size without missing collisions.
Advanced time integration algorithms for dislocation dynamics simulations of work hardening
Sills, Ryan B.; Aghaei, Amin; Cai, Wei
2016-04-25
Efficient time integration is a necessity for dislocation dynamics simulations of work hardening to achieve experimentally relevant strains. In this work, an efficient time integration scheme using a high order explicit method with time step subcycling and a newly-developed collision detection algorithm are evaluated. First, time integrator performance is examined for an annihilating Frank–Read source, showing the effects of dislocation line collision. The integrator with subcycling is found to significantly out-perform other integration schemes. The performance of the time integration and collision detection algorithms is then tested in a work hardening simulation. The new algorithms show a 100-fold speed-up relative to traditional schemes. As a result, subcycling is shown to improve efficiency significantly while maintaining an accurate solution, and the new collision algorithm allows an arbitrarily large time step size without missing collisions.
Discrete event simulation in an artificial intelligence environment: Some examples
Roberts, D.J.; Farish, T.
1991-01-01
Several Los Alamos National Laboratory (LANL) object-oriented discrete-event simulation efforts have been completed during the past three years. One of these systems has been put into production and has a growing customer base. Another (started two years earlier than the first project) was completed but has not yet been used. This paper will describe these simulation projects. Factors which were pertinent to the success of the one project, and to the failure of the second project will be discussed (success will be measured as the extent to which the simulation model was used as originally intended). 5 figs.
Mapping Strain-rate Dependent Dislocation-Defect Interactions by Atomistic Simulations
Fan, Yue; Osetskiy, Yury N; Yip, Sidney; Yildiz-Botterud, Bilge
2013-01-01
Probing the mechanisms of defect-defect interactions at strain rates lower than 106 s-1 is an unresolved challenge to date to molecular dynamics (MD) techniques. Here we propose a novel atomistic approach based on transition state theory and the concept of a strain-dependent effective activation barrier that is capable of simulating the kinetics of dislocation-defect interactions at virtually any strain rate, exemplified within 10-7 to 107 s-1. We apply this approach to the problem of an edge dislocation colliding with a cluster of self-interstitial atoms (SIA) under shear deformation. Using an activation-relaxation algorithm (1), we uncover a unique strain-rate dependent trigger mechanism that allows the SIA cluster to be absorbed during the process leading to dislocation climb. Guided by this finding, we determine the activation barrier of the trigger mechanism as a function of shear strain, and use that in a coarse-graining rate equation formulation for constructing a mechanism map in the phase space of strain-rate and temperature. Our predictions of a crossover from a defect recovery at the low strain rate regime to defect absorption behavior in the high strain-rate regime are validated against our own independent, direct MD simulations at 105 to 107 s-1. Implications of the present approach for probing molecular-level mechanisms in strain-rate regimes previously considered inaccessible to atomistic simulations are discussed.
Spurious haloes and discreteness-driven relaxation in cosmological simulations
NASA Astrophysics Data System (ADS)
Power, C.; Robotham, A. S. G.; Obreschkow, D.; Hobbs, A.; Lewis, G. F.
2016-10-01
There is strong evidence that cosmological N-body simulations dominated by warm dark matter (WDM) contain spurious or unphysical haloes, most readily apparent as regularly spaced low-mass haloes strung along filaments. We show that spurious haloes are a feature of traditional N-body simulations of cosmological structure formation models, including WDM and cold dark matter models, in which gravitational collapse proceeds in an initially anisotropic fashion, and arises naturally as a consequence of discreteness-driven relaxation. We demonstrate this using controlled N-body simulations of plane-symmetric collapse and show that spurious haloes are seeded at shell crossing by localized velocity perturbations induced by the discrete nature of the density field, and that their characteristic separation should be approximately the mean inter-particle separation of the N-body simulation, which is fixed by the mass resolution within the volume. Using cosmological N-body simulations in which particles are split into two collisionless components of fixed mass ratio, we find that the spatial distribution of the two components show signatures of discreteness-driven relaxation on both large and small scales. Adopting a spline kernel gravitational softening that is of order the comoving mean inter-particle separation helps to suppress the effect of discreteness-driven relaxation, but cannot eliminate it completely. These results provide further motivation for recent developments of new algorithms, which include, for example, revisions of the traditional N-body approach by means of spatially adaptive anistropric gravitational softenings or explicit solution of the evolution of dark matter in phase space.
Chen, Qian
2008-01-01
The generation, motion, and interaction of dislocations play key roles during the plastic deformation process of crystalline solids. 3D Dislocation Dynamics has been employed as a mesoscale simulation algorithm to investigate the collective and cooperative behavior of dislocations. Most current research on 3D Dislocation Dynamics is based on the solutions available in the framework of classical isotropic elasticity. However, due to some degree of elastic anisotropy in almost all crystalline solids, it is very necessary to extend 3D Dislocation Dynamics into anisotropic elasticity. In this study, first, the details of efficient and accurate incorporation of the fully anisotropic elasticity into 3D discrete Dislocation Dynamics by numerically evaluating the derivatives of Green's functions are described. Then the intrinsic properties of perfect dislocations, including their stability, their core properties and disassociation characteristics, in newly discovered rare earth-based intermetallics and in conventional intermetallics are investigated, within the framework of fully anisotropic elasticity supplemented with the atomistic information obtained from the ab initio calculations. Moreover, the evolution and interaction of dislocations in these intermetallics as well as the role of solute segregation are presented by utilizing fully anisotropic 3D dislocation dynamics. The results from this work clearly indicate the role and the importance of elastic anisotropy on the evolution of dislocation microstructures, the overall ductility and the hardening behavior in these systems.
NASA Astrophysics Data System (ADS)
Chen, Qian
The generation, motion, and interaction of dislocations play key roles during the plastic deformation process of crystalline solids. 3D Dislocation Dynamics has been employed as a mesoscale simulation algorithm to investigate the collective and cooperative behavior of dislocations. Most current research on 3D Dislocation Dynamics is based on the solutions available in the framework of classical isotropic elasticity. However, due to some degree of elastic anisotropy in almost all crystalline solids, it is very necessary to extend 3D Dislocation Dynamics into anisotropic elasticity. In this study, first, the details of efficient and accurate incorporation of the fully anisotropic elasticity into 3D discrete Dislocation Dynamics by numerically evaluating the derivatives of Green's functions are described. Then the intrinsic properties of perfect dislocations, including their stability, their core properties and disassociation characteristics, in newly discovered rare earth-based intermetallics and in conventional intermetallics are investigated, within the framework of fully anisotropic elasticity supplemented with the atomistic information obtained from the ab initio calculations. Moreover, the evolution and interaction of dislocations in these intermetallics as well as the role of solute segregation are presented by utilizing fully anisotropic 3D dislocation dynamics. The results from this work clearly indicate the role and the importance of elastic anisotropy on the evolution of dislocation microstructures, the overall ductility and the hardening behavior in these systems.
Farkas, D.; Schon, C.G.; Lima, M.S.F. de; Goldenstein, H.
1996-01-01
The atomistic structure of dislocation cores of <111> screw dislocations in disordered Fe-Cr b.c.c. alloys was simulated using embedded atom method potentials and molecular statics computer simulation. The mixed Fe-Cr interatomic potentials used were derived by fitting to the thermodynamic data of the disordered system and the measured lattice parameter changes of Fe upon Cr additions. The potentials predict phase separation as the most stable configuration for the central region of the phase diagram. The next most stable situation is the disordered b.c.c. phase. The structure of the screw 1/2 <111> dislocation core was studied using atomistic computer simulation and an improved visualization method for the representation of the resulting structures. The structure of the dislocation core is different from that typical of 1/2 <111> dislocations in pure b.c.c. materials. The core structure in the alloy tends to lose the threefold symmetry seen in pure b.c.c. materials and the stress necessary to initiate dislocation motion increases with Cr content. The mobility of kinks in these screw dislocations was also simulated and it was found that while the critical stress for kink motion in pure Fe is extremely low, it increases significantly with the addition of Cr. The implications of these differences for mechanical behavior are discussed.
Parallel discrete-event simulation of FCFS stochastic queueing networks
NASA Technical Reports Server (NTRS)
Nicol, David M.
1988-01-01
Physical systems are inherently parallel. Intuition suggests that simulations of these systems may be amenable to parallel execution. The parallel execution of a discrete-event simulation requires careful synchronization of processes in order to ensure the execution's correctness; this synchronization can degrade performance. Largely negative results were recently reported in a study which used a well-known synchronization method on queueing network simulations. Discussed here is a synchronization method (appointments), which has proven itself to be effective on simulations of FCFS queueing networks. The key concept behind appointments is the provision of lookahead. Lookahead is a prediction on a processor's future behavior, based on an analysis of the processor's simulation state. It is shown how lookahead can be computed for FCFS queueing network simulations, give performance data that demonstrates the method's effectiveness under moderate to heavy loads, and discuss performance tradeoffs between the quality of lookahead, and the cost of computing lookahead.
Discrete dipole approximation simulation of bead enhanced diffraction grating biosensor
NASA Astrophysics Data System (ADS)
Arif, Khalid Mahmood
2016-08-01
We present the discrete dipole approximation simulation of light scattering from bead enhanced diffraction biosensor and report the effect of bead material, number of beads forming the grating and spatial randomness on the diffraction intensities of 1st and 0th orders. The dipole models of gratings are formed by volume slicing and image processing while the spatial locations of the beads on the substrate surface are randomly computed using discrete probability distribution. The effect of beads reduction on far-field scattering of 632.8 nm incident field, from fully occupied gratings to very coarse gratings, is studied for various bead materials. Our findings give insight into many difficult or experimentally impossible aspects of this genre of biosensors and establish that bead enhanced grating may be used for rapid and precise detection of small amounts of biomolecules. The results of simulations also show excellent qualitative similarities with experimental observations.
Dislocation mechanisms in stressed crystals with surface effects
NASA Astrophysics Data System (ADS)
Wu, Chi-Chin; Crone, Joshua; Munday, Lynn; Discrete Dislocation Dynamics Team
2014-03-01
Understanding dislocation properties in stressed crystals is the key for important processes in materials science, including the strengthening of metals and the stress relaxation during the growth of hetero-epitaxial structures. Despite existing experimental approaches and theories, many dislocation mechanisms with surface effects still remain elusive in experiments. Even though discrete dislocation dynamics (DDD) simulations are commonly employed to study dislocations, few demonstrate sufficient computational capabilities for massive dislocations with the combined effects of surfaces and stresses. Utilizing the Army's newly developed FED3 code, a DDD computation code coupled with finite elements, this work presents several dislocation mechanisms near different types of surfaces in finite domains. Our simulation models include dislocations in a bended metallic cantilever beam, near voids in stressed metals, as well as threading and misfit dislocations in as-grown semiconductor epitaxial layers and their quantitative inter-correlations to stress relaxation and surface instability. Our studies provide not only detailed physics of individual dislocation mechanisms, but also important collective dislocation properties such as dislocation densities and strain-stress profiles and their interactions with surfaces.
Three-phase flow simulations in discrete fracture networks
NASA Astrophysics Data System (ADS)
Geiger, S.; Niessner, J.; Matthai, S. K.; Helmig, R.
2006-12-01
Fractures are often the key conduits for fluid flow in otherwise low permeability rocks. Their presence in hydrocarbon reservoirs leads to complex production histories, unpredictable coupling of wells, rapidly changing flow rates, possibly early water breakthrough, and low final recovery. Recently, it has been demonstrated that a combination of finite volume and finite element discretization is well suited to model incompressible, immiscible two-phase flow in 3D discrete fracture networks (DFN) representing complexly fractured rocks. Such an approach has been commercialized in Golder Associates' FracMan Reservoir Edition software. For realistic reservoir simulations, however, it would be desirable if a third compressible gas phase can be included which is often present at reservoir conditions. Here we present the extension of an existing node-centred finite volume - finite element (FEFV) discretization for the efficient and accurate simulations of three-component - three-phase flow in geologically realistic representations of fractured porous media. Two possible types of fracture networks can be used: In 2D, they are detailed geometrical representations of fractured rock masses mapped in field studies. In 3D, they are geologically constrained, stochastically generated discrete fracture networks. Flow and transport can be simulated for fractures only or for fractures and matrix combined. The governing equations are solved decoupled using an implicit-pressure, explicit-saturation (IMPES) approach. Flux and concentration terms can be treated with higher-order accuracy in the finite volume scheme to preserve shock fronts. The method is locally mass conservative and works on unstructured, spatially refined grids. Flash calculations are carried out by a new description of the Black-Oil model. Capillary and gravity effects are included in this formulation. The robustness and accuracy of this formulation is shown in several applications. First, grid convergence is
Three dimensional discrete particle simulation of an AOTV
NASA Technical Reports Server (NTRS)
Feiereisen, William J.; Mcdonald, Jeffrey D.
1989-01-01
A discrete Particle Simulation method, recently formulated by Baganoff, is discussed in the context of its application to the simulation of the flow field about the Aeroassisted Flight Experiment (AFE). As a basis for discussion the current algorithm is first described. Because of the use of a cubic Cartesian mesh, the representation of the geometry is different than that of other particle methods and an algorithm for its generation is discussed. The method is applied to test problems and then to the AFE calculation with the use of 9.52 million particles and 432,000 cells.
Three dimensional Discrete Particle Simulation about the AFE geometry
NASA Technical Reports Server (NTRS)
Feiereisen, William J.; Mcdonald, Jeffrey D.; Fallavollita, Michael A.
1990-01-01
The Discrete Particle Simulation method, due to Baganoff, has recently been extended to allow representation of gases composed of multiple species, to general power-law molecular interactions and to permit flows in thermal non-equilibrium. Particular attention has been paid to the implementation of this physics while retaining the efficiency of the original algorithm. Here, the enhanced algorithm is applied to the simulation of the flow field about the Aeroassisted Flight Experiment (AFE) vehicle with the same flight parameters as in a previous paper. The enhancements to the algorithm are introduced and comparisons are made to the previous calculation.
Quenched pinning and collective dislocation dynamics
Ovaska, Markus; Laurson, Lasse; Alava, Mikko J.
2015-01-01
Several experiments show that crystalline solids deform in a bursty and intermittent fashion. Power-law distributed strain bursts in compression experiments of micron-sized samples, and acoustic emission energies from larger-scale specimens, are the key signatures of the underlying critical-like collective dislocation dynamics - a phenomenon that has also been seen in discrete dislocation dynamics (DDD) simulations. Here we show, by performing large-scale two-dimensional DDD simulations, that the character of the dislocation avalanche dynamics changes upon addition of sufficiently strong randomly distributed quenched pinning centres, present e.g. in many alloys as immobile solute atoms. For intermediate pinning strength, our results adhere to the scaling picture of depinning transitions, in contrast to pure systems where dislocation jamming dominates the avalanche dynamics. Still stronger disorder quenches the critical behaviour entirely. PMID:26024505
Quality Improvement With Discrete Event Simulation: A Primer for Radiologists.
Booker, Michael T; O'Connell, Ryan J; Desai, Bhushan; Duddalwar, Vinay A
2016-04-01
The application of simulation software in health care has transformed quality and process improvement. Specifically, software based on discrete-event simulation (DES) has shown the ability to improve radiology workflows and systems. Nevertheless, despite the successful application of DES in the medical literature, the power and value of simulation remains underutilized. For this reason, the basics of DES modeling are introduced, with specific attention to medical imaging. In an effort to provide readers with the tools necessary to begin their own DES analyses, the practical steps of choosing a software package and building a basic radiology model are discussed. In addition, three radiology system examples are presented, with accompanying DES models that assist in analysis and decision making. Through these simulations, we provide readers with an understanding of the theory, requirements, and benefits of implementing DES in their own radiology practices.
The effects of parallel processing architectures on discrete event simulation
NASA Astrophysics Data System (ADS)
Cave, William; Slatt, Edward; Wassmer, Robert E.
2005-05-01
As systems become more complex, particularly those containing embedded decision algorithms, mathematical modeling presents a rigid framework that often impedes representation to a sufficient level of detail. Using discrete event simulation, one can build models that more closely represent physical reality, with actual algorithms incorporated in the simulations. Higher levels of detail increase simulation run time. Hardware designers have succeeded in producing parallel and distributed processor computers with theoretical speeds well into the teraflop range. However, the practical use of these machines on all but some very special problems is extremely limited. The inability to use this power is due to great difficulties encountered when trying to translate real world problems into software that makes effective use of highly parallel machines. This paper addresses the application of parallel processing to simulations of real world systems of varying inherent parallelism. It provides a brief background in modeling and simulation validity and describes a parameter that can be used in discrete event simulation to vary opportunities for parallel processing at the expense of absolute time synchronization and is constrained by validity. It focuses on the effects of model architecture, run-time software architecture, and parallel processor architecture on speed, while providing an environment where modelers can achieve sufficient model accuracy to produce valid simulation results. It describes an approach to simulation development that captures subject area expert knowledge to leverage inherent parallelism in systems in the following ways: * Data structures are separated from instructions to track which instruction sets share what data. This is used to determine independence and thus the potential for concurrent processing at run-time. * Model connectivity (independence) can be inspected visually to determine if the inherent parallelism of a physical system is properly
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.
2013-01-01
results. 2. Simulation technique The atomistic simulations described here employed the three-dimensional (3-D) parallel molecular dynamics (MD) code...dislocation takes to spontaneously cross-slip at the mildly-repulsive 120o intersection, molecular dynamics constant NVT simulations at a low temperature of...of the screw dislocation at the intersection. Similar molecular dynamics simulations were Figure 2. (colour online) A dislocation line representation
Bachlechner; Omeltchenko; Nakano; Kalia; Vashishta; Ebbsjo; Madhukar
2000-01-10
Mechanical behavior of the Si(111)/Si(3)N4(0001) interface is studied using million atom molecular dynamics simulations. At a critical value of applied strain parallel to the interface, a crack forms on the silicon nitride surface and moves toward the interface. The crack does not propagate into the silicon substrate; instead, dislocations are emitted when the crack reaches the interface. The dislocation loop propagates in the (1; 1;1) plane of the silicon substrate with a speed of 500 (+/-100) m/s. Time evolution of the dislocation emission and nature of defects is studied.
NASA Astrophysics Data System (ADS)
Bachlechner, Martina E.; Omeltchenko, Andrey; Nakano, Aiichiro; Kalia, Rajiv K.; Vashishta, Priya; Ebbsjö, Ingvar; Madhukar, Anupam
2000-01-01
Mechanical behavior of the Si\\(111\\)/Si3N4\\(0001\\) interface is studied using million atom molecular dynamics simulations. At a critical value of applied strain parallel to the interface, a crack forms on the silicon nitride surface and moves toward the interface. The crack does not propagate into the silicon substrate; instead, dislocations are emitted when the crack reaches the interface. The dislocation loop propagates in the \\(1¯ 1¯1\\) plane of the silicon substrate with a speed of 500 \\(+/-100\\) m/s. Time evolution of the dislocation emission and nature of defects is studied.
Discrete Particle Method for Simulating Hypervelocity Impact Phenomena
Watson, Erkai; Steinhauser, Martin O.
2017-01-01
In this paper, we introduce a computational model for the simulation of hypervelocity impact (HVI) phenomena which is based on the Discrete Element Method (DEM). Our paper constitutes the first application of DEM to the modeling and simulating of impact events for velocities beyond 5 kms−1. We present here the results of a systematic numerical study on HVI of solids. For modeling the solids, we use discrete spherical particles that interact with each other via potentials. In our numerical investigations we are particularly interested in the dynamics of material fragmentation upon impact. We model a typical HVI experiment configuration where a sphere strikes a thin plate and investigate the properties of the resulting debris cloud. We provide a quantitative computational analysis of the resulting debris cloud caused by impact and a comprehensive parameter study by varying key parameters of our model. We compare our findings from the simulations with recent HVI experiments performed at our institute. Our findings are that the DEM method leads to very stable, energy–conserving simulations of HVI scenarios that map the experimental setup where a sphere strikes a thin plate at hypervelocity speed. Our chosen interaction model works particularly well in the velocity range where the local stresses caused by impact shock waves markedly exceed the ultimate material strength. PMID:28772739
Discrete Particle Method for Simulating Hypervelocity Impact Phenomena.
Watson, Erkai; Steinhauser, Martin O
2017-04-02
In this paper, we introduce a computational model for the simulation of hypervelocity impact (HVI) phenomena which is based on the Discrete Element Method (DEM). Our paper constitutes the first application of DEM to the modeling and simulating of impact events for velocities beyond 5 kms(-1). We present here the results of a systematic numerical study on HVI of solids. For modeling the solids, we use discrete spherical particles that interact with each other via potentials. In our numerical investigations we are particularly interested in the dynamics of material fragmentation upon impact. We model a typical HVI experiment configuration where a sphere strikes a thin plate and investigate the properties of the resulting debris cloud. We provide a quantitative computational analysis of the resulting debris cloud caused by impact and a comprehensive parameter study by varying key parameters of our model. We compare our findings from the simulations with recent HVI experiments performed at our institute. Our findings are that the DEM method leads to very stable, energy-conserving simulations of HVI scenarios that map the experimental setup where a sphere strikes a thin plate at hypervelocity speed. Our chosen interaction model works particularly well in the velocity range where the local stresses caused by impact shock waves markedly exceed the ultimate material strength.
Dislocation glide in the presence of either solute atoms or glissile loops.
Bacon, David J; Osetskiy, Yury N; Rong, Zhouwen; Tapasa, Kanit
2004-01-01
Atomic-scale computer simulation is used to investigate obstacle effects on the dynamics of glide of an edge dislocation in two situations. In one, a dislocation in {alpha}-iron encounters copper atoms in solution and it is found that the effect on dislocation velocity under constant stress below the static Peierls stress is strong. In the other, drag of glissile interstitial loops with the same Burgers vector as the dislocation is considered for iron and copper. The drag coefficient of a loop is determined for the first time, and is shown to be related to the diffusivity of clusters of interstitials via a model of dislocation drag of discrete pinning points.
Monet, Giath; Bacon, David J; Osetskiy, Yury N
2010-01-01
Given the time and length scales in molecular dynamics (MD) simulations of dislocation-defect interactions, quantitative MD results cannot be used directly in larger scale simulations or compared directly with experiment. A method to extract fundamental quantities from MD simulations is proposed here. The first quantity is a critical stress defined to characterise the obstacle resistance. This mesoscopic parameter, rather than the obstacle 'strength' designed for a point obstacle, is to be used for an obstacle of finite size. At finite temperature, our analyses of MD simulations allow the activation energy to be determined as a function of temperature. The results confirm the proportionality between activation energy and temperature that is frequently observed by experiment. By coupling the data for the activation energy and the critical stress as functions of temperature, we show how the activation energy can be deduced at a given value of the critical stress.
Desktop Modeling and Simulation: Parsimonious, yet Effective Discrete-Event Simulation Analysis
NASA Technical Reports Server (NTRS)
Bradley, James R.
2012-01-01
This paper evaluates how quickly students can be trained to construct useful discrete-event simulation models using Excel The typical supply chain used by many large national retailers is described, and an Excel-based simulation model is constructed of it The set of programming and simulation skills required for development of that model are then determined we conclude that six hours of training are required to teach the skills to MBA students . The simulation presented here contains all fundamental functionallty of a simulation model, and so our result holds for any discrete-event simulation model. We argue therefore that Industry workers with the same technical skill set as students having completed one year in an MBA program can be quickly trained to construct simulation models. This result gives credence to the efficacy of Desktop Modeling and Simulation whereby simulation analyses can be quickly developed, run, and analyzed with widely available software, namely Excel.
Advances in Discrete-Event Simulation for MSL Command Validation
NASA Technical Reports Server (NTRS)
Patrikalakis, Alexander; O'Reilly, Taifun
2013-01-01
In the last five years, the discrete event simulator, SEQuence GENerator (SEQGEN), developed at the Jet Propulsion Laboratory to plan deep-space missions, has greatly increased uplink operations capacity to deal with increasingly complicated missions. In this paper, we describe how the Mars Science Laboratory (MSL) project makes full use of an interpreted environment to simulate change in more than fifty thousand flight software parameters and conditional command sequences to predict the result of executing a conditional branch in a command sequence, and enable the ability to warn users whenever one or more simulated spacecraft states change in an unexpected manner. Using these new SEQGEN features, operators plan more activities in one sol than ever before.
Advances in Discrete-Event Simulation for MSL Command Validation
NASA Technical Reports Server (NTRS)
Patrikalakis, Alexander; O'Reilly, Taifun
2013-01-01
In the last five years, the discrete event simulator, SEQuence GENerator (SEQGEN), developed at the Jet Propulsion Laboratory to plan deep-space missions, has greatly increased uplink operations capacity to deal with increasingly complicated missions. In this paper, we describe how the Mars Science Laboratory (MSL) project makes full use of an interpreted environment to simulate change in more than fifty thousand flight software parameters and conditional command sequences to predict the result of executing a conditional branch in a command sequence, and enable the ability to warn users whenever one or more simulated spacecraft states change in an unexpected manner. Using these new SEQGEN features, operators plan more activities in one sol than ever before.
Multi-threaded, discrete event simulation of distributed computing systems
NASA Astrophysics Data System (ADS)
Legrand, Iosif; MONARC Collaboration
2001-10-01
The LHC experiments have envisaged computing systems of unprecedented complexity, for which is necessary to provide a realistic description and modeling of data access patterns, and of many jobs running concurrently on large scale distributed systems and exchanging very large amounts of data. A process oriented approach for discrete event simulation is well suited to describe various activities running concurrently, as well the stochastic arrival patterns specific for such type of simulation. Threaded objects or "Active Objects" can provide a natural way to map the specific behaviour of distributed data processing into the simulation program. The simulation tool developed within MONARC is based on Java (TM) technology which provides adequate tools for developing a flexible and distributed process oriented simulation. Proper graphics tools, and ways to analyze data interactively, are essential in any simulation project. The design elements, status and features of the MONARC simulation tool are presented. The program allows realistic modeling of complex data access patterns by multiple concurrent users in large scale computing systems in a wide range of possible architectures, from centralized to highly distributed. Comparison between queuing theory and realistic client-server measurements is also presented.
Ring discretization of the wave spectrum for sea surface simulation.
Varela, Jose Miguel; Guedes Soares, Carlos
2014-01-01
Although interactive computer-generated ocean scenes based on real wave spectra are impressively realistic, they usually don't exhibit the original sea state's statistical properties. This might be unacceptable for applications in which the sea surface height field's correctness is important, such as 3D ship simulators for training professionals. Researchers have developed a discretization of the wave spectrum that obtains a sea state statistically more equivalent to the original. This method can also improve the scene's visual realism and real-time performance.
Combined simulated annealing algorithm for the discrete facility location problem.
Qin, Jin; Ni, Ling-Lin; Shi, Feng
2012-01-01
The combined simulated annealing (CSA) algorithm was developed for the discrete facility location problem (DFLP) in the paper. The method is a two-layer algorithm, in which the external subalgorithm optimizes the decision of the facility location decision while the internal subalgorithm optimizes the decision of the allocation of customer's demand under the determined location decision. The performance of the CSA is tested by 30 instances with different sizes. The computational results show that CSA works much better than the previous algorithm on DFLP and offers a new reasonable alternative solution method to it.
Mapping strain rate dependence of dislocation-defect interactions by atomistic simulations
Fan, Yue; Osetskiy, Yuri N.; Yip, Sidney; Yildiz, Bilge
2013-01-01
Probing the mechanisms of defect–defect interactions at strain rates lower than 106 s−1 is an unresolved challenge to date to molecular dynamics (MD) techniques. Here we propose an original atomistic approach based on transition state theory and the concept of a strain-dependent effective activation barrier that is capable of simulating the kinetics of dislocation–defect interactions at virtually any strain rate, exemplified within 10−7 to 107 s−1. We apply this approach to the problem of an edge dislocation colliding with a cluster of self-interstitial atoms (SIAs) under shear deformation. Using an activation–relaxation algorithm [Kushima A, et al. (2009) J Chem Phys 130:224504], we uncover a unique strain-rate–dependent trigger mechanism that allows the SIA cluster to be absorbed during the process, leading to dislocation climb. Guided by this finding, we determine the activation barrier of the trigger mechanism as a function of shear strain, and use that in a coarse-graining rate equation formulation for constructing a mechanism map in the phase space of strain rate and temperature. Our predictions of a crossover from a defect recovery at the low strain-rate regime to defect absorption behavior in the high strain-rate regime are validated against our own independent, direct MD simulations at 105 to 107 s−1. Implications of the present approach for probing molecular-level mechanisms in strain-rate regimes previously considered inaccessible to atomistic simulations are discussed. PMID:24114271
NASA Astrophysics Data System (ADS)
Rajgarhia, Rahul K.; Spearot, Douglas E.; Saxena, Ashok
2010-04-01
Recent experimental and simulation results have indicated that high-temperature grain growth in nanocrystalline (NC) materials can be suppressed by introducing dopant atoms at the grain boundaries. However, the influence of grain boundary dopants on the mechanical behavior of stabilized NC materials is less clear. In this work, molecular dynamics (MD) simulations are used to study the impact of very low dopant concentrations (<1.0 at. pct Sb) on plastic deformation in single-crystal and NC Cu. A new interatomic potential for low Sb concentration Cu-Sb solid-solution alloys is used to model dopant/host and dopant/dopant interatomic interactions within the MD framework. In single-crystal models, the strained regions around the Sb atoms act as heterogeneous sources for partial dislocation nucleation; the stress associated with this process decreases with increasing Sb concentration. In NC models, MD simulations indicate that Sb dopants randomly dispersed at the grain boundaries cause an increase in the flow stress in NC Cu, implying that Sb atoms at the grain boundaries retard both grain boundary sliding and dislocation nucleation from grain boundary regions.
Simulation of tantalum nanocrystals under shock-wave loading: Dislocations and twinning
NASA Astrophysics Data System (ADS)
Tramontina, D. R.; Hahn, E. N.; Meyers, M. A.; Bringa, E. M.
2017-01-01
We simulate strong shock waves in nanocrystalline tantalum using atomistic molecular dynamics simulations, for particle velocities in the range 0.35-2.0 km s-1, which induce pressures in the range 20-195 GPa. Our simulations explore strain rates in the range 108 s-1 - 1010 s-1, and lead to a peak strength in the range 3-15 GPa. Nanocrystalline tantalum exposed to strong shock waves demonstrates deformation enabled by concomitant dislocations, twinning, and grain boundary activity at a variety of particle velocities. Twinning is observed for a mean grain size of 7 nm, starting at around 32 GPa, in disagreement with models which predict a Hall-Petch behavior for twinning, i.e. a twinning stress scaling with grain size d as d-0.5, and supporting the presence of an inverse Hall-Petch effect for twinning at small grain sizes.
Novel discretization schemes for the numerical simulation of membrane dynamics
NASA Astrophysics Data System (ADS)
Kolsti, Kyle F.
Motivated by the demands of simulating flapping wings of Micro Air Vehicles, novel numerical methods were developed and evaluated for the dynamic simulation of membranes. For linear membranes, a mixed-form time-continuous Galerkin method was employed using trilinear space-time elements. Rather than time-marching, the entire space-time domain was discretized and solved simultaneously. Second-order rates of convergence in both space and time were observed in numerical studies. Slight high-frequency noise was filtered during post-processing. For geometrically nonlinear membranes, the model incorporated two new schemes that were independently developed and evaluated. Time marching was performed using quintic Hermite polynomials uniquely determined by end-point jerk constraints. The single-step, implicit scheme was significantly more accurate than the most common Newmark schemes. For a simple harmonic oscillator, the scheme was found to be symplectic, frequency-preserving, and conditionally stable. Time step size was limited by accuracy requirements rather than stability. The spatial discretization scheme employed a staggered grid, grouping of nonlinear terms, and polygon shape functions in a strong-form point collocation formulation. The observed rate of convergence was two for both displacement and strain. Validation against existing experimental data showed the method to be accurate until hyperelastic effects dominate.
a Discrete Mathematical Model to Simulate Malware Spreading
NASA Astrophysics Data System (ADS)
Del Rey, A. Martin; Sánchez, G. Rodriguez
2012-10-01
With the advent and worldwide development of Internet, the study and control of malware spreading has become very important. In this sense, some mathematical models to simulate malware propagation have been proposed in the scientific literature, and usually they are based on differential equations exploiting the similarities with mathematical epidemiology. The great majority of these models study the behavior of a particular type of malware called computer worms; indeed, to the best of our knowledge, no model has been proposed to simulate the spreading of a computer virus (the traditional type of malware which differs from computer worms in several aspects). In this sense, the purpose of this work is to introduce a new mathematical model not based on continuous mathematics tools but on discrete ones, to analyze and study the epidemic behavior of computer virus. Specifically, cellular automata are used in order to design such model.
Performance Analysis of Cloud Computing Architectures Using Discrete Event Simulation
NASA Technical Reports Server (NTRS)
Stocker, John C.; Golomb, Andrew M.
2011-01-01
Cloud computing offers the economic benefit of on-demand resource allocation to meet changing enterprise computing needs. However, the flexibility of cloud computing is disadvantaged when compared to traditional hosting in providing predictable application and service performance. Cloud computing relies on resource scheduling in a virtualized network-centric server environment, which makes static performance analysis infeasible. We developed a discrete event simulation model to evaluate the overall effectiveness of organizations in executing their workflow in traditional and cloud computing architectures. The two part model framework characterizes both the demand using a probability distribution for each type of service request as well as enterprise computing resource constraints. Our simulations provide quantitative analysis to design and provision computing architectures that maximize overall mission effectiveness. We share our analysis of key resource constraints in cloud computing architectures and findings on the appropriateness of cloud computing in various applications.
Towards effective flow simulations in realistic discrete fracture networks
NASA Astrophysics Data System (ADS)
Berrone, Stefano; Pieraccini, Sandra; Scialò, Stefano
2016-04-01
We focus on the simulation of underground flow in fractured media, modeled by means of Discrete Fracture Networks. Focusing on a new recent numerical approach proposed by the authors for tackling the problem avoiding mesh generation problems, we further improve the new family of methods making a step further towards effective simulations of large, multi-scale, heterogeneous networks. Namely, we tackle the imposition of Dirichlet boundary conditions in weak form, in such a way that geometrical complexity of the DFN is not an issue; we effectively solve DFN problems with fracture transmissivities spanning many orders of magnitude and approaching zero; furthermore, we address several numerical issues for improving the numerical solution also in quite challenging networks.
Interfacial dislocation motion and interactions in single-crystal superalloys
Liu, B.; Raabe, D.; Roters, F.; Arsenlis, A.
2014-10-01
The early stage of high-temperature low-stress creep in single-crystal superalloys is characterized by the rapid development of interfacial dislocation networks. Although interfacial motion and dynamic recovery of these dislocation networks have long been expected to control the subsequent creep behavior, direct observation and hence in-depth understanding of such processes has not been achieved. Incorporating recent developments of discrete dislocation dynamics models, we simulate interfacial dislocation motion in the channel structures of single-crystal superalloys, and investigate how interfacial dislocation motion and dynamic recovery are affected by interfacial dislocation interactions and lattice misfit. Different types of dislocation interactions are considered: self, collinear, coplanar, Lomer junction, glissile junction, and Hirth junction. The simulation results show that strong dynamic recovery occurs due to the short-range reactions of collinear annihilation and Lomer junction formation. The misfit stress is found to induce and accelerate dynamic recovery of interfacial dislocation networks involving self-interaction and Hirth junction formation, but slow down the steady interfacial motion of coplanar and glissile junction forming dislocation networks. The insights gained from these simulations on high-temperature low-stress creep of single-crystal superalloys are also discussed.
Synchronous Parallel System for Emulation and Discrete Event Simulation
NASA Technical Reports Server (NTRS)
Steinman, Jeffrey S. (Inventor)
2001-01-01
A synchronous parallel system for emulation and discrete event simulation having parallel nodes responds to received messages at each node by generating event objects having individual time stamps, stores only the changes to the state variables of the simulation object attributable to the event object and produces corresponding messages. The system refrains from transmitting the messages and changing the state variables while it determines whether the changes are superseded, and then stores the unchanged state variables in the event object for later restoral to the simulation object if called for. This determination preferably includes sensing the time stamp of each new event object and determining which new event object has the earliest time stamp as the local event horizon, determining the earliest local event horizon of the nodes as the global event horizon, and ignoring events whose time stamps are less than the global event horizon. Host processing between the system and external terminals enables such a terminal to query, monitor, command or participate with a simulation object during the simulation process.
Synchronous parallel system for emulation and discrete event simulation
NASA Technical Reports Server (NTRS)
Steinman, Jeffrey S. (Inventor)
1992-01-01
A synchronous parallel system for emulation and discrete event simulation having parallel nodes responds to received messages at each node by generating event objects having individual time stamps, stores only the changes to state variables of the simulation object attributable to the event object, and produces corresponding messages. The system refrains from transmitting the messages and changing the state variables while it determines whether the changes are superseded, and then stores the unchanged state variables in the event object for later restoral to the simulation object if called for. This determination preferably includes sensing the time stamp of each new event object and determining which new event object has the earliest time stamp as the local event horizon, determining the earliest local event horizon of the nodes as the global event horizon, and ignoring the events whose time stamps are less than the global event horizon. Host processing between the system and external terminals enables such a terminal to query, monitor, command or participate with a simulation object during the simulation process.
Simulating Sea Ice Floes Using the Discrete Element Method
NASA Astrophysics Data System (ADS)
Bateman, S. P.; Shi, F.; Orzech, M.; Veeramony, J.; Calantoni, J.
2016-02-01
We developed a model for simulating ice floe dynamics using LIGGGHTS, an open source discrete element method (DEM) software package. Each ice floe is composed of a collection of smaller (0.1 - 1.0 m) individual elements bonded together. The bond matrix is constructed by randomly packing nearly identical spherical elements into the desired size and shape of the ice floe. Consequently, the number of bonds in the matrix is determined by the coordination number of the random packing of nearly identical spherical elements. Bonds break when the stress exceeds a critical bond stress. The Voronoi volume of each element is used to conserve mass of sea ice in the model. An ensemble of simulations was performed to calibrate the critical bond stress using existing laboratory measurements for the compressive, tensile, and flexural strength of sea ice. Simulations of ice floes quantitatively exhibit seemingly realistic fracture behavior. Through coupling the DEM with the non-hydrostatic wave model, NHWAVE, simulations of the interaction between sea ice and ocean waves in the marginal ice zone were performed. Simulations containing O(104 - 109) elements were performed to quantify and demonstrate model scalability.
NASA Astrophysics Data System (ADS)
Carvalho Resende, T.; Balan, T.; Abed-Meraim, F.; Bouvier, S.; Sablin, S.-S.
2010-06-01
With a view to environmental, economic and safety concerns, car manufacturers need to design lighter and safer vehicles in ever shorter development times. In recent years, High Strength Steels (HSS) like Interstitial Free (IF) steels which have higher ratios of yield strength to elastic modulus, are increasingly used for sheet metal parts in automotive industry to meet the demands. Moreover, the application of sheet metal forming simulations has proven to be beneficial to reduce tool costs in the design stage and to optimize current processes. The Finite Element Method (FEM) is quite successful to simulate metal forming processes but accuracy largely depends on the quality of the material properties provided as input to the material model. Common phenomenological models roughly consist in the fitting of functions on experimental results and do not provide any predictive character for different metals from the same grade. Therefore, the use of accurate plasticity models based on physics would increase predictive capability, reduce parameter identification cost and allow for robust and time-effective finite element simulations. For this purpose, a 3D physically based model at large strain with dislocation density evolution approach was presented in IDDRG2009 by the authors [1]. This model allows the description of work-hardening's behavior for different loading paths (i.e. uni-axial tensile, simple shear and Bauschinger tests) taking into account several data from microstructure (i.e. grain size, texture, etc…). The originality of this model consists in the introduction of microstructure data in a classical phenomenological model in order to achieve work-hardening's predictive character for different metals from the same grade. Indeed, thanks to a microstructure parameter set for an Interstitial Free steel, it is possible to describe work-hardening behavior for different loading paths of other IF steels by only changing the mean grain size and the chemical
NASA Astrophysics Data System (ADS)
Jansen Van Rensburg, G. J.; Kok, S.; Wilke, D. N.
2017-07-01
Different roll pass reduction schedules have different effects on the through-thickness properties of hot-rolled metal slabs. In order to assess or improve a reduction schedule using the finite element method, a material model is required that captures the relevant deformation mechanisms and physics. The model should also report relevant field quantities to assess variations in material state through the thickness of a simulated rolled metal slab. In this paper, a dislocation density-based material model with recrystallization is presented and calibrated on the material response of a high-strength low-alloy steel. The model has the ability to replicate and predict material response to a fair degree thanks to the physically motivated mechanisms it is built on. An example study is also presented to illustrate the possible effect different reduction schedules could have on the through-thickness material state and the ability to assess these effects based on finite element simulations.
NASA Astrophysics Data System (ADS)
Jansen Van Rensburg, G. J.; Kok, S.; Wilke, D. N.
2017-10-01
Different roll pass reduction schedules have different effects on the through-thickness properties of hot-rolled metal slabs. In order to assess or improve a reduction schedule using the finite element method, a material model is required that captures the relevant deformation mechanisms and physics. The model should also report relevant field quantities to assess variations in material state through the thickness of a simulated rolled metal slab. In this paper, a dislocation density-based material model with recrystallization is presented and calibrated on the material response of a high-strength low-alloy steel. The model has the ability to replicate and predict material response to a fair degree thanks to the physically motivated mechanisms it is built on. An example study is also presented to illustrate the possible effect different reduction schedules could have on the through-thickness material state and the ability to assess these effects based on finite element simulations.
Molecular dynamics simulations of dislocations in TlBr crystals under an electrical field
Zhou, X. W.; Foster, M. E.; Yang, P.; Doty, F. P.
2016-07-13
TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. As a result, this discovery can lead to new understanding of TlBr aging mechanisms under external fields.
Molecular dynamics simulations of dislocations in TlBr crystals under an electrical field
Zhou, X. W.; Foster, M. E.; Yang, P.; ...
2016-07-13
TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. As a result, this discovery can lead to new understanding of TlBr agingmore » mechanisms under external fields.« less
Molecular dynamics simulations of dislocations in TlBr crystals under an electrical field
Zhou, X. W.; Foster, M. E.; Yang, P.; Doty, F. P.
2016-07-13
TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. As a result, this discovery can lead to new understanding of TlBr aging mechanisms under external fields.
Simulation of dry granular flows using discrete element methods
NASA Astrophysics Data System (ADS)
Martin, Hugo; Lefebvre, Aline; Maday, Yvon; Mangeney, Anne; Maury, Bertrand; Sainte-Marie, Jacques
2017-04-01
Granular flows are composed of interacting particles (for instance sand grains). While natural flow simulations at the field scale are generally based on continuum models, discrete element methods are very useful to get insight into the detailed contact interactions between the particles involved. We shall consider here both well known molecular dynamics (MD) and contact dynamics (CD) methods to simulate granular particle interaction. The difference between these methods is the linearisation of contact forces in MD. We are interested to compare these methods, and especially the effects of the linearisation in simulations. In the present work, we introduce a new rigid bodies model at the scale of the particles and its resolution by contact dynamics. The interesting aspect of our CD method is to treat the contacts in all the material system in one step without any iterative process required when the contacts are dealt with one after the other. All contacts are calculated here at the same time in just one iteration and the normal and tangential constraints are treated simultaneously. The present model follows from a convex optimization problem presented in [1] by B. Maury in which we add a frictional behaviour to the contact law between the particles. To analyse the behaviour of this model, we compare our results to analytical solutions when we can compute them and otherwise to simulations with molecular dynamics method. [1] A time-stepping scheme for inelastic collisions. Numerical handling of the nonoverlapping constraint, B. Maury, Numerische Mathematik, 17 january 2006.
The cost of conservative synchronization in parallel discrete event simulations
NASA Technical Reports Server (NTRS)
Nicol, David M.
1990-01-01
The performance of a synchronous conservative parallel discrete-event simulation protocol is analyzed. The class of simulation models considered is oriented around a physical domain and possesses a limited ability to predict future behavior. A stochastic model is used to show that as the volume of simulation activity in the model increases relative to a fixed architecture, the complexity of the average per-event overhead due to synchronization, event list manipulation, lookahead calculations, and processor idle time approach the complexity of the average per-event overhead of a serial simulation. The method is therefore within a constant factor of optimal. The analysis demonstrates that on large problems--those for which parallel processing is ideally suited--there is often enough parallel workload so that processors are not usually idle. The viability of the method is also demonstrated empirically, showing how good performance is achieved on large problems using a thirty-two node Intel iPSC/2 distributed memory multiprocessor.
NASA Astrophysics Data System (ADS)
Zhu, Bida; Huang, Minsheng; Li, Zhenhuan
2017-04-01
High concentrations of vacancies tend to be formed inside the metal materials under irradiation, and then accumulate and cluster together gradually to promote the formation of nanovoids. Generally, these voids act as obstacles for dislocation glide and thereby change/degrade the mechanical behavior of irradiated materials. In this work, the interaction between ellipsoidal nanovoids with edge dislocations in alpha-iron has been studied by atomic simulations. The results illuminate that the ellipsoidal void's semi-major axis on the slip plane and parallel to the dislocation line is the dominant factor controlling the obstacle strength of ellipsoidal nanovoids. Two other semi-major axes, which are perpendicular to the glide plane and parallel to the Burgers vector, respectively, can also influence the critical resolved shear stress (CRSS) for dislocation shearing the ellipsoidal void. The intrinsic atomic mechanisms controlling above phenomena, such as nanovoid-geometry spatial constraint and nanovoid-surface curvature on dislocation evolution, have been discussed carefully. The classical continuum model has been amended to describe the dislocation-ellipsoidal nanovoid interaction base on current results. In addition, the influence of temperature on the CRSS of ellipsoidal nanovoids has also been investigated.
Study of Flapping Flight Using Discrete Vortex Method Based Simulations
NASA Astrophysics Data System (ADS)
Devranjan, S.; Jalikop, Shreyas V.; Sreenivas, K. R.
2013-12-01
In recent times, research in the area of flapping flight has attracted renewed interest with an endeavor to use this mechanism in Micro Air vehicles (MAVs). For a sustained and high-endurance flight, having larger payload carrying capacity we need to identify a simple and efficient flapping-kinematics. In this paper, we have used flow visualizations and Discrete Vortex Method (DVM) based simulations for the study of flapping flight. Our results highlight that simple flapping kinematics with down-stroke period (tD) shorter than the upstroke period (tU) would produce a sustained lift. We have identified optimal asymmetry ratio (Ar = tD/tU), for which flapping-wings will produce maximum lift and find that introducing optimal wing flexibility will further enhances the lift.
Properties of discrete breathers in graphane from ab initio simulations
NASA Astrophysics Data System (ADS)
Chechin, G. M.; Dmitriev, S. V.; Lobzenko, I. P.; Ryabov, D. S.
2014-07-01
A density functional theory (DFT) study of the discrete breathers (DBs) in graphane (fully hydrogenated graphene) was performed. To the best of our knowledge, this is the first demonstration of the existence of DBs in a crystalline body from the first-principle simulations. It is found that the DB is a robust, highly localized vibrational mode with one hydrogen atom oscillating with a large amplitude along the direction normal to the graphane plane with all neighboring atoms having much smaller vibration amplitudes. DB frequency decreases with increase in its amplitude, and it can take any value within the phonon gap and can even enter the low-frequency phonon band. The concept of DB is then used to propose an explanation to the recent experimental results on the nontrivial kinetics of graphane dehydrogenation at elevated temperatures.
Discrete-time pilot model. [human dynamics and digital simulation
NASA Technical Reports Server (NTRS)
Cavalli, D.
1978-01-01
Pilot behavior is considered as a discrete-time process where the decision making has a sequential nature. This model differs from both the quasilinear model which follows from classical control theory and from the optimal control model which considers the human operator as a Kalman estimator-predictor. An additional factor considered is that the pilot's objective may not be adequately formulated as a quadratic cost functional to be minimized, but rather as a more fuzzy measure of the closeness with which the aircraft follows a reference trajectory. All model parameters, in the digital program simulating the pilot's behavior, were successfully compared in terms of standard-deviation and performance with those of professional pilots in IFR configuration. The first practical application of the model was in the study of its performance degradation when the aircraft model static margin decreases.
Including dislocation flux in a continuum crystal plasticity model to produce size scale effects
Becker, R; Arsenlis, A; Bulatov, V V; Parks, D M
2004-02-13
A novel model has been developed to capture size scale and gradient effects within the context of continuum crystal plasticity by explicitly incorporating details of dislocation transport, coupling dislocation transport to slip, evolving spatial distributions of dislocations consistent with the flux, and capturing the interactions among various dislocation populations. Dislocation flux and density are treated as nodal degrees of freedom in the finite element model, and they are determined as part of the global system of equations. The creation, annihilation and flux of dislocations between elements are related by transport equations. Crystallographic slip is coupled to the dislocation flux and the stress state. The resultant gradients in dislocation density and local lattice rotations are analyzed for geometrically necessary and statistically stored dislocation contents that contribute to strength and hardening. Grain boundaries are treated as surfaces where dislocation flux is restricted depending on the relative orientations of the neighboring grains. Numerical results show different behavior near free surfaces and non-deforming surfaces resulting from differing levels of dislocation transmission. Simulations also show development of dislocation pile-ups at grain boundaries and an increase in flow strength reminiscent of the Hall-Petch model. The dislocation patterns have a characteristic size independent of the numerical discretization.
Microcanonical ensemble simulation method applied to discrete potential fluids
NASA Astrophysics Data System (ADS)
Sastre, Francisco; Benavides, Ana Laura; Torres-Arenas, José; Gil-Villegas, Alejandro
2015-09-01
In this work we extend the applicability of the microcanonical ensemble simulation method, originally proposed to study the Ising model [A. Hüller and M. Pleimling, Int. J. Mod. Phys. C 13, 947 (2002), 10.1142/S0129183102003693], to the case of simple fluids. An algorithm is developed by measuring the transition rates probabilities between macroscopic states, that has as advantage with respect to conventional Monte Carlo NVT (MC-NVT) simulations that a continuous range of temperatures are covered in a single run. For a given density, this new algorithm provides the inverse temperature, that can be parametrized as a function of the internal energy, and the isochoric heat capacity is then evaluated through a numerical derivative. As an illustrative example we consider a fluid composed of particles interacting via a square-well (SW) pair potential of variable range. Equilibrium internal energies and isochoric heat capacities are obtained with very high accuracy compared with data obtained from MC-NVT simulations. These results are important in the context of the application of the Hüller-Pleimling method to discrete-potential systems, that are based on a generalization of the SW and square-shoulder fluids properties.
Stochastic discrete event simulation of germinal center reactions
NASA Astrophysics Data System (ADS)
Figge, Marc Thilo
2005-05-01
We introduce a generic reaction-diffusion model for germinal center reactions and perform numerical simulations within a stochastic discrete event approach. In contrast to the frequently used deterministic continuum approach, each single reaction event is monitored in space and time in order to simulate the correct time evolution of this complex biological system. Germinal centers play an important role in the immune system by performing a reaction that aims at improving the affinity between antibodies and antigens. Our model captures experimentally observed features of this reaction, such as the development of the remarkable germinal center morphology and the maturation of antibody-antigen affinity in the course of time. We model affinity maturation within a simple affinity class picture and study it as a function of the distance between the initial antibody-antigen affinity and the highest possible affinity. The model reveals that this mutation distance may be responsible for the experimentally observed all-or-none behavior of germinal centers; i.e., they generate either mainly output cells of high affinity or no high-affinity output cells at all. Furthermore, the exact simulation of the system dynamics allows us to study the hypothesis of cell recycling in germinal centers as a mechanism for affinity optimization. A comparison of three possible recycling pathways indicates that affinity maturation is optimized by a recycling pathway that has previously not been taken into account in deterministic continuum models.
Microcanonical ensemble simulation method applied to discrete potential fluids.
Sastre, Francisco; Benavides, Ana Laura; Torres-Arenas, José; Gil-Villegas, Alejandro
2015-09-01
In this work we extend the applicability of the microcanonical ensemble simulation method, originally proposed to study the Ising model [A. Hüller and M. Pleimling, Int. J. Mod. Phys. C 13, 947 (2002)0129-183110.1142/S0129183102003693], to the case of simple fluids. An algorithm is developed by measuring the transition rates probabilities between macroscopic states, that has as advantage with respect to conventional Monte Carlo NVT (MC-NVT) simulations that a continuous range of temperatures are covered in a single run. For a given density, this new algorithm provides the inverse temperature, that can be parametrized as a function of the internal energy, and the isochoric heat capacity is then evaluated through a numerical derivative. As an illustrative example we consider a fluid composed of particles interacting via a square-well (SW) pair potential of variable range. Equilibrium internal energies and isochoric heat capacities are obtained with very high accuracy compared with data obtained from MC-NVT simulations. These results are important in the context of the application of the Hüller-Pleimling method to discrete-potential systems, that are based on a generalization of the SW and square-shoulder fluids properties.
NASA Astrophysics Data System (ADS)
Staikov, P.; Djourelov, N.
2013-03-01
Dislocations in BCC metals are of crucial importance for understanding behavior of fusion materials. In this study model positron lifetime quantum-mechanical calculations have been carried out in the two-component density functional theory (DFT) in local density approximation (LDA) for perfect iron and tungsten lattices, lattices with <1 0 0> edge and 1/2<1 1 1> screw dislocations and several cases in which dislocations interact with a vacancy, bi-vacancy and vacancies containing hydrogen or helium atoms. The core structures of the dislocations have been obtained by MD-simulations using Mendelev and Ackland potentials for iron and Finnis-Sinclair potential for tungsten. The calculated values for iron are 153 ps for edge dislocation and 124 ps for screw dislocation, while for tungsten are 161 and 130 ps, respectively. We report new results for screw dislocation showing that minor dilation of the lattice volume associated with second-order elasticity theory influences the calculated positron lifetime.
Alhafez, Iyad Alabd; Ruestes, Carlos J; Gao, Yu; Urbassek, Herbert M
2016-01-29
Using molecular dynamics simulation, we study the nanoindentation of three hcp metals: Mg, Ti, and Zr. Both the basal and two prismatic surface planes are considered. We focus on the characterization of the plasticity generated in the crystal. The similarities to, and the differences from, the behavior of the more commonly investigated fcc and bcc metals are highlighted. We find that hcp metals show a larger variety than the fcc and bcc metals studied up until now. The prolific emission of prismatic loops can lead to extended plastic zones. The size of the plastic zone is quantified by the ratio f of the plastic zone radius to the radius of the contact area. We find values of between 1.6 (an almost collapsed zone) and >5; in the latter case, complex dislocation networks build up which are extended in the direction of easy glide.
NASA Astrophysics Data System (ADS)
Alabd Alhafez, Iyad; Ruestes, Carlos J.; Gao, Yu; Urbassek, Herbert M.
2016-01-01
Using molecular dynamics simulation, we study the nanoindentation of three hcp metals: Mg, Ti, and Zr. Both the basal and two prismatic surface planes are considered. We focus on the characterization of the plasticity generated in the crystal. The similarities to, and the differences from, the behavior of the more commonly investigated fcc and bcc metals are highlighted. We find that hcp metals show a larger variety than the fcc and bcc metals studied up until now. The prolific emission of prismatic loops can lead to extended plastic zones. The size of the plastic zone is quantified by the ratio f of the plastic zone radius to the radius of the contact area. We find values of between 1.6 (an almost collapsed zone) and >5 in the latter case, complex dislocation networks build up which are extended in the direction of easy glide.
Cazamias, J; Lassila, D; Shehadeh, M; Zbib, H
2004-02-19
Dynamically loaded gas gun experiments were performed to validate the predictive capabilities of 3-D dislocation dynamics (DD) code simulations at very high strain rates and dislocation velocities where the phonon drag mechanism will be dominant. Experiments were performed in the weak-shock regime on high-purity Mo single crystals with [001] compression axes. We have also performed shock-recovery experiments and are in the process of analyzing the dislocation structure generated by the weak-shock using transmission electron microscopy (TEM), which will also be used to validate the dislocation structure predicted by the DD simulations. The DD simulations being performed at Washington State University by Prof. H. Zbib and co-workers will be compared to the experimentally measured wave profiles, thereby validating mechanisms of dislocation generation and motion. Some DD simulation results are presented to demonstrate the feasibility of using a combined experimental/simulation effort for the validation of dislocation generation and mobility physics issues in the phonon drag regime.
NASA Astrophysics Data System (ADS)
Gröger, R.; Dudeck, K. J.; Nellist, P. D.; Vitek, V.; Hirsch, P. B.; Cockayne, D. J. H.
2011-06-01
This paper addresses the question as to whether the core structure of screw dislocations in Mo in the bulk can be obtained from high-resolution electron microscopy (HREM) images of such dislocations viewed end-on in a thin foil. Atomistic simulations of the core structure of screw dislocations in elastically anisotropic Mo were carried out using bond order potentials. These simulations take account automatically of the effects of the surface relaxation displacements (anisotropic Eshelby twist). They show that the differential displacements of the atoms at the surface are different with components perpendicular to the Burgers vector about five times larger than those in the middle of the foil, the latter being characteristic of the bulk. Nye tensor plots show that the surface relaxation stresses strongly affect the incompatible distortions. HREM simulations of the computed structure reflect the displacements at the exit surface, modified by interband scattering and the microscope transfer function. Nye tensor plots obtained from the HREM images show that interband scattering also affects the incompatible distortions. It is concluded that it would be very difficult to obtain information on the core structure of screw dislocations in the bulk Mo from HREM images, even under ideal experimental conditions, and that quantitative comparisons between experimental and simulated images from assumed model structures would be essential.
The strength and dislocation microstructure evolution in superalloy microcrystals
NASA Astrophysics Data System (ADS)
Hussein, Ahmed M.; Rao, Satish I.; Uchic, Michael D.; Parthasarathay, Triplicane A.; El-Awady, Jaafar A.
2017-02-01
In this work, the evolution of the dislocations microstructure in single crystal two-phase superalloy microcrystals under monotonic loading has been studied using the three-dimensional discrete dislocation dynamics (DDD) method. The DDD framework has been extended to properly handle the collective behavior of dislocations and their interactions with large collections of arbitrary shaped precipitates. Few constraints are imposed on the initial distribution of the dislocations or the precipitates, and the extended DDD framework can support experimentally-obtained precipitate geometries. Full tracking of the creation and destruction of anti-phase boundaries (APB) is accounted for. The effects of the precipitate volume fraction, APB energy, precipitate size, and crystal size on the deformation of superalloy microcrystals have been quantified. Correlations between the precipitate microstructure and the dominant deformation features, such as dislocation looping versus precipitate shearing, are also discussed. It is shown that the mechanical strength is independent of the crystal size, increases linearly with increasing the volume fraction, follows a near square-root relationship with the APB energy and an inverse square-root relationship with the precipitate size. Finally, the flow strength in simulations having initial dislocation pair sources show a flow strength that is about one half of that predicted from simulations starting with single dislocation sources. The method developed can be used, with minimal extensions, to simulate dislocation microstructure evolution in general multiphase materials.
Dislocation mean free paths and strain hardening of crystals.
Devincre, B; Hoc, T; Kubin, L
2008-06-27
Predicting the strain hardening properties of crystals constitutes a long-standing challenge for dislocation theory. The main difficulty resides in the integration of dislocation processes through a wide range of time and length scales, up to macroscopic dimensions. In the present multiscale approach, dislocation dynamics simulations are used to establish a dislocation-based continuum model incorporating discrete and intermittent aspects of plastic flow. This is performed through the modeling of a key quantity, the mean free path of dislocations. The model is then integrated at the scale of bulk crystals, which allows for the detailed reproduction of the complex deformation curves of face-centered cubic crystals. Because of its predictive ability, the proposed framework has a large potential for further applications.
Predicting Liver Transplant Capacity Using Discrete Event Simulation
Diaz, Hector Toro; Mayorga, Maria; Barritt, A. Sidney; Orman, Eric S.; Wheeler, Stephanie B.
2014-01-01
The number of liver transplants (LTs) performed in the US increased until 2006, but has since declined despite an ongoing increase in demand. This decline may be due in part to decreased donor liver quality and increasing discard of poor quality livers. We constructed a Discrete Event Simulation (DES) model informed by current donor characteristics to predict future LT trends through the year 2030. The data source for our model is the United Network for Organ Sharing database, which contains patient level information on all organ transplants performed in the US. Previous analysis showed that liver discard is increasing and that discarded organs are more often from donors who are older, obese, have diabetes, and donated after cardiac death. Given that the prevalence of these factors is increasing, the DES model quantifies the reduction in the number of LTs performed through 2030. In addition, the model estimates the total number of future donors needed to maintain the current volume of LTs, and the effect of a hypothetical scenario of improved reperfusion technology. We also forecast the number of patients on the waiting list and compare this to the estimated number of LTs to illustrate the impact that decreased LTs will have on patients needing transplants. By altering assumptions about the future donor pool, this model can be used to develop policy interventions to prevent a further decline in this life saving therapy. To our knowledge, there are no similar predictive models of future LT use based on epidemiologic trends. PMID:25391681
Simulating discrete models of pattern formation by ion beam sputtering.
Hartmann, Alexander K; Kree, Reiner; Yasseri, Taha
2009-06-03
A class of simple, (2+1)-dimensional, discrete models is reviewed, which allow us to study the evolution of surface patterns on solid substrates during ion beam sputtering (IBS). The models are based on the same assumptions about the erosion process as the existing continuum theories. Several distinct physical mechanisms of surface diffusion are added, which allow us to study the interplay of erosion-driven and diffusion-driven pattern formation. We present results from our own work on evolution scenarios of ripple patterns, especially for longer timescales, where nonlinear effects become important. Furthermore we review kinetic phase diagrams, both with and without sample rotation, which depict the systematic dependence of surface patterns on the shape of energy depositing collision cascades after ion impact. Finally, we discuss some results from more recent work on surface diffusion with Ehrlich-Schwoebel barriers as the driving force for pattern formation during IBS and on Monte Carlo simulations of IBS with codeposition of surfactant atoms.
Enhancing Complex System Performance Using Discrete-Event Simulation
Allgood, Glenn O; Olama, Mohammed M; Lake, Joe E
2010-01-01
In this paper, we utilize discrete-event simulation (DES) merged with human factors analysis to provide the venue within which the separation and deconfliction of the system/human operating principles can occur. A concrete example is presented to illustrate the performance enhancement gains for an aviation cargo flow and security inspection system achieved through the development and use of a process DES. The overall performance of the system is computed, analyzed, and optimized for the different system dynamics. Various performance measures are considered such as system capacity, residual capacity, and total number of pallets waiting for inspection in the queue. These metrics are performance indicators of the system's ability to service current needs and respond to additional requests. We studied and analyzed different scenarios by changing various model parameters such as the number of pieces per pallet ratio, number of inspectors and cargo handling personnel, number of forklifts, number and types of detection systems, inspection modality distribution, alarm rate, and cargo closeout time. The increased physical understanding resulting from execution of the queuing model utilizing these vetted performance measures identified effective ways to meet inspection requirements while maintaining or reducing overall operational cost and eliminating any shipping delays associated with any proposed changes in inspection requirements. With this understanding effective operational strategies can be developed to optimally use personnel while still maintaining plant efficiency, reducing process interruptions, and holding or reducing costs.
NASA Astrophysics Data System (ADS)
Xu, Ke; Niu, Liang-Liang; Jin, Shuo; Shu, Xiaolin; Xie, Hongxian; Wang, Lifang; Lu, Guang-Hong
2017-02-01
Atomistic simulations have been used to investigate the core structures, static properties of isolated 1/2 <1 1 1> screw dislocations, and their interaction with vacancies in bcc tungsten (W) based on three empirical interatomic potentials. Differential displacement maps show that only one embedded atom method potential is able to reproduce the compact non-degenerate core as evidenced by ab initio calculations. The obtained strain energy and stress distribution from atomistic simulations are, in general, consistent with elasticity theory predictions. In particular, one component of the calculated shear stress, which is not present according to elasticity theory, is non-negligible in the core region of our dislocation model. The differences between the results calculated from three interatomic potentials are in details, such as the specific value and the symmetry, but the trend of spatial distributions of static properties in the long range are close to each other. By calculating the binding energies between the dislocations and vacancies, we demonstrate that the dislocations act as vacancy sinks, which may be important for the nucleation and growth of hydrogen bubbles in W under irradiation.
Dislocation Dynamics Simulations of Junctions in Hexagonal Close-Packed Crystals
Wu, C; Aubry, S; Chung, P; Arsenlis, A
2011-12-05
The formation and strength of dislocations in the hexagonal closed packed material beryllium are studied through dislocation junctions and the critical stress required to break them. Dislocation dynamics calculations (using the code ParaDiS) of junction maps are compared to an analytical line tension approximation in order to validate our model. Results show that the two models agree very well. Also the critical shear stress necessary to break 30{sup o} - 30{sup o} and 30{sup o} - 90{sup o} dislocation junctions is computed numerically. Yield surfaces are mapped out for these junctions to describe their stability regions as function of resolved shear stresses on the glide planes. The example of two non-coplanar binary dislocation junctions with slip planes [2-1-10] (01-10) and [-12-10] (0001) corresponding to a prismatic and basal slip respectively is chosen to verify and validate our implementation.
de Pablo Márquez, B; Castillón Bernal, P; Bernaus Johnson, M C; Ibañez Aparicio, N M
2017-03-09
Elbow dislocation is the most frequent dislocation in the upper limb after shoulder dislocation. Closed reduction is feasible in outpatient care when there is no associated fracture. A review is presented of the different reduction procedures.
Flexible Ab initio boundary conditions: simulating isolated dislocations in bcc Mo and Ta.
Woodward, C; Rao, S I
2002-05-27
We report the first ab initio density-functional study of the strain field and Peierls stress of isolated <111> screw dislocations in bcc Mo and Ta. The local dislocation strain field is self-consistently coupled to the long-range elastic field using a flexible boundary condition method. This reduces the mesoscopic atomistic calculation to one involving only degrees of freedom near the dislocation core. The predicted equilibrium core for Mo is significantly different from previous atomistic results and the Peierls stress shows significant non-Schmid behavior as expected for the bcc metals.
NASA Astrophysics Data System (ADS)
Zhou, Tianyi; Raghothamachar, Balaji; Wu, Fangzhen; Dalmau, Rafael; Moody, Baxter; Craft, Spalding; Schlesser, Raoul; Dudley, Michael; Sitar, Zlatko
2014-04-01
Threading dislocations in aluminum nitride boules grown by physical vapor transport method were systematically studied via synchrotron x-ray topography (white beam and monochromatic) in conjunction with ray tracing simulations. Two major types of threading dislocations were observed in the c-axis-grown boules: threading screw dislocations (TSDs) and threading edge dislocations (TEDs) with Burgers vectors along the [0001] and directions, respectively. TSDs were typically observed in the middle of the boule while TEDs were commonly observed to aggregate into arrays along the and directions in various parts of the boule on basal plane oriented wafers. By comparison with ray tracing simulations, the absolute Burgers vectors of both TSDs and TEDs in the arrays could be unambiguously determined. TEDs comprise over 90 % of all threading dislocations observed. The relationships between TED arrays and low angle grain boundaries and their possible formation mechanisms are discussed.
NASA Technical Reports Server (NTRS)
Repa, B. S.; Zucker, R. S.; Wierwille, W. W.
1977-01-01
The influence of vehicle transient response characteristics on driver-vehicle performance in discrete maneuvers as measured by integral performance criteria was investigated. A group of eight ordinary drivers was presented with a series of eight vehicle transfer function configurations in a driving simulator. Performance in two discrete maneuvers was analyzed by means of integral performance criteria. Results are presented.
Validation of a DICE Simulation Against a Discrete Event Simulation Implemented Entirely in Code.
Möller, Jörgen; Davis, Sarah; Stevenson, Matt; Caro, J Jaime
2017-07-01
Modeling is an essential tool for health technology assessment, and various techniques for conceptualizing and implementing such models have been described. Recently, a new method has been proposed-the discretely integrated condition event or DICE simulation-that enables frequently employed approaches to be specified using a common, simple structure that can be entirely contained and executed within widely available spreadsheet software. To assess if a DICE simulation provides equivalent results to an existing discrete event simulation, a comparison was undertaken. A model of osteoporosis and its management programmed entirely in Visual Basic for Applications and made public by the National Institute for Health and Care Excellence (NICE) Decision Support Unit was downloaded and used to guide construction of its DICE version in Microsoft Excel(®). The DICE model was then run using the same inputs and settings, and the results were compared. The DICE version produced results that are nearly identical to the original ones, with differences that would not affect the decision direction of the incremental cost-effectiveness ratios (<1% discrepancy), despite the stochastic nature of the models. The main limitation of the simple DICE version is its slow execution speed. DICE simulation did not alter the results and, thus, should provide a valid way to design and implement decision-analytic models without requiring specialized software or custom programming. Additional efforts need to be made to speed up execution.
2006-12-01
NAVAL POSTGRADUATE SCHOOL MONTEREY, CALIFORNIA MBA PROFESSIONAL REPORT Discrete-Event Simulation Modeling of the Repairable...TYPE AND DATES COVERED MBA Professional Report 4. TITLE AND SUBTITLE: Discrete-Event Simulation Modeling of the Repairable Inventory Process to...Advanced Concept Technology Demonstration; Agile Rapid Global Combat Support; Discrete- Event Simulation Modeling of the Repairable Inventory Process to
Icy Curtain Eruptions on Enceladus Create an Illusion of Discrete Jets Simulation
2015-05-06
This simulation, which begins and ends with a real image from NASA Cassini spacecraft, demonstrates how the appearance of discrete jets could be an optical illusion that varies based on viewing geometry.
Brown, D.; Freitag, L.; Glimm, J.
2002-03-28
We present an overview of the technical objectives of the Terascale Simulation Tools and Technologies center. The primary goal of this multi-institution collaboration is to develop technologies that enable application scientists to easily use multiple mesh and discretization strategies within a single simulation on terascale computers. The discussion focuses on our efforts to create interoperable mesh generation tools, high-order discretization techniques, and adaptive meshing strategies.
Simulated capsulolabral lesion in cadavers: dislocation does not result from a bankart lesion only.
Pouliart, Nicole; Marmor, Simon; Gagey, Olivier
2006-07-01
Although an anteroinferior capsulolabral detachment (typical Bankart lesion) has been evaluated in other experimental studies, it has not yet been tested with an apprehension test in an intact shoulder model. Adjacent combinations of 4 zones of the capsuloligamentous complex were sequentially detached from the glenoid neck in 50 cadaveric shoulders. Stability was tested before and after each resection step: inferior stability with a sulcus test and anterior stability with an anterior drawer test and with a load-and-shift test in the apprehension position. A metastable anteroinferior dislocation occurred in 18 specimens after section of 3 zones and in 14 only after section of 4 zones. A locked dislocation occurred after section of all 4 zones in 33 specimens and in the other 17 shoulders only after the posterior capsule was also cut. The humeral head cannot dislocate anteroinferiorly when there only is a Bankart lesion. In our study superior and posterior extension was necessary before the tensioning mechanism in external rotation and abduction failed enough for dislocation to occur. Because the Bankart lesion is most likely not the only lesion present in patients with recurrent dislocation, a careful search for other lesions needs to be done when one is attempting surgical treatment. These lesions would need to be treated as well if one wants to avoid the risk of residual instability.
Non-basal dislocations should be accounted for in simulating ice mass flow
NASA Astrophysics Data System (ADS)
Chauve, T.; Montagnat, M.; Piazolo, S.; Journaux, B.; Wheeler, J.; Barou, F.; Mainprice, D.; Tommasi, A.
2017-09-01
Prediction of ice mass flow and associated dynamics is pivotal at a time of climate change. Ice flow is dominantly accommodated by the motion of crystal defects - the dislocations. In the specific case of ice, their observation is not always accessible by means of the classical tools such as X-ray diffraction or transmission electron microscopy (TEM). Part of the dislocation population, the geometrically necessary dislocations (GNDs) can nevertheless be constrained using crystal orientation measurements via electron backscattering diffraction (EBSD) associated with appropriate analyses based on the Nye (1950) approach. The present study uses the Weighted Burgers Vectors, a reduced formulation of the Nye theory that enables the characterization of GNDs. Applied to ice, this method documents, for the first time, the presence of dislocations with non-basal [ c ] or < c + a > Burgers vectors. These [ c ] or < c + a > dislocations represent up to 35% of the GNDs observed in laboratory-deformed ice samples. Our findings offer a more complex and comprehensive picture of the key plasticity processes responsible for polycrystalline ice creep and provide better constraints on the constitutive mechanical laws implemented in ice sheet flow models used to predict the response of Earth ice masses to climate change.
Zheng, Weihua; Andrec, Michael; Gallicchio, Emilio; Levy, Ronald M
2008-05-15
The efficiency of temperature replica exchange (RE) simulations hinge on their ability to enhance conformational sampling at physiological temperatures by taking advantage of more rapid conformational interconversions at higher temperatures. While temperature RE is a parallel simulation technique that is relatively straightforward to implement, kinetics in the RE ensemble is complicated, and there is much to learn about how best to employ RE simulations in computational biophysics. Protein folding rates often slow down above a certain temperature due to entropic bottlenecks. This "anti-Arrhenius" behavior represents a challenge for RE. However, it is far from straightforward to systematically explore the impact of this on RE by brute force molecular simulations, since RE simulations of protein folding are very difficult to converge. To understand some of the basic mechanisms that determine the efficiency of RE, it is useful to study simplified low dimensionality systems that share some of the key characteristics of molecular systems. Results are presented concerning the efficiency of temperature RE on a continuous two-dimensional potential that contains an entropic bottleneck. Optimal efficiency was obtained when the temperatures of the replicas did not exceed the temperature at which the harmonic mean of the folding and unfolding rates is maximized. This confirms a result we previously obtained using a discrete network model of RE. Comparison of the efficiencies obtained using the continuous and discrete models makes it possible to identify non-Markovian effects, which slow down equilibration of the RE ensemble on the more complex continuous potential. In particular, the rate of temperature diffusion and also the efficiency of RE is limited by the time scale of conformational rearrangements within free energy basins.
Supersonic Dislocation Bursts in Silicon
Hahn, E. N.; Zhao, S.; Bringa, E. M.; ...
2016-06-06
Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolutionmore » we successfully predict a dislocation density of 1.5 x 10(12) cm(-2) within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon.« less
Supersonic Dislocation Bursts in Silicon
Hahn, E. N.; Zhao, S.; Bringa, E. M.; Meyers, M. A.
2016-06-06
Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolution we successfully predict a dislocation density of 1.5 x 10(12) cm(-2) within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon.
Supersonic Dislocation Bursts in Silicon
NASA Astrophysics Data System (ADS)
Hahn, E. N.; Zhao, S.; Bringa, E. M.; Meyers, M. A.
2016-06-01
Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolution we successfully predict a dislocation density of 1.5 × 1012 cm-2 within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon.
On constructing optimistic simulation algorithms for the discrete event system specification
Nutaro, James J
2008-01-01
This article describes a Time Warp simulation algorithm for discrete event models that are described in terms of the Discrete Event System Specification (DEVS). The article shows how the total state transition and total output function of a DEVS atomic model can be transformed into an event processing procedure for a logical process. A specific Time Warp algorithm is constructed around this logical process, and it is shown that the algorithm correctly simulates a DEVS coupled model that consists entirely of interacting atomic models. The simulation algorithm is presented abstractly; it is intended to provide a basis for implementing efficient and scalable parallel algorithms that correctly simulate DEVS models.
Dislocation-density based description of the deformation of a composite material
NASA Astrophysics Data System (ADS)
Schulz, K.; Sudmanns, M.; Gumbsch, P.
2017-09-01
Composite materials consisting of hard particles in a ductile metallic matrix are of major interest since their strength and deformability can be dramatically changed by varying volume fraction, size and shape of the particles. Understanding dislocation motion in composite materials as the cause of plastic deformation therefore is an important task. Recently, advanced dislocation-based continuum theories of plasticity have been developed for performing meaningful averages over systems of straight and curved dislocation lines in a continuum approach. In this paper, we focus on a single slip heterogeneous microstructure and investigate how the dislocation interactions can be represented in an averaged dislocation density based continuum description. The representation of strong dislocation density gradients is discussed in the context of a formulation, which aims at a coarse-grained resolution. We introduce a set of dislocation density evolution equations which account for the formation and dissolution of dislocation dipoles. By applying the model to a composite structure, we demonstrate that the dislocation density based description can well describe the physical processes in the microstructure and a comparison to discrete dislocation dynamics simulations shows good agreement for the relaxation behavior of the considered composites.
Truncated Gaussian simulation of discrete-valued, ordinal coregionalized variables
NASA Astrophysics Data System (ADS)
Emery, Xavier; Cornejo, Javier
2010-10-01
This paper deals with the modeling and cosimulation of ordinal coregionalized variables, such as indicators, counts or continuous-valued variables discretized into a limited number of classes. The proposed model relies on truncations of a set of cross-correlated stationary Gaussian random fields. We provide guidelines and algorithms for inferring and validating the structural model (direct and cross variograms of the underlying Gaussian random fields) and constructing realizations conditioned to data. The algorithms are implemented in a set of computer programs and are illustrated with applications to datasets in pest management and mineral resources evaluation.
Use proper techniques when exercising or playing sports. Keep your knee strong and flexible. Some cases of knee dislocation may not be preventable, especially if physical factors make you more likely to dislocate your knee.
NASA Astrophysics Data System (ADS)
Huang, Min-Sheng; Zhu, Ya-Xin; Li, Zhen-Huan
2014-04-01
The influence of dislocation dissociation on the evolution of Frank—Read (F-R) sources is studied using a three-dimensional discrete dislocation dynamics simulation (3D-DDD). The classical Orowan nucleation stress and recently proposed Benzerga nucleation time models for F-R sources are improved. This work shows that it is necessary to introduce the dislocation dissociation scheme into 3D-DDD simulation, especially for simulations on micro-plasticity of small sized materials with low stacking fault energy.
NASA Technical Reports Server (NTRS)
Kuo, C. T. K.; Arsenault, R. J.
1977-01-01
An investigation was undertaken to determine if the size and modulus interaction of a solute atom with a screw dislocation and the modulus interaction with an edge dislocation contributed to strengthening, in addition to the size interaction with an edge dislocation. The results indicate that the size interaction between solute atom and an edge dislocation accounts for most of the solid solution strengthening in f.c.c. alloys. The contribution to the yield stress from the modulus interaction with an edge dislocation is less than 15%. The interaction between a solute atom and a screw dislocation is much less than that between a solute atom and an edge dislocation.
NASA Astrophysics Data System (ADS)
Neogi, Anupam; Mitra, Nilanjan
Deformation and observation of different types of plasticity mechanisms of FCC metals (e.g. Copper) under shock loading of various intensities has been investigated by several groups of researchers around the globe through different types of experiments and/or atomistic simulations. However, there still exists lacuna in this well researched area. In this study the temporal details of dislocation dynamics are provided. Simulations also demonstrate different types of temporal evolution of different loops observed for single crystal Cu under different intensities of shock loading. Observance of formation of twins and their temporal evolution at higher intensities of shock loading are also demonstrated as part of this study. Comparisons of these NEMD simulations using EAM potential are discussed with regards to different experimental and simulation studies in literature.
Conservative parallel discrete-event simulation: Principles and practice
Wagner, D.B.
1989-01-01
Simulation is one of the most important computational technologies in use today. Unfortunately, its importance is matched by its appetite for computational resources. These factors make parallel simulation a topic with far-reaching consequences in all fields of science and engineering. This thesis is concerned with one approach to this problem, conservative loose event-driven parallel simulation, the objective of which is to apply multiple processors to a single simulation run in an effort to reduce its time-to-completion. There are several factors that make parallel simulation difficult. First, the fact that a physical system has a high degree of concurrency does not necessarily mean that a simulation of that system will benefit from parallelism. The author introduces two simple analytic techniques that can be used to bound from above the speedup potential of parallel simulations. Second, a parallel simulation requires synchronization to ensure that the results obtained are equivalent to those of a sequential simulation of the problem. He argues that the availability of inexpensive, medium-scale and shared-memory multiprocessors mandates a re-examination of synchronization algorithms for conservative loose event-driven parallel simulation. His investigations lead to a novel synchronization technique called lazy blocking avoidance. His measurements show that lazy blocking avoidance performs at least as well as, and often substantially better than, two other synchronization methods that have been widely discussed in the literature (deadlock detection and recovery), and eager blocking avoidance.
A discrete event simulation tool to support and predict hospital and clinic staffing.
DeRienzo, Christopher M; Shaw, Ryan J; Meanor, Phillip; Lada, Emily; Ferranti, Jeffrey; Tanaka, David
2017-06-01
We demonstrate how to develop a simulation tool to help healthcare managers and administrators predict and plan for staffing needs in a hospital neonatal intensive care unit using administrative data. We developed a discrete event simulation model of nursing staff needed in a neonatal intensive care unit and then validated the model against historical data. The process flow was translated into a discrete event simulation model. Results demonstrated that the model can be used to give a respectable estimate of annual admissions, transfers, and deaths based upon two different staffing levels. The discrete event simulation tool model can provide healthcare managers and administrators with (1) a valid method of modeling patient mix, patient acuity, staffing needs, and costs in the present state and (2) a forecast of how changes in a unit's staffing, referral patterns, or patient mix would affect a unit in a future state.
Theory and simulation of discrete kinetic beta induced Alfvén eigenmode in tokamak plasmas
NASA Astrophysics Data System (ADS)
Wang, X.; Zonca, F.; Chen, L.
2010-11-01
It is shown, both analytically and by numerical simulations, that, in the presence of thermal ion kinetic effects, the beta induced Alfvén eigenmode (BAE)-shear Alfvén wave continuous spectrum can be discretized into radially trapped eigenstates known as kinetic BAE (KBAE). While thermal ion compressibility gives rise to finite BAE accumulation point frequency, the discretization occurs via the finite Larmor radius and finite orbit width effects. Simulations and analytical theories agree both qualitatively and quantitatively. Simulations also demonstrate that KBAE can be readily excited by the finite radial gradients of energetic particles.
Electronic properties of dislocations
NASA Astrophysics Data System (ADS)
Reiche, M.; Kittler, M.; Uebensee, H.; Pippel, E.; Haehnel, A.; Birner, S.
2016-04-01
Dislocations exhibit a number of exceptional electronic properties resulting in a significant increase in the drain current of MOSFETs if defined numbers of these defects are placed in the channel. Measurements on individual dislocations in Si refer to a supermetallic conductivity. A model of the electronic structure of dislocations is proposed based on experimental measurements and tight-binding simulations. It is shown that the high strain level on the dislocation core—exceeding 10 % or more—causes locally dramatic changes in the band structure and results in the formation of a quantum well along the dislocation line. This explains experimental findings (two-dimensional electron gas, single-electron transitions). The energy quantization within the quantum well is most important for supermetallic conductivity.
Modeling Anti-Air Warfare With Discrete Event Simulation and Analyzing Naval Convoy Operations
2016-06-01
Simkit, Component Based Approach, Layered Defense Systems, Formation Movements , Design of Experiments, Simulation Output Analysis 15. NUMBER OF PAGES...5 2. Simple Movement and Detection in Discrete Event Simulation Using Simkit...Work ........................................... 6 SIMPLE MOVEMENT AND DETECTION .................................................. 7 II. A
Acceleration of discrete stochastic biochemical simulation using GPGPU.
Sumiyoshi, Kei; Hirata, Kazuki; Hiroi, Noriko; Funahashi, Akira
2015-01-01
For systems made up of a small number of molecules, such as a biochemical network in a single cell, a simulation requires a stochastic approach, instead of a deterministic approach. The stochastic simulation algorithm (SSA) simulates the stochastic behavior of a spatially homogeneous system. Since stochastic approaches produce different results each time they are used, multiple runs are required in order to obtain statistical results; this results in a large computational cost. We have implemented a parallel method for using SSA to simulate a stochastic model; the method uses a graphics processing unit (GPU), which enables multiple realizations at the same time, and thus reduces the computational time and cost. During the simulation, for the purpose of analysis, each time course is recorded at each time step. A straightforward implementation of this method on a GPU is about 16 times faster than a sequential simulation on a CPU with hybrid parallelization; each of the multiple simulations is run simultaneously, and the computational tasks within each simulation are parallelized. We also implemented an improvement to the memory access and reduced the memory footprint, in order to optimize the computations on the GPU. We also implemented an asynchronous data transfer scheme to accelerate the time course recording function. To analyze the acceleration of our implementation on various sizes of model, we performed SSA simulations on different model sizes and compared these computation times to those for sequential simulations with a CPU. When used with the improved time course recording function, our method was shown to accelerate the SSA simulation by a factor of up to 130.
NASA Astrophysics Data System (ADS)
Wang, Chao-Ying; Yang, Li-Jun; Zhao, Wei; Meng, Qing-Yuan; Wu, Guo-Xun; Wang, Bao-Lai; Li, Chen-Liang
2014-12-01
The stable positions, binding energies, and dynamic properties of Li impurity in the presence of a 90° partial dislocation in Si have been studied by using the multi-scale simulation method. The corresponding results are compared with the defect-free Si crystal in order to reflect how the dislocation defect affects the performances of Li-ion batteries (LIBs) at the atomic level. It is found that the inserted Li atom in the dislocation core and nearest regions is more stable, since the binding energies are 0.13 eV to 0.52 eV larger than the bulk Si. Moreover, it is easier for Li atom to diffuse into those defect areas and harder to diffuse out. Thus, Li dopant may tend to congregate in the dislocation core and nearest regions. On the other side, the 90° partial dislocation can glide in the {111} plane accompanied by the diffusion of Li impurity along the pentagon ring of core. In addition, the spacious heptagon ring of dislocation core can lower the migration barrier of Li atom from 0.63 eV to 0.34 eV, which will enhance the motion of the dopant. Therefore, the presence of 90° partial dislocations may provide a fast and favorable diffusion path for the congregated Li impurity, which finally facilitates the lithiation of LIBs.
Strength and Dislocation Structure Evolution of Small Metals under Vibrations
NASA Astrophysics Data System (ADS)
Ngan, Alfonso
2015-03-01
It is well-known that ultrasonic vibration can soften metals, and this phenomenon has been widely exploited in industrial applications concerning metal forming and bonding. In this work, we explore the effects of a superimposed small oscillatory load on metal plasticity, from the nano- to macro-size range, and from audible to ultrasonic frequency ranges. Macroscopic and nano-indentation were performed on aluminum, copper and molybdenum, and the results show that the simultaneous application of oscillatory stresses can lower the hardness of these samples. More interestingly, EBSD and TEM observations show that subgrain formation and reduction in dislocation density generally occurred when stress oscillations were applied. These findings point to an important knowledge gap in metal plasticity - the existing understanding of ultrasound softening in terms of the vibrations either imposing additional stress waves to augment the quasi-static applied load, or heating up the metal, whereas the metal's intrinsic deformation resistance or dislocation interactive processes are assumed unaltered by the ultrasound, is proven wrong by the present results. Furthermore, in the case of nanoindentation, the Continuous Stiffness Measurement technique for contact stiffness measurement assumes that the imposed signal-carrier oscillations do not intrinsically alter the material properties of the specimen, and again, the present results prove that this can be wrong. To understand the enhanced subgrain formation and dislocation annihilation, Discrete Dislocation Dynamics (DDD) simulations were carried out and these show that when an oscillatory stress is superimposed on a quasi-static applied stress, reversals of motion of dislocations may occur, and these allow the dislocations to revisit repeatedly suitable configurations for annihilation. DDD, however, was unable to predict the observed subgrain formation presumably because the number of dislocations that can be handled is not large
Dislocation Multiplication by Single Cross Slip for FCC at Submicron Scales
NASA Astrophysics Data System (ADS)
Cui, Yi-Nan; Liu, Zhan-Li; Zhuang, Zhuo
2013-04-01
The operation mechanism of single cross slip multiplication (SCSM) is investigated by studying the response of one dislocation loop expanding in face-centered-cubic (FCC) single crystal using three-dimensional discrete dislocation dynamic (3D-DDD) simulation. The results show that SCSM can trigger highly correlated dislocation generation in a short time, which may shed some light on understanding the large strain burst observed experimentally. Furthermore, we find that there is a critical stress and material size for the operation of SCSM, which agrees with that required to trigger large strain burst in the compression tests of FCC micropillars.
NASA Astrophysics Data System (ADS)
Baker, K. L.; Curtin, W. A.
2016-07-01
In many problems of interest to materials scientists and engineers, the evolution of crystalline extended defects (dislocations, cracks, grain boundaries, interfaces, voids, precipitates) is controlled by the flow of point defects (interstitial/substitutional atoms and/or vacancies) through the crystal into the extended defect. Precise modeling of this behavior requires fully atomistic methods in and around the extended defect, but the flow of point defects entering the defect region can be treated by coarse-grained methods. Here, a multiscale algorithm is presented to provide this coupling. Specifically, direct accelerated molecular dynamics (AMD) of extended defect evolution is coupled to a diffusing point defect concentration field that captures the long spatial and temporal scales of point defect motion in the presence of the internal stress fields generated by the evolving defect. The algorithm is applied to study vacancy absorption into an edge dislocation in aluminum where vacancy accumulation in the core leads to nucleation of a double-jog that then operates as a sink for additional vacancies; this corresponds to the initial stages of dislocation climb modeled with explicit atomistic resolution. The method is general and so can be applied to many other problems associated with nucleation, growth, and reaction due to accumulation of point defects in crystalline materials.
Representing Dynamic Social Networks in Discrete Event Social Simulation
2010-12-01
applied settings in the areas of marketing and behavior modification programs (exercise adoption, smoking cessation) ( Icek Ajzen 2006). The model has an...society. The action choice component of the conceptual model is based on the theory of planned behavior (TPB) (I. Ajzen 1991). The TPB states that an...information networks into military simulations. In Pro- ceedings of the 40th Conference on Winter Simulation. pp. 133–144. Ajzen , I. 1991. The theory of
Chen, Cheng; Meng, Fanchao; Song, Jun
2015-05-21
The core structures and slip characteristics of (a+c)-edge dislocations on pyramidal planes in wurtzite GaN were investigated employing molecular dynamics simulations. Multiple stable core configurations are identified for dislocations along the glide and shuffle planes. The corresponding generalized-stacking-fault energy (GSFE) curves for the glide and shuffle slips are calculated. The GSFE curves, combined with the Peierls–Nabarro model, demonstrate that the shuffle slip is favored over the glide slip given the markedly lower Peierls energy and stress of the shuffle slip. Our findings also indicate that in general slip motions for (a+c)-edge dislocations are only possible at elevated temperature, and the necessity of further studies of thermally activated processes to better understand the dynamics of (a+c) dislocations in GaN.
Dislocation climb models from atomistic scheme to dislocation dynamics
NASA Astrophysics Data System (ADS)
Niu, Xiaohua; Luo, Tao; Lu, Jianfeng; Xiang, Yang
2017-02-01
We develop a mesoscopic dislocation dynamics model for vacancy-assisted dislocation climb by upscalings from a stochastic model on the atomistic scale. Our models incorporate microscopic mechanisms of (i) bulk diffusion of vacancies, (ii) vacancy exchange dynamics between bulk and dislocation core, (iii) vacancy pipe diffusion along the dislocation core, and (iv) vacancy attachment-detachment kinetics at jogs leading to the motion of jogs. Our mesoscopic model consists of the vacancy bulk diffusion equation and a dislocation climb velocity formula. The effects of these microscopic mechanisms are incorporated by a Robin boundary condition near the dislocations for the bulk diffusion equation and a new contribution in the dislocation climb velocity due to vacancy pipe diffusion driven by the stress variation along the dislocation. Our climb formulation is able to quantitatively describe the translation of prismatic loops at low temperatures when the bulk diffusion is negligible. Using this new formulation, we derive analytical formulas for the climb velocity of a straight edge dislocation and a prismatic circular loop. Our dislocation climb formulation can be implemented in dislocation dynamics simulations to incorporate all the above four microscopic mechanisms of dislocation climb.
A simulator for discrete quantum walks on lattices
NASA Astrophysics Data System (ADS)
Rodrigues, J.; Paunković, N.; Mateus, P.
In this paper, we present a simulator for two-particle quantum walks on the line and one-particle on a two-dimensional squared lattice. It can be used to investigate the equivalence between the two cases (one- and two-particle walks) for various boundary conditions (open, circular, reflecting, absorbing and their combinations). For the case of a single walker on a two-dimensional lattice, the simulator can also implement the Möbius strip. Furthermore, other topologies for the walker are also simulated by the proposed tool, like certain types of planar graphs with degree up to 4, by considering missing links over the lattice. The main purpose of the simulator is to study the genuinely quantum effects on the global properties of the two-particle joint probability distribution on the entanglement between the walkers/axis. For that purpose, the simulator is designed to compute various quantities such as: the entanglement and classical correlations, (classical and quantum) mutual information, the average distance between the two walkers, different hitting times and quantum discord. These quantities are of vital importance in designing possible algorithmic applications of quantum walks, namely in search, 3-SAT problems, etc. The simulator can also implement the static partial measurements of particle(s) positions and dynamic breaking of the links between certain nodes, both of which can be used to investigate the effects of decoherence on the walker(s). Finally, the simulator can be used to investigate the dynamic Anderson-like particle localization by varying the coin operators of certain nodes on the line/lattice. We also present some illustrative and relevant examples of one- and two-particle quantum walks in various scenarios. The tool was implemented in C and is available on-line at http://qwsim.weebly.com/.
DISCRETE EVENT SIMULATION OF OPTICAL SWITCH MATRIX PERFORMANCE IN COMPUTER NETWORKS
Imam, Neena; Poole, Stephen W
2013-01-01
In this paper, we present application of a Discrete Event Simulator (DES) for performance modeling of optical switching devices in computer networks. Network simulators are valuable tools in situations where one cannot investigate the system directly. This situation may arise if the system under study does not exist yet or the cost of studying the system directly is prohibitive. Most available network simulators are based on the paradigm of discrete-event-based simulation. As computer networks become increasingly larger and more complex, sophisticated DES tool chains have become available for both commercial and academic research. Some well-known simulators are NS2, NS3, OPNET, and OMNEST. For this research, we have applied OMNEST for the purpose of simulating multi-wavelength performance of optical switch matrices in computer interconnection networks. Our results suggest that the application of DES to computer interconnection networks provides valuable insight in device performance and aids in topology and system optimization.
Effect of interatomic potential on the behavior of dislocation-defect interaction simulation in α-Fe
NASA Astrophysics Data System (ADS)
Hafez Haghighat, S. M.; Fikar, J.; Schäublin, R.
2008-12-01
Molecular dynamics simulation is one of the most useful methods to model defect generation and subsequent change in mechanical properties in material that will suffer irradiation in the future fusion reactors. This work is aimed at showing the influence of the empirical interatomic potential for the Fe-Fe interaction on the simulated shearing of α-Fe containing one edge dislocation interacting with one nanometric void sitting on its glide plane. The recent potentials derived by Ackland et al. [G.J. Ackland, D.J. Bacon, A.F. Calder, T. Harry, Philosophical magazine a-physics of condensed matter structure defects and mechanical properties 75 (1997) 713], Mendelev et al. [M.I. Mendelev, S. Han, D.J. Srolovitz, G.J. Ackland, D.Y. Sun, M. Asta, Philos. Mag. 83 (2003) 3977] and Dudarev-Derlet [S.L. Dudarev, P.M. Derlet, J. Phys. Condens. Matter 17 (2005) 7097] are used to identify critical parameters. The stress-strain responses are obtained under imposed strain rate and at temperatures ranging from 10 to 700 K at constant volume. It appears that different potentials give different strengths and rates of decrease of obstacle strength with increasing temperature. Results are analyzed in terms of dislocation core structure and thermal expansion. Implications for the choice of the potential are given.
Mennemann, Jan-Frederik Jüngel, Ansgar
2014-10-15
Discrete transparent boundary conditions (DTBC) and the Perfectly Matched Layers (PML) method for the realization of open boundary conditions in quantum device simulations are compared, based on the stationary and time-dependent Schrödinger equation. The comparison includes scattering state, wave packet, and transient scattering state simulations in one and two space dimensions. The Schrödinger equation is discretized by a second-order Crank–Nicolson method in case of DTBC. For the discretization with PML, symmetric second-, fourth-, and sixth-order spatial approximations as well as Crank–Nicolson and classical Runge–Kutta time-integration methods are employed. In two space dimensions, a ring-shaped quantum waveguide device is simulated in the stationary and transient regime. As an application, a simulation of the Aharonov–Bohm effect in this device is performed, showing the excitation of bound states localized in the ring region. The numerical experiments show that the results obtained from PML are comparable to those obtained using DTBC, while keeping the high numerical efficiency and flexibility as well as the ease of implementation of the former method. -- Highlights: •In-depth comparison between discrete transparent boundary conditions (DTBC) and PML. •First 2-D transient scattering state simulations using DTBC. •First 2-D transient scattering state simulations of the Aharonov–Bohm effect.
Analyzing Noncombatant Evacuation Operations Using Discrete Event Simulation
2013-12-01
hotels, stadiums , and other locations where NCEs can stay until transportation to the United States is available (Joint Chiefs of Staff 2007). NCEs...applying analytical techniques such as simulation, optimization, and decision analysis to solve real world problems. He is an avid sports fan and is known
The core structure and recombination energy of a copper screw dislocation: a Peierls study
Szajewski, B. A.; Hunter, A.; Beyerlein, I. J.
2017-05-19
The recombination process of dislocations is central to cross-slip, and transmission through Σ3 grain boundaries among other fundamental plastic deformation processes. Despite its importance, a detailed mechanistic understanding remains lacking. In this paper, we apply a continuous dislocation model, inspired by Peierls and Nabarro, complete with an ab-initio computed -surface and continuous units of infinitesimal dislocation slip, towards computing the stress-dependent recombination path of both an isotropic and anisotropic Cu screw dislocation. Under no applied stress, our model reproduces the stacking fault width between Shockley partial dislocations as predicted by discrete linear elasticity. Upon application of a compressive Escaig stress,more » the two partial dislocations coalesce to a separation of ~|b|. Upon increased loading the edge components of each partial dislocation recede, leaving behind a spread Peierls screw dislocation, indicating the recombined state. We demonstrate that the critical stress required to achieve the recombined state is independent of the shear modulus. Rather the critical recombination stress depends on an energy difference between an unstable fault energy (γτ) and the intrinsic stacking fault energy (γτ-γisf). We report recombination energies of ΔW = 0.168 eV/Å and ΔW = 0.084 eV/Å, respectively, for the Cu screw dislocation within isotropic and anisotropic media. Finally, we develop an analytic model which provides insight into our simulation results which compare favourably with other (similar) models.« less
Simple Movement and Detection in Discrete Event Simulation
2005-12-01
with a description of uniform linear motion in the following section. We will then con- sider the simplest kind of sensing, the “ cookie -cutter.” A... cookie -cutter sensor sees everything that is within its range R, and must be notified at the precise time a target enters it range. In a time-step...simulation, cookie -cutter detection is very easy. Simply compute the distance between the sensor and the target at each time step. If the target is
Unravelling the physics of size-dependent dislocation-mediated plasticity
NASA Astrophysics Data System (ADS)
El-Awady, Jaafar A.
2015-01-01
Size-affected dislocation-mediated plasticity is important in a wide range of materials and technologies. Here we develop a generalized size-dependent dislocation-based model that predicts strength as a function of crystal/grain size and the dislocation density. Three-dimensional (3D) discrete dislocation dynamics (DDD) simulations reveal the existence of a well-defined relationship between strength and dislocation microstructure at all length scales for both single crystals and polycrystalline materials. The results predict a transition from dislocation-source strengthening to forest-dominated strengthening at a size-dependent critical dislocation density. It is also shown that the Hall-Petch relationship can be physically interpreted by coupling with an appropriate kinetic equation of the evolution of the dislocation density in polycrystals. The model is shown to be in remarkable agreement with experiments. This work presents a micro-mechanistic framework to predict and interpret strength size-scale effects, and provides an avenue towards performing multiscale simulations without ad hoc assumptions.
Effects of Computer Simulation Training on In Vivo Discrete Trial Teaching
ERIC Educational Resources Information Center
Eldevik, Sigmund; Ondire, Iwona; Hughes, J. Carl; Grindle, Corinna F.; Randell, Tom; Remington, Bob
2013-01-01
Although Discrete-trial Teaching (DTT) is effective in teaching a many skills to children with autism, its proper implementation requires rigorous staff training. This study used an interactive computer simulation program ("DTkid") to teach staff relevant DTT skills. Participants (N = 12) completed two sets of pre-tests either once (n = 7) or…
Effects of Computer Simulation Training on In Vivo Discrete Trial Teaching
ERIC Educational Resources Information Center
Eldevik, Sigmund; Ondire, Iwona; Hughes, J. Carl; Grindle, Corinna F.; Randell, Tom; Remington, Bob
2013-01-01
Although Discrete-trial Teaching (DTT) is effective in teaching a many skills to children with autism, its proper implementation requires rigorous staff training. This study used an interactive computer simulation program ("DTkid") to teach staff relevant DTT skills. Participants (N = 12) completed two sets of pre-tests either once (n = 7) or…
Computer simulation of a wind tunnel test section with discrete finite-length wall slots
NASA Technical Reports Server (NTRS)
Kemp, W. B., Jr.
1986-01-01
A computer simulation of a slotted wind tunnel test section which includes a discrete, finite-length wall slot representation with plenum chamber constraints and accounts for the nonlinear effects of the dynamic pressure of the slot outflow jet and of the low energy of slot inflow air was developed. The simulation features were selected to be those appropriate for the intended subsequent use of the simulation in a wall interference assessment procedure using sparsely located wall pressure measurements. It is demonstrated that accounting for slot discreteness is important in interpreting wall pressure measured between slots, and that accounting for nonlinear slot flow effects produces significant changes in tunnel-induced velocity distributions and, in particular, produces a longitudinal component of tunnel-induced velocity due to model lift. A characteristic mode of tunnel flow interaction with constraints imposed by the plenum chamber and diffuser entrance is apparent in simulation results and is derived analytically through a simplified analysis.
Collective dynamics of dislocations interacting with mobile solute atoms
NASA Astrophysics Data System (ADS)
Ovaska, Markus; Paananen, Topi; Laurson, Lasse; Alava, Mikko J.
2016-04-01
We study the effect of diffusing solute atoms on the collective dynamics of dislocations in plastically deforming crystals, by simulating a two-dimensional discrete dislocation dynamics model with solute atoms included. We employ various protocols to apply the external stress, including constant, oscillatory and quasistatically increasing stress, and study the resulting dynamics for various values of the solute mobility, temperature, and interaction strength with the dislocations. The values of these parameters dictate if Cottrell clouds are formed around the dislocations, and whether the dislocations are able to drag them along as they move. The relevant solute-induced processes include a temporally increasing average Cottrell cloud size due to cloud merging during the evolution of the dislocation structures subject to constant stresses, and a crossover between a solute-free ‘phase’ and a regime where solute drag is important for cyclic stresses, controlled by the solute mobility and temperature. Statistics of deformation bursts under quasistatic loading exhibit atypical scaling where the average burst size is directly proportional to its duration, and are also affected by solute-induced strain hardening in the high-stress regime.
NASA Astrophysics Data System (ADS)
Yang, L. M.; Shu, C.; Wang, Y.; Sun, Y.
2016-08-01
The sphere function-based gas kinetic scheme (GKS), which was presented by Shu and his coworkers [23] for simulation of inviscid compressible flows, is extended to simulate 3D viscous incompressible and compressible flows in this work. Firstly, we use certain discrete points to represent the spherical surface in the phase velocity space. Then, integrals along the spherical surface for conservation forms of moments, which are needed to recover 3D Navier-Stokes equations, are approximated by integral quadrature. The basic requirement is that these conservation forms of moments can be exactly satisfied by weighted summation of distribution functions at discrete points. It was found that the integral quadrature by eight discrete points on the spherical surface, which forms the D3Q8 discrete velocity model, can exactly match the integral. In this way, the conservative variables and numerical fluxes can be computed by weighted summation of distribution functions at eight discrete points. That is, the application of complicated formulations resultant from integrals can be replaced by a simple solution process. Several numerical examples including laminar flat plate boundary layer, 3D lid-driven cavity flow, steady flow through a 90° bending square duct, transonic flow around DPW-W1 wing and supersonic flow around NACA0012 airfoil are chosen to validate the proposed scheme. Numerical results demonstrate that the present scheme can provide reasonable numerical results for 3D viscous flows.
Discrete Particle Dynamics Simulations of Adhesive Systems with Thermostatting
NASA Astrophysics Data System (ADS)
Pierce, Flint; Lechman, Jeremy; Hewson, John
2012-02-01
Aggregation/coagulation/flocculation processes are ubiquitous in modern industry from fields as diverse as waste water treatment, the food industry, algae biofuel production, and materials processing where control of the size and morphology of aggregates is paramount to the application of interest. Population balance models have historically been used with success in predicting aggregation kinetics and size distributions for these processes. However, even the most robust population balance schemes can lack an exact description of the underlying physical processes governing attractive or adhesive particulate matter suspended in a background medium, including finite aggregate strength and yield stress, restructuring length and time scales, and response to hydrodynamic forces. In order to elucidate these phenomena, We develop and use a JKR type model for simulating adhesive particulate matter in a background medium varying from dilute gas to liquid. We evaluate the time and length scales for restructuring/fragmentation that result from this model as a function of aggregate size and fractal dimension. We additionally introduce a method for pairwise thermostatting of the adhesive potential and discuss the applicability of this model to various adhesive systems.
The Investigation of Optimal Discrete Approximations for Real Time Flight Simulations
NASA Technical Reports Server (NTRS)
Parrish, E. A.; Mcvey, E. S.; Cook, G.; Henderson, K. C.
1976-01-01
The results are presented of an investigation of discrete approximations for real time flight simulation. Major topics discussed include: (1) consideration of the particular problem of approximation of continuous autopilots by digital autopilots; (2) use of Bode plots and synthesis of transfer functions by asymptotic fits in a warped frequency domain; (3) an investigation of the various substitution formulas, including the effects of nonlinearities; (4) use of pade approximation to the solution of the matrix exponential arising from the discrete state equations; and (5) an analytical integration of the state equation using interpolated input.
Chick, S E; Adams, A L; Koopman, J S
2000-07-01
Deterministic differential equation models indicate that partnership concurrency and non-homogeneous mixing patterns play an important role in the spread of sexually transmitted infections. Stochastic discrete-individual simulation studies arrive at similar conclusions, but from a very different modeling perspective. This paper presents a stochastic discrete-individual infection model that helps to unify these two approaches to infection modeling. The model allows for both partnership concurrency, as well as the infection, recovery, and reinfection of an individual from repeated contact with a partner, as occurs with many mucosal infections. The simplest form of the model is a network-valued Markov chain, where the network's nodes are individuals and arcs represent partnerships. Connections between the differential equation and discrete-individual approaches are constructed with large-population limits that approximate endemic levels and equilibrium probability distributions that describe partnership concurrency. A more general form of the discrete-individual model that allows for semi-Markovian dynamics and heterogeneous contact patterns is implemented in simulation software. Analytical and simulation results indicate that the basic reproduction number R(0) increases when reinfection is possible, and the epidemic rate of rise and endemic levels are not related by 1-1/R(0), when partnerships are not point-time processes.
NASA Technical Reports Server (NTRS)
Joshi, R. P.
2001-01-01
The objective of this work was to conduct a modeling study of SiC P-N junction diodes operating under high reverse biased conditions. Analytical models and numerical simulation capabilities were to be developed for self-consistent electro-thermal analysis of the diode current-voltage (I-V) characteristics. Data from GRC indicate that screw dislocations are unavoidable in large area SiC devices, and lead to changes in the SiC diode electrical response characteristics under high field conditions. For example, device instability and failures linked to internal current filamentation have been observed. The physical origin of these processes is not well understood, and quantitative projections of the electrical behavior under high field and temperature conditions are lacking. Thermal calculations for SiC devices have not been reported in the literature either. So estimates or projections of peak device temperatures and power limitations do not exist. This numerical study and simulation analysis was aimed at resolving some of the above issues. The following tasks were successfully accomplished: (1) Development of physically based models using one- and two-dimensional drift-diffusion theory for the transport behavior and I-V characteristics; (2) One- and two-dimensional heat flow to account for internal device heating. This led to calculations of the internal temperature profiles, which in turn, were used to update the electrical transport parameters for a self-consistent analysis. The temperature profiles and the peak values were thus obtainable for a given device operating condition; (3) Inclusion of traps assumed to model the presence of internal screw dislocations running along the longitudinal direction; (4) Predictions of the operating characteristics with and without heating as a function of applied bias with and without traps. Both one and two-dimensional cases were implemented; (5) Assessment of device stability based on the operating characteristics. The
Discrete event model-based simulation for train movement on a single-line railway
NASA Astrophysics Data System (ADS)
Xu, Xiao-Ming; Li, Ke-Ping; Yang, Li-Xing
2014-08-01
The aim of this paper is to present a discrete event model-based approach to simulate train movement with the considered energy-saving factor. We conduct extensive case studies to show the dynamic characteristics of the traffic flow and demonstrate the effectiveness of the proposed approach. The simulation results indicate that the proposed discrete event model-based simulation approach is suitable for characterizing the movements of a group of trains on a single railway line with less iterations and CPU time. Additionally, some other qualitative and quantitative characteristics are investigated. In particular, because of the cumulative influence from the previous trains, the following trains should be accelerated or braked frequently to control the headway distance, leading to more energy consumption.
Bürger, Raimund; Diehl, Stefan; Mejías, Camilo
2016-01-01
The main purpose of the recently introduced Bürger-Diehl simulation model for secondary settling tanks was to resolve spatial discretization problems when both hindered settling and the phenomena of compression and dispersion are included. Straightforward time integration unfortunately means long computational times. The next step in the development is to introduce and investigate time-integration methods for more efficient simulations, but where other aspects such as implementation complexity and robustness are equally considered. This is done for batch settling simulations. The key findings are partly a new time-discretization method and partly its comparison with other specially tailored and standard methods. Several advantages and disadvantages for each method are given. One conclusion is that the new linearly implicit method is easier to implement than another one (semi-implicit method), but less efficient based on two types of batch sedimentation tests.
A discrete geometric approach for simulating the dynamics of thin viscous threads
Audoly, B.; Clauvelin, N.; Brun, P.-T.; Bergou, M.; Grinspun, E.; Wardetzky, M.
2013-11-15
We present a numerical model for the dynamics of thin viscous threads based on a discrete, Lagrangian formulation of the smooth equations. The model makes use of a condensed set of coordinates, called the centerline/spin representation: the kinematic constraints linking the centerline's tangent to the orientation of the material frame is used to eliminate two out of three degrees of freedom associated with rotations. Based on a description of twist inspired from discrete differential geometry and from variational principles, we build a full-fledged discrete viscous thread model, which includes in particular a discrete representation of the internal viscous stress. Consistency of the discrete model with the classical, smooth equations for thin threads is established formally. Our numerical method is validated against reference solutions for steady coiling. The method makes it possible to simulate the unsteady behavior of thin viscous threads in a robust and efficient way, including the combined effects of inertia, stretching, bending, twisting, large rotations and surface tension.
Dislocation Multi-junctions and Strain Hardening
Bulatov, V; Hsiung, L; Tang, M; Arsenlis, A; Bartelt, M; Cai, W; Florando, J; Hiratani, M; Rhee, M; Hommes, G; Pierce, T; Diaz de la Rubia, T
2006-06-20
At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects--dislocations. First theorized in 1934 to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed only two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening: a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions tying dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed hereafter multi-junctions. The existence of multi-junctions is first predicted by Dislocation Dynamics (DD) and atomistic simulations and then confirmed by the transmission electron microscopy (TEM) experiments in single crystal molybdenum. In large-scale Dislocation Dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in BCC crystals.
Dislocation multi-junctions and strain hardening.
Bulatov, Vasily V; Hsiung, Luke L; Tang, Meijie; Arsenlis, Athanasios; Bartelt, Maria C; Cai, Wei; Florando, Jeff N; Hiratani, Masato; Rhee, Moon; Hommes, Gregg; Pierce, Tim G; de la Rubia, Tomas Diaz
2006-04-27
At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects called dislocations. First proposed theoretically in 1934 (refs 1-3) to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening, a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions that tie the dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed 'multi-junctions'. We first predict the existence of multi-junctions using dislocation dynamics and atomistic simulations and then confirm their existence by transmission electron microscopy experiments in single-crystal molybdenum. In large-scale dislocation dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication, thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in body-centred cubic crystals.
NASA Technical Reports Server (NTRS)
Steinman, Jeffrey S. (Inventor)
1998-01-01
The present invention is embodied in a method of performing object-oriented simulation and a system having inter-connected processor nodes operating in parallel to simulate mutual interactions of a set of discrete simulation objects distributed among the nodes as a sequence of discrete events changing state variables of respective simulation objects so as to generate new event-defining messages addressed to respective ones of the nodes. The object-oriented simulation is performed at each one of the nodes by assigning passive self-contained simulation objects to each one of the nodes, responding to messages received at one node by generating corresponding active event objects having user-defined inherent capabilities and individual time stamps and corresponding to respective events affecting one of the passive self-contained simulation objects of the one node, restricting the respective passive self-contained simulation objects to only providing and receiving information from die respective active event objects, requesting information and changing variables within a passive self-contained simulation object by the active event object, and producing corresponding messages specifying events resulting therefrom by the active event objects.
Automated identification and indexing of dislocations in crystal interfaces
Stukowski, Alexander; Bulatov, Vasily V.; Arsenlis, Athanasios
2012-10-31
Here, we present a computational method for identifying partial and interfacial dislocations in atomistic models of crystals with defects. Our automated algorithm is based on a discrete Burgers circuit integral over the elastic displacement field and is not limited to specific lattices or dislocation types. Dislocations in grain boundaries and other interfaces are identified by mapping atomic bonds from the dislocated interface to an ideal template configuration of the coherent interface to reveal incompatible displacements induced by dislocations and to determine their Burgers vectors. Additionally, the algorithm generates a continuous line representation of each dislocation segment in the crystal andmore » also identifies dislocation junctions.« less
Automated identification and indexing of dislocations in crystal interfaces
Stukowski, Alexander; Bulatov, Vasily V.; Arsenlis, Athanasios
2012-10-31
Here, we present a computational method for identifying partial and interfacial dislocations in atomistic models of crystals with defects. Our automated algorithm is based on a discrete Burgers circuit integral over the elastic displacement field and is not limited to specific lattices or dislocation types. Dislocations in grain boundaries and other interfaces are identified by mapping atomic bonds from the dislocated interface to an ideal template configuration of the coherent interface to reveal incompatible displacements induced by dislocations and to determine their Burgers vectors. Additionally, the algorithm generates a continuous line representation of each dislocation segment in the crystal and also identifies dislocation junctions.
NASA Astrophysics Data System (ADS)
Xie, Zheng; Ye, Zheng; Ma, Yu-Jie
2009-12-01
Numerical simulation of antennae is a topic in computational electromagnetism, which is concerned with the numerical study of Maxwell equations. By discrete exterior calculus and the lattice gauge theory with coefficient R, we obtain the Bianchi identity on prism lattice. By defining an inner product of discrete differential forms, we derive the source equation and continuity equation. Those equations compose the discrete Maxwell equations in vacuum case on discrete manifold, which are implemented on Java development platform to simulate the Gaussian pulse radiation on antennaes.
Ghany, Ahmad; Vassanji, Karim; Kuziemsky, Craig; Keshavjee, Karim
2013-01-01
Electronic prescribing (e-prescribing) is expected to bring many benefits to Canadian healthcare, such as a reduction in errors and adverse drug reactions. As there currently is no functioning e-prescribing system in Canada that is completely electronic, we are unable to evaluate the performance of a live system. An alternative approach is to use simulation modeling for evaluation. We developed two discrete-event simulation models, one of the current handwritten prescribing system and one of a proposed e-prescribing system, to compare the performance of these two systems. We were able to compare the number of processes in each model, workflow efficiency, and the distribution of patients or prescriptions. Although we were able to compare these models to each other, using discrete-event simulation software was challenging. We were limited in the number of variables we could measure. We discovered non-linear processes and feedback loops in both models that could not be adequately represented using discrete-event simulation software. Finally, interactions between entities in both models could not be modeled using this type of software. We have come to the conclusion that a more appropriate approach to modeling both the handwritten and electronic prescribing systems would be to use a complex adaptive systems approach using agent-based modeling or systems-based modeling.
NASA Astrophysics Data System (ADS)
Bhatti, Ghulam M.; Vakili, Pirooz
1997-06-01
There are significant opportunities for the development of parallel/distributed simulation algorithms in the context of parametric study of discrete event systems. In such studies, simulation of multiple (often a large number of) parametric variants is required in order to, for example, identify significant parameters (factor screening), determine directions for response improvement (gradient estimation), find optimal parameter settings (response optimization), or construct a model of the response (meta-modeling). The computational burden in this case is to a large extent due to the large number of alternatives that need to be simulated. An effective strategy in this context is to concurrently simulate a number of parametric variants: the structural similarity of the variants often allows for significant amount of sharing of the simulation work, and the code for concurrent simulation of the variants can often be implemented in a parallel/distributed environment. In this paper, we describe two methods of parallel/distributed/concurrent simulation called the standard clock (SC) and the general shared clock (GSC) simulation. Both approaches rely on an event-reservation approach: by contrast to most discrete-event simulation approaches that are based on an event-scheduling approach, in the SC and GSC simulation, the occurrence instances of all events are reserved on the time axis. These instances may or may not be used. This event-reservation approach frees the clock mechanism of the simulation from needing feedback from the state-update mechanism. Due to this autonomy of the clock mechanism, a single clock can be used to drive a number (possibly large) of variants concurrently and in parallel. The autonomy of the clock mechanism is also the key to the different implementation strategies we adopt. To illustrate, we describe the simulation of parametric versions of wireless communication networks on message passing and shared memory environments.
A continuum theory of edge dislocations
NASA Astrophysics Data System (ADS)
Berdichevsky, V. L.
2017-09-01
Continuum theory of dislocation aims to describe the behavior of large ensembles of dislocations. This task is far from completion, and, most likely, does not have a ;universal solution;, which is applicable to any dislocation ensemble. In this regards it is important to have guiding lines set by benchmark cases, where the transition from a discrete set of dislocations to a continuum description is made rigorously. Two such cases have been considered recently: equilibrium of dislocation walls and screw dislocations in beams. In this paper one more case is studied, equilibrium of a large set of 2D edge dislocations placed randomly in a 2D bounded region. The major characteristic of interest is energy of dislocation ensemble, because it determines the structure of continuum equations. The homogenized energy functional is obtained for the periodic dislocation ensembles with a random contents of the periodic cell. Parameters of the periodic structure can change slowly on distances of order of the size of periodic cells. The energy functional is obtained by the variational-asymptotic method. Equilibrium positions are local minima of energy. It is confirmed the earlier assertion that energy density of the system is the sum of elastic energy of averaged elastic strains and microstructure energy, which is elastic energy of the neutralized dislocation system, i.e. the dislocation system placed in a constant dislocation density field making the averaged dislocation density zero. The computation of energy is reduced to solution of a variational cell problem. This problem is solved analytically. The solution is used to investigate stability of simple dislocation arrays, i.e. arrays with one dislocation in the periodic cell. The relations obtained yield two outcomes: First, there is a state parameter of the system, dislocation polarization; averaged stresses affect only dislocation polarization and cannot change other characteristics of the system. Second, the structure of
Application of Parallel Discrete Event Simulation to the Space Surveillance Network
NASA Astrophysics Data System (ADS)
Jefferson, D.; Leek, J.
2010-09-01
In this paper we describe how and why we chose parallel discrete event simulation (PDES) as the paradigm for modeling the Space Surveillance Network (SSN) in our modeling framework, TESSA (Testbed Environment for Space Situational Awareness). DES is a simulation paradigm appropriate for systems dominated by discontinuous state changes at times that must be calculated dynamically. It is used primarily for complex man-made systems like telecommunications, vehicular traffic, computer networks, economic models etc., although it is also useful for natural systems that are not described by equations, such as particle systems, population dynamics, epidemics, and combat models. It is much less well known than simple time-stepped simulation methods, but has the great advantage of being time scale independent, so that one can freely mix processes that operate at time scales over many orders of magnitude with no runtime performance penalty. In simulating the SSN we model in some detail: (a) the orbital dynamics of up to 105 objects, (b) their reflective properties, (c) the ground- and space-based sensor systems in the SSN, (d) the recognition of orbiting objects and determination of their orbits, (e) the cueing and scheduling of sensor observations, (f) the 3-d structure of satellites, and (g) the generation of collision debris. TESSA is thus a mixed continuous-discrete model. But because many different types of discrete objects are involved with such a wide variation in time scale (milliseconds for collisions, hours for orbital periods) it is suitably described using discrete events. The PDES paradigm is surprising and unusual. In any instantaneous runtime snapshot some parts my be far ahead in simulation time while others lag behind, yet the required causal relationships are always maintained and synchronized correctly, exactly as if the simulation were executed sequentially. The TESSA simulator is custom-built, conservatively synchronized, and designed to scale to
A high precision dual feedback discrete control system designed for satellite trajectory simulator
NASA Astrophysics Data System (ADS)
Liu, Ximin; Liu, Liren; Sun, Jianfeng; Xu, Nan
2005-08-01
Cooperating with the free-space laser communication terminals, the satellite trajectory simulator is used to test the acquisition, pointing, tracking and communicating performances of the terminals. So the satellite trajectory simulator plays an important role in terminal ground test and verification. Using the double-prism, Sun etc in our group designed a satellite trajectory simulator. In this paper, a high precision dual feedback discrete control system designed for the simulator is given and a digital fabrication of the simulator is made correspondingly. In the dual feedback discrete control system, Proportional- Integral controller is used in velocity feedback loop and Proportional- Integral- Derivative controller is used in position feedback loop. In the controller design, simplex method is introduced and an improvement to the method is made. According to the transfer function of the control system in Z domain, the digital fabrication of the simulator is given when it is exposed to mechanism error and moment disturbance. Typically, when the mechanism error is 100urad, the residual standard error of pitching angle, azimuth angle, x-coordinate position and y-coordinate position are 0.49urad, 6.12urad, 4.56urad, 4.09urad respectively. When the moment disturbance is 0.1rad, the residual standard error of pitching angle, azimuth angle, x-coordinate position and y-coordinate position are 0.26urad, 0.22urad, 0.16urad, 0.15urad respectively. The digital fabrication results demonstrate that the dual feedback discrete control system designed for the simulator can achieve the anticipated high precision performance.
The role of the mobility law of dislocations in the plastic response of shock loaded pure metals
NASA Astrophysics Data System (ADS)
Gurrutxaga-Lerma, Beñat
2016-08-01
This article examines the role that the choice of a dislocation mobility law has in the study of plastic relaxation at shock fronts. Five different mobility laws, two of them phenomenological fits to data, and three more based on physical models of dislocation inertia, are tested by employing dynamic discrete dislocation plasticity (D3P) simulations of a shock loaded aluminium thin foil. It is found that inertial laws invariably entail very short acceleration times for dislocations changing their kinematic state. As long as the mobility laws describe the same regime of terminal speeds, all mobility laws predict the same degree of plastic relaxation at the shock front. This is used to show that the main factor affecting plastic relaxation at the shock front is in fact the speed of dislocations.
Wilke, Jeremiah J; Kenny, Joseph P.
2015-02-01
Discrete event simulation provides a powerful mechanism for designing and testing new extreme- scale programming models for high-performance computing. Rather than debug, run, and wait for results on an actual system, design can first iterate through a simulator. This is particularly useful when test beds cannot be used, i.e. to explore hardware or scales that do not yet exist or are inaccessible. Here we detail the macroscale components of the structural simulation toolkit (SST). Instead of depending on trace replay or state machines, the simulator is architected to execute real code on real software stacks. Our particular user-space threading framework allows massive scales to be simulated even on small clusters. The link between the discrete event core and the threading framework allows interesting performance metrics like call graphs to be collected from a simulated run. Performance analysis via simulation can thus become an important phase in extreme-scale programming model and runtime system design via the SST macroscale components.
Parallel Discrete Molecular Dynamics Simulation With Speculation and In-Order Commitment*†
Khan, Md. Ashfaquzzaman; Herbordt, Martin C.
2011-01-01
Discrete molecular dynamics simulation (DMD) uses simplified and discretized models enabling simulations to advance by event rather than by timestep. DMD is an instance of discrete event simulation and so is difficult to scale: even in this multi-core era, all reported DMD codes are serial. In this paper we discuss the inherent difficulties of scaling DMD and present our method of parallelizing DMD through event-based decomposition. Our method is microarchitecture inspired: speculative processing of events exposes parallelism, while in-order commitment ensures correctness. We analyze the potential of this parallelization method for shared-memory multiprocessors. Achieving scalability required extensive experimentation with scheduling and synchronization methods to mitigate serialization. The speed-up achieved for a variety of system sizes and complexities is nearly 6× on an 8-core and over 9× on a 12-core processor. We present and verify analytical models that account for the achieved performance as a function of available concurrency and architectural limitations. PMID:21822327
Parallel Discrete Molecular Dynamics Simulation With Speculation and In-Order Commitment.
Khan, Md Ashfaquzzaman; Herbordt, Martin C
2011-07-20
Discrete molecular dynamics simulation (DMD) uses simplified and discretized models enabling simulations to advance by event rather than by timestep. DMD is an instance of discrete event simulation and so is difficult to scale: even in this multi-core era, all reported DMD codes are serial. In this paper we discuss the inherent difficulties of scaling DMD and present our method of parallelizing DMD through event-based decomposition. Our method is microarchitecture inspired: speculative processing of events exposes parallelism, while in-order commitment ensures correctness. We analyze the potential of this parallelization method for shared-memory multiprocessors. Achieving scalability required extensive experimentation with scheduling and synchronization methods to mitigate serialization. The speed-up achieved for a variety of system sizes and complexities is nearly 6× on an 8-core and over 9× on a 12-core processor. We present and verify analytical models that account for the achieved performance as a function of available concurrency and architectural limitations.
Parallel discrete molecular dynamics simulation with speculation and in-order commitment
NASA Astrophysics Data System (ADS)
Khan, Md. Ashfaquzzaman; Herbordt, Martin C.
2011-07-01
Discrete molecular dynamics simulation (DMD) uses simplified and discretized models enabling simulations to advance by event rather than by timestep. DMD is an instance of discrete event simulation and so is difficult to scale: even in this multi-core era, all reported DMD codes are serial. In this paper we discuss the inherent difficulties of scaling DMD and present our method of parallelizing DMD through event-based decomposition. Our method is microarchitecture inspired: speculative processing of events exposes parallelism, while in-order commitment ensures correctness. We analyze the potential of this parallelization method for shared-memory multiprocessors. Achieving scalability required extensive experimentation with scheduling and synchronization methods to mitigate serialization. The speed-up achieved for a variety of system sizes and complexities is nearly 6× on an 8-core and over 9× on a 12-core processor. We present and verify analytical models that account for the achieved performance as a function of available concurrency and architectural limitations.
GPU accelerated simulations of 3D deterministic particle transport using discrete ordinates method
NASA Astrophysics Data System (ADS)
Gong, Chunye; Liu, Jie; Chi, Lihua; Huang, Haowei; Fang, Jingyue; Gong, Zhenghu
2011-07-01
Graphics Processing Unit (GPU), originally developed for real-time, high-definition 3D graphics in computer games, now provides great faculty in solving scientific applications. The basis of particle transport simulation is the time-dependent, multi-group, inhomogeneous Boltzmann transport equation. The numerical solution to the Boltzmann equation involves the discrete ordinates ( Sn) method and the procedure of source iteration. In this paper, we present a GPU accelerated simulation of one energy group time-independent deterministic discrete ordinates particle transport in 3D Cartesian geometry (Sweep3D). The performance of the GPU simulations are reported with the simulations of vacuum boundary condition. The discussion of the relative advantages and disadvantages of the GPU implementation, the simulation on multi GPUs, the programming effort and code portability are also reported. The results show that the overall performance speedup of one NVIDIA Tesla M2050 GPU ranges from 2.56 compared with one Intel Xeon X5670 chip to 8.14 compared with one Intel Core Q6600 chip for no flux fixup. The simulation with flux fixup on one M2050 is 1.23 times faster than on one X5670.
Computer simulation and discrete-event models in the analysis of a mammography clinic patient flow.
Coelli, Fernando C; Ferreira, Rodrigo B; Almeida, Renan Moritz V R; Pereira, Wagner Coelho A
2007-09-01
This work develops a discrete-event computer simulation model for the analysis of a mammography clinic performance. Two mammography clinic computer simulation models were developed, based on an existing public sector clinic of the Brazilian Cancer Institute, located in Rio de Janeiro city, Brazil. Two clinics in a total of seven configurations (number of equipment units and working personnel) were studied. Models tried to simulate changes in patient arrival rates, number of equipment units, available personnel (technicians and physicians), equipment maintenance scheduling schemes and exam repeat rates. Model parameters were obtained by direct measurements and literature reviews. A commercially-available simulation software was used for model building. The best patient scheduling (patient arrival rate) for the studied configurations had an average of 29 min for Clinic 1 (consisting of one mammography equipment, one to three technicians and one physician) and 21 min for Clinic 2 (two mammography equipment units, one to four technicians and one physician). The exam repeat rates and equipment maintenance scheduling simulations indicated that a large impact over patient waiting time would appear in the smaller capacity configurations. Discrete-event simulation was a useful tool for defining optimal operating conditions for the studied clinics, indicating the most adequate capacity configurations and equipment maintenance schedules.
GPU accelerated simulations of 3D deterministic particle transport using discrete ordinates method
Gong Chunye; Liu Jie; Chi Lihua; Huang Haowei; Fang Jingyue; Gong Zhenghu
2011-07-01
Graphics Processing Unit (GPU), originally developed for real-time, high-definition 3D graphics in computer games, now provides great faculty in solving scientific applications. The basis of particle transport simulation is the time-dependent, multi-group, inhomogeneous Boltzmann transport equation. The numerical solution to the Boltzmann equation involves the discrete ordinates (S{sub n}) method and the procedure of source iteration. In this paper, we present a GPU accelerated simulation of one energy group time-independent deterministic discrete ordinates particle transport in 3D Cartesian geometry (Sweep3D). The performance of the GPU simulations are reported with the simulations of vacuum boundary condition. The discussion of the relative advantages and disadvantages of the GPU implementation, the simulation on multi GPUs, the programming effort and code portability are also reported. The results show that the overall performance speedup of one NVIDIA Tesla M2050 GPU ranges from 2.56 compared with one Intel Xeon X5670 chip to 8.14 compared with one Intel Core Q6600 chip for no flux fixup. The simulation with flux fixup on one M2050 is 1.23 times faster than on one X5670.
Discrete-event simulation for the design and evaluation of physical protection systems
Jordan, S.E.; Snell, M.K.; Madsen, M.M.; Smith, J.S.; Peters, B.A.
1998-08-01
This paper explores the use of discrete-event simulation for the design and control of physical protection systems for fixed-site facilities housing items of significant value. It begins by discussing several modeling and simulation activities currently performed in designing and analyzing these protection systems and then discusses capabilities that design/analysis tools should have. The remainder of the article then discusses in detail how some of these new capabilities have been implemented in software to achieve a prototype design and analysis tool. The simulation software technology provides a communications mechanism between a running simulation and one or more external programs. In the prototype security analysis tool, these capabilities are used to facilitate human-in-the-loop interaction and to support a real-time connection to a virtual reality (VR) model of the facility being analyzed. This simulation tool can be used for both training (in real-time mode) and facility analysis and design (in fast mode).
A Framework for the Optimization of Discrete-Event Simulation Models
NASA Technical Reports Server (NTRS)
Joshi, B. D.; Unal, R.; White, N. H.; Morris, W. D.
1996-01-01
With the growing use of computer modeling and simulation, in all aspects of engineering, the scope of traditional optimization has to be extended to include simulation models. Some unique aspects have to be addressed while optimizing via stochastic simulation models. The optimization procedure has to explicitly account for the randomness inherent in the stochastic measures predicted by the model. This paper outlines a general purpose framework for optimization of terminating discrete-event simulation models. The methodology combines a chance constraint approach for problem formulation, together with standard statistical estimation and analyses techniques. The applicability of the optimization framework is illustrated by minimizing the operation and support resources of a launch vehicle, through a simulation model.
Biess, Armin; Nagurka, Mark; Flash, Tamar
2006-07-01
An optimization approach applied to mechanical linkage models is used to simulate human arm movements. Predicted arm trajectories are the result of minimizing a nonlinear performance index that depends on kinematic or dynamic variables of the movement. A robust optimization algorithm is presented that computes trajectories which satisfy the necessary conditions with high accuracy. It is especially adapted to the analysis of discrete and rhythmic movements. The optimization problem is solved by parameterizing each generalized coordinate (e.g., joint angular displacement) in terms of Jacobi polynomials and Fourier series, depending on whether discrete or rhythmic movements are considered, combined with a multiple shooting algorithm. The parameterization of coordinates has two advantages. First, it provides an initial guess for the multiple shooting algorithm which solves the optimization problem with high accuracy. Second, it leads to a low dimensional representation of discrete and rhythmic movements in terms of expansion coefficients. The selection of a suitable feature space is an important prerequisite for comparison, recognition and classification of movements. In addition, the separate computational analysis of discrete and rhythmic movements is motivated by their distinct neurophysiological realizations in the cortex. By investigating different performance indices subject to different boundary conditions, the approach can be used to examine possible strategies that humans adopt in selecting specific arm motions for the performance of different tasks in a plane and in three-dimensional space.
An extension of the OpenModelica compiler for using Modelica models in a discrete event simulation
Nutaro, James
2014-11-03
In this article, a new back-end and run-time system is described for the OpenModelica compiler. This new back-end transforms a Modelica model into a module for the adevs discrete event simulation package, thereby extending adevs to encompass complex, hybrid dynamical systems. The new run-time system that has been built within the adevs simulation package supports models with state-events and time-events and that comprise differential-algebraic systems with high index. Finally, although the procedure for effecting this transformation is based on adevs and the Discrete Event System Specification, it can be adapted to any discrete event simulation package.
Eisenhauer, Philipp; Heckman, James J.; Mosso, Stefano
2015-01-01
We compare the performance of maximum likelihood (ML) and simulated method of moments (SMM) estimation for dynamic discrete choice models. We construct and estimate a simplified dynamic structural model of education that captures some basic features of educational choices in the United States in the 1980s and early 1990s. We use estimates from our model to simulate a synthetic dataset and assess the ability of ML and SMM to recover the model parameters on this sample. We investigate the performance of alternative tuning parameters for SMM. PMID:26494926
Simulation of Feynman-alpha measurements from SILENE reactor using a discrete ordinates code
Humbert, P.; Mechitoua, B.; Verrey, B.
2006-07-01
In this paper we present the simulation of Feynman-{alpha} measurements from SILENE reactor using the discrete ordinates code PANDA. A 2-D cylindrical model of SILENE reactor is designed for computer simulations. Two methods are implemented for variance to mean calculation. In the first method we used the Feynman point reactor formula where the parameters (Diven factor, reactivity, detector efficiency and alpha eigenvalue) are obtained by 2-D PANDA calculations. In the second method the time dependent adjoint equations for the first two moments are solved. The calculated results are compared to the measurements. Both methods are in excellent agreement with the experimental data. (authors)
Discrete event simulation of the Defense Waste Processing Facility (DWPF) analytical laboratory
Shanahan, K.L.
1992-02-01
A discrete event simulation of the Savannah River Site (SRS) Defense Waste Processing Facility (DWPF) analytical laboratory has been constructed in the GPSS language. It was used to estimate laboratory analysis times at process analytical hold points and to study the effect of sample number on those times. Typical results are presented for three different simultaneous representing increasing levels of complexity, and for different sampling schemes. Example equipment utilization time plots are also included. SRS DWPF laboratory management and chemists found the simulations very useful for resource and schedule planning.
Incorporating discrete event simulation into quality improvement efforts in health care systems.
Rutberg, Matthew Harris; Wenczel, Sharon; Devaney, John; Goldlust, Eric Jonathan; Day, Theodore Eugene
2015-01-01
Quality improvement (QI) efforts are an indispensable aspect of health care delivery, particularly in an environment of increasing financial and regulatory pressures. The ability to test predictions of proposed changes to flow, policy, staffing, and other process-level changes using discrete event simulation (DES) has shown significant promise and is well reported in the literature. This article describes how to incorporate DES into QI departments and programs in order to support QI efforts, develop high-fidelity simulation models, conduct experiments, make recommendations, and support adoption of results. The authors describe how DES-enabled QI teams can partner with clinical services and administration to plan, conduct, and sustain QI investigations.
The Discrete Multi-Hybrid System for the Simulation of Solid-Liquid Flows
Alexiadis, Alessio
2015-01-01
This study proposes a model based on the combination of Smoothed Particle Hydrodynamics, Coarse Grained Molecular Dynamics and the Discrete Element Method for the simulation of dispersed solid-liquid flows. The model can deal with a large variety of particle types (non-spherical, elastic, breakable, melting, solidifying, swelling), flow conditions (confined, free-surface, microscopic), and scales (from microns to meters). Various examples, ranging from biological fluids to lava flows, are simulated and discussed. In all cases, the model captures the most important features of the flow. PMID:25961561
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.
Visual Data-Analytics of Large-Scale Parallel Discrete-Event Simulations
Ross, Caitlin; Carothers, Christopher D.; Mubarak, Misbah; Carns, Philip; Ross, Robert; Li, Jianping Kelvin; Ma, Kwan-Liu
2016-11-13
Parallel discrete-event simulation (PDES) is an important tool in the codesign of extreme-scale systems because PDES provides a cost-effective way to evaluate designs of highperformance computing systems. Optimistic synchronization algorithms for PDES, such as Time Warp, allow events to be processed without global synchronization among the processing elements. A rollback mechanism is provided when events are processed out of timestamp order. Although optimistic synchronization protocols enable the scalability of large-scale PDES, the performance of the simulations must be tuned to reduce the number of rollbacks and provide an improved simulation runtime. To enable efficient large-scale optimistic simulations, one has to gain insight into the factors that affect the rollback behavior and simulation performance. We developed a tool for ROSS model developers that gives them detailed metrics on the performance of their large-scale optimistic simulations at varying levels of simulation granularity. Model developers can use this information for parameter tuning of optimistic simulations in order to achieve better runtime and fewer rollbacks. In this work, we instrument the ROSS optimistic PDES framework to gather detailed statistics about the simulation engine. We have also developed an interactive visualization interface that uses the data collected by the ROSS instrumentation to understand the underlying behavior of the simulation engine. The interface connects real time to virtual time in the simulation and provides the ability to view simulation data at different granularities. We demonstrate the usefulness of our framework by performing a visual analysis of the dragonfly network topology model provided by the CODES simulation framework built on top of ROSS. The instrumentation needs to minimize overhead in order to accurately collect data about the simulation performance. To ensure that the instrumentation does not introduce unnecessary overhead, we perform a
A statistical analysis of the elastic distortion and dislocation density fields in deformed crystals
NASA Astrophysics Data System (ADS)
Mohamed, Mamdouh S.; Larson, Bennett C.; Tischler, Jonathan Z.; El-Azab, Anter
2015-09-01
The statistical properties of the elastic distortion fields of dislocations in deforming crystals are investigated using the method of discrete dislocation dynamics to simulate dislocation structures and dislocation density evolution under tensile loading. Probability distribution functions (PDF) and pair correlation functions (PCF) of the simulated internal elastic strains and lattice rotations are generated for tensile strain levels up to 0.85%. The PDFs of simulated lattice rotation are compared with sub-micrometer resolution three-dimensional X-ray microscopy measurements of rotation magnitudes and deformation length scales in 1.0% and 2.3% compression strained Cu single crystals to explore the linkage between experiment and the theoretical analysis. The statistical properties of the deformation simulations are analyzed through determinations of the Nye and Kröner dislocation density tensors. The significance of the magnitudes and the length scales of the elastic strain and the rotation parts of dislocation density tensors are demonstrated, and their relevance to understanding the fundamental aspects of deformation is discussed.
A statistical analysis of the elastic distortion and dislocation density fields in deformed crystals
Mohamed, Mamdouh S.; Larson, Bennett C.; Tischler, Jonathan Z.; ...
2015-05-18
The statistical properties of the elastic distortion fields of dislocations in deforming crystals are investigated using the method of discrete dislocation dynamics to simulate dislocation structures and dislocation density evolution under tensile loading. Probability distribution functions (PDF) and pair correlation functions (PCF) of the simulated internal elastic strains and lattice rotations are generated for tensile strain levels up to 0.85%. The PDFs of simulated lattice rotation are compared with sub-micrometer resolution three-dimensional X-ray microscopy measurements of rotation magnitudes and deformation length scales in 1.0% and 2.3% compression strained Cu single crystals to explore the linkage between experiment and the theoreticalmore » analysis. The statistical properties of the deformation simulations are analyzed through determinations of the Nye and Kr ner dislocation density tensors. The significance of the magnitudes and the length scales of the elastic strain and the rotation parts of dislocation density tensors are demonstrated, and their relevance to understanding the fundamental aspects of deformation is discussed.« less
A statistical analysis of the elastic distortion and dislocation density fields in deformed crystals
Mohamed, Mamdouh S.; Larson, Bennett C.; Tischler, Jonathan Z.; El-Azab, Anter
2015-05-18
The statistical properties of the elastic distortion fields of dislocations in deforming crystals are investigated using the method of discrete dislocation dynamics to simulate dislocation structures and dislocation density evolution under tensile loading. Probability distribution functions (PDF) and pair correlation functions (PCF) of the simulated internal elastic strains and lattice rotations are generated for tensile strain levels up to 0.85%. The PDFs of simulated lattice rotation are compared with sub-micrometer resolution three-dimensional X-ray microscopy measurements of rotation magnitudes and deformation length scales in 1.0% and 2.3% compression strained Cu single crystals to explore the linkage between experiment and the theoretical analysis. The statistical properties of the deformation simulations are analyzed through determinations of the Nye and Kr ner dislocation density tensors. The significance of the magnitudes and the length scales of the elastic strain and the rotation parts of dislocation density tensors are demonstrated, and their relevance to understanding the fundamental aspects of deformation is discussed.
A practical discrete-adjoint method for high-fidelity compressible turbulence simulations
NASA Astrophysics Data System (ADS)
Vishnampet, Ramanathan; Bodony, Daniel J.; Freund, Jonathan B.
2015-03-01
Methods and computing hardware advances have enabled accurate predictions of complex compressible turbulence phenomena, such as the generation of jet noise that motivates the present effort. However, limited understanding of underlying physical mechanisms restricts the utility of such predictions since they do not, by themselves, indicate a route to design improvements. Gradient-based optimization using adjoints can circumvent the flow complexity to guide designs, though this is predicated on the availability of a sufficiently accurate solution of the forward and adjoint systems. These are challenging to obtain, since both the chaotic character of the turbulence and the typical use of discretizations near their resolution limits in order to efficiently represent its smaller scales will amplify any approximation errors made in the adjoint formulation. Formulating a practical exact adjoint that avoids such errors is especially challenging if it is to be compatible with state-of-the-art simulation methods used for the turbulent flow itself. Automatic differentiation (AD) can provide code to calculate a nominally exact adjoint, but existing general-purpose AD codes are inefficient to the point of being prohibitive for large-scale turbulence simulations. Here, we analyze the compressible flow equations as discretized using the same high-order workhorse methods used for many high-fidelity compressible turbulence simulations, and formulate a practical space-time discrete-adjoint method without changing the basic discretization. A key step is the definition of a particular discrete analog of the continuous norm that defines our cost functional; our selection leads directly to an efficient Runge-Kutta-like scheme, though it would be just first-order accurate if used outside the adjoint formulation for time integration, with finite-difference spatial operators for the adjoint system. Its computational cost only modestly exceeds that of the flow equations. We confirm that its
A practical discrete-adjoint method for high-fidelity compressible turbulence simulations
Vishnampet, Ramanathan; Bodony, Daniel J.; Freund, Jonathan B.
2015-03-15
Methods and computing hardware advances have enabled accurate predictions of complex compressible turbulence phenomena, such as the generation of jet noise that motivates the present effort. However, limited understanding of underlying physical mechanisms restricts the utility of such predictions since they do not, by themselves, indicate a route to design improvements. Gradient-based optimization using adjoints can circumvent the flow complexity to guide designs, though this is predicated on the availability of a sufficiently accurate solution of the forward and adjoint systems. These are challenging to obtain, since both the chaotic character of the turbulence and the typical use of discretizations near their resolution limits in order to efficiently represent its smaller scales will amplify any approximation errors made in the adjoint formulation. Formulating a practical exact adjoint that avoids such errors is especially challenging if it is to be compatible with state-of-the-art simulation methods used for the turbulent flow itself. Automatic differentiation (AD) can provide code to calculate a nominally exact adjoint, but existing general-purpose AD codes are inefficient to the point of being prohibitive for large-scale turbulence simulations. Here, we analyze the compressible flow equations as discretized using the same high-order workhorse methods used for many high-fidelity compressible turbulence simulations, and formulate a practical space–time discrete-adjoint method without changing the basic discretization. A key step is the definition of a particular discrete analog of the continuous norm that defines our cost functional; our selection leads directly to an efficient Runge–Kutta-like scheme, though it would be just first-order accurate if used outside the adjoint formulation for time integration, with finite-difference spatial operators for the adjoint system. Its computational cost only modestly exceeds that of the flow equations. We confirm that
Mechanical discrete simulator of the electro-mechanical lift with n:1 roping
NASA Astrophysics Data System (ADS)
Alonso, F. J.; Herrera, I.
2016-05-01
The design process of new products in lift engineering is a difficult task due to, mainly, the complexity and slenderness of the lift system, demanding a predictive tool for the lift mechanics. A mechanical ad-hoc discrete simulator, as an alternative to ‘general purpose’ mechanical simulators is proposed. Firstly, the synthesis and experimentation process that has led to establish a suitable model capable of simulating accurately the response of the electromechanical lift is discussed. Then, the equations of motion are derived. The model comprises a discrete system of 5 vertically displaceable masses (car, counterweight, car frame, passengers/loads and lift drive), an inertial mass of the assembly tension pulley-rotor shaft which can rotate about the machine axis and 6 mechanical connectors with 1:1 suspension layout. The model is extended to any n:1 roping lift by setting 6 equivalent mechanical components (suspension systems for car and counterweight, lift drive silent blocks, tension pulley-lift drive stator and passengers/load equivalent spring-damper) by inductive inference from 1:1 and generalized 2:1 roping system. The application to simulate real elevator systems is proposed by numeric time integration of the governing equations using the Kutta-Meden algorithm and implemented in a computer program for ad-hoc elevator simulation called ElevaCAD.
Discrete event simulation as a tool in optimization of a professional complex adaptive system.
Nielsen, Anders Lassen; Hilwig, Helmer; Kissoon, Niranjan; Teelucksingh, Surujpal
2008-01-01
Similar urgent needs for improvement of health care systems exist in the developed and developing world. The culture and the organization of an emergency department in developing countries can best be described as a professional complex adaptive system, where each agent (employee) are ignorant of the behavior of the system as a whole; no one understands the entire system. Each agent's action is based on the state of the system at the moment (i.e. lack of medicine, unavailable laboratory investigation, lack of beds and lack of staff in certain functions). An important question is how one can improve the emergency service within the given constraints. The use of simulation signals is one new approach in studying issues amenable to improvement. Discrete event simulation was used to simulate part of the patient flow in an emergency department. A simple model was built using a prototyping approach. The simulation showed that a minor rotation among the nurses could reduce the mean number of visitors that had to be refereed to alternative flows within the hospital from 87 to 37 on a daily basis with a mean utilization of the staff between 95.8% (the nurses) and 87.4% (the doctors). We conclude that even faced with resource constraints and lack of accessible data discrete event simulation is a tool that can be used successfully to study the consequences of changes in very complex and self organizing professional complex adaptive systems.
Discrete Particle Simulation Techniques for the Analysis of Colliding and Flowing Particulate Media
NASA Astrophysics Data System (ADS)
Mukherjee, Debanjan
Flowing particulate media are ubiquitous in a wide spectrum of applications that include transport systems, fluidized beds, manufacturing and materials processing technologies, energy conversion and propulsion technologies, sprays, jets, slurry flows, and biological flows. The discrete nature of the media, along with their underlying coupled multi-physical interactions can lead to a variety of interesting phenomena, many of which are unique to such media - for example, turbulent diffusion and preferential concentration in particle laden flows, and soliton like excitation patterns in a vibrated pile of granular material. This dissertation explores the utility of numerical simulations based on the discrete element method and collision driven particle dynamics methods for analyzing flowing particulate media. Such methods are well-suited to handle phenomena involving particulate, granular, and discontinuous materials, and often provide abilities to tackle complicated physical phenomena, for which pursuing continuum based approaches might be difficult or sometimes insufficient. A detailed discussion on hierarchically representing coupled, multi-physical phenomena through simple models for underlying physical interactions is presented. Appropriate physical models for mechanical contact, conductive and convective heat exchange, fluid-particle interactions, adhesive and near-field effects, and interaction with applied electromagnetic fields are presented. Algorithmic details on assembling the interaction models into a large-scale simulation framework have been elaborated with illustrations. The assembled frameworks were used to develop a computer simulation library (named `Software Library for Discrete Element Simulations' (SLIDES) for the sake of reference and continued future development efforts) and aspects of the architecture and development of this library have also been addressed. This is an object-oriented discrete particle simulation library developed in Fortran
NASA Astrophysics Data System (ADS)
Abushaikha, Ahmad S.; Voskov, Denis V.; Tchelepi, Hamdi A.
2017-10-01
We present a new fully-implicit, mixed-hybrid, finite-element (MHFE) discretization scheme for general-purpose compositional reservoir simulation. The locally conservative scheme solves the coupled momentum and mass balance equations simultaneously, and the fluid system is modeled using a cubic equation-of-state. We introduce a new conservative flux approach for the mass balance equations for this fully-implicit approach. We discuss the nonlinear solution procedure for the proposed approach, and we present extensive numerical tests to demonstrate the convergence and accuracy of the MHFE method using tetrahedral elements. We also compare the method to other advanced discretization schemes for unstructured meshes and tensor permeability. Finally, we illustrate the applicability and robustness of the method for highly heterogeneous reservoirs with unstructured grids.
Discrete element simulation of powder compaction in cold uniaxial pressing with low pressure
NASA Astrophysics Data System (ADS)
Rojek, Jerzy; Nosewicz, Szymon; Jurczak, Kamila; Chmielewski, Marcin; Bochenek, Kamil; Pietrzak, Katarzyna
2016-11-01
This paper presents numerical studies of powder compaction in cold uniaxial pressing. The powder compaction in this work is considered as an initial stage of a hot pressing process so it is realized with relatively low pressure (up to 50 MPa). Hence the attention has been focused on the densification mechanisms at this range of pressure and models suitable for these conditions. The discrete element method employing spherical particles has been used in the numerical studies. Numerical simulations have been performed for two different contact models—the elastic Hertz-Mindlin-Deresiewicz model and the plastic Storåkers model. Numerical results have been compared with the results of laboratory tests of the die compaction of the NiAl powder. Comparisons have shown that the discrete element method is capable to represent properly the densification mechanisms by the particle rearrangement and particle deformation.
Discrete simulation of the dynamics of spread of extreme opinions in a society
NASA Astrophysics Data System (ADS)
Stauffer, Dietrich; Sahimi, Muhammad
2006-05-01
We propose a discrete model for how opinions about a given “extreme” subject, about which various groups of a population have different degrees of enthusiasm for or susceptibility to, such as fanaticism, extreme social and political positions, and terrorism, may spread. The model, in a certain limit, is the discrete analogue of a deterministic continuum model suggested by others. We carry out extensive computer simulation of the model by utilizing it on lattices with infinite- or short-range interactions, and on symmetric and hierarchical (or directed) Barabási-Albert scale-free networks. Several interesting features of the model are demonstrated, and comparison is made with the deterministic continuum model.
Schenck, Robert C.; Richter, Dustin L.; Wascher, Daniel C.
2014-01-01
Background: Traumatic knee dislocation is becoming more prevalent because of improved recognition and increased exposure to high-energy trauma, but long-term results are lacking. Purpose: To present 2 cases with minimum 20-year follow-up and a review of the literature to illustrate some of the fundamental principles in the management of the dislocated knee. Study Design: Review and case reports. Methods: Two patients with knee dislocations who underwent multiligamentous knee reconstruction were reviewed, with a minimum 20-year follow-up. These patients were brought back for a clinical evaluation using both subjective and objective measures. Subjective measures include the following scales: Lysholm, Tegner activity, visual analog scale (VAS), Short Form–36 (SF-36), International Knee Documentation Committee (IKDC), and a psychosocial questionnaire. Objective measures included ligamentous examination, radiographic evaluation (including Telos stress radiographs), and physical therapy assessment of function and stability. Results: The mean follow-up was 22 years. One patient had a vascular injury requiring repair prior to ligament reconstruction. The average assessment scores were as follows: SF-36 physical health, 52; SF-36 mental health, 59; Lysholm, 92; IKDC, 86.5; VAS involved, 10.5 mm; and VAS uninvolved, 2.5 mm. Both patients had excellent stability and were functioning at high levels of activity for their age (eg, hiking, skydiving). Both patients had radiographic signs of arthritis, which lowered 1 subject’s IKDC score to “C.” Conclusion: Knee dislocations have rare long-term excellent results, and most intermediate-term studies show fair to good functional results. By following fundamental principles in the management of a dislocated knee, patients can be given the opportunity to function at high levels. Hopefully, continued advances in the evaluation and treatment of knee dislocations will improve the long-term outcomes for these patients in the
Discrete event simulation tool for analysis of qualitative models of continuous processing systems
NASA Technical Reports Server (NTRS)
Malin, Jane T. (Inventor); Basham, Bryan D. (Inventor); Harris, Richard A. (Inventor)
1990-01-01
An artificial intelligence design and qualitative modeling tool is disclosed for creating computer models and simulating continuous activities, functions, and/or behavior using developed discrete event techniques. Conveniently, the tool is organized in four modules: library design module, model construction module, simulation module, and experimentation and analysis. The library design module supports the building of library knowledge including component classes and elements pertinent to a particular domain of continuous activities, functions, and behavior being modeled. The continuous behavior is defined discretely with respect to invocation statements, effect statements, and time delays. The functionality of the components is defined in terms of variable cluster instances, independent processes, and modes, further defined in terms of mode transition processes and mode dependent processes. Model construction utilizes the hierarchy of libraries and connects them with appropriate relations. The simulation executes a specialized initialization routine and executes events in a manner that includes selective inherency of characteristics through a time and event schema until the event queue in the simulator is emptied. The experimentation and analysis module supports analysis through the generation of appropriate log files and graphics developments and includes the ability of log file comparisons.
Dislocation dynamics and core structure
NASA Astrophysics Data System (ADS)
Lin, Karin Shu
2000-10-01
Understanding the dynamics of dislocations is essential to the accurate prediction of the mechanical properties of materials. In recent years, considerable progress has been made in this area through the development of large computer simulations which seek to model plastic deformation by considering the interactions of many dislocations. However, the many-body nature of the problem, as well as the limitations inherent in the elasticity theory used to describe dislocation interactions, requires that such simulations make certain simplifying assumptions. The work reported here seeks to examine some of the issues relevant to these simulations in two ways. First, the dynamics of a single dislocation are studied through the development and analysis of a mesoscopic, two-dimensional kinetic Monte Carlo simulation of dislocation motion. The stress and temperature dependence of the dislocation velocity is studied, and finite-size effects are discussed. Through a simple analogy to models of crystal growth, it is shown that the simulated dislocations exhibit kinetic roughening with scaling exponents predicted by the Kardar-Parisi-Zhang equation. Second, the structure of dislocation cores is studied at the atomic level in diamond cubic materials. These studies are necessary for understanding dislocation properties at small distances, and can provide accurate parameters for use in larger scale continuum simulations. The first study uses periodic supercells and ab initio techniques to compare two possible reconstructions of the 90° partial dislocation core in diamond. The relative energies are found to depend upon the stress field experienced by a dislocation in the periodic array. By fitting the energies to an isotropic elasticity theory expression for the dislocation core energy, values for the core radius and shear modulus of diamond are extracted and found to agree well with theoretical estimates and experimental observations. A similar analysis using empirical potentials
Derlet, P. M.; Gilbert, M. R.; Dudarev, S. L.
2011-10-01
Nanoscale prismatic loops are modeled via a partial stochastic differential equation that describes an overdamped continuum elastic string, with a view to describing both the internal and collective dynamics of the loop as a function of temperature. Within the framework of the Langevin equation, expressions are derived that relate the empirical parameters of the model, the friction per unit length, and the elastic stiffness per unit length, to observables that can be obtained directly via molecular-dynamics simulations of interstitial or vacancy prismatic loop mobility. The resulting expressions naturally exhibit the properties that the collective diffusion coefficient of the loop (i) scales inversely with the square root of the number of interstitials, a feature that has been observed in both atomistic simulation and in situ TEM investigations of loop mobility, and (ii) the collective diffusion coefficient is not at all dependent on the internal interactions within the loop, thus qualitatively rationalizing past simulation results showing that the characteristic migration energy barrier is comparable to that of a single interstitial, and cluster migration is a result of individual (but correlated) interstitial activity.
DeMO: An Ontology for Discrete-event Modeling and Simulation
Silver, Gregory A; Miller, John A; Hybinette, Maria; Baramidze, Gregory; York, William S
2011-01-01
Several fields have created ontologies for their subdomains. For example, the biological sciences have developed extensive ontologies such as the Gene Ontology, which is considered a great success. Ontologies could provide similar advantages to the Modeling and Simulation community. They provide a way to establish common vocabularies and capture knowledge about a particular domain with community-wide agreement. Ontologies can support significantly improved (semantic) search and browsing, integration of heterogeneous information sources, and improved knowledge discovery capabilities. This paper discusses the design and development of an ontology for Modeling and Simulation called the Discrete-event Modeling Ontology (DeMO), and it presents prototype applications that demonstrate various uses and benefits that such an ontology may provide to the Modeling and Simulation community. PMID:22919114
DeMO: An Ontology for Discrete-event Modeling and Simulation.
Silver, Gregory A; Miller, John A; Hybinette, Maria; Baramidze, Gregory; York, William S
2011-09-01
Several fields have created ontologies for their subdomains. For example, the biological sciences have developed extensive ontologies such as the Gene Ontology, which is considered a great success. Ontologies could provide similar advantages to the Modeling and Simulation community. They provide a way to establish common vocabularies and capture knowledge about a particular domain with community-wide agreement. Ontologies can support significantly improved (semantic) search and browsing, integration of heterogeneous information sources, and improved knowledge discovery capabilities. This paper discusses the design and development of an ontology for Modeling and Simulation called the Discrete-event Modeling Ontology (DeMO), and it presents prototype applications that demonstrate various uses and benefits that such an ontology may provide to the Modeling and Simulation community.
Discrete fracture simulations of the hydrogeology at Koongarra, Northern Territory, Australia
Smoot, J.L.
1992-04-01
The US Department of Energy is studying the Alligator Rivers Natural Analogue Project site at Koongarra, Northern Territory, Australia to investigate and simulate radionuclide migration in fractured rocks. Discrete fracture simulations were conducted within a cubic volume (180-m edge length) of fractured Cahill Formation schist oriented with one major axis parallel to the trend of the Koongarra Fault. Five hundred fractures are simulated within this domain. The fractures have a mean orientation parallel to the idealized plane of the Koongarra Fault dipping 55{degrees} SE. Simple flow modeling of this fracture network was conducted by assigning constant head boundaries to upgradient and downgradient vertical faces of the cube, which trend parallel to the fault. No-flow boundaries were assigned to all other faces. The fracture network allows hydraulic communication across the block, in spite of relatively low fracture density across the block.
Discrete fracture simulations of the hydrogeology at Koongarra, Northern Territory, Australia
Smoot, J.L.
1992-04-01
The US Department of Energy is studying the Alligator Rivers Natural Analogue Project site at Koongarra, Northern Territory, Australia to investigate and simulate radionuclide migration in fractured rocks. Discrete fracture simulations were conducted within a cubic volume (180-m edge length) of fractured Cahill Formation schist oriented with one major axis parallel to the trend of the Koongarra Fault. Five hundred fractures are simulated within this domain. The fractures have a mean orientation parallel to the idealized plane of the Koongarra Fault dipping 55{degrees} SE. Simple flow modeling of this fracture network was conducted by assigning constant head boundaries to upgradient and downgradient vertical faces of the cube, which trend parallel to the fault. No-flow boundaries were assigned to all other faces. The fracture network allows hydraulic communication across the block, in spite of relatively low fracture density across the block.
Statistical and Probabilistic Extensions to Ground Operations' Discrete Event Simulation Modeling
NASA Technical Reports Server (NTRS)
Trocine, Linda; Cummings, Nicholas H.; Bazzana, Ashley M.; Rychlik, Nathan; LeCroy, Kenneth L.; Cates, Grant R.
2010-01-01
NASA's human exploration initiatives will invest in technologies, public/private partnerships, and infrastructure, paving the way for the expansion of human civilization into the solar system and beyond. As it is has been for the past half century, the Kennedy Space Center will be the embarkation point for humankind's journey into the cosmos. Functioning as a next generation space launch complex, Kennedy's launch pads, integration facilities, processing areas, launch and recovery ranges will bustle with the activities of the world's space transportation providers. In developing this complex, KSC teams work through the potential operational scenarios: conducting trade studies, planning and budgeting for expensive and limited resources, and simulating alternative operational schemes. Numerous tools, among them discrete event simulation (DES), were matured during the Constellation Program to conduct such analyses with the purpose of optimizing the launch complex for maximum efficiency, safety, and flexibility while minimizing life cycle costs. Discrete event simulation is a computer-based modeling technique for complex and dynamic systems where the state of the system changes at discrete points in time and whose inputs may include random variables. DES is used to assess timelines and throughput, and to support operability studies and contingency analyses. It is applicable to any space launch campaign and informs decision-makers of the effects of varying numbers of expensive resources and the impact of off nominal scenarios on measures of performance. In order to develop representative DES models, methods were adopted, exploited, or created to extend traditional uses of DES. The Delphi method was adopted and utilized for task duration estimation. DES software was exploited for probabilistic event variation. A roll-up process was used, which was developed to reuse models and model elements in other less - detailed models. The DES team continues to innovate and expand
NASA Astrophysics Data System (ADS)
Maginnis, P. A.; West, M.; Dullerud, G. E.
2016-10-01
We propose an algorithm to accelerate Monte Carlo simulation for a broad class of stochastic processes. Specifically, the class of countable-state, discrete-time Markov chains driven by additive Poisson noise, or lattice discrete-time Markov chains. In particular, this class includes simulation of reaction networks via the tau-leaping algorithm. To produce the speedup, we simulate pairs of fair-draw trajectories that are negatively correlated. Thus, when averaged, these paths produce an unbiased Monte Carlo estimator that has reduced variance and, therefore, reduced error. Numerical results for three example systems included in this work demonstrate two to four orders of magnitude reduction of mean-square error. The numerical examples were chosen to illustrate different application areas and levels of system complexity. The areas are: gene expression (affine state-dependent rates), aerosol particle coagulation with emission and human immunodeficiency virus infection (both with nonlinear state-dependent rates). Our algorithm views the system dynamics as a ;black-box;, i.e., we only require control of pseudorandom number generator inputs. As a result, typical codes can be retrofitted with our algorithm using only minor changes. We prove several analytical results. Among these, we characterize the relationship of covariances between paths in the general nonlinear state-dependent intensity rates case, and we prove variance reduction of mean estimators in the special case of affine intensity rates.
Jahn, Beate; Theurl, Engelbert; Siebert, Uwe; Pfeiffer, Karl-Peter
2010-01-01
In most decision-analytic models in health care, it is assumed that there is treatment without delay and availability of all required resources. Therefore, waiting times caused by limited resources and their impact on treatment effects and costs often remain unconsidered. Queuing theory enables mathematical analysis and the derivation of several performance measures of queuing systems. Nevertheless, an analytical approach with closed formulas is not always possible. Therefore, simulation techniques are used to evaluate systems that include queuing or waiting, for example, discrete event simulation. To include queuing in decision-analytic models requires a basic knowledge of queuing theory and of the underlying interrelationships. This tutorial introduces queuing theory. Analysts and decision-makers get an understanding of queue characteristics, modeling features, and its strength. Conceptual issues are covered, but the emphasis is on practical issues like modeling the arrival of patients. The treatment of coronary artery disease with percutaneous coronary intervention including stent placement serves as an illustrative queuing example. Discrete event simulation is applied to explicitly model resource capacities, to incorporate waiting lines and queues in the decision-analytic modeling example.
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)
Ryerson, F. J.; Ezzedine, S. M.; Glascoe, L. G.; Antoun, T. H.
2011-12-01
Fractures and fracture networks are the principle pathways for migration of water, heat and mass in enhanced geothermal systems, oil and gas reservoirs, CO2 leakage from saline aquifers, and radioactive and toxic industrial wastes from underground storage repositories. A major issue to overcome when characterizing a fractured reservoir is that of data limitation due to accessibility and affordability. Moreover, the ability to map discontinuities in the rock with available geological and geophysical tools tends to decrease particularly as the scale of the discontinuity goes down. Data collected are often reduced to probability distribution functions for predictive modeling and simulation in a stochastic framework such as stochastic discrete fracture network. Stochastic discrete fracture network models enable probabilistic assessment of flow, transport and geomechanical phenomena that are not adequately captured using continuum models. Despite the fundamental uncertainties inherited within the probabilistic reduction of the sparse data collected, very little work has been conducted on quantifying uncertainty on the reduced probabilistic distribution functions. In the current study, we investigate the impact of parameter uncertainties of the distribution functions that characterize discrete fracture networks on the flow, heat and mass transport and geomechanics. Numerical results of first, second and third moments, normalized to a base case scenario, are presented and compared to theoretical results extended from percolation theory. (Prepared by LLNL under Contract DE-AC52-07NA27344)
Armstrong, April
2015-11-01
Simple elbow dislocation refers to those elbow dislocations that do not involve an osseous injury. A complex elbow dislocation refers to an elbow that has dislocated with an osseous injury. Most simple elbow dislocations are treated nonoperatively. Understanding the importance of the soft tissue injury following a simple elbow dislocation is a key to being successful with treatment.
Ho, C K
2009-01-01
Simulations of UV disinfection systems require accurate models of UV radiation within the reactor. Processes such as reflection and refraction at surfaces within the reactor can impact the intensity of the simulated radiation field, which in turn impacts the simulated dose and performance of the UV reactor. This paper describes a detailed discrete ordinates radiation model and comparisons to a test that recorded the UV radiation distribution around a low pressure UV lamp in a water-filled chamber with a UV transmittance of 88%. The effects of reflection and refraction at the quartz sleeve were investigated, along with the impact of wall reflection from the interior surfaces of the chamber. Results showed that the inclusion of wall reflection improved matches between predicted and measured values of incident radiation throughout the chamber. The difference between simulations with and without reflection ranged from several percent near the lamp to nearly 40% further away from the lamp. Neglecting reflection and refraction at the quartz sleeve increased the simulated radiation near the lamp and reduced the simulated radiation further away from the lamp. However, the distribution and trends in the simulated radiation field both with and without the effects of reflection and refraction at the quartz sleeve were consistent with the measured data distributions.
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.
Using Discrete Event Simulation to predict KPI's at a Projected Emergency Room.
Concha, Pablo; Neriz, Liliana; Parada, Danilo; Ramis, Francisco
2015-01-01
Discrete Event Simulation (DES) is a powerful factor in the design of clinical facilities. DES enables facilities to be built or adapted to achieve the expected Key Performance Indicators (KPI's) such as average waiting times according to acuity, average stay times and others. Our computational model was built and validated using expert judgment and supporting statistical data. One scenario studied resulted in a 50% decrease in the average cycle time of patients compared to the original model, mainly by modifying the patient's attention model.
Coupled discrete element and smoothed particle hydrodynamics simulations of the die filling process
NASA Astrophysics Data System (ADS)
Breinlinger, Thomas; Kraft, Torsten
2016-11-01
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.
Thulasidasan, Sunil; Kasiviswanathan, Shiva; Eidenbenz, Stephan; Romero, Philip
2010-01-01
We re-examine the problem of load balancing in conservatively synchronized parallel, discrete-event simulations executed on high-performance computing clusters, focusing on simulations where computational and messaging load tend to be spatially clustered. Such domains are frequently characterized by the presence of geographic 'hot-spots' - regions that generate significantly more simulation events than others. Examples of such domains include simulation of urban regions, transportation networks and networks where interaction between entities is often constrained by physical proximity. Noting that in conservatively synchronized parallel simulations, the speed of execution of the simulation is determined by the slowest (i.e most heavily loaded) simulation process, we study different partitioning strategies in achieving equitable processor-load distribution in domains with spatially clustered load. In particular, we study the effectiveness of partitioning via spatial scattering to achieve optimal load balance. In this partitioning technique, nearby entities are explicitly assigned to different processors, thereby scattering the load across the cluster. This is motivated by two observations, namely, (i) since load is spatially clustered, spatial scattering should, intuitively, spread the load across the compute cluster, and (ii) in parallel simulations, equitable distribution of CPU load is a greater determinant of execution speed than message passing overhead. Through large-scale simulation experiments - both of abstracted and real simulation models - we observe that scatter partitioning, even with its greatly increased messaging overhead, significantly outperforms more conventional spatial partitioning techniques that seek to reduce messaging overhead. Further, even if hot-spots change over the course of the simulation, if the underlying feature of spatial clustering is retained, load continues to be balanced with spatial scattering leading us to the observation that
Chen,Y.; Dudley, M.; Sanchez, E.; Macmillan, M.
2008-01-01
Detailed comparison has been drawn between synchrotron white beam X-ray topographic images of micropipes in 4H-SiC, recorded using pyramidal plane reflections in grazing-incidence geometry, and images simulated using the ray-tracing method. The simulations were carried out with and without the influence of surface relaxation effects. The images simulated in the absence of surface relaxation effects appear as white elliptical shaped features, canted to one side or other of the g-vector (depending on the dislocation sense), surrounded by a dark contrast perimeter which thickens at both ends of the major axis and which exhibits two fold symmetry axes parallel to the major and minor axes. On the other hand, on the images simulated taking into account the effects of surface relaxation, the features are again canted to one side or other of the g-vector (depending on the dislocation sense) but do not exhibit the same symmetry in that one of the sides of the oval shape is flattened (along the major axis) and the distribution of intensity around the perimeter no longer possesses the two fold symmetry axes parallel to the major and minor axes. While the details of the distribution of the dark perimeter contrast observed on the recorded images are not so easy to correlate with the simulated distributions, the sense of cant of the roughly elliptical white features on the observed images can be easily correlated with the simulations. Since the sense of cant has the same behavior as a function of dislocation sense for simulations carried out with and without surface relaxation, this provides a high level of confidence that the sense of cant of these features can be readily used to determine the senses of the micropipes.
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.
Improving Energy Efficiency for the Vehicle Assembly Industry: A Discrete Event Simulation Approach
NASA Astrophysics Data System (ADS)
Oumer, Abduaziz; Mekbib Atnaw, Samson; Kie Cheng, Jack; Singh, Lakveer
2016-11-01
This paper presented a Discrete Event Simulation (DES) model for investigating and improving energy efficiency in vehicle assembly line. The car manufacturing industry is one of the highest energy consuming industries. Using Rockwell Arena DES package; a detailed model was constructed for an actual vehicle assembly plant. The sources of energy considered in this research are electricity and fuel; which are the two main types of energy sources used in a typical vehicle assembly plant. The model depicts the performance measurement for process- specific energy measures of painting, welding, and assembling processes. Sound energy efficiency model within this industry has two-fold advantage: reducing CO2 emission and cost reduction associated with fuel and electricity consumption. The paper starts with an overview of challenges in energy consumption within the facilities of automotive assembly line and highlights the parameters for energy efficiency. The results of the simulation model indicated improvements for energy saving objectives and reduced costs.
NASA Technical Reports Server (NTRS)
Malin, Jane T.; Basham, Bryan D.
1989-01-01
CONFIG is a modeling and simulation tool prototype for analyzing the normal and faulty qualitative behaviors of engineered systems. Qualitative modeling and discrete-event simulation have been adapted and integrated, to support early development, during system design, of software and procedures for management of failures, especially in diagnostic expert systems. Qualitative component models are defined in terms of normal and faulty modes and processes, which are defined by invocation statements and effect statements with time delays. System models are constructed graphically by using instances of components and relations from object-oriented hierarchical model libraries. Extension and reuse of CONFIG models and analysis capabilities in hybrid rule- and model-based expert fault-management support systems are discussed.
A conceptual modeling framework for discrete event simulation using hierarchical control structures
Furian, N.; O’Sullivan, M.; Walker, C.; Vössner, S.; Neubacher, D.
2015-01-01
Conceptual Modeling (CM) is a fundamental step in a simulation project. Nevertheless, it is only recently that structured approaches towards the definition and formulation of conceptual models have gained importance in the Discrete Event Simulation (DES) community. As a consequence, frameworks and guidelines for applying CM to DES have emerged and discussion of CM for DES is increasing. However, both the organization of model-components and the identification of behavior and system control from standard CM approaches have shortcomings that limit CM’s applicability to DES. Therefore, we discuss the different aspects of previous CM frameworks and identify their limitations. Further, we present the Hierarchical Control Conceptual Modeling framework that pays more attention to the identification of a models’ system behavior, control policies and dispatching routines and their structured representation within a conceptual model. The framework guides the user step-by-step through the modeling process and is illustrated by a worked example. PMID:26778940
A conceptual modeling framework for discrete event simulation using hierarchical control structures.
Furian, N; O'Sullivan, M; Walker, C; Vössner, S; Neubacher, D
2015-08-01
Conceptual Modeling (CM) is a fundamental step in a simulation project. Nevertheless, it is only recently that structured approaches towards the definition and formulation of conceptual models have gained importance in the Discrete Event Simulation (DES) community. As a consequence, frameworks and guidelines for applying CM to DES have emerged and discussion of CM for DES is increasing. However, both the organization of model-components and the identification of behavior and system control from standard CM approaches have shortcomings that limit CM's applicability to DES. Therefore, we discuss the different aspects of previous CM frameworks and identify their limitations. Further, we present the Hierarchical Control Conceptual Modeling framework that pays more attention to the identification of a models' system behavior, control policies and dispatching routines and their structured representation within a conceptual model. The framework guides the user step-by-step through the modeling process and is illustrated by a worked example.
PowderSim: Lagrangian Discrete and Mesh-Free Continuum Simulation Code for Cohesive Soils
NASA Technical Reports Server (NTRS)
Johnson, Scott; Walton, Otis; Settgast, Randolph
2013-01-01
PowderSim is a calculation tool that combines a discrete-element method (DEM) module, including calibrated interparticle-interaction relationships, with a mesh-free, continuum, SPH (smoothed-particle hydrodynamics) based module that utilizes enhanced, calibrated, constitutive models capable of mimicking both large deformations and the flow behavior of regolith simulants and lunar regolith under conditions anticipated during in situ resource utilization (ISRU) operations. The major innovation introduced in PowderSim is to use a mesh-free method (SPH-based) with a calibrated and slightly modified critical-state soil mechanics constitutive model to extend the ability of the simulation tool to also address full-scale engineering systems in the continuum sense. The PowderSim software maintains the ability to address particle-scale problems, like size segregation, in selected regions with a traditional DEM module, which has improved contact physics and electrostatic interaction models.
NASA Technical Reports Server (NTRS)
Malin, Jane T.; Basham, Bryan D.
1989-01-01
CONFIG is a modeling and simulation tool prototype for analyzing the normal and faulty qualitative behaviors of engineered systems. Qualitative modeling and discrete-event simulation have been adapted and integrated, to support early development, during system design, of software and procedures for management of failures, especially in diagnostic expert systems. Qualitative component models are defined in terms of normal and faulty modes and processes, which are defined by invocation statements and effect statements with time delays. System models are constructed graphically by using instances of components and relations from object-oriented hierarchical model libraries. Extension and reuse of CONFIG models and analysis capabilities in hybrid rule- and model-based expert fault-management support systems are discussed.
2.5D discrete-dual-porosity model for simulating geoelectrical experiments in fractured rock
NASA Astrophysics Data System (ADS)
Caballero Sanz, Victor; Roubinet, Delphine; Demirel, Serdar; Irving, James
2017-02-01
Previous work has demonstrated that geoelectrical measurements, acquired either along the Earth's surface or in boreholes, can be sensitive to the presence of fractures. However, a lack of numerical approaches that are well suited to modeling electric current flow in fractured media prevents us from systematically exploring the links between geoelectrical measurements and fractured rock properties. To address this issue, we present a highly computationally efficient methodology for the numerical simulation of geoelectrical data in 2.5 dimensions in complex fractured domains. Our approach is based upon a discrete-dual-porosity formulation, whereby the fractures and rock matrix are treated separately and coupled through the exchange of electric current between them. We first validate our methodology against standard analytical and finite-element solutions. Subsequent use of the approach to simulate geoelectrical data for a variety of different fracture configurations demonstrates the sensitivity of these data to important parameters such as the fracture density, depth, and orientation.
NASA Astrophysics Data System (ADS)
Park, Chuljin; Telci, Ilker T.; Kim, Seong-Hee; Aral, Mustafa M.
2014-01-01
The problem of designing a water quality monitoring network for river systems is to find the optimal location of a finite number of monitoring devices that minimizes the expected detection time of a contaminant spill event while guaranteeing good detection reliability. When uncertainties in spill and rain events are considered, both the expected detection time and detection reliability need to be estimated by stochastic simulation. This problem is formulated as a stochastic discrete optimization via simulation (OvS) problem on the expected detection time with a stochastic constraint on detection reliability; and it is solved with an OvS algorithm combined with a recently proposed method called penalty function with memory (PFM). The performance of the algorithm is tested on the Altamaha River and compared with that of a genetic algorithm due to Telci, Nam, Guan and Aral (2009).
Examining Passenger Flow Choke Points at Airports Using Discrete Event Simulation
NASA Technical Reports Server (NTRS)
Brown, Jeremy R.; Madhavan, Poomima
2011-01-01
The movement of passengers through an airport quickly, safely, and efficiently is the main function of the various checkpoints (check-in, security. etc) found in airports. Human error combined with other breakdowns in the complex system of the airport can disrupt passenger flow through the airport leading to lengthy waiting times, missing luggage and missed flights. In this paper we present a model of passenger flow through an airport using discrete event simulation that will provide a closer look into the possible reasons for breakdowns and their implications for passenger flow. The simulation is based on data collected at Norfolk International Airport (ORF). The primary goal of this simulation is to present ways to optimize the work force to keep passenger flow smooth even during peak travel times and for emergency preparedness at ORF in case of adverse events. In this simulation we ran three different scenarios: real world, increased check-in stations, and multiple waiting lines. Increased check-in stations increased waiting time and instantaneous utilization. while the multiple waiting lines decreased both the waiting time and instantaneous utilization. This simulation was able to show how different changes affected the passenger flow through the airport.
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
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.
SPEEDES - A multiple-synchronization environment for parallel discrete-event simulation
NASA Technical Reports Server (NTRS)
Steinman, Jeff S.
1992-01-01
Synchronous Parallel Environment for Emulation and Discrete-Event Simulation (SPEEDES) is a unified parallel simulation environment. It supports multiple-synchronization protocols without requiring users to recompile their code. When a SPEEDES simulation runs on one node, all the extra parallel overhead is removed automatically at run time. When the same executable runs in parallel, the user preselects the synchronization algorithm from a list of options. SPEEDES currently runs on UNIX networks and on the California Institute of Technology/Jet Propulsion Laboratory Mark III Hypercube. SPEEDES also supports interactive simulations. Featured in the SPEEDES environment is a new parallel synchronization approach called Breathing Time Buckets. This algorithm uses some of the conservative techniques found in Time Bucket synchronization, along with the optimism that characterizes the Time Warp approach. A mathematical model derived from first principles predicts the performance of Breathing Time Buckets. Along with the Breathing Time Buckets algorithm, this paper discusses the rules for processing events in SPEEDES, describes the implementation of various other synchronization protocols supported by SPEEDES, describes some new ones for the future, discusses interactive simulations, and then gives some performance results.
SPEEDES - A multiple-synchronization environment for parallel discrete-event simulation
NASA Technical Reports Server (NTRS)
Steinman, Jeff S.
1992-01-01
Synchronous Parallel Environment for Emulation and Discrete-Event Simulation (SPEEDES) is a unified parallel simulation environment. It supports multiple-synchronization protocols without requiring users to recompile their code. When a SPEEDES simulation runs on one node, all the extra parallel overhead is removed automatically at run time. When the same executable runs in parallel, the user preselects the synchronization algorithm from a list of options. SPEEDES currently runs on UNIX networks and on the California Institute of Technology/Jet Propulsion Laboratory Mark III Hypercube. SPEEDES also supports interactive simulations. Featured in the SPEEDES environment is a new parallel synchronization approach called Breathing Time Buckets. This algorithm uses some of the conservative techniques found in Time Bucket synchronization, along with the optimism that characterizes the Time Warp approach. A mathematical model derived from first principles predicts the performance of Breathing Time Buckets. Along with the Breathing Time Buckets algorithm, this paper discusses the rules for processing events in SPEEDES, describes the implementation of various other synchronization protocols supported by SPEEDES, describes some new ones for the future, discusses interactive simulations, and then gives some performance results.
The use of discrete-event simulation modelling to improve radiation therapy planning processes.
Werker, Greg; Sauré, Antoine; French, John; Shechter, Steven
2009-07-01
The planning portion of the radiation therapy treatment process at the British Columbia Cancer Agency is efficient but nevertheless contains room for improvement. The purpose of this study is to show how a discrete-event simulation (DES) model can be used to represent this complex process and to suggest improvements that may reduce the planning time and ultimately reduce overall waiting times. A simulation model of the radiation therapy (RT) planning process was constructed using the Arena simulation software, representing the complexities of the system. Several types of inputs feed into the model; these inputs come from historical data, a staff survey, and interviews with planners. The simulation model was validated against historical data and then used to test various scenarios to identify and quantify potential improvements to the RT planning process. Simulation modelling is an attractive tool for describing complex systems, and can be used to identify improvements to the processes involved. It is possible to use this technique in the area of radiation therapy planning with the intent of reducing process times and subsequent delays for patient treatment. In this particular system, reducing the variability and length of oncologist-related delays contributes most to improving the planning time.
PetriScape - A plugin for discrete Petri net simulations in Cytoscape.
Almeida, Diogo; Azevedo, Vasco; Silva, Artur; Baumbach, Jan
2016-06-04
Systems biology plays a central role for biological network analysis in the post-genomic era. Cytoscape is the standard bioinformatics tool offering the community an extensible platform for computational analysis of the emerging cellular network together with experimental omics data sets. However, only few apps/plugins/tools are available for simulating network dynamics in Cytoscape 3. Many approaches of varying complexity exist but none of them have been integrated into Cytoscape as app/plugin yet. Here, we introduce PetriScape, the first Petri net simulator for Cytoscape. Although discrete Petri nets are quite simplistic models, they are capable of modeling global network properties and simulating their behaviour. In addition, they are easily understood and well visualizable. PetriScape comes with the following main functionalities: (1) import of biological networks in SBML format, (2) conversion into a Petri net, (3) visualization as Petri net, and (4) simulation and visualization of the token flow in Cytoscape. PetriScape is the first Cytoscape plugin for Petri nets. It allows a straightforward Petri net model creation, simulation and visualization with Cytoscape, providing clues about the activity of key components in biological networks.
A parallel program for numerical simulation of discrete fracture network and groundwater flow
NASA Astrophysics Data System (ADS)
Huang, Ting-Wei; Liou, Tai-Sheng; Kalatehjari, Roohollah
2017-04-01
The ability of modeling fluid flow in Discrete Fracture Network (DFN) is critical to various applications such as exploration of reserves in geothermal and petroleum reservoirs, geological sequestration of carbon dioxide and final disposal of spent nuclear fuels. Although several commerical or acdametic DFN flow simulators are already available (e.g., FracMan and DFNWORKS), challenges in terms of computational efficiency and three-dimensional visualization still remain, which therefore motivates this study for developing a new DFN and flow simulator. A new DFN and flow simulator, DFNbox, was written in C++ under a cross-platform software development framework provided by Qt. DFNBox integrates the following capabilities into a user-friendly drop-down menu interface: DFN simulation and clipping, 3D mesh generation, fracture data analysis, connectivity analysis, flow path analysis and steady-state grounwater flow simulation. All three-dimensional visualization graphics were developed using the free OpenGL API. Similar to other DFN simulators, fractures are conceptualized as random point process in space, with stochastic characteristics represented by orientation, size, transmissivity and aperture. Fracture meshing was implemented by Delaunay triangulation for visualization but not flow simulation purposes. Boundary element method was used for flow simulations such that only unknown head or flux along exterior and interection bounaries are needed for solving the flow field in the DFN. Parallel compuation concept was taken into account in developing DFNbox for calculations that such concept is possible. For example, the time-consuming seqential code for fracture clipping calculations has been completely replaced by a highly efficient parallel one. This can greatly enhance compuational efficiency especially on multi-thread platforms. Furthermore, DFNbox have been successfully tested in Windows and Linux systems with equally-well performance.
NASA Astrophysics Data System (ADS)
Lampoudi, Sotiria; Gillespie, Dan T.; Petzold, Linda R.
2009-03-01
The Inhomogeneous Stochastic Simulation Algorithm (ISSA) is a variant of the stochastic simulation algorithm in which the spatially inhomogeneous volume of the system is divided into homogeneous subvolumes, and the chemical reactions in those subvolumes are augmented by diffusive transfers of molecules between adjacent subvolumes. The ISSA can be prohibitively slow when the system is such that diffusive transfers occur much more frequently than chemical reactions. In this paper we present the Multinomial Simulation Algorithm (MSA), which is designed to, on the one hand, outperform the ISSA when diffusive transfer events outnumber reaction events, and on the other, to handle small reactant populations with greater accuracy than deterministic-stochastic hybrid algorithms. The MSA treats reactions in the usual ISSA fashion, but uses appropriately conditioned binomial random variables for representing the net numbers of molecules diffusing from any given subvolume to a neighbor within a prescribed distance. Simulation results illustrate the benefits of the algorithm.
A Study on Discrete Event Simulation (DES) in a High-Level Architecture (HLA) Networked Simulation
2010-12-01
MONITORING AGENCY NAME(S) AND ADDRESS(ES) Singapore Technologies Electronics (Training & Simulation System) Pte Ltd 24 Ang Mo Kio St 65 Singapore...Thesis Co-Advisor Mathias Kölsch Chairman, MOVES Academic Committee Peter J. Denning Chairman, Department of Computer Science iv...and Simulation and broaden his academic spectrum. xiv THIS PAGE INTENTIONALLY LEFT BLANK 1 I. INTRODUCTION A. OVERVIEW Since the evolution of
Buckling of dislocation in graphene
NASA Astrophysics Data System (ADS)
Yao, Yin; Wang, Shaofeng; Bai, Jianhui; Wang, Rui
2016-10-01
The buckling of dislocation in graphene is discussed through the lattice theory of dislocation and elastic theory. The approximate solution of the buckling is obtained based on the inner stress distribution caused by different structure of dislocations and is proved to be suitable by the simulation. The position of the highest buckling is predicted to be at the vertex of the pentagon far away from the heptagon. The buckling is strongly influenced by the internal stress and the distance between the extrusive area and stretching area, as well as the critical stress σc. The SW defect is proved to be unbuckled due to its strong interaction between extrusion and stretching.
The effects of indoor environmental exposures on pediatric asthma: a discrete event simulation model
2012-01-01
Background In the United States, asthma is the most common chronic disease of childhood across all socioeconomic classes and is the most frequent cause of hospitalization among children. Asthma exacerbations have been associated with exposure to residential indoor environmental stressors such as allergens and air pollutants as well as numerous additional factors. Simulation modeling is a valuable tool that can be used to evaluate interventions for complex multifactorial diseases such as asthma but in spite of its flexibility and applicability, modeling applications in either environmental exposures or asthma have been limited to date. Methods We designed a discrete event simulation model to study the effect of environmental factors on asthma exacerbations in school-age children living in low-income multi-family housing. Model outcomes include asthma symptoms, medication use, hospitalizations, and emergency room visits. Environmental factors were linked to percent predicted forced expiratory volume in 1 second (FEV1%), which in turn was linked to risk equations for each outcome. Exposures affecting FEV1% included indoor and outdoor sources of NO2 and PM2.5, cockroach allergen, and dampness as a proxy for mold. Results Model design parameters and equations are described in detail. We evaluated the model by simulating 50,000 children over 10 years and showed that pollutant concentrations and health outcome rates are comparable to values reported in the literature. In an application example, we simulated what would happen if the kitchen and bathroom exhaust fans were improved for the entire cohort, and showed reductions in pollutant concentrations and healthcare utilization rates. Conclusions We describe the design and evaluation of a discrete event simulation model of pediatric asthma for children living in low-income multi-family housing. Our model simulates the effect of environmental factors (combustion pollutants and allergens), medication compliance, seasonality
NASA Astrophysics Data System (ADS)
Wellman, T. P.; Poeter, E. P.
2003-12-01
Fractured aquifers serve as primary water resources throughout the western United States. In light of diminishing water supply, management practices must be improved to promote resource sustainability. Ground-water flow models are often the preferred management tool, but can be computationally expensive and difficult to implement in large-scale fractured environments. Discrete feature network (DFN) simulation is a robust approach for modeling fluid movement in fractured architecture, but numerically expensive for large-scale models. By using an equivalent continuum model (ECM) numerical expense may be substantially reduced. An intrinsic assumption of the ECM approach is that the geologic media is represented accurately as a continuum, requiring that grid scale discretization correspond to representative elementary scale (RES) at each location within a fractured aquifer. Heterogeneity and compartmentalization likely cause regions with large differences in fracture permeability and connectivity, resulting in spatially variable RES. Thus, while regional flow may be honored using essentially any grid pattern, failure to properly represent spatially variable RES could lead to erroneous predictions of local flow and transport, especially in highly heterogeneous zones. The purpose of our study is to determine whether head predictions from DFN flow simulations can delineate spatially variable RES in fractured aquifers. Provided there is a correlation of simulated hydraulic head to continuum scale, we hypothesize that RES can be identified using spatially disperse water level observations within a fractured aquifer watershed. Preliminary results suggest there is potential for using hydraulic head data to determine the RES. Ongoing research is necessary to confirm these preliminary results and our hypothesis.
Developing Flexible Discrete Event Simulation Models in an Uncertain Policy Environment
NASA Technical Reports Server (NTRS)
Miranda, David J.; Fayez, Sam; Steele, Martin J.
2011-01-01
On February 1st, 2010 U.S. President Barack Obama submitted to Congress his proposed budget request for Fiscal Year 2011. This budget included significant changes to the National Aeronautics and Space Administration (NASA), including the proposed cancellation of the Constellation Program. This change proved to be controversial and Congressional approval of the program's official cancellation would take many months to complete. During this same period an end-to-end discrete event simulation (DES) model of Constellation operations was being built through the joint efforts of Productivity Apex Inc. (PAl) and Science Applications International Corporation (SAIC) teams under the guidance of NASA. The uncertainty in regards to the Constellation program presented a major challenge to the DES team, as to: continue the development of this program-of-record simulation, while at the same time remain prepared for possible changes to the program. This required the team to rethink how it would develop it's model and make it flexible enough to support possible future vehicles while at the same time be specific enough to support the program-of-record. This challenge was compounded by the fact that this model was being developed through the traditional DES process-orientation which lacked the flexibility of object-oriented approaches. The team met this challenge through significant pre-planning that led to the "modularization" of the model's structure by identifying what was generic, finding natural logic break points, and the standardization of interlogic numbering system. The outcome of this work resulted in a model that not only was ready to be easily modified to support any future rocket programs, but also a model that was extremely structured and organized in a way that facilitated rapid verification. This paper discusses in detail the process the team followed to build this model and the many advantages this method provides builders of traditional process-oriented discrete
Modeling a Million-Node Slim Fly Network Using Parallel Discrete-Event Simulation
Wolfe, Noah; Carothers, Christopher; Mubarak, Misbah; Ross, Robert; Carns, Philip
2016-05-15
As supercomputers close in on exascale performance, the increased number of processors and processing power translates to an increased demand on the underlying network interconnect. The Slim Fly network topology, a new lowdiameter and low-latency interconnection network, is gaining interest as one possible solution for next-generation supercomputing interconnect systems. In this paper, we present a high-fidelity Slim Fly it-level model leveraging the Rensselaer Optimistic Simulation System (ROSS) and Co-Design of Exascale Storage (CODES) frameworks. We validate our Slim Fly model with the Kathareios et al. Slim Fly model results provided at moderately sized network scales. We further scale the model size up to n unprecedented 1 million compute nodes; and through visualization of network simulation metrics such as link bandwidth, packet latency, and port occupancy, we get an insight into the network behavior at the million-node scale. We also show linear strong scaling of the Slim Fly model on an Intel cluster achieving a peak event rate of 36 million events per second using 128 MPI tasks to process 7 billion events. Detailed analysis of the underlying discrete-event simulation performance shows that a million-node Slim Fly model simulation can execute in 198 seconds on the Intel cluster.
Towards High Performance Discrete-Event Simulations of Smart Electric Grids
Perumalla, Kalyan S; Nutaro, James J; Yoginath, Srikanth B
2011-01-01
Future electric grid technology is envisioned on the notion of a smart grid in which responsive end-user devices play an integral part of the transmission and distribution control systems. Detailed simulation is often the primary choice in analyzing small network designs, and the only choice in analyzing large-scale electric network designs. Here, we identify and articulate the high-performance computing needs underlying high-resolution discrete event simulation of smart electric grid operation large network scenarios such as the entire Eastern Interconnect. We focus on the simulator's most computationally intensive operation, namely, the dynamic numerical solution for the electric grid state, for both time-integration as well as event-detection. We explore solution approaches using general-purpose dense and sparse solvers, and propose a scalable solver specialized for the sparse structures of actual electric networks. Based on experiments with an implementation in the THYME simulator, we identify performance issues and possible solution approaches for smart grid experimentation in the large.
Capacity planning for maternal-fetal medicine using discrete event simulation.
Ferraro, Nicole M; Reamer, Courtney B; Reynolds, Thomas A; Howell, Lori J; Moldenhauer, Julie S; Day, Theodore Eugene
2015-07-01
Maternal-fetal medicine is a rapidly growing field requiring collaboration from many subspecialties. We provide an evidence-based estimate of capacity needs for our clinic, as well as demonstrate how simulation can aid in capacity planning in similar environments. A Discrete Event Simulation of the Center for Fetal Diagnosis and Treatment and Special Delivery Unit at The Children's Hospital of Philadelphia was designed and validated. This model was then used to determine the time until demand overwhelms inpatient bed availability under increasing capacity. No significant deviation was found between historical inpatient censuses and simulated censuses for the validation phase (p = 0.889). Prospectively increasing capacity was found to delay time to balk (the inability of the center to provide bed space for a patient in need of admission). With current capacity, the model predicts mean time to balk of 276 days. Adding three beds delays mean time to first balk to 762 days; an additional six beds to 1,335 days. Providing sufficient access is a patient safety issue, and good planning is crucial for targeting infrastructure investments appropriately. Computer-simulated analysis can provide an evidence base for both medical and administrative decision making in a complex clinical environment. Thieme Medical Publishers 333 Seventh Avenue, New York, NY 10001, USA.
NASA Astrophysics Data System (ADS)
El-Sayed, A. M. A.; Elsonbaty, A.; Elsadany, A. A.; Matouk, A. E.
2016-12-01
This paper presents an analytical framework to investigate the dynamical behavior of a new fractional-order hyperchaotic circuit system. A sufficient condition for existence, uniqueness and continuous dependence on initial conditions of the solution of the proposed system is derived. The local stability of all the system’s equilibrium points are discussed using fractional Routh-Hurwitz test. Then the analytical conditions for the existence of a pitchfork bifurcation in this system with fractional-order parameter less than 1/3 are provided. Conditions for the existence of Hopf bifurcation in this system are also investigated. The dynamics of discretized form of our fractional-order hyperchaotic system are explored. Chaos control is also achieved in discretized system using delay feedback control technique. The numerical simulation are presented to confirm our theoretical analysis via phase portraits, bifurcation diagrams and Lyapunov exponents. A text encryption algorithm is presented based on the proposed fractional-order system. The results show that the new system exhibits a rich variety of dynamical behaviors such as limit cycles, chaos and transient phenomena where fractional-order derivative represents a key parameter in determining system qualitative behavior.
Simulation of Healing Threshold in Strain-Induced Inflammation through a Discrete Informatics Model.
Ibrahim, Israr; Oruganti, Sanjay Venkata; Pidaparti, Ramana
2017-02-15
Respiratory diseases such as asthma and acute respiratory distress syndrome as well as acute lung injury involve inflammation at the cellular level. The inflammation process is very complex and is characterized by the emergence of cytokines along with other changes in cellular processes. Due to the complexity of the various constituents that makes up the inflammation dynamics, it is necessary to develop models that can complement experiments to fully understand inflammatory diseases. In this study, we developed a discrete informatics model based on cellular automata (CA) approach to investigate the influence of elastic field (stretch/strain) on the dynamics of inflammation and account for probabilistic adaptation based on statistical interpretation of existing experimental data. Our simulation model investigated the effects of low, medium and high strain conditions on inflammation dynamics. Results suggest that the model is able to indicate the threshold of innate healing of tissue as a response to strain experienced by the tissue. When strain is under the threshold, the tissue is still capable of adapting its structure to heal the damaged part. However, there exists a strain threshold where healing capability breaks down. The results obtained demonstrate that the developed discrete informatics based CA model is capable of modeling and giving insights into inflammation dynamics parameters under various mechanical strain/stretch environments.
Discrete event simulation for exploring strategies: an urban water management case.
Huang, Dong-Bin; Scholz, Roland W; Gujer, Willi; Chitwood, Derek E; Loukopoulos, Peter; Schertenleib, Roland; Siegrist, Hansruedi
2007-02-01
This paper presents a model structure aimed at offering an overview of the various elements of a strategy and exploring their multidimensional effects through time in an efficient way. It treats a strategy as a set of discrete events planned to achieve a certain strategic goal and develops a new form of causal networks as an interfacing component between decision makers and environment models, e.g., life cycle inventory and material flow models. The causal network receives a strategic plan as input in a discrete manner and then outputs the updated parameter sets to the subsequent environmental models. Accordingly, the potential dynamic evolution of environmental systems caused by various strategies can be stepwise simulated. It enables a way to incorporate discontinuous change in models for environmental strategy analysis, and enhances the interpretability and extendibility of a complex model by its cellular constructs. It is exemplified using an urban water management case in Kunming, a major city in Southwest China. By utilizing the presented method, the case study modeled the cross-scale interdependencies of the urban drainage system and regional water balance systems, and evaluated the effectiveness of various strategies for improving the situation of Dianchi Lake.
NASA Astrophysics Data System (ADS)
Ezzedine, S. M.; Vorobiev, O.; Herbold, E. B.; Glenn, L. A.; Antoun, T.
2013-12-01
algorithm. It is also suitable for evaluating the bounds of possible shear motion due to uncertainties in the joints distribution. Details of this uncertainty quantification study are presented in a separate abstract (Vorobiev, et.al). In the present work using both the continuum and the discrete approaches we study the effects of the surface spall, in-situ stress and joint orientation on the observed near-field motion. Three dimensional numerical simulations are performed for different burial depths and yields to investigate scalability of both radial and shear motions. The motion calculated in the near-field is then propagated into a far field. Results of the far field study are presented in an accompanied work (Pitarka, et al). This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Theory of interacting dislocations on cylinders.
Amir, Ariel; Paulose, Jayson; Nelson, David R
2013-04-01
We study the mechanics and statistical physics of dislocations interacting on cylinders, motivated by the elongation of rod-shaped bacterial cell walls and cylindrical assemblies of colloidal particles subject to external stresses. The interaction energy and forces between dislocations are solved analytically, and analyzed asymptotically. The results of continuum elastic theory agree well with numerical simulations on finite lattices even for relatively small systems. Isolated dislocations on a cylinder act like grain boundaries. With colloidal crystals in mind, we show that saddle points are created by a Peach-Koehler force on the dislocations in the circumferential direction, causing dislocation pairs to unbind. The thermal nucleation rate of dislocation unbinding is calculated, for an arbitrary mobility tensor and external stress, including the case of a twist-induced Peach-Koehler force along the cylinder axis. Surprisingly rich phenomena arise for dislocations on cylinders, despite their vanishing Gaussian curvature.
Theory of interacting dislocations on cylinders
NASA Astrophysics Data System (ADS)
Amir, Ariel; Paulose, Jayson; Nelson, David R.
2013-04-01
We study the mechanics and statistical physics of dislocations interacting on cylinders, motivated by the elongation of rod-shaped bacterial cell walls and cylindrical assemblies of colloidal particles subject to external stresses. The interaction energy and forces between dislocations are solved analytically, and analyzed asymptotically. The results of continuum elastic theory agree well with numerical simulations on finite lattices even for relatively small systems. Isolated dislocations on a cylinder act like grain boundaries. With colloidal crystals in mind, we show that saddle points are created by a Peach-Koehler force on the dislocations in the circumferential direction, causing dislocation pairs to unbind. The thermal nucleation rate of dislocation unbinding is calculated, for an arbitrary mobility tensor and external stress, including the case of a twist-induced Peach-Koehler force along the cylinder axis. Surprisingly rich phenomena arise for dislocations on cylinders, despite their vanishing Gaussian curvature.
Discharge flow of a bidisperse granular media from a silo: Discrete particle simulations
NASA Astrophysics Data System (ADS)
Zhou, Y.; Ruyer, P.; Aussillous, P.
2015-12-01
Discrete particle simulations are used to study two-dimensional discharge flow from a silo using both monodisperse and bidisperse mixtures. The density and the velocity profiles through the aperture are measured. In the monodisperse case, two particles' diameters are studied for different outlet diameters. In the bidisperse case, we varied the fine mass fraction of the mixture. In all cases, the density and the velocity profiles are found to follow the same self-similar law. Based on these observations and the previous work of Benyamine et al., a physical model is proposed to describe the flow of bidisperse mixtures giving an explicit expression for the flow rate that is in good agreement with the results.
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.
NASA Astrophysics Data System (ADS)
Spellings, Matthew; Marson, Ryan L.; Anderson, Joshua A.; Glotzer, Sharon C.
2017-04-01
Faceted shapes, such as polyhedra, are commonly found in systems of nanoscale, colloidal, and granular particles. Many interesting physical phenomena, like crystal nucleation and growth, vacancy motion, and glassy dynamics are challenging to model in these systems because they require detailed dynamical information at the individual particle level. Within the granular materials community the Discrete Element Method has been used extensively to model systems of anisotropic particles under gravity, with friction. We provide an implementation of this method intended for simulation of hard, faceted nanoparticles, with a conservative Weeks-Chandler-Andersen (WCA) interparticle potential, coupled to a thermodynamic ensemble. This method is a natural extension of classical molecular dynamics and enables rigorous thermodynamic calculations for faceted particles.
A hybrid mortar virtual element method for discrete fracture network simulations
NASA Astrophysics Data System (ADS)
Benedetto, Matías Fernando; Berrone, Stefano; Borio, Andrea; Pieraccini, Sandra; Scialò, Stefano
2016-02-01
The most challenging issue in performing underground flow simulations in Discrete Fracture Networks (DFN) is to effectively tackle the geometrical difficulties of the problem. In this work we put forward a new application of the Virtual Element Method combined with the Mortar method for domain decomposition: we exploit the flexibility of the VEM in handling polygonal meshes in order to easily construct meshes conforming to the traces on each fracture, and we resort to the mortar approach in order to "weakly" impose continuity of the solution on intersecting fractures. The resulting method replaces the need for matching grids between fractures, so that the meshing process can be performed independently for each fracture. Numerical results show optimal convergence and robustness in handling very complex geometries.
Butts, Carter T
Stochastic models for finite binary vectors are widely used in sociology, with examples ranging from social influence models on dichotomous behaviors or attitudes to models for random graphs. Exact sampling for such models is difficult in the presence of dependence, leading to the use of Markov chain Monte Carlo (MCMC) as an approximation technique. While often effective, MCMC methods have variable execution time, and the quality of the resulting draws can be difficult to assess. Here, we present a novel alternative method for approximate sampling from binary discrete exponential families having fixed execution time and well-defined quality guarantees. We demonstrate the use of this sampling procedure in the context of random graph generation, with an application to the simulation of a large-scale social network using both geographical covariates and dyadic dependence mechanisms.
NASA Astrophysics Data System (ADS)
Zhan, Qiwei; Sun, Qingtao; Ren, Qiang; Fang, Yuan; Wang, Hua; Liu, Qing Huo
2017-08-01
We develop a non-conformal mesh discontinuous Galerkin (DG) pseudospectral time domain (PSTD) method for 3-D elastic wave scattering problems with arbitrary fracture inclusions. In contrast to directly meshing the exact thin-layer fracture, we use the linear-slip model, one kind of transmission boundary condition, for the DG scheme. Intrinsically, we can efficiently impose a jump-boundary condition by defining a new numerical flux for the surface integration in the DG framework. This transmission boundary condition in the DG-PSTD method significantly reduces the computational cost. 3-D DG simulations and accurate waveform comparisons validate our results for arbitrary discrete fractures. Numerical results indicate that fractures have a significant influence on wave propagation.
The impact of inpatient boarding on ED efficiency: a discrete-event simulation study.
Bair, Aaron E; Song, Wheyming T; Chen, Yi-Chun; Morris, Beth A
2010-10-01
In this study, a discrete-event simulation approach was used to model Emergency Department's (ED) patient flow to investigate the effect of inpatient boarding on the ED efficiency in terms of the National Emergency Department Crowding Scale (NEDOCS) score and the rate of patients who leave without being seen (LWBS). The decision variable in this model was the boarder-released-ratio defined as the ratio of admitted patients whose boarding time is zero to all admitted patients. Our analysis shows that the Overcrowded(+) (a NEDOCS score over 100) ratio decreased from 88.4% to 50.4%, and the rate of LWBS patients decreased from 10.8% to 8.4% when the boarder-released-ratio changed from 0% to 100%. These results show that inpatient boarding significantly impacts both the NEDOCS score and the rate of LWBS patient and this analysis provides a quantification of the impact of boarding on emergency department patient crowding.
NASA Astrophysics Data System (ADS)
Andreev, Victor P.; Head, Trajen; Johnson, Neil; Deo, Sapna K.; Daunert, Sylvia; Goldschmidt-Clermont, Pascal J.
2013-05-01
Sudden Cardiac Death (SCD) is responsible for at least 180,000 deaths a year and incurs an average cost of $286 billion annually in the United States alone. Herein, we present a novel discrete event simulation model of SCD, which quantifies the chains of events associated with the formation, growth, and rupture of atheroma plaques, and the subsequent formation of clots, thrombosis and on-set of arrhythmias within a population. The predictions generated by the model are in good agreement both with results obtained from pathological examinations on the frequencies of three major types of atheroma, and with epidemiological data on the prevalence and risk of SCD. These model predictions allow for identification of interventions and importantly for the optimal time of intervention leading to high potential impact on SCD risk reduction (up to 8-fold reduction in the number of SCDs in the population) as well as the increase in life expectancy.
NASA Technical Reports Server (NTRS)
Park, K. C.; Alvin, K. F.; Belvin, W. Keith
1991-01-01
A second-order form of discrete Kalman filtering equations is proposed as a candidate state estimator for efficient simulations of control-structure interactions in coupled physical coordinate configurations as opposed to decoupled modal coordinates. The resulting matrix equation of the present state estimator consists of the same symmetric, sparse N x N coupled matrices of the governing structural dynamics equations as opposed to unsymmetric 2N x 2N state space-based estimators. Thus, in addition to substantial computational efficiency improvement, the present estimator can be applied to control-structure design optimization for which the physical coordinates associated with the mass, damping and stiffness matrices of the structure are needed instead of modal coordinates.
Butts, Carter T.
2015-01-01
Stochastic models for finite binary vectors are widely used in sociology, with examples ranging from social influence models on dichotomous behaviors or attitudes to models for random graphs. Exact sampling for such models is difficult in the presence of dependence, leading to the use of Markov chain Monte Carlo (MCMC) as an approximation technique. While often effective, MCMC methods have variable execution time, and the quality of the resulting draws can be difficult to assess. Here, we present a novel alternative method for approximate sampling from binary discrete exponential families having fixed execution time and well-defined quality guarantees. We demonstrate the use of this sampling procedure in the context of random graph generation, with an application to the simulation of a large-scale social network using both geographical covariates and dyadic dependence mechanisms. PMID:26586920
Lattice-free models of cell invasion: discrete simulations and travelling waves.
Plank, Michael J; Simpson, Matthew J
2013-11-01
Invasion waves of cells play an important role in development, disease, and repair. Standard discrete models of such processes typically involve simulating cell motility, cell proliferation, and cell-to-cell crowding effects in a lattice-based framework. The continuum-limit description is often given by a reaction-diffusion equation that is related to the Fisher-Kolmogorov equation. One of the limitations of a standard lattice-based approach is that real cells move and proliferate in continuous space and are not restricted to a predefined lattice structure. We present a lattice-free model of cell motility and proliferation, with cell-to-cell crowding effects, and we use the model to replicate invasion wave-type behaviour. The continuum-limit description of the discrete model is a reaction-diffusion equation with a proliferation term that is different from lattice-based models. Comparing lattice-based and lattice-free simulations indicates that both models lead to invasion fronts that are similar at the leading edge, where the cell density is low. Conversely, the two models make different predictions in the high-density region of the domain, well behind the leading edge. We analyse the continuum-limit description of the lattice-based and lattice-free models to show that both give rise to invasion wave type solutions that move with the same speed but have very different shapes. We explore the significance of these differences by calibrating the parameters in the standard Fisher-Kolmogorov equation using data from the lattice-free model. We conclude that estimating parameters using this kind of standard procedure can produce misleading results.
Discrete exterior calculus for numerical simulation of meteor head-echo radar reflections
NASA Astrophysics Data System (ADS)
Räbinä, J.; Mönkölä, S.; Rossi, T.; Penttilä, A.; Markkanen, J.; Muinonen, K.
2014-07-01
The meteor head-echo feature has been studied by high-power large-aperture (HPLA) radars since 1960's (see Evans 1965). Based on the observations conducted by the different radar systems and post-processing techniques, there exist several models for the meteor head-echo simulations. One reason for this is the characteristics of the radar system, e.g., in terms of frequency and antenna geometry (see Kero et al. 2012). It is also worth mentioning that there are significant differences in the meteor sizes. According to the observations reported by, e.g., Vertatschitsch et al. (2011) and Wannberg et al. (2011), the head echo can be modeled as overdense scatter from a plasma layer, surrounding the meteor, with a certain density distribution. In these models, the plasmatic object is assumed to be a conducting spherical object, and the electromagnetic phenomenon can be presented by partial differential equations coupling the electric and magnetic fields. The traditional way of solving electromagnetic problems presented in space-time-domain as partial differential equations is to use the finite-difference time-domain method (FDTD; see Dyrud et al. 2008). In this study, we use more generalized finite differences by applying the discrete exterior calculus (DEC) to the numerical simulation of meteor head-echo radar reflections. The properties and calculus of differential forms is provided in a natural way at the discretization stage, and we associate the degrees of freedom of the electric and magnetic fields to the primal and dual mesh structures, respectively. The connection between the primal and dual forms is obtained by the discrete Hodge operator, the quality of which depends on the mesh construction. Our generalized formulation of the DEC for the Maxwell equations (see Pauly and Rossi 2011) works basically on unstructured grids, and it covers both the classical Yee's FDTD scheme and the Bossavit-Kettunen approach (Bossavit and Kettunen 1999). The method has been shown
NASA Astrophysics Data System (ADS)
Mendoza-Torres, F.; Diaz-Viera, M. A.
2015-12-01
In many natural fractured porous media, such as aquifers, soils, oil and geothermal reservoirs, fractures play a crucial role in their flow and transport properties. An approach that has recently gained popularity for modeling fracture systems is the Discrete Fracture Network (DFN) model. This approach consists in applying a stochastic boolean simulation method, also known as object simulation method, where fractures are represented as simplified geometric objects (line segments in 2D and polygons in 3D). One of the shortcomings of this approach is that it usually does not consider the dependency relationships that may exist between the geometric properties of fractures (direction, length, aperture, etc), that is, each property is simulated independently. In this work a method for modeling such dependencies by copula theory is introduced. In particular, a nonparametric model using Bernstein copulas for direction-length fracture dependency in 2D is presented. The application of this method is illustrated in a case study for a fractured rock sample from a carbonate reservoir outcrop.
Numazawa, Satoshi; Smith, Roger
2011-10-01
Classical harmonic transition state theory is considered and applied in discrete lattice cells with hierarchical transition levels. The scheme is then used to determine transitions that can be applied in a lattice-based kinetic Monte Carlo (KMC) atomistic simulation model. The model results in an effective reduction of KMC simulation steps by utilizing a classification scheme of transition levels for thermally activated atomistic diffusion processes. Thermally activated atomistic movements are considered as local transition events constrained in potential energy wells over certain local time periods. These processes are represented by Markov chains of multidimensional Boolean valued functions in three-dimensional lattice space. The events inhibited by the barriers under a certain level are regarded as thermal fluctuations of the canonical ensemble and accepted freely. Consequently, the fluctuating system evolution process is implemented as a Markov chain of equivalence class objects. It is shown that the process can be characterized by the acceptance of metastable local transitions. The method is applied to a problem of Au and Ag cluster growth on a rippled surface. The simulation predicts the existence of a morphology-dependent transition time limit from a local metastable to stable state for subsequent cluster growth by accretion. Excellent agreement with observed experimental results is obtained.
NASA Astrophysics Data System (ADS)
Numazawa, Satoshi; Smith, Roger
2011-10-01
Classical harmonic transition state theory is considered and applied in discrete lattice cells with hierarchical transition levels. The scheme is then used to determine transitions that can be applied in a lattice-based kinetic Monte Carlo (KMC) atomistic simulation model. The model results in an effective reduction of KMC simulation steps by utilizing a classification scheme of transition levels for thermally activated atomistic diffusion processes. Thermally activated atomistic movements are considered as local transition events constrained in potential energy wells over certain local time periods. These processes are represented by Markov chains of multidimensional Boolean valued functions in three-dimensional lattice space. The events inhibited by the barriers under a certain level are regarded as thermal fluctuations of the canonical ensemble and accepted freely. Consequently, the fluctuating system evolution process is implemented as a Markov chain of equivalence class objects. It is shown that the process can be characterized by the acceptance of metastable local transitions. The method is applied to a problem of Au and Ag cluster growth on a rippled surface. The simulation predicts the existence of a morphology-dependent transition time limit from a local metastable to stable state for subsequent cluster growth by accretion. Excellent agreement with observed experimental results is obtained.
Monte Carlo simulation of breast tumor imaging properties with compact, discrete gamma cameras
Gruber, G.J.; Moses, W.W.; Derenzo, S.E.
1999-12-01
The authors describe Monte Carlo simulation results for breast tumor imaging using a compact, discrete gamma camera. The simulations were designed to analyze and optimize camera design, particularly collimator configuration and detector pixel size. Simulated planar images of 5--15 mm diameter tumors in a phantom patient (including a breast, torso, and heart) were generated for imaging distances of 5--55 mm, pixel sizes of 2 x 2--4 x 4 mm{sup 2}, and hexagonal and square hole collimators with sensitivities from 4,000 to 16,000 counts/mCi/sec. Other factors considered included T/B (tumor-to-background tissue uptake ratio) and detector energy resolution. Image properties were quantified by computing the observed tumor fwhm (full-width at half-maximum) and S/N (sum of detected tumor events divided by the statistical noise). Results suggest that hexagonal and square hole collimators perform comparably, that higher sensitivity collimators provide higher tumor S/N with little increase in the observed tumor fwhm, that smaller pixels only slightly improve tumor fwhm and S/N, and that improved detector energy resolution has little impact on either the observed tumor fwhm or the observed tumor S/N.
Rau, Chi-Lun; Tsai, Pei-Fang Jennifer; Liang, Sheau-Farn Max; Tan, Jhih-Cian; Syu, Hong-Cheng; Jheng, Yue-Ling; Ciou, Ting-Syuan; Jaw, Fu-Shan
2013-12-01
This study uses a simulation model as a tool for strategic capacity planning for an outpatient physical therapy clinic in Taipei, Taiwan. The clinic provides a wide range of physical treatments, with 6 full-time therapists in each session. We constructed a discrete-event simulation model to study the dynamics of patient mixes with realistic treatment plans, and to estimate the practical capacity of the physical therapy room. The changes in time-related and space-related performance measurements were used to evaluate the impact of various strategies on the capacity of the clinic. The simulation results confirmed that the clinic is extremely patient-oriented, with a bottleneck occurring at the traction units for Intermittent Pelvic Traction (IPT), with usage at 58.9 %. Sensitivity analysis showed that attending to more patients would significantly increase the number of patients staying for overtime sessions. We found that pooling the therapists produced beneficial results. The average waiting time per patient could be reduced by 45 % when we pooled 2 therapists. We found that treating up to 12 new patients per session had no significantly negative impact on returning patients. Moreover, we found that the average waiting time for new patients decreased if they were given priority over returning patients when called by the therapists.
NASA Astrophysics Data System (ADS)
Cui, Yinan; Po, Giacomo; Ghoniem, Nasr
2017-02-01
Through three-dimensional discrete dislocation dynamics simulations, we show that by tuning the mode of external loading, the collective dynamics of dislocations undergo a transition from driven avalanches under stress control to quasiperiodic oscillations under strain control. We directly correlate measured intermittent plastic events with internal dislocation activities and collective dynamics. Under different loading modes, the roles of the weakest dislocation source and the defect population trend are significantly different. This finding raises new possibilities of controlling correlated dislocation activities and obtaining a low defect density in nanostructured devices by tuning external constraints. In addition, the effect of machine stiffness comes to light. The statistical analysis of the burst magnitude is revisited and carefully discussed. Self-organized criticality and scale-free statistics of strain bursts are obeyed under stress control. However, this behavior is shown to break down under strain control. Rapid stress drops under pure strain control force truncation of dislocation avalanches, leading to a dynamical transition to quasiperiodic oscillations.
NASA Astrophysics Data System (ADS)
Zhang, Xiaohan; Acharya, Amit; Walkington, Noel J.; Bielak, Jacobo
2015-11-01
We describe a model based on continuum mechanics that reduces the study of a significant class of problems of discrete dislocation dynamics to questions of the modern theory of continuum plasticity. As applications, we explore the questions of the existence of a Peierls stress in a continuum theory, dislocation annihilation, dislocation dissociation, finite-speed-of-propagation effects of elastic waves vis-a-vis dynamic dislocation fields, supersonic dislocation motion, and short-slip duration in rupture dynamics.
A Two-Level, Discrete Particle Approach for Large-Scale Simulation of Colloidal Aggregates
NASA Astrophysics Data System (ADS)
Dzwinel, Witold; Yuen, David A.
Most numerical techniques employed for aggregation simulation are based on equilibrium growth assumption and Smoluchowski theory. We present a new two-level discrete particle model, which can be employed in simulating large colloidal clusters in highly nonequilibrium physical conditions. We consider the system of colloidal particles (CP) interacting via conservative CP-CP repulsive-attractive two-body forces, which is initially mixed in a dissipative solvent. In order to obtain a high-resolution picture of colloidal dynamics, we employ around 20 million particles consisting of two kinds of particles. For bridging the spatio-temporal scales between nanoscale colloidal and the solvent particles (SP), the solvent is modeled by dissipative particle dynamics (DPD) fluid. We focus on the systems size for which the CP-SP interactions can also be described by the DPD forces. Unlike previous numerical techniques, the two-level particle model can display much more realistic physics, thus allowing for the simulation of aggregation for various types of colloids and solvent liquids in a broad range of conditions. We show that not only large and static clusters but also the initial stages of aggregation evolution can be better scrutinized. The large-scale simulation results obtained in two-dimensions show that the mean cluster size grows with time t according to the power law tκ. Because of the time-dependence of growth mechanism, the value of κ necessarily must change. We have first κ=1 with a value of 1 achieved asymptotically with time. We can also discern intermediate-scale structures. We emphasize that the method developed here can be easily extended to algorithms dealing with multi-level hierarchy and multiphase fluid dynamics.
Random vs. Combinatorial Methods for Discrete Event Simulation of a Grid Computer Network
NASA Technical Reports Server (NTRS)
Kuhn, D. Richard; Kacker, Raghu; Lei, Yu
2010-01-01
This study compared random and t-way combinatorial inputs of a network simulator, to determine if these two approaches produce significantly different deadlock detection for varying network configurations. Modeling deadlock detection is important for analyzing configuration changes that could inadvertently degrade network operations, or to determine modifications that could be made by attackers to deliberately induce deadlock. Discrete event simulation of a network may be conducted using random generation, of inputs. In this study, we compare random with combinatorial generation of inputs. Combinatorial (or t-way) testing requires every combination of any t parameter values to be covered by at least one test. Combinatorial methods can be highly effective because empirical data suggest that nearly all failures involve the interaction of a small number of parameters (1 to 6). Thus, for example, if all deadlocks involve at most 5-way interactions between n parameters, then exhaustive testing of all n-way interactions adds no additional information that would not be obtained by testing all 5-way interactions. While the maximum degree of interaction between parameters involved in the deadlocks clearly cannot be known in advance, covering all t-way interactions may be more efficient than using random generation of inputs. In this study we tested this hypothesis for t = 2, 3, and 4 for deadlock detection in a network simulation. Achieving the same degree of coverage provided by 4-way tests would have required approximately 3.2 times as many random tests; thus combinatorial methods were more efficient for detecting deadlocks involving a higher degree of interactions. The paper reviews explanations for these results and implications for modeling and simulation.
Teleradiology system analysis using a discrete event-driven block-oriented network simulator
NASA Astrophysics Data System (ADS)
Stewart, Brent K.; Dwyer, Samuel J., III
1992-07-01
Performance evaluation and trade-off analysis are the central issues in the design of communication networks. Simulation plays an important role in computer-aided design and analysis of communication networks and related systems, allowing testing of numerous architectural configurations and fault scenarios. We are using the Block Oriented Network Simulator (BONeS, Comdisco, Foster City, CA) software package to perform discrete, event- driven Monte Carlo simulations in capacity planning, tradeoff analysis and evaluation of alternate architectures for a high-speed, high-resolution teleradiology project. A queuing network model of the teleradiology system has been devise, simulations executed and results analyzed. The wide area network link uses a switched, dial-up N X 56 kbps inverting multiplexer where the number of digital voice-grade lines (N) can vary from one (DS-0) through 24 (DS-1). The proposed goal of such a system is 200 films (2048 X 2048 X 12-bit) transferred between a remote and local site in an eight hour period with a mean delay time less than five minutes. It is found that: (1) the DS-1 service limit is around 100 films per eight hour period with a mean delay time of 412 +/- 39 seconds, short of the goal stipulated above; (2) compressed video teleconferencing can be run simultaneously with image data transfer over the DS-1 wide area network link without impacting the performance of the described teleradiology system; (3) there is little sense in upgrading to a higher bandwidth WAN link like DS-2 or DS-3 for the current system; and (4) the goal of transmitting 200 films in an eight hour period with a mean delay time less than five minutes can be achieved simply if the laser printer interface is updated from the current DR-11W interface to a much faster SCSI interface.
Simulation of River Bluffs and Slip-Off Slopes With a Discrete Particle-Based Model
NASA Astrophysics Data System (ADS)
Lancaster, S. T.; Zunka, J. P.; Tucker, G. E.
2013-12-01
A discrete particle-based model simulates evolution of two-dimensional valley cross sections similar to those produced by bedrock meandering rivers and thereby suggests that characteristic features such as overhanging cliffs and talus slopes are dependent on specific relationships among process rates. Discrete coordinates on a gridded cross-section define locations of particles of intact bedrock, sediment (loose material with half the bulk density of bedrock), water, or air on that grid, and each particle of rock or sediment has a unique (or zero) concentration of terrestrial cosmogenic nuclides (TCNs). Stochastic processes determine both the possible locations of process actions and the results of those actions. Stochastic discharges generate boundary shear stresses, calculated by an approximation to the ray-isovel model, that determine removal probabilities for candidate particles of bedrock or sediment from the boundary of a self-formed channel. An asymmetric probability distribution governs the selection of candidate particles on the wetted perimeter and drives asymmetric fluvial erosion and transport that can undermine adjacent slopes, so that the channel migrates laterally. Sediment is produced from intact bedrock by weathering and rock fall. The latter acts only on candidate bedrock particles that are undermined and exposed at the surface. Weathering produces two sediment particles from one of bedrock, and thereby inflates the surface, when slope-normal random walks from candidate sites on the surface end at bedrock particles, so that the sediment-bedrock interface is irregular and discontinuous. Diffusive transport moves candidate particles on random walks in random directions along the surface, where transition probabilities depend on local topography. TCNs are produced when the randomly situated and oriented random walks of cosmic rays end at bedrock or sediment, and not water, particles. The model produces asymmetric channels and valley cross sections
Discrete-vortex simulation of pulsating flow on a turbulent leading-edge separation bubble
NASA Technical Reports Server (NTRS)
Sung, Hyung Jin; Rhim, Jae Wook; Kiya, Masaru
1992-01-01
Studies are made of the turbulent separation bubble in a two-dimensional semi-infinite blunt plate aligned to a uniform free stream with a pulsating component. The discrete-vortex method is applied to simulate this flow situation because this approach is effective for representing the unsteady motions of the turbulent shear layer and the effect of viscosity near the solid surface. The numerical simulation provides reasonable predictions when compared with the experimental results. A particular frequency with a minimum reattachment is related to the drag reduction. The most effective frequency is dependent on the amplified shedding frequency. The turbulent flow structure is scrutinized. This includes the time-mean and fluctuations of the velocity and the surface pressure, together with correlations between the fluctuating components. A comparison between the pulsating flow and the non-pulsating flow at the particular frequency of the minimum reattachment length of the separation bubble suggests that the large-scale vortical structure is associated with the shedding frequency and the flow instabilities.
Perkins, Casey; Muller, George
2015-10-08
The number of connections between physical and cyber security systems is rapidly increasing due to centralized control from automated and remotely connected means. As the number of interfaces between systems continues to grow, the interactions and interdependencies between them cannot be ignored. Historically, physical and cyber vulnerability assessments have been performed independently. This independent evaluation omits important aspects of the integrated system, where the impacts resulting from malicious or opportunistic attacks are not easily known or understood. Here, we describe a discrete event simulation model that uses information about integrated physical and cyber security systems, attacker characteristics and simple responsemore » rules to identify key safeguards that limit an attacker's likelihood of success. Key features of the proposed model include comprehensive data generation to support a variety of sophisticated analyses, and full parameterization of safeguard performance characteristics and attacker behaviours to evaluate a range of scenarios. Lastly, we also describe the core data requirements and the network of networks that serves as the underlying simulation structure.« less
Perkins, Casey; Muller, George
2015-10-08
The number of connections between physical and cyber security systems is rapidly increasing due to centralized control from automated and remotely connected means. As the number of interfaces between systems continues to grow, the interactions and interdependencies between them cannot be ignored. Historically, physical and cyber vulnerability assessments have been performed independently. This independent evaluation omits important aspects of the integrated system, where the impacts resulting from malicious or opportunistic attacks are not easily known or understood. Here, we describe a discrete event simulation model that uses information about integrated physical and cyber security systems, attacker characteristics and simple response rules to identify key safeguards that limit an attacker's likelihood of success. Key features of the proposed model include comprehensive data generation to support a variety of sophisticated analyses, and full parameterization of safeguard performance characteristics and attacker behaviours to evaluate a range of scenarios. Lastly, we also describe the core data requirements and the network of networks that serves as the underlying simulation structure.
A non-discrete method for computation of residence time in fluid mechanics simulations
NASA Astrophysics Data System (ADS)
Esmaily-Moghadam, Mahdi; Hsia, Tain-Yen; Marsden, Alison L.
2013-11-01
Cardiovascular simulations provide a promising means to predict risk of thrombosis in grafts, devices, and surgical anatomies in adult and pediatric patients. Although the pathways for platelet activation and clot formation are not yet fully understood, recent findings suggest that thrombosis risk is increased in regions of flow recirculation and high residence time (RT). Current approaches for calculating RT are typically based on releasing a finite number of Lagrangian particles into the flow field and calculating RT by tracking their positions. However, special care must be taken to achieve temporal and spatial convergence, often requiring repeated simulations. In this work, we introduce a non-discrete method in which RT is calculated in an Eulerian framework using the advection-diffusion equation. We first present the formulation for calculating residence time in a given region of interest using two alternate definitions. The physical significance and sensitivity of the two measures of RT are discussed and their mathematical relation is established. An extension to a point-wise value is also presented. The methods presented here are then applied in a 2D cavity and two representative clinical scenarios, involving shunt placement for single ventricle heart defects and Kawasaki disease. In the second case study, we explored the relationship between RT and wall shear stress, a parameter of particular importance in cardiovascular disease.
Discrete Event Simulation Models for CT Examination Queuing in West China Hospital
Luo, Li; Tang, Shijun; Shi, Yingkang; Guo, Huili
2016-01-01
In CT examination, the emergency patients (EPs) have highest priorities in the queuing system and thus the general patients (GPs) have to wait for a long time. This leads to a low degree of satisfaction of the whole patients. The aim of this study is to improve the patients' satisfaction by designing new queuing strategies for CT examination. We divide the EPs into urgent type and emergency type and then design two queuing strategies: one is that the urgent patients (UPs) wedge into the GPs' queue with fixed interval (fixed priority model) and the other is that the patients have dynamic priorities for queuing (dynamic priority model). Based on the data from Radiology Information Database (RID) of West China Hospital (WCH), we develop some discrete event simulation models for CT examination according to the designed strategies. We compare the performance of different strategies on the basis of the simulation results. The strategy that patients have dynamic priorities for queuing makes the waiting time of GPs decrease by 13 minutes and the degree of satisfaction increase by 40.6%. We design a more reasonable CT examination queuing strategy to decrease patients' waiting time and increase their satisfaction degrees. PMID:27547237
StratBAM: A Discrete-Event Simulation Model to Support Strategic Hospital Bed Capacity Decisions.
Devapriya, Priyantha; Strömblad, Christopher T B; Bailey, Matthew D; Frazier, Seth; Bulger, John; Kemberling, Sharon T; Wood, Kenneth E
2015-10-01
The ability to accurately measure and assess current and potential health care system capacities is an issue of local and national significance. Recent joint statements by the Institute of Medicine and the Agency for Healthcare Research and Quality have emphasized the need to apply industrial and systems engineering principles to improving health care quality and patient safety outcomes. To address this need, a decision support tool was developed for planning and budgeting of current and future bed capacity, and evaluating potential process improvement efforts. The Strategic Bed Analysis Model (StratBAM) is a discrete-event simulation model created after a thorough analysis of patient flow and data from Geisinger Health System's (GHS) electronic health records. Key inputs include: timing, quantity and category of patient arrivals and discharges; unit-level length of care; patient paths; and projected patient volume and length of stay. Key outputs include: admission wait time by arrival source and receiving unit, and occupancy rates. Electronic health records were used to estimate parameters for probability distributions and to build empirical distributions for unit-level length of care and for patient paths. Validation of the simulation model against GHS operational data confirmed its ability to model real-world data consistently and accurately. StratBAM was successfully used to evaluate the system impact of forecasted patient volumes and length of stay in terms of patient wait times, occupancy rates, and cost. The model is generalizable and can be appropriately scaled for larger and smaller health care settings.
Shirvanyants, David; Ding, Feng; Tsao, Douglas; Ramachandran, Srinivas; Dokholyan, Nikolay V
2012-07-26
Until now it has been impractical to observe protein folding in silico for proteins larger than 50 residues. Limitations of both force field accuracy and computational efficiency make the folding problem very challenging. Here we employ discrete molecular dynamics (DMD) simulations with an all-atom force field to fold fast-folding proteins. We extend the DMD force field by introducing long-range electrostatic interactions to model salt-bridges and a sequence-dependent semiempirical potential accounting for natural tendencies of certain amino acid sequences to form specific secondary structures. We enhance the computational performance by parallelizing the DMD algorithm. Using a small number of commodity computers, we achieve sampling quality and folding accuracy comparable to the explicit-solvent simulations performed on high-end hardware. We demonstrate that DMD can be used to observe equilibrium folding of villin headpiece and WW domain, study two-state folding kinetics, and sample near-native states in ab initio folding of proteins of ∼100 residues.
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.
A non-discrete method for computation of residence time in fluid mechanics simulations.
Esmaily-Moghadam, Mahdi; Hsia, Tain-Yen; Marsden, Alison L
2013-11-01
Cardiovascular simulations provide a promising means to predict risk of thrombosis in grafts, devices, and surgical anatomies in adult and pediatric patients. Although the pathways for platelet activation and clot formation are not yet fully understood, recent findings suggest that thrombosis risk is increased in regions of flow recirculation and high residence time (RT). Current approaches for calculating RT are typically based on releasing a finite number of Lagrangian particles into the flow field and calculating RT by tracking their positions. However, special care must be taken to achieve temporal and spatial convergence, often requiring repeated simulations. In this work, we introduce a non-discrete method in which RT is calculated in an Eulerian framework using the advection-diffusion equation. We first present the formulation for calculating residence time in a given region of interest using two alternate definitions. The physical significance and sensitivity of the two measures of RT are discussed and their mathematical relation is established. An extension to a point-wise value is also presented. The methods presented here are then applied in a 2D cavity and two representative clinical scenarios, involving shunt placement for single ventricle heart defects and Kawasaki disease. In the second case study, we explored the relationship between RT and wall shear stress, a parameter of particular importance in cardiovascular disease.
Discrete element simulation of charging and mixed layer formation in the ironmaking blast furnace
NASA Astrophysics Data System (ADS)
Mitra, Tamoghna; Saxén, Henrik
2016-11-01
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.
Discrete Event Simulation Models for CT Examination Queuing in West China Hospital.
Luo, Li; Liu, Hangjiang; Liao, Huchang; Tang, Shijun; Shi, Yingkang; Guo, Huili
2016-01-01
In CT examination, the emergency patients (EPs) have highest priorities in the queuing system and thus the general patients (GPs) have to wait for a long time. This leads to a low degree of satisfaction of the whole patients. The aim of this study is to improve the patients' satisfaction by designing new queuing strategies for CT examination. We divide the EPs into urgent type and emergency type and then design two queuing strategies: one is that the urgent patients (UPs) wedge into the GPs' queue with fixed interval (fixed priority model) and the other is that the patients have dynamic priorities for queuing (dynamic priority model). Based on the data from Radiology Information Database (RID) of West China Hospital (WCH), we develop some discrete event simulation models for CT examination according to the designed strategies. We compare the performance of different strategies on the basis of the simulation results. The strategy that patients have dynamic priorities for queuing makes the waiting time of GPs decrease by 13 minutes and the degree of satisfaction increase by 40.6%. We design a more reasonable CT examination queuing strategy to decrease patients' waiting time and increase their satisfaction degrees.
ERIC Educational Resources Information Center
Blackstone, Barbara
A study was conducted to determine the effectiveness of "Discretion vs. Valor," a simulation game designed to give North American players a chance to: (1) identify with "believers" (Christians) in the Soviet Union in order to form new images of these persons; (2) gain empathy for Christians by understanding the dilemmas they…
In this paper, the capability of two methods of modelling detonation of high explosives (HE) buried in soil viz., (1) coupled discrete element and...blast simulation method. The main focus of this study is to understand the strengths of DEM_PGM and identify the limitations /strengths compared to the ALE
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
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
Coupled large eddy simulation and discrete element model of bedload motion
NASA Astrophysics Data System (ADS)
Furbish, D.; Schmeeckle, M. W.
2011-12-01
We combine a three-dimensional large eddy simulation of turbulence to a three-dimensional discrete element model of turbulence. The large eddy simulation of the turbulent fluid is extended into the bed composed of non-moving particles by adding resistance terms to the Navier-Stokes equations in accordance with the Darcy-Forchheimer law. This allows the turbulent velocity and pressure fluctuations to penetrate the bed of discrete particles, and this addition of a porous zone results in turbulence structures above the bed that are similar to previous experimental and numerical results for hydraulically-rough beds. For example, we reproduce low-speed streaks that are less coherent than those over smooth-beds due to the episodic outflow of fluid from the bed. Local resistance terms are also added to the Navier-Stokes equations to account for the drag of individual moving particles. The interaction of the spherical particles utilizes a standard DEM soft-sphere Hertz model. We use only a simple drag model to calculate the fluid forces on the particles. The model reproduces an exponential distribution of bedload particle velocities that we have found experimentally using high-speed video of a flat bed of moving sand in a recirculating water flume. The exponential distribution of velocity results from the motion of many particles that are nearly constantly in contact with other bed particles and come to rest after short distances, in combination with a relatively few particles that are entrained further above the bed and have velocities approaching that of the fluid. Entrainment and motion "hot spots" are evident that are not perfectly correlated with the local, instantaneous fluid velocity. Zones of the bed that have recently experienced motion are more susceptible to motion because of the local configuration of particle contacts. The paradigm of a characteristic saltation hop length in riverine bedload transport has infused many aspects of geomorphic thought, including
Solute atmospheres at dislocations
Hirth, John P.; Barnett, David M.; Hoagland, Richard G.
2017-06-01
In this study, a two-dimensional plane strain elastic solution is determined for the Cottrell solute atmosphere around an edge dislocation in an infinitely long cylinder of finite radius (the matrix), in which rows of solutes are represented by cylindrical rods with in-plane hydrostatic misfit (axial misfit is also considered). The periphery of the matrix is traction-free, thus introducing an image solute field which generates a solute-solute interaction energy that has not been considered previously. The relevant energy for the field of any distribution of solutes coexistent with a single edge dislocation along the (matrix) cylinder axis is determined, and coherencymore » effects are discussed and studied. Monte Carlo simulations accounting for all pertinent interactions over a range of temperatures are found to yield solute distributions different from classical results, namely, (1) Fermi-Dirac condensations at low temperatures at the free surface, (2) the majority of the atmosphere lying within an unexpectedly large non-linear interaction region near the dislocation core, and (3) temperature-dependent asymmetrical solute arrangements that promote bending. The solute distributions at intermediate temperatures show a 1/r dependence in agreement with previous linearized approximations. With a standard state of solute corresponding to a mean concentration, c0, the relevant interaction energy expression presented in this work is valid when extended to large concentrations for which Henry's Law and Vegard's Law do not apply.« less
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
Jia, Hao; Chen, Bin; Li, Dong; Zhang, Yong
2015-02-01
To adapt the complex tissue structure, laser propagation in a two-layered skin model is simulated to compare voxel-based Monte Carlo (VMC) and tetrahedron-based MC (TMC) methods with a geometry-based MC (GMC) method. In GMC, the interface is mathematically defined without any discretization. GMC is the most accurate but is not applicable to complicated domains. The implementation of VMC is simple because of its structured voxels. However, unavoidable errors are expected because of the zigzag polygonal interface. Compared with GMC and VMC, TMC provides a balance between accuracy and flexibility by the tetrahedron cells. In the present TMC, the body-fitted tetrahedra are generated in different tissues. No interface tetrahedral cells exist, thereby avoiding the photon reflection error in the interface cells in VMC. By introducing a distance threshold, the error caused by confused optical parameters between neighboring cells when photons are incident along the cell boundary can be avoided. The results show that the energy deposition error by TMC in the interfacial region is one-tenth to one-fourth of that by VMC, yielding more accurate computations of photon reflection, refraction, and energy deposition. The results of multilayered and n-shaped vessels indicate that a laser with a 1064-nm wavelength should be introduced to clean deep-buried vessels. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)
Efficiency of endoscopy units can be improved with use of discrete event simulation modeling.
Sauer, Bryan G; Singh, Kanwar P; Wagner, Barry L; Vanden Hoek, Matthew S; Twilley, Katherine; Cohn, Steven M; Shami, Vanessa M; Wang, Andrew Y
2016-11-01
Background and study aims: The projected increased demand for health services obligates healthcare organizations to operate efficiently. Discrete event simulation (DES) is a modeling method that allows for optimization of systems through virtual testing of different configurations before implementation. The objective of this study was to identify strategies to improve the daily efficiencies of an endoscopy center with the use of DES. Methods: We built a DES model of a five procedure room endoscopy unit at a tertiary-care university medical center. After validating the baseline model, we tested alternate configurations to run the endoscopy suite and evaluated outcomes associated with each change. The main outcome measures included adequate number of preparation and recovery rooms, blocked inflow, delay times, blocked outflows, and patient cycle time. Results: Based on a sensitivity analysis, the adequate number of preparation rooms is eight and recovery rooms is nine for a five procedure room unit (total 3.4 preparation and recovery rooms per procedure room). Simple changes to procedure scheduling and patient arrival times led to a modest improvement in efficiency. Increasing the preparation/recovery rooms based on the sensitivity analysis led to significant improvements in efficiency. Conclusions: By applying tools such as DES, we can model changes in an environment with complex interactions and find ways to improve the medical care we provide. DES is applicable to any endoscopy unit and would be particularly valuable to those who are trying to improve on the efficiency of care and patient experience.
Dorn, Martin; Hekmat, Dariusch
2016-03-01
Preparative packed-bed chromatography using polymer-based, compressible, porous resins is a powerful method for purification of macromolecular bioproducts. During operation, a complex, hysteretic, thus, history-dependent packed bed behavior is often observed but theoretical understanding of the causes is limited. Therefore, a rigorous modeling approach of the chromatography column on the particle scale has been made which takes into account interparticle micromechanics and fluid-particle interactions for the first time. A three-dimensional deterministic model was created by applying Computational Fluid Dynamics (CFD) coupled with the Discrete Element Method (DEM). The column packing behavior during either flow or mechanical compression was investigated in-silico and in laboratory experiments. A pronounced axial compression-relaxation profile was identified that differed for both compression strategies. Void spaces were clearly visible in the packed bed after compression. It was assumed that the observed bed inhomogeneity was because of a force-chain network at the particle scale. The simulation satisfactorily reproduced the measured behavior regarding packing compression as well as pressure-flow dependency. Furthermore, the particle Young's modulus and particle-wall friction as well as interparticle friction were identified as crucial parameters affecting packing dynamics. It was concluded that compaction of the chromatographic bed is rather because of particle rearrangement than particle deformation. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 32:363-371, 2016. © 2015 American Institute of Chemical Engineers.
NASA Astrophysics Data System (ADS)
Jia, Hao; Chen, Bin; Li, Dong; Zhang, Yong
2015-02-01
To adapt the complex tissue structure, laser propagation in a two-layered skin model is simulated to compare voxel-based Monte Carlo (VMC) and tetrahedron-based MC (TMC) methods with a geometry-based MC (GMC) method. In GMC, the interface is mathematically defined without any discretization. GMC is the most accurate but is not applicable to complicated domains. The implementation of VMC is simple because of its structured voxels. However, unavoidable errors are expected because of the zigzag polygonal interface. Compared with GMC and VMC, TMC provides a balance between accuracy and flexibility by the tetrahedron cells. In the present TMC, the body-fitted tetrahedra are generated in different tissues. No interface tetrahedral cells exist, thereby avoiding the photon reflection error in the interface cells in VMC. By introducing a distance threshold, the error caused by confused optical parameters between neighboring cells when photons are incident along the cell boundary can be avoided. The results show that the energy deposition error by TMC in the interfacial region is one-tenth to one-fourth of that by VMC, yielding more accurate computations of photon reflection, refraction, and energy deposition. The results of multilayered and n-shaped vessels indicate that a laser with a 1064-nm wavelength should be introduced to clean deep-buried vessels.
Efficiency of endoscopy units can be improved with use of discrete event simulation modeling
Sauer, Bryan G.; Singh, Kanwar P.; Wagner, Barry L.; Vanden Hoek, Matthew S.; Twilley, Katherine; Cohn, Steven M.; Shami, Vanessa M.; Wang, Andrew Y.
2016-01-01
Background and study aims: The projected increased demand for health services obligates healthcare organizations to operate efficiently. Discrete event simulation (DES) is a modeling method that allows for optimization of systems through virtual testing of different configurations before implementation. The objective of this study was to identify strategies to improve the daily efficiencies of an endoscopy center with the use of DES. Methods: We built a DES model of a five procedure room endoscopy unit at a tertiary-care university medical center. After validating the baseline model, we tested alternate configurations to run the endoscopy suite and evaluated outcomes associated with each change. The main outcome measures included adequate number of preparation and recovery rooms, blocked inflow, delay times, blocked outflows, and patient cycle time. Results: Based on a sensitivity analysis, the adequate number of preparation rooms is eight and recovery rooms is nine for a five procedure room unit (total 3.4 preparation and recovery rooms per procedure room). Simple changes to procedure scheduling and patient arrival times led to a modest improvement in efficiency. Increasing the preparation/recovery rooms based on the sensitivity analysis led to significant improvements in efficiency. Conclusions: By applying tools such as DES, we can model changes in an environment with complex interactions and find ways to improve the medical care we provide. DES is applicable to any endoscopy unit and would be particularly valuable to those who are trying to improve on the efficiency of care and patient experience. PMID:27853739
Discrete event simulation for healthcare organizations: a tool for decision making.
Hamrock, Eric; Paige, Kerrie; Parks, Jennifer; Scheulen, James; Levin, Scott
2013-01-01
Healthcare organizations face challenges in efficiently accommodating increased patient demand with limited resources and capacity. The modern reimbursement environment prioritizes the maximization of operational efficiency and the reduction of unnecessary costs (i.e., waste) while maintaining or improving quality. As healthcare organizations adapt, significant pressures are placed on leaders to make difficult operational and budgetary decisions. In lieu of hard data, decision makers often base these decisions on subjective information. Discrete event simulation (DES), a computerized method of imitating the operation of a real-world system (e.g., healthcare delivery facility) over time, can provide decision makers with an evidence-based tool to develop and objectively vet operational solutions prior to implementation. DES in healthcare commonly focuses on (1) improving patient flow, (2) managing bed capacity, (3) scheduling staff, (4) managing patient admission and scheduling procedures, and (5) using ancillary resources (e.g., labs, pharmacies). This article describes applicable scenarios, outlines DES concepts, and describes the steps required for development. An original DES model developed to examine crowding and patient flow for staffing decision making at an urban academic emergency department serves as a practical example.
The Impact of Inpatient Boarding on ED Efficiency: A Discrete-Event Simulation Study
Bair, Aaron E.; Chen, Yi-Chun; Morris, Beth A.
2009-01-01
In this study, a discrete-event simulation approach was used to model Emergency Department’s (ED) patient flow to investigate the effect of inpatient boarding on the ED efficiency in terms of the National Emergency Department Crowding Scale (NEDOCS) score and the rate of patients who leave without being seen (LWBS). The decision variable in this model was the boarder-released-ratio defined as the ratio of admitted patients whose boarding time is zero to all admitted patients. Our analysis shows that the Overcrowded+ (a NEDOCS score over 100) ratio decreased from 88.4% to 50.4%, and the rate of LWBS patients decreased from 10.8% to 8.4% when the boarder-released-ratio changed from 0% to 100%. These results show that inpatient boarding significantly impacts both the NEDOCS score and the rate of LWBS patient and this analysis provides a quantification of the impact of boarding on emergency department patient crowding. PMID:20703616
Discrete Event Simulation-Based Resource Modelling in Health Technology Assessment.
Salleh, Syed; Thokala, Praveen; Brennan, Alan; Hughes, Ruby; Dixon, Simon
2017-07-03
The objective of this article was to conduct a systematic review of published research on the use of discrete event simulation (DES) for resource modelling (RM) in health technology assessment (HTA). RM is broadly defined as incorporating and measuring effects of constraints on physical resources (e.g. beds, doctors, nurses) in HTA models. Systematic literature searches were conducted in academic databases (JSTOR, SAGE, SPRINGER, SCOPUS, IEEE, Science Direct, PubMed, EMBASE) and grey literature (Google Scholar, NHS journal library), enhanced by manual searchers (i.e. reference list checking, citation searching and hand-searching techniques). The search strategy yielded 4117 potentially relevant citations. Following the screening and manual searches, ten articles were included. Reviewing these articles provided insights into the applications of RM: firstly, different types of economic analyses, model settings, RM and cost-effectiveness analysis (CEA) outcomes were identified. Secondly, variation in the characteristics of the constraints such as types and nature of constraints and sources of data for the constraints were identified. Thirdly, it was found that including the effects of constraints caused the CEA results to change in these articles. The review found that DES proved to be an effective technique for RM but there were only a small number of studies applied in HTA. However, these studies showed the important consequences of modelling physical constraints and point to the need for a framework to be developed to guide future applications of this approach.
Anomalous transport in discrete arcs and simulation of double layers in a model auroral circuit
NASA Technical Reports Server (NTRS)
Smith, Robert A.
1987-01-01
The evolution and long-time stability of a double layer in a discrete auroral arc requires that the parallel current in the arc, which may be considered uniform at the source, be diverted within the arc to charge the flanks of the U-shaped double-layer potential structure. A simple model is presented in which this current re-distribution is effected by anomalous transport based on electrostatic lower hybrid waves driven by the flank structure itself. This process provides the limiting constraint on the double-layer potential. The flank charging may be represented as that of a nonlinear transmission. A simplified model circuit, in which the transmission line is represented by a nonlinear impedance in parallel with a variable resistor, is incorporated in a 1-d simulation model to give the current density at the DL boundaries. Results are presented for the scaling of the DL potential as a function of the width of the arc and the saturation efficiency of the lower hybrid instability mechanism.
Coupled Large Eddy Simulation and Discrete Element Model for Particle Saltation
NASA Astrophysics Data System (ADS)
Liu, X.; Liu, D.; Fu, X.
2016-12-01
Particle saltation is the major mode of motion for sediment transport. The quantification of the characteristics of saltation, either as an individual particle or as a group, is of great importance to our understanding of the transport process. In the past, experiments and numerical models have been performed to study the saltation length, height, and velocity under different turbulent flow and rough bed conditions. Most previous numerical models have very restrictive assumptions. For example, many models assumed Log-law flow velocity profiles to drive the motion of particles. Others assumed some "splash-function" which assigns the reflection angle for the rebounding of the saltating particle after each collision with bed. This research aims to relax these restrictions by a coupled eddy-resolving flow solver and a discrete element model. The model simulates the fully four-way coupling among fluid, particles, and wall. The model is extensively validated on both the turbulent flow field and saltation statistics. The results show that the two controlling factors for particle saltation are turbulent fluctuations and bed collision. Detailed quantification of these two factors will be presented. Through the statistics of incidence reflection angles, a more physical "splash-function" is obtained in which the reflection angle follows an asymmetric bimodal distribution for a given incidence angle. The higher mode is always located on the upstream side of the bed particle, while the lower one is always on the downstream surface.
Anomalous transport in discrete arcs and simulation of double layers in a model auroral circuit
NASA Technical Reports Server (NTRS)
Smith, Robert A.
1987-01-01
The evolution and long-time stability of a double layer (DL) in a discrete auroral arc requires that the parallel current in the arc, which may be considered uniform at the source, be diverted within the arc to charge the flanks of the U-shaped double layer potential structure. A simple model is presented in which this current redistribution is effected by anomalous transport based on electrostatic lower hybrid waves driven by the flank structure itself. This process provides the limiting constraint on the double layer potential. The flank charging may be represented as that of a nonlinear transmission line. A simplified model circuit, in which the transmission line is represented by a nonlinear impedance in parallel with a variable resistor, is incorporated in a one-dimensional simulation model to give the current density at the DL boundaries. Results are presented for the scaling of the DL potential as a function of the width of the arc and the saturation efficiency of the lower hybrid instability mechanism.
Anomalous transport in discrete arcs and simulation of double layers in a model auroral circuit
NASA Technical Reports Server (NTRS)
Smith, Robert A.
1987-01-01
The evolution and long-time stability of a double layer in a discrete auroral arc requires that the parallel current in the arc, which may be considered uniform at the source, be diverted within the arc to charge the flanks of the U-shaped double-layer potential structure. A simple model is presented in which this current re-distribution is effected by anomalous transport based on electrostatic lower hybrid waves driven by the flank structure itself. This process provides the limiting constraint on the double-layer potential. The flank charging may be represented as that of a nonlinear transmission. A simplified model circuit, in which the transmission line is represented by a nonlinear impedance in parallel with a variable resistor, is incorporated in a 1-d simulation model to give the current density at the DL boundaries. Results are presented for the scaling of the DL potential as a function of the width of the arc and the saturation efficiency of the lower hybrid instability mechanism.
NASA Astrophysics Data System (ADS)
Borodin, V. A.; Kevorkyan, Yu. R.; Kolomytkin, V. V.; Ryazanov, A. I.
1992-04-01
The effect of external loading on vacancy pipe diffusion along a dislocation line is studied. It is shown that the external load leads to orientational dependence of dislocation sink strengths due to polarization of vacancies in the dislocation core. Computer calculations of the dipole tensor for vacancy diffusion in the core of a [100](010) dislocation in α-Fe are performed to obtain an estimate of the irradiation creep compliance value.
Glide dislocation nucleation from dislocation nodes at semi-coherent {111} Cu–Ni interfaces
Shao, Shuai; Wang, Jian; Beyerlein, Irene J.; Misra, Amit
2015-07-23
Using atomistic simulations and dislocation theory on a model system of semi-coherent {1 1 1} interfaces, we show that misfit dislocation nodes adopt multiple atomic arrangements corresponding to the creation and redistribution of excess volume at the nodes. We identified four distinctive node structures: volume-smeared nodes with (i) spiral or (ii) straight dislocation patterns, and volume-condensed nodes with (iii) triangular or (iv) hexagonal dislocation patterns. Volume-smeared nodes contain interfacial dislocations lying in the Cu–Ni interface but volume-condensed nodes contain two sets of interfacial dislocations in the two adjacent interfaces and jogs across the atomic layer between the two adjacent interfaces. Finally, under biaxial tension/compression applied parallel to the interface, we show that the nucleation of lattice dislocations is preferred at the nodes and is correlated with the reduction of excess volume at the nodes.
Glide dislocation nucleation from dislocation nodes at semi-coherent {111} Cu–Ni interfaces
Shao, Shuai; Wang, Jian; Beyerlein, Irene J.; ...
2015-07-23
Using atomistic simulations and dislocation theory on a model system of semi-coherent {1 1 1} interfaces, we show that misfit dislocation nodes adopt multiple atomic arrangements corresponding to the creation and redistribution of excess volume at the nodes. We identified four distinctive node structures: volume-smeared nodes with (i) spiral or (ii) straight dislocation patterns, and volume-condensed nodes with (iii) triangular or (iv) hexagonal dislocation patterns. Volume-smeared nodes contain interfacial dislocations lying in the Cu–Ni interface but volume-condensed nodes contain two sets of interfacial dislocations in the two adjacent interfaces and jogs across the atomic layer between the two adjacent interfaces.more » Finally, under biaxial tension/compression applied parallel to the interface, we show that the nucleation of lattice dislocations is preferred at the nodes and is correlated with the reduction of excess volume at the nodes.« less
Using Discrete Event Computer Simulation to Improve Patient Flow in a Ghanaian Acute Care Hospital
Best, Allyson M.; Dixon, Cinnamon A.; Kelton, W. David; Lindsell, Christopher J.
2014-01-01
Objectives Crowding and limited resources have increased the strain on acute care facilities and emergency departments (EDs) worldwide. These problems are particularly prevalent in developing countries. Discrete event simulation (DES) is a computer-based tool that can be used to estimate how changes to complex healthcare delivery systems, such as EDs, will affect operational performance. Using this modality, our objective was to identify operational interventions that could potentially improve patient throughput of one acute care setting in a developing country. Methods We developed a simulation model of acute care at a district level hospital in Ghana to test the effects of resource-neutral (e.g. modified staff start times and roles) and resource-additional (e.g. increased staff) operational interventions on patient throughput. Previously captured, de-identified time-and-motion data from 487 acute care patients were used to develop and test the model. The primary outcome was the modeled effect of interventions on patient length of stay (LOS). Results The base-case (no change) scenario had a mean LOS of 292 minutes (95% CI 291, 293). In isolation, neither adding staffing, changing staff roles, nor varying shift times affected overall patient LOS. Specifically, adding two registration workers, history takers, and physicians resulted in a 23.8 (95% CI 22.3, 25.3) minute LOS decrease. However, when shift start-times were coordinated with patient arrival patterns, potential mean LOS was decreased by 96 minutes (95% CI 94, 98); and with the simultaneous combination of staff roles (Registration and History-taking) there was an overall mean LOS reduction of 152 minutes (95% CI 150, 154). Conclusions Resource-neutral interventions identified through DES modeling have the potential to improve acute care throughput in this Ghanaian municipal hospital. DES offers another approach to identifying potentially effective interventions to improve patient flow in emergency and acute
Markov modeling and discrete event simulation in health care: a systematic comparison.
Standfield, Lachlan; Comans, Tracy; Scuffham, Paul
2014-04-01
The aim of this study was to assess if the use of Markov modeling (MM) or discrete event simulation (DES) for cost-effectiveness analysis (CEA) may alter healthcare resource allocation decisions. A systematic literature search and review of empirical and non-empirical studies comparing MM and DES techniques used in the CEA of healthcare technologies was conducted. Twenty-two pertinent publications were identified. Two publications compared MM and DES models empirically, one presented a conceptual DES and MM, two described a DES consensus guideline, and seventeen drew comparisons between MM and DES through the authors' experience. The primary advantages described for DES over MM were the ability to model queuing for limited resources, capture individual patient histories, accommodate complexity and uncertainty, represent time flexibly, model competing risks, and accommodate multiple events simultaneously. The disadvantages of DES over MM were the potential for model overspecification, increased data requirements, specialized expensive software, and increased model development, validation, and computational time. Where individual patient history is an important driver of future events an individual patient simulation technique like DES may be preferred over MM. Where supply shortages, subsequent queuing, and diversion of patients through other pathways in the healthcare system are likely to be drivers of cost-effectiveness, DES modeling methods may provide decision makers with more accurate information on which to base resource allocation decisions. Where these are not major features of the cost-effectiveness question, MM remains an efficient, easily validated, parsimonious, and accurate method of determining the cost-effectiveness of new healthcare interventions.
Sub-discretized surface model with application to contact mechanics in multi-body simulation
Johnson, S; Williams, J
2008-02-28
The mechanics of contact between rough and imperfectly spherical adhesive powder grains are often complicated by a variety of factors, including several which vary over sub-grain length scales. These include several traction factors that vary spatially over the surface of the individual grains, including high energy electron and acceptor sites (electrostatic), hydrophobic and hydrophilic sites (electrostatic and capillary), surface energy (general adhesion), geometry (van der Waals and mechanical), and elasto-plastic deformation (mechanical). For mechanical deformation and reaction, coupled motions, such as twisting with bending and sliding, as well as surface roughness add an asymmetry to the contact force which invalidates assumptions for popular models of contact, such as the Hertzian and its derivatives, for the non-adhesive case, and the JKR and DMT models for adhesive contacts. Though several contact laws have been offered to ameliorate these drawbacks, they are often constrained to particular loading paths (most often normal loading) and are relatively complicated for computational implementation. This paper offers a simple and general computational method for augmenting contact law predictions in multi-body simulations through characterization of the contact surfaces using a hierarchically-defined surface sub-discretization. For the case of adhesive contact between powder grains in low stress regimes, this technique can allow a variety of existing contact laws to be resolved across scales, allowing for moments and torques about the contact area as well as normal and tangential tractions to be resolved. This is especially useful for multi-body simulation applications where the modeler desires statistical distributions and calibration for parameters in contact laws commonly used for resolving near-surface contact mechanics. The approach is verified against analytical results for the case of rough, elastic spheres.
Collective Dislocation Dynamics and Avalanches during Fatigue of Aluminum
NASA Astrophysics Data System (ADS)
Rhouma, W. Ben; Deschanel, S.; Weiss, J.
2011-09-01
We present a study of collective dislocation dynamics and plasticity during fatigue of pure Aluminum from the analysis of continuous and discrete acoustic emission (AE). The three stages of macroscopic fatigue behavior (strain-hardening, shakedown, and strain softening) are clearly differentiated in terms of AE. During the first loading cycles, collective dislocation dynamics consists in dislocation avalanches of various sizes and clustered in time. Once a microstructure of dislocation cells and walls is formed, the spreading of such avalanches is restrained, and the discrete AE strongly decreases. Instead, a symmetrical (tension-compression) continuous AE, maximal at plastic yield, is observed, likely associated to a superposition of numerous, small and uncorrelated motions such as dislocation loops initiation from cell walls. However, some discrete AE activity remains during shakedown, a possible signature of sudden rearrangements of the microstructure occurring at scales larger than its wavelength. Finally, the onset of strain softening is associated to a strong increase of discrete AE, in relation with microcracking. Our results suggest that collective dislocation instabilities and the emergence of a dislocation microstructure are interrelated, and challenge future numerical modeling developments of dislocation assemblies.
ERIC Educational Resources Information Center
Arnal, Lindsay; Fazzio, Daniela; Martin, Garry L.; Yu, C. T.; Keilback, Lukas; Starke, Mandy
2007-01-01
An essential component of applied behavior analysis programs for teaching children with autism is discrete trials teaching. Experiment 1 investigated the effectiveness of a self-instructional manual for teaching university students to correctly apply discrete-trials teaching to teach three tasks to confederates role-playing children with autism.…
Multiscale Theory of Dislocation Climb.
Geslin, Pierre-Antoine; Appolaire, Benoît; Finel, Alphonse
2015-12-31
Dislocation climb is a ubiquitous mechanism playing a major role in the plastic deformation of crystals at high temperature. We propose a multiscale approach to model quantitatively this mechanism at mesoscopic length and time scales. First, we analyze climb at a nanoscopic scale and derive an analytical expression of the climb rate of a jogged dislocation. Next, we deduce from this expression the activation energy of the process, bringing valuable insights to experimental studies. Finally, we show how to rigorously upscale the climb rate to a mesoscopic phase-field model of dislocation climb. This upscaling procedure opens the way to large scale simulations where climb processes are quantitatively reproduced even though the mesoscopic length scale of the simulation is orders of magnitude larger than the atomic one.
NASA Astrophysics Data System (ADS)
Meyers, J.; Geurts, B. J.; Sagaut, P.
2007-11-01
We present a framework for the computational assessment and comparison of large-eddy simulation methods. We apply this to large-eddy simulation of homogeneous isotropic decaying turbulence using a Smagorinsky subgrid model and investigate the combined effect of discretization and model errors at coarse subgrid resolutions. We compare four different central finite-volume methods. These discretization methods arise from the four possible combinations that can be made with a second-order and a fourth-order central scheme for either the convective and the viscous fluxes. By systematically varying the simulation resolution and the Smagorinsky coefficient, we determine parameter regions for which a desired number of flow properties is simultaneously predicted with approximately minimal error. We include both physics-based and mathematics-based error definitions, leading to different error-measures designed to emphasize either errors in large- or in small-scale flow properties. It is shown that the evaluation of simulations based on a single physics-based error may lead to inaccurate perceptions on quality. We demonstrate however that evaluations based on a range of errors yields robust conclusions on accuracy, both for physics-based and mathematics-based errors. Parameter regions where all considered errors are simultaneously near-optimal are referred to as 'multi-objective optimal' parameter regions. The effects of discretization errors are particularly important at marginal spatial resolution. Such resolutions reflect local simulation conditions that may also be found in parts of more complex flow simulations. Under these circumstances, the asymptotic error-behavior as expressed by the order of the spatial discretization is no longer characteristic for the total dynamic consequences of discretization errors. We find that the level of overall simulation errors for a second-order central discretization of both the convective and viscous fluxes (the '2-2' method), and the
NASA Astrophysics Data System (ADS)
Kulchitsky, A. V.; Johnson, J.; Duvoy, P.; Wilkinson, A.; Creager, C. M.
2012-12-01
For in situ resource utilization on the Moon, asteroids, Mars, or other space body it is necessary to be able to simulate the interaction of mobile platforms and excavation machines with the regolith for engineering design, planning, and operations. For accurate simulations, tools designed to measure regolith properties will need to be deployed and interpreted. Two such tools are the penetrometer, used to measure a soil strength index as a function of depth, and the bevameter, used to characterize regolith surface properties of strength, friction and sinkage. The penetrometer interrogates regolith properties from the surface to a depth limited only by the capabilities of the instrument to penetrate the regolith while a bevameter interrogates only the upper few centimeters needed to describe a mobility platform's traction and sinkage. Interpretation of penetrometer and bevameter data can be difficult, especially on low gravity objects. We use the discrete element method (DEM) model to simulate the large regolith deformations and failures associated with the tests to determine regolith properties. The DEM simulates granular material behavior using large aggregates of distinct particles. Realistic physics of particle-particle interaction introduces many granular specific phenomena such as interlocking and force chain formation that cannot be represented using continuum methods. In this work, experiments using a cone penetrometer test (CPT) and bevameter on lunar simulants JSC-1A and GRC-1 were performed at NASA Glenn Research Center. These tests were used to validate the physics in the COUPi DEM model. COUPi 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. The experimental results were used in this work to build an accurate model to simulate the lunar regolith. The CPT consists of driving an instrumented cone with opening angle of 60
Discretization effects and the scalar meson correlator in mixed-action lattice simulations
Aubin, C.; Laiho, Jack; Van de Water, Ruth S.
2008-06-01
We study discretization effects in a mixed-action lattice theory with domain-wall valence quarks and Asqtad-improved staggered sea quarks. At the level of the chiral effective Lagrangian, discretization effects in the mixed-action theory give rise to two new parameters as compared to the lowest order Lagrangian for rooted-staggered fermions - the residual quark mass m{sub res} and the mixed valence-sea meson mass splitting {delta}{sub mix}. We find that m{sub res}, which parametrizes explicit chiral symmetry breaking in the mixed-action theory, is approximately one-quarter the size of our lightest valence quark mass on our coarser lattice spacing and of comparable size to that of simulations by the RBC and UKQCD Collaborations. We also find that the size of {delta}{sub mix} is comparable to the size of the smallest of the staggered meson taste splittings measured by the MILC Collaboration. Because lattice artifacts are different in the valence and sea sectors of the mixed-action theory, they give rise to unitarity-violating effects that disappear in the continuum limit, some of which should be described by mixed-action chiral perturbation theory (MA{chi}PT). Such effects are expected to be mild for many quantities of interest but are expected to be significant in the case of the isovector scalar (a{sub 0}) correlator. Specifically, once the parameters m{sub res}, {delta}{sub mix}, and two others that can be determined from the light pseudoscalar meson spectrum are known, the two-particle intermediate state 'bubble' contribution to the scalar correlator is completely predicted within MA{chi}PT. We find that the behavior of the scalar meson correlator is quantitatively consistent with the MA{chi}PT prediction; this supports the claim that MA{chi}PT describes the dominant unitarity-violating effects in the mixed-action theory and can therefore be used to remove lattice artifacts and recover physical quantities.
Fracture mechanics of propagating 3-D fatigue cracks with parametric dislocations
NASA Astrophysics Data System (ADS)
Takahashi, Akiyuki; Ghoniem, Nasr M.
2013-07-01
Propagation of 3-D fatigue cracks is analyzed using a discrete dislocation representation of the crack opening displacement. Three dimensional cracks are represented with Volterra dislocation loops in equilibrium with the applied external load. The stress intensity factor (SIF) is calculated using the Peach-Koehler (PK) force acting on the crack tip dislocation loop. Loading mode decomposition of the SIF is achieved by selection of Burgers vector components to correspond to each fracture mode in the PK force calculations. The interaction between 3-D cracks and free surfaces is taken into account through application of the superposition principle. A boundary integral solution of an elasticity problem in a finite domain is superposed onto the elastic field solution of the discrete dislocation method in an infinite medium. The numerical accuracy of the SIF is ascertained by comparison with known analytical solution of a 3-D crack problem in pure mode I, and for mixed-mode loading. Finally, fatigue crack growth simulations are performed with the Paris law, showing that 3-D cracks do not propagate in a self-similar shape, but they re-configure as a result of their interaction with external boundaries. A specific numerical example of fatigue crack growth is presented to demonstrate the utility of the developed method for studies of 3-D crack growth during fatigue.
NASA Astrophysics Data System (ADS)
Ruggles, Timothy J.
Modeling of plasticity is often hampered by the difficulty in accurately characterizing dislocation density on the microscale for real samples. It is particularly difficult to resolve measured dislocation content onto individual dislocation systems at the length scales most commonly of interest in plasticity studies. Traditionally, dislocation content is analyzed at the continuum level using the Nye tensor and the fundamental relation of continuum dislocation theory to interpret information measured by diffraction techniques, typically EBSD or High Resolution EBSD. In this work the established Nye-Kroner method for resolving measured geometrically necessary dislocation content onto individual slip systems is assessed and extended. Two new methods are also presented to relieve the ambiguity of the Nye-Kroner method. One of these methods uses modified classical dislocation equations to bypass the Nye-Kroner relation, and the other estimates the bulk dislocation density via the entry-wise one-norm of the Nye tensor. These methods are validated via a novel simulation of distortion fields around continuum fields of dislocation density based on classical lattice mechanics and then applied to actual HR-EBSD scans of a micro-indented single crystals of nickel and tantalum. Finally, a detailed analysis of the effect of the spacing between points in an EBSD scan (which is related to the step size of the numerical derivatives used in EBSD dislocation microscopy) on geometrically necessary dislocation measurements is conducted.
Initial dislocation structure and dynamic dislocation multiplication in Mo single crystals
Hsiung, L M; Lassila, D H
2000-03-22
Initial dislocation structure in annealed high-purity Mo single crystals and deformation substructure in a crystal subjected to 1% compression have been examined and studied in order to investigate dislocation multiplication mechanisms in the early stages of plastic deformation. The initial dislocation density is in a range of 10{sup 6} {approx} 10{sup 7} cm{sup -2}, and the dislocation structure is found to contain many grown-in superjogs along dislocation lines. The dislocation density increases to a range of 10{sup 8} {approx} 10{sup 9} cm{sup -2}, and the average jog height is also found to increase after compressing for a total strain of 1%. It is proposed that the preexisting jogged screw dislocations can act as (multiple) dislocation multiplication sources when deformed under quasi-static conditions. Both the jog height and length of link segment (between jogs) can increase by stress-induced jog coalescence, which takes place via the lateral migration (drift) of superjogs driven by unbalanced line-tension partials acting on link segments of unequal lengths. Applied shear stress begins to push each link segment to precede dislocation multiplication when link length and jog height are greater than critical lengths. This dynamic dislocation multiplication source is subsequently verified by direct simulations of dislocation dynamics under stress to be crucial in the early stages of plastic deformation in Mo single crystals.
Anderson, Gillian H; Jenkins, Paul J; McDonald, David A; Van Der Meer, Robert; Morton, Alec; Nugent, Margaret; Rymaszewski, Lech A
2017-09-07
Healthcare faces the continual challenge of improving outcome while aiming to reduce cost. The aim of this study was to determine the micro cost differences of the Glasgow non-operative trauma virtual pathway in comparison to a traditional pathway. Discrete event simulation was used to model and analyse cost and resource utilisation with an activity-based costing approach. Data for a full comparison before the process change was unavailable so we used a modelling approach, comparing a virtual fracture clinic (VFC) with a simulated traditional fracture clinic (TFC). The orthopaedic unit VFC pathway pioneered at Glasgow Royal Infirmary has attracted significant attention and interest and is the focus of this cost study. Our study focused exclusively on patients with non-operative trauma attending emergency department or the minor injuries unit and the subsequent step in the patient pathway. Retrospective studies of patient outcomes as a result of the protocol introductions for specific injuries are presented in association with activity costs from the models. Patients are satisfied with the new pathway, the information provided and the outcome of their injuries (Evidence Level IV). There was a 65% reduction in the number of first outpatient face-to-face (f2f) attendances in orthopaedics. In the VFC pathway, the resources required per day were significantly lower for all staff groups (p≤0.001). The overall cost per patient of the VFC pathway was £22.84 (95% CI 21.74 to 23.92) per patient compared with £36.81 (95% CI 35.65 to 37.97) for the TFC pathway. Our results give a clearer picture of the cost comparison of the virtual pathway over a wholly traditional f2f clinic system. The use of simulation-based stochastic costings in healthcare economic analysis has been limited to date, but this study provides evidence for adoption of this method as a basis for its application in other healthcare settings. © Article author(s) (or their employer(s) unless otherwise
NASA Astrophysics Data System (ADS)
Wang, Yueying; Yao, Jun; Fu, Shuaishi; Lv, Aimin; Sun, Zhixue; Bongole, Kelvin
2017-08-01
Isolated fractures usually exist in fractured media systems, where the capillary pressure in the fracture is lower than that of the matrix, causing the discrepancy in oil recoveries between fractured and non-fractured porous media. Experiments, analytical solutions and conventional simulation methods based on the continuum model approach are incompetent or insufficient in describing media containing isolated fractures. In this paper, the simulation of the counter-current imbibition in fractured media is based on the discrete-fracture model (DFM). The interlocking or arrangement of matrix and fracture system within the model resembles the traditional discrete fracture network model and the hybrid-mixed-finite-element method is employed to solve the associated equations. The Behbahani experimental data validates our simulation solution for consistency. The simulation results of the fractured media show that the isolated-fractures affect the imbibition in the matrix block. Moreover, the isolated fracture parameters such as fracture length and fracture location influence the trend of the recovery curves. Thus, the counter-current imbibition behavior of media with isolated fractures can be predicted using this method based on the discrete-fracture model.
Direct determination of discrete harmonic bath parameters from molecular dynamics simulations.
Walters, Peter L; Allen, Thomas C; Makri, Nancy
2017-01-15
We present a direct procedure for determining the parameters of a discrete harmonic bath modeling the influence of a complex condensed phase environment on the system of interest. The procedure employs an efficient discretization of the spectral density into modes that correspond to equal fractions of the reorganization energy. The new procedure uses directly the classical correlation function (available from molecular dynamics calculations) as input, avoiding numerical computation of the spectral density by means of a discrete Fourier transform. Convergence is obtained using a shorter time length of the correlation function, leading to significant computational savings. © 2016 Wiley Periodicals, Inc.
Sharda, Praveen; DuFosse, Julian
2008-07-01
Subtalar dislocations are rare in routine orthopedic practice. While many of these dislocations are a result of high-energy injuries such as fall from a height or traffic accidents, it is not uncommon for patients to present after slipping down a few stairs. Two types of dislocation have been described, medial and lateral. The type of dislocation is described according to the position of the foot. In lateral subtalar dislocation the head of talus is found medially and the calcaneus is dislocated laterally. The navicular may lie dorsolateral to the talus. The reverse is true of lateral dislocation. Medial dislocation has been referred to as "basketball foot" due to its preponderance in basketball players.4 The deciding factor is the inverted or everted position of the foot when the force is dissipated through the weak talonavicular and talocalcaneal ligaments. This article presents a case of an adult with lateral subtalar dislocation following a fall.
NASA Astrophysics Data System (ADS)
Lessmann, Johann-Sebastian; Schoeppner, Volker
2016-03-01
The goal of this contribution is to describe a method of simulating solids-conveying processes in single screw extruders which include a defined back pressure leading to a resulting pressure buildup in the screw channel. To do so, use is made of the Discrete Element Method. Material parameters are presented, as well as details concerning the contact model used and the simulation tool EDEM. Additionally, a test setup is presented which has been used to validate the solids-conveying simulations. Results are shown for both simulations and experimental tests. Comparing the results from simulations and measurements shows acceptable conformity. Such simulations and experimental tests are crucial in order to better understand the buildup of pressure in high-speed single-screw extruders.
NASA Astrophysics Data System (ADS)
Qi, Chenkun; Zhao, Xianchao; Gao, Feng; Ren, Anye; Hu, Yan
2016-11-01
The hardware-in-the-loop (HIL) contact simulation for flying objects in space is challenging due to the divergence caused by the time delay. In this study, a divergence compensation approach is proposed for the stiffness-varying discrete contact. The dynamic response delay of the motion simulator and the force measurement delay are considered. For the force measurement delay, a phase lead based force compensation approach is used. For the dynamic response delay of the motion simulator, a response error based force compensation approach is used, where the compensation force is obtained from the real-time identified contact stiffness and real-time measured position response error. The dynamic response model of the motion simulator is not required. The simulations and experiments show that the simulation divergence can be compensated effectively and satisfactorily by using the proposed approach.
Dislocation pileup as a representation of strain accumulation on a strike-slip fault
Savage, J.C.
2006-01-01
The conventional model of strain accumulation on a vertical transform fault is a discrete screw dislocation in an elastic half-space with the Burgers vector of the dislocation increasing at the rate of relative plate motion. It would be more realistic to replace that discrete dislocation by a dislocation distribution, presumably a pileup in which the individual dislocations are in equilibrium. The length of the pileup depends upon the applied stress and the amount of slip that has occurred at depth. I argue here that the dislocation pileup (the transition on the fault from no slip to slip at the full plate rate) occupies a substantial portion of the lithosphere thickness. A discrete dislocation at an adjustable depth can reproduce the surface deformation profile predicted by a pileup so closely that it will be difficult to distinguish between the two models. The locking depth (dislocation depth) of that discrete dislocation approximation is substantially (???30%) larger than that (depth to top of the pileup) in the pileup model. Thus, in inverting surface deformation data using the discrete dislocation model, the locking depth in the model should not be interpreted as the true locking depth. Although dislocation pileup models should provide a good explanation of the surface deformation near the fault trace, that explanation may not be adequate at greater distances from the fault trace because approximating the expected horizontally distributed deformation at subcrustal depths by uniform slip concentrated on the fault is not justified.
Reexamination of Lunar Exospheric Dust Estimates Using Discrete Dipole Scattering Simulations
NASA Astrophysics Data System (ADS)
Stubbs, T. J.; Glenar, D. A.; Richard, D. T.; Feldman, P. D.; Retherford, K. D.
2014-12-01
Analysis of Apollo regolith samples showed that lunar dust grains consist of a diverse set of shapes. Consequently, the optical scattering properties of these grains will differ from those predicted using the Mie approximation, which strictly applies only for spheres. Because it is analytically convenient and without shape ambiguity, Mie theory has been used routinely to estimate the concentration of dust or it's upper limits in the lunar exosphere from brightness measurements acquired during orbital dust searches. Utilizing the Discrete Dipole Approximation (DDA), we have computed a more realistic set of scattering parameters for a collection of sub-micron grain shapes that represents the ultra-fine fraction of lunar soil. Included in this suite are spheroids (oblate and prolate) and irregular geometries resembling isolated grains observed in Apollo samples. A subset of these models includes the addition of nanophase iron, in order to examine the influence of space weathering. Wavelength coverage of the DDA scattering computations extends from far-UV to near-IR. This range is diagnostic of grain size and shape, since scattering efficiency depends on both of these parameters. This collection of grain scattering models is used, together with an observing simulation code, to reexamine some prior estimates of exospheric dust concentration derived from Apollo-era limb brightness measurements (e.g., Apollo 15 coronal photography), as well as the subsequent Clementine star tracker search and a search for lunar horizon glow by LRO Lyman Alpha Mapping Project (LAMP). We compare our revised estimates of exospheric dust abundance with the results of these previous dust searches.
Numerical simulation on ferrofluid flow in fractured porous media based on discrete-fracture model
NASA Astrophysics Data System (ADS)
Huang, Tao; Yao, Jun; Huang, Zhaoqin; Yin, Xiaolong; Xie, Haojun; Zhang, Jianguang
2017-06-01
Water flooding is an efficient approach to maintain reservoir pressure and has been widely used to enhance oil recovery. However, preferential water pathways such as fractures can significantly decrease the sweep efficiency. Therefore, the utilization ratio of injected water is seriously affected. How to develop new flooding technology to further improve the oil recovery in this situation is a pressing problem. For the past few years, controllable ferrofluid has caused the extensive concern in oil industry as a new functional material. In the presence of a gradient in the magnetic field strength, a magnetic body force is produced on the ferrofluid so that the attractive magnetic forces allow the ferrofluid to be manipulated to flow in any desired direction through the control of the external magnetic field. In view of these properties, the potential application of using the ferrofluid as a new kind of displacing fluid for flooding in fractured porous media is been studied in this paper for the first time. Considering the physical process of the mobilization of ferrofluid through porous media by arrangement of strong external magnetic fields, the magnetic body force was introduced into the Darcy equation and deals with fractures based on the discrete-fracture model. The fully implicit finite volume method is used to solve mathematical model and the validity and accuracy of numerical simulation, which is demonstrated through an experiment with ferrofluid flowing in a single fractured oil-saturated sand in a 2-D horizontal cell. At last, the water flooding and ferrofluid flooding in a complex fractured porous media have been studied. The results showed that the ferrofluid can be manipulated to flow in desired direction through control of the external magnetic field, so that using ferrofluid for flooding can raise the scope of the whole displacement. As a consequence, the oil recovery has been greatly improved in comparison to water flooding. Thus, the ferrofluid
Koester, Martin; García, R Edwin; Thommes, Markus
2014-12-30
Spheronization is an important pharmaceutical manufacturing technique to produce spherical agglomerates of 0.5-2mm diameter. These pellets have a narrow size distribution and a spherical shape. During the spheronization process, the extruded cylindrical strands break in short cylinders and evolve from a cylindrical to a spherical state by deformation and attrition/agglomeration mechanisms. Using the discrete element method, an integrated modeling-experimental framework is presented, that captures the particle motion during the spheronization process. Simulations were directly compared and validated against particle image velocimetry (PIV) experiments with monodisperse spherical and dry γ-Al2O3 particles. demonstrate a characteristic torus like flow pattern, with particle velocities about three times slower than the rotation speed of the friction plate. Five characteristic zones controlling the spheronization process are identified: Zone I, where particles undergo shear forces that favors attrition and contributes material to the agglomeration process; Zone II, where the static wall contributes to the mass exchange between particles; Zone III, where gravitational forces combined with particle motion induce particles to collide with the moving plate and re-enter Zone I; Zone IV, where a subpopulation of particles are ejected into the air when in contact with the friction plate structure; and Zone V where the low poloidal velocity favors a stagnant particle population and is entirely controlled by the batch size. These new insights in to the particle motion are leading to deeper process understanding, e.g., the effect of load and rotation speed to the pellet formation kinetics. This could be beneficial for the optimization of a manufacturing process as well as for the development of new formulations. Copyright © 2014 Elsevier B.V. All rights reserved.
Stable grid refinement and singular source discretization for seismic wave simulations
Petersson, N A; Sjogreen, B
2009-10-30
An energy conserving discretization of the elastic wave equation in second order formulation is developed for a composite grid, consisting of a set of structured rectangular component grids with hanging nodes on the grid refinement interface. Previously developed summation-by-parts properties are generalized to devise a stable second order accurate coupling of the solution across mesh refinement interfaces. The discretization of singular source terms of point force and point moment tensor type are also studied. Based on enforcing discrete moment conditions that mimic properties of the Dirac distribution and its gradient, previous single grid formulas are generalized to work in the vicinity of grid refinement interfaces. These source discretization formulas are shown to give second order accuracy in the solution, with the error being essentially independent of the distance between the source and the grid refinement boundary. Several numerical examples are given to illustrate the properties of the proposed method.
Dislocation motion and instability
NASA Astrophysics Data System (ADS)
Zhu, Yichao; Chapman, Stephen Jonathan; Acharya, Amit
2013-08-01
The Peach-Koehler expression for the stress generated by a single (non-planar) curvilinear dislocation is evaluated to calculate the dislocation self stress. This is combined with a law of motion to give the self-induced motion of a general dislocation curve. A stability analysis of a rectilinear, uniformly translating dislocation is then performed. The dislocation is found to be susceptible to a helical instability, with the maximum growth rate occurring when the dislocation is almost, but not exactly, pure screw. The non-linear evolution of the instability is determined numerically, and implications for slip band formation and non-Schmid behavior in yielding are discussed.
Bloomer, I.; Charap, J.M.
1985-09-01
Dislocation continuity is derived from the Bilby--Kondo theory of dislocations using exterior calculus. Dislocation density is represented by the torsion vector-valued two-form. Burgers vectors are associated with the vector part of the torsion while dislocation lines are associated with the two-form part. The exterior derivative of the torsion is shown to vanish when the crystal curvature vanishes. This implies two simultaneous continuity conditions: Burgers vector conservation and continuity of dislocation lines. On the other hand, dislocation continuity is violated when the curvature does not vanish. Since this can occur on grain boundaries it is inferred that grain boundaries are regions where crystal curvature is concentrated.
NASA Astrophysics Data System (ADS)
Ye, Zheng; Xie, Zheng; Ma, Yu-Jie
2009-08-01
We show how to construct discrete Maxwell equations by discrete exterior calculus. The new scheme has many virtues compared to the traditional Yee's scheme: it is a multisymplectic scheme and keeps geometric properties. Moreover, it can be applied on triangular mesh and thus is more adaptive to handle domains with irregular shapes. We have implemented this scheme on a Java platform successfully and our experimental results show that this scheme works well.
Zheng, Zhongquan C.; Wei, Zhenglun A.; Bennett, James S.; Yang, Xiaofan
2012-12-11
In simulating fluid/solid-particle multiphase -flows, various methods are available. One approach is the combined Euler-Lagrange method, which simulates the fluid phase flow in the Eulerian framework and the discrete phase (particle) motion in the Lagrangian framework simultaneously. The Lagrangian approach, where particle motion is determined by the current state of the fluid phase flow, is also called the discrete phase model (DPM), in the context of numerical flow simulation. In this method, the influence of the particle motions on the fluid flow can be included (two-way interactions) but are more commonly excluded (one-way interactions, when the discrete phase concentration is dilute. The other approach is to treat the particle number concentration as a continuous species, a necessarily passive quantity determined by the fluid flow, with no influences from the particles on the fluid flow (one-way interactions only), except to the extent the discrete phase “continuum” alters the overall fluid properties, such as density. In this paper, we compare these two methods with experimental data for an indoor environmental chamber. The effects of injection particle numbers and the related boundary conditions are investigated. In the Euler-Lagrange interaction or DPM model for incompressible flow, the Eulerian continuous phase is governed by the Reynolds-averaged N-S (RANS) equations. The motions of particles are governed by Newton’s second law. The effects of particle motions are communicated to the continuous phase through a force term in the RANS equations. The second formulation is a pure Eulerian type, where only the particle-number concentration is addressed, rather than the motion of each individual particle. The fluid flow is governed by the same RANS equations without the particle force term. The particle-number concentration is simulated by a species transport equation. Comparisons among the models and with experimental and literature data are presented
Discrete-State Simulated Annealing For Traveling-Wave Tube Slow-Wave Circuit Optimization
NASA Technical Reports Server (NTRS)
Wilson, Jeffrey D.; Bulson, Brian A.; Kory, Carol L.; Williams, W. Dan (Technical Monitor)
2001-01-01
Algorithms based on the global optimization technique of simulated annealing (SA) have proven useful in designing traveling-wave tube (TWT) slow-wave circuits for high RF power efficiency. The characteristic of SA that enables it to determine a globally optimized solution is its ability to accept non-improving moves in a controlled manner. In the initial stages of the optimization, the algorithm moves freely through configuration space, accepting most of the proposed designs. This freedom of movement allows non-intuitive designs to be explored rather than restricting the optimization to local improvement upon the initial configuration. As the optimization proceeds, the rate of acceptance of non-improving moves is gradually reduced until the algorithm converges to the optimized solution. The rate at which the freedom of movement is decreased is known as the annealing or cooling schedule of the SA algorithm. The main disadvantage of SA is that there is not a rigorous theoretical foundation for determining the parameters of the cooling schedule. The choice of these parameters is highly problem dependent and the designer needs to experiment in order to determine values that will provide a good optimization in a reasonable amount of computational time. This experimentation can absorb a large amount of time especially when the algorithm is being applied to a new type of design. In order to eliminate this disadvantage, a variation of SA known as discrete-state simulated annealing (DSSA), was recently developed. DSSA provides the theoretical foundation for a generic cooling schedule which is problem independent, Results of similar quality to SA can be obtained, but without the extra computational time required to tune the cooling parameters. Two algorithm variations based on DSSA were developed and programmed into a Microsoft Excel spreadsheet graphical user interface (GUI) to the two-dimensional nonlinear multisignal helix traveling-wave amplifier analysis program TWA3
2013-01-01
Background Computer simulation studies of the emergency department (ED) are often patient driven and consider the physician as a human resource whose primary activity is interacting directly with the patient. In many EDs, physicians supervise delegates such as residents, physician assistants and nurse practitioners each with different skill sets and levels of independence. The purpose of this study is to present an alternative approach where physicians and their delegates in the ED are modeled as interacting pseudo-agents in a discrete event simulation (DES) and to compare it with the traditional approach ignoring such interactions. Methods The new approach models a hierarchy of heterogeneous interacting pseudo-agents in a DES, where pseudo-agents are entities with embedded decision logic. The pseudo-agents represent a physician and delegate, where the physician plays a senior role to the delegate (i.e. treats high acuity patients and acts as a consult for the delegate). A simple model without the complexity of the ED is first created in order to validate the building blocks (programming) used to create the pseudo-agents and their interaction (i.e. consultation). Following validation, the new approach is implemented in an ED model using data from an Ontario hospital. Outputs from this model are compared with outputs from the ED model without the interacting pseudo-agents. They are compared based on physician and delegate utilization, patient waiting time for treatment, and average length of stay. Additionally, we conduct sensitivity analyses on key parameters in the model. Results In the hospital ED model, comparisons between the approach with interaction and without showed physician utilization increase from 23% to 41% and delegate utilization increase from 56% to 71%. Results show statistically significant mean time differences for low acuity patients between models. Interaction time between physician and delegate results in increased ED length of stay and longer
Anteromedial subtalar dislocation.
Stafford, Harry; Boggess, Blake; Toth, Alison; Berkoff, David
2013-01-25
Subtalar dislocation is the simultaneous dislocation of the talocalcaneal and talonavicular joints of the foot, typically caused by falls from heights, twisting leg injuries and motor vehicle accidents. The dislocation can occur medially, lateral, anterior or posterior, but most commonly occurs from inversion injury producing a medial dislocation. These dislocations may be accompanied by fractures. Careful physical examination must be performed to assess for neurovascular compromise. Most subtalar dislocations can be treated with closed reduction under sedation. However, if the dislocation is associated with an open fracture it may require reduction in the operating room. Treatment should include postreduction plain x-ray and CT scan to evaluate for proper alignment and for fractures. This article presents a case of medial subtalar dislocation in a 23-year-old football player.
NASA Astrophysics Data System (ADS)
Samtaney, Ravi; Mohamed, Mamdouh; Hirani, Anil
2015-11-01
We present examples of numerical solutions of incompressible flow on 2D curved domains. The Navier-Stokes equations are first rewritten using the exterior calculus notation, replacing vector calculus differential operators by the exterior derivative, Hodge star and wedge product operators. A conservative discretization of Navier-Stokes equations on simplicial meshes is developed based on discrete exterior calculus (DEC). The discretization is then carried out by substituting the corresponding discrete operators based on the DEC framework. By construction, the method is conservative in that both the discrete divergence and circulation are conserved up to machine precision. The relative error in kinetic energy for inviscid flow test cases converges in a second order fashion with both the mesh size and the time step. Numerical examples include Taylor vortices on a sphere, Stuart vortices on a sphere, and flow past a cylinder on domains with varying curvature. Supported by the KAUST Office of Competitive Research Funds under Award No. URF/1/1401-01.
Hudson, Christopher D; Huxley, Jonathan N; Green, Martin J
2014-01-01
The ever-growing volume of data routinely collected and stored in everyday life presents researchers with a number of opportunities to gain insight and make predictions. This study aimed to demonstrate the usefulness in a specific clinical context of a simulation-based technique called probabilistic sensitivity analysis (PSA) in interpreting the results of a discrete time survival model based on a large dataset of routinely collected dairy herd management data. Data from 12,515 dairy cows (from 39 herds) were used to construct a multilevel discrete time survival model in which the outcome was the probability of a cow becoming pregnant during a given two day period of risk, and presence or absence of a recorded lameness event during various time frames relative to the risk period amongst the potential explanatory variables. A separate simulation model was then constructed to evaluate the wider clinical implications of the model results (i.e. the potential for a herd's incidence rate of lameness to influence its overall reproductive performance) using PSA. Although the discrete time survival analysis revealed some relatively large associations between lameness events and risk of pregnancy (for example, occurrence of a lameness case within 14 days of a risk period was associated with a 25% reduction in the risk of the cow becoming pregnant during that risk period), PSA revealed that, when viewed in the context of a realistic clinical situation, a herd's lameness incidence rate is highly unlikely to influence its overall reproductive performance to a meaningful extent in the vast majority of situations. Construction of a simulation model within a PSA framework proved to be a very useful additional step to aid contextualisation of the results from a discrete time survival model, especially where the research is designed to guide on-farm management decisions at population (i.e. herd) rather than individual level.
Hudson, Christopher D.; Huxley, Jonathan N.; Green, Martin J.
2014-01-01
The ever-growing volume of data routinely collected and stored in everyday life presents researchers with a number of opportunities to gain insight and make predictions. This study aimed to demonstrate the usefulness in a specific clinical context of a simulation-based technique called probabilistic sensitivity analysis (PSA) in interpreting the results of a discrete time survival model based on a large dataset of routinely collected dairy herd management data. Data from 12,515 dairy cows (from 39 herds) were used to construct a multilevel discrete time survival model in which the outcome was the probability of a cow becoming pregnant during a given two day period of risk, and presence or absence of a recorded lameness event during various time frames relative to the risk period amongst the potential explanatory variables. A separate simulation model was then constructed to evaluate the wider clinical implications of the model results (i.e. the potential for a herd’s incidence rate of lameness to influence its overall reproductive performance) using PSA. Although the discrete time survival analysis revealed some relatively large associations between lameness events and risk of pregnancy (for example, occurrence of a lameness case within 14 days of a risk period was associated with a 25% reduction in the risk of the cow becoming pregnant during that risk period), PSA revealed that, when viewed in the context of a realistic clinical situation, a herd’s lameness incidence rate is highly unlikely to influence its overall reproductive performance to a meaningful extent in the vast majority of situations. Construction of a simulation model within a PSA framework proved to be a very useful additional step to aid contextualisation of the results from a discrete time survival model, especially where the research is designed to guide on-farm management decisions at population (i.e. herd) rather than individual level. PMID:25101997
Quantum dislocations in solid Helium-4
NASA Astrophysics Data System (ADS)
Aleinikava, Darya
In this thesis the following problems on properties of solid 4He are considered: (i) the role of long-range interactions in suppression of dislocation roughening at T = 0; (ii) the combined effect of 3He impurities and Peierls potential on shear modulus softening; (iii) the dislocation superclimb and its connection to the phenomenon of "giant isochoric compressibility"; (iv) non-linear dislocation response to the applied stress and stress-induces dislocation roughening as a I-order phase transition in 1D at finite temperature. First we investigate the effect of long-range interactions on the state of edge dislocation at T = 0. Such interactions are induced by elastic forces of the solid. We found that quantum roughening transition of a dislocation at T = 0 is completely suppressed by arbitrarily small long-range interactions between kinks. A heuristic argument is presented and the result has been verified by numerical Monte-Carlo simulations using Worm Algorithm in J-current model. It was shown that the Peierls potential plays a crucial role in explaining the elastic properties of dislocations, namely shear modulus softening phenomenon. The crossover from T = 0 to finite temperatures leads to intrinsic softening of the shear modulus and is solely controlled by kink typical energy. It was demonstrated that the mechanism, involving only the binding of 3He impurities to the dislocations, requires an unrealistically high concentrations of defects (or impurities) in order to explain the shear modulus phenomenon and therefore an inclusion of Peierls potential in consideration is required. Superclimbing dislocations, that is the edge dislocations with the superfluidity along the core, were investigated. The theoretical prediction that superclimb is responsible for the phenomenon of "giant isochoric compressibility" was confirmed by Monte-Carlo simulations. It was demonstrated that the isochoric compressibility is suppressed at low temperatures. The dependence of
Dislocation dynamics in hexagonal close-packed crystals
Aubry, S.; Rhee, M.; Hommes, G.; ...
2016-04-14
Extensions of the dislocation dynamics methodology necessary to enable accurate simulations of crystal plasticity in hexagonal close-packed (HCP) metals are presented. They concern the introduction of dislocation motion in HCP crystals through linear and non-linear mobility laws, as well as the treatment of composite dislocation physics. Formation, stability and dissociation of and other dislocations with large Burgers vectors defined as composite dislocations are examined and a new topological operation is proposed to enable their dissociation. Furthermore, the results of our simulations suggest that composite dislocations are omnipresent and may play important roles both in specific dislocation mechanisms and in bulkmore » crystal plasticity in HCP materials. While fully microscopic, our bulk DD simulations provide wealth of data that can be used to develop and parameterize constitutive models of crystal plasticity at the mesoscale.« less
Dislocation dynamics in hexagonal close-packed crystals
Aubry, S.; Rhee, M.; Hommes, G.; Bulatov, V. V.; Arsenlis, A.
2016-04-14
Extensions of the dislocation dynamics methodology necessary to enable accurate simulations of crystal plasticity in hexagonal close-packed (HCP) metals are presented. They concern the introduction of dislocation motion in HCP crystals through linear and non-linear mobility laws, as well as the treatment of composite dislocation physics. Formation, stability and dissociation of
Dislocation dynamics in hexagonal close-packed crystals
Aubry, S.; Rhee, M.; Hommes, G.; Bulatov, V. V.; Arsenlis, A.
2016-04-14
Extensions of the dislocation dynamics methodology necessary to enable accurate simulations of crystal plasticity in hexagonal close-packed (HCP) metals are presented. They concern the introduction of dislocation motion in HCP crystals through linear and non-linear mobility laws, as well as the treatment of composite dislocation physics. Formation, stability and dissociation of
NASA Astrophysics Data System (ADS)
Mandal, Sandip; Khakhar, D. V.
2016-10-01
Granular materials handled in industries are typically non-spherical in shape and understanding the flow of such materials is important. The steady flow of mono-disperse, frictional, inelastic dumbbells in two-dimensions is studied by soft sphere, discrete element method simulations for chute flow and shear cell flow. The chute flow data are in the dense flow regime, while the shear cell data span a wide range of solid fractions. Results of a detailed parametric study for both systems are presented. In chute flow, increase in the aspect ratio of the dumbbells results in significant slowing of the flow at a fixed inclination and in the shear cell it results in increase in the shear stress and pressure for a fixed shear rate. The flow is well-described by the μ-I scaling for inertial numbers as high as I = 1, corresponding to solid fractions as low as ϕ = 0.3, where μ is the effective friction (the ratio of shear stress to pressure) and I is the inertial number (a dimensionless shear rate scaled with the time scale obtained from the local pressure). For a fixed inertial number, the effective friction increases by 60%-70% when aspect ratio is increased from 1.0 (sphere) to 1.9. At low values of the inertial number, there is little change in the solid fraction with aspect ratio of the dumbbells, whereas at high values of the inertial number, there is a significant increase in solid fraction with increase in aspect ratio. The dense flow data are well-described by the Jop-Forterre-Pouliquen model [P. Jop et al., Nature 441, 727-730 (2006)] with the model parameters dependent on the dumbbell aspect ratio. The variation of μ with I over the extended range shows a maximum in the range I ∈ (0.4, 0.5), while the solid fraction shows a faster than linear decrease with inertial number. A modified version of the JFP model for μ(I) and a power law model for ϕ(I) is shown to describe the combined data over the extended range of I.
Genuis, Emerson D; Doan, Quynh
2013-11-01
Providing patient care and medical education are both important missions of teaching hospital emergency departments (EDs). With medical school enrollment rising, and ED crowding becoming an increasing prevalent issue, it is important for both pediatric EDs (PEDs) and general EDs to find a balance between these two potentially competing goals. The objective was to determine how the number of trainees in a PED affects patient wait time, total ED length of stay (LOS), and rates of patients leaving without being seen (LWBS) for PED patients overall and stratified by acuity level as defined by the Pediatric Canadian Triage and Acuity Scale (CTAS) using discrete event simulation (DES) modeling. A DES model of an urban tertiary care PED, which receives approximately 40,000 visits annually, was created and validated. Thirteen different trainee schedules, which ranged from averaging zero to six trainees per shift, were input into the DES model and the outcome measures were determined using the combined output of five model iterations. An increase in LOS of approximately 7 minutes was noted to be associated with each additional trainee per attending emergency physician working in the PED. The relationship between the number of trainees and wait time varied with patients' level of acuity and with the degree of PED utilization. Patient wait time decreased as the number of trainees increased for low-acuity visits and when the PED was not operating at full capacity. With rising numbers of trainees, the PED LWBS rate decreased in the whole department and in the CTAS 4 and 5 patient groups, but it rose in patients triaged CTAS 3 or higher. A rising numbers of trainees was not associated with any change to flow outcomes for CTAS 1 patients. The results of this study demonstrate that trainees in PEDs have an impact mainly on patient LOS and that the effect on wait time differs between patients presenting with varying degrees of acuity. These findings will assist PEDs in finding a
NASA Astrophysics Data System (ADS)
Cranmer, Steven R.; Owocki, Stanley P.
1996-05-01
We present two-dimensional hydrodynamical simulations of corotating interaction regions (CIRs) in the wind from a rotating 0 star, together with resulting synthetic line profiles showing discrete absorption components (DACs). For computational tractability, we use a local, Sobolev treatment of the radiative force, which suppresses the small-scale instability intrinsic to line driving but still allows us to model the dynamics of large-scale wind structure. As a first step toward modeling the wind response to large-scale base perturbations (e.g., from surface magnetic fields or nonradial pulsations), the structure here is explicitly induced by localized increases or decreases in the radiative force, as would result from a bright or dark "star spot" near the star's equator. We find that bright spots with enhanced driving generate high-density, low-speed streams, while dark spots generate low-density, high-speed streams. CIRs form where fast material collides with slow material; e.g., at the leading (trailing) edge of a stream from a dark (bright) spot, often steepening into shocks. The unperturbed supersonic wind obliquely impacts the high-density CIR and sends back a nonlinear signal that takes the form of a sharp propagating discontinuity ("kink" or "plateau") in the radial velocity gradient. In the wind's comoving frame, these features propagate inward at the fast characteristic speed derived by Abbott for radiatively modified acoustic waves, but because this is generally only slightly less than the outward wind speed, the features evolve only slowly outward in the star's frame. We find that these slow kinks, rather than the CIRs themselves, are more likely to result in DACs in the absorption troughs of unsaturated P Cygni line profiles. Because the hydrodynamic structure settles to a steady state in a frame corotating with the star, the more tightly spiraled kinks sweep by an observer on a longer timescale than material moving with the wind itself. This is in
Ishii, Akio; Li, Ju; Ogata, Shigenobu
2013-01-01
Dislocation pipe diffusion seems to be a well-established phenomenon. Here we demonstrate an unexpected effect, that the migration of interstitials such as carbon in iron may be accelerated not in the dislocation line direction ξ, but in a conjugate diffusion direction. This accelerated random walk arises from a simple crystallographic channeling effect. c is a function of the Burgers vector b, but not ξ, thus a dislocation loop possesses the same everywhere. Using molecular dynamics and accelerated dynamics simulations, we further show that such dislocation-core-coupled carbon diffusion in iron has temperature-dependent activation enthalpy like a fragile glass. The 71° mixed dislocation is the only case in which we see straightforward pipe diffusion that does not depend on dislocation mobility.
“Conjugate Channeling” Effect in Dislocation Core Diffusion: Carbon Transport in Dislocated BCC Iron
Ishii, Akio; Li, Ju; Ogata, Shigenobu
2013-01-01
Dislocation pipe diffusion seems to be a well-established phenomenon. Here we demonstrate an unexpected effect, that the migration of interstitials such as carbon in iron may be accelerated not in the dislocation line direction , but in a conjugate diffusion direction. This accelerated random walk arises from a simple crystallographic channeling effect. is a function of the Burgers vector b, but not , thus a dislocation loop possesses the same everywhere. Using molecular dynamics and accelerated dynamics simulations, we further show that such dislocation-core-coupled carbon diffusion in iron has temperature-dependent activation enthalpy like a fragile glass. The 71° mixed dislocation is the only case in which we see straightforward pipe diffusion that does not depend on dislocation mobility. PMID:23593255
Boriskina, Svetlana V; Sewell, Phillip; Benson, Trevor M; Nosich, Alexander I
2004-03-01
A fast and accurate method is developed to compute the natural frequencies and scattering characteristics of arbitrary-shape two-dimensional dielectric resonators. The problem is formulated in terms of a uniquely solvable set of second-kind boundary integral equations and discretized by the Galerkin method with angular exponents as global test and trial functions. The log-singular term is extracted from one of the kernels, and closed-form expressions are derived for the main parts of all the integral operators. The resulting discrete scheme has a very high convergence rate. The method is used in the simulation of several optical microcavities for modern dense wavelength-division-multiplexed systems.
Modified symplectic schemes with nearly-analytic discrete operators for acoustic wave simulations
NASA Astrophysics Data System (ADS)
Liu, Shaolin; Yang, Dinghui; Lang, Chao; Wang, Wenshuai; Pan, Zhide
2017-04-01
Using a structure-preserving algorithm significantly increases the computational efficiency of solving wave equations. However, only a few explicit symplectic schemes are available in the literature, and the capabilities of these symplectic schemes have not been sufficiently exploited. Here, we propose a modified strategy to construct explicit symplectic schemes for time advance. The acoustic wave equation is transformed into a Hamiltonian system. The classical symplectic partitioned Runge-Kutta (PRK) method is used for the temporal discretization. Additional spatial differential terms are added to the PRK schemes to form the modified symplectic methods and then two modified time-advancing symplectic methods with all of positive symplectic coefficients are then constructed. The spatial differential operators are approximated by nearly-analytic discrete (NAD) operators, and we call the fully discretized scheme modified symplectic nearly analytic discrete (MSNAD) method. Theoretical analyses show that the MSNAD methods exhibit less numerical dispersion and higher stability limits than conventional methods. Three numerical experiments are conducted to verify the advantages of the MSNAD methods, such as their numerical accuracy, computational cost, stability, and long-term calculation capability.
'Inverse' temporomandibular joint dislocation.
Alemán Navas, R M; Martínez Mendoza, M G
2011-08-01
Temporomandibular joint (TMJ) dislocation can be classified into four groups (anterior, posterior, lateral, and superior) depending on the direction of displacement and the location of the condylar head. All the groups are rare except for anterior dislocation. 'Inverse' TMJ dislocation is a bilateral anterior and superior dislocation with impaction of the mandible over the maxilla; to the authors' knowledge only two cases have previously been reported in the literature. Inverse TMJ dislocation has unique clinical and radiographic findings, which are described for this case. Copyright © 2011 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Vescovi, D.; Berzi, D.; Richard, P.
2014-05-15
We use existing 3D Discrete Element simulations of simple shear flows of spheres to evaluate the radial distribution function at contact that enables kinetic theory to correctly predict the pressure and the shear stress, for different values of the collisional coefficient of restitution. Then, we perform 3D Discrete Element simulations of plane flows of frictionless, inelastic spheres, sheared between walls made bumpy by gluing particles in a regular array, at fixed average volume fraction and distance between the walls. The results of the numerical simulations are used to derive boundary conditions appropriated in the cases of large and small bumpiness. Those boundary conditions are, then, employed to numerically integrate the differential equations of Extended Kinetic Theory, where the breaking of the molecular chaos assumption at volume fraction larger than 0.49 is taken into account in the expression of the dissipation rate. We show that the Extended Kinetic Theory is in very good agreement with the numerical simulations, even for coefficients of restitution as low as 0.50. When the bumpiness is increased, we observe that some of the flowing particles are stuck in the gaps between the wall spheres. As a consequence, the walls are more dissipative than expected, and the flows resemble simple shear flows, i.e., flows of rather constant volume fraction and granular temperature.
Statistics of dislocation pinning at localized obstacles
Dutta, A.; Bhattacharya, M. Barat, P.
2014-10-14
Pinning of dislocations at nanosized obstacles like precipitates, voids, and bubbles is a crucial mechanism in the context of phenomena like hardening and creep. The interaction between such an obstacle and a dislocation is often studied at fundamental level by means of analytical tools, atomistic simulations, and finite element methods. Nevertheless, the information extracted from such studies cannot be utilized to its maximum extent on account of insufficient information about the underlying statistics of this process comprising a large number of dislocations and obstacles in a system. Here, we propose a new statistical approach, where the statistics of pinning of dislocations by idealized spherical obstacles is explored by taking into account the generalized size-distribution of the obstacles along with the dislocation density within a three-dimensional framework. Starting with a minimal set of material parameters, the framework employs the method of geometrical statistics with a few simple assumptions compatible with the real physical scenario. The application of this approach, in combination with the knowledge of fundamental dislocation-obstacle interactions, has successfully been demonstrated for dislocation pinning at nanovoids in neutron irradiated type 316-stainless steel in regard to the non-conservative motion of dislocations. An interesting phenomenon of transition from rare pinning to multiple pinning regimes with increasing irradiation temperature is revealed.
NASA Astrophysics Data System (ADS)
Chang, Hyung-Jun; Segurado, Javier; Molina-Aldareguía, Jon M.; Soler, Rafael; LLorca, Javier
2016-03-01
The mechanical behavior in compression of [1 1 1] LiF micropillars with diameters in the range 0.5 μm to 2.0 μm was analyzed by means of discrete dislocation dynamics at ambient and elevated temperature. The dislocation velocity was obtained from the Peach-Koehler force acting on the dislocation segments from a thermally-activated model that accounted for the influence of temperature on the lattice resistance. A size effect of the type ‘smaller is stronger’ was predicted by the simulations, which was in quantitative agreement with previous experimental results by the authors [1]. The contribution of the different physical deformation mechanisms to the size effect (namely, nucleation of dislocations, dislocation exhaustion and forest hardening) could be ascertained from the simulations and the dominant deformation mode could be assessed as a function of the specimen size and temperature. These results shed light into the complex interaction among size, lattice resistance and dislocation mobility in the mechanical behavior of μm-sized single crystals.
NASA Technical Reports Server (NTRS)
Dubos, Gregory F.; Cornford, Steven
2012-01-01
While the ability to model the state of a space system over time is essential during spacecraft operations, the use of time-based simulations remains rare in preliminary design. The absence of the time dimension in most traditional early design tools can however become a hurdle when designing complex systems whose development and operations can be disrupted by various events, such as delays or failures. As the value delivered by a space system is highly affected by such events, exploring the trade space for designs that yield the maximum value calls for the explicit modeling of time.This paper discusses the use of discrete-event models to simulate spacecraft development schedule as well as operational scenarios and on-orbit resources in the presence of uncertainty. It illustrates how such simulations can be utilized to support trade studies, through the example of a tool developed for DARPA's F6 program to assist the design of "fractionated spacecraft".
NASA Astrophysics Data System (ADS)
Liu, Chun; Pollard, David D.; Gu, Kai; Shi, Bin
2015-12-01
Wiggly compaction bands in porous aeolian sandstone vary from chevron shape to wavy shape to nearly straight. In some outcrops these variations occur along a single band. A bonded close-packed discrete element model is used to investigate what mechanical properties control the formation of wiggly compaction bands (CBs). To simulate the volumetric yielding failure of porous sandstone, a discrete element shrinks when the force state of one of its bonds reaches the yielding cap defined by the failure force and the aspect ratio (k) of the yielding ellipse. A Matlab code "MatDEM3D" has been developed on the basis of this enhanced discrete element method. Mechanical parameters of elements are chosen according to the elastic properties and the strengths of porous sandstone. In numerical simulations, the failure angle between the band segment and maximum principle stress decreases from 90° to approximately 45° as k increases from 0.5 to 2, and compaction bands vary from straight to chevron shape. With increasing strain, subsequent compaction occurs inside or beside compacted elements, which leads to further compaction and thickening of bands. The simulations indicate that a greater yielding stress promotes chevron CBs, and a greater cement strength promotes straight CBs. Combined with the microscopic analysis introduced in the companion paper, we conclude that the shape of wiggly CBs is controlled by the mechanical properties of sandstone, including the aspect ratio of the yielding ellipse, the critical yielding stress, and the cement strength, which are determined primarily by petrophysical attributes, e.g., grain sorting, porosity, and cementation.
Te homogeneous precipitation in Ge dislocation loop vicinity
Perrin Toinin, J.; Portavoce, A. Texier, M.; Bertoglio, M.; Hoummada, K.
2016-06-06
High resolution microscopies were used to study the interactions of Te atoms with Ge dislocation loops, after a standard n-type doping process in Ge. Te atoms neither segregate nor precipitate on dislocation loops, but form Te-Ge clusters at the same depth as dislocation loops, in contradiction with usual dopant behavior and thermodynamic expectations. Atomistic kinetic Monte Carlo simulations show that Te atoms are repulsed from dislocation loops due to elastic interactions, promoting homogeneous Te-Ge nucleation between dislocation loops. This phenomenon is enhanced by coulombic interactions between activated Te{sup 2+} or Te{sup 1+} ions.
Lai, Po-Yen; Chen, Liu; Lin-Liu, Y. R.; Chen, Shih-Hung
2015-09-15
The thermal relaxation time of a one-dimensional plasma has been demonstrated to scale with N{sub D}{sup 2} due to discrete particle effects by collisionless particle-in-cell (PIC) simulations, where N{sub D} is the particle number in a Debye length. The N{sub D}{sup 2} scaling is consistent with the theoretical analysis based on the Balescu-Lenard-Landau kinetic equation. However, it was found that the thermal relaxation time is anomalously shortened to scale with N{sub D} while externally introducing the Krook type collision model in the one-dimensional electrostatic PIC simulation. In order to understand the discrete particle effects enhanced by the Krook type collision model, the superposition principle of dressed test particles was applied to derive the modified Balescu-Lenard-Landau kinetic equation. The theoretical results are shown to be in good agreement with the simulation results when the collisional effects dominate the plasma system.
NASA Astrophysics Data System (ADS)
Shadwick, Bradley; Evstatiev, Evstati
2013-10-01
We formulate finite-size particle plasma simulation methods from a time-discretized action principle viewpoint. Using Low's Lagrangian as a starting point, we first discretized in spatially the continuous fields and formulate a time-continuous action for the self-consistent system of particles and fields in the electrostatic (ES) plasma approximation. We then utilize a technique due to Lew et al. to formulate time-discrete action and its variation to obtain a particular time integrator. Such time integrators are symplectic (provided there is symmetry with respect to time inversion); they do not conserve energy exactly but the energy variation is bounded and its magnitude depends on the time step. These general time integrators can be of any order of accuracy, however as a rule, beyond second order they are implicit. Time-implicit schemes are easy to formulate for the general cases of electromagnetic and magnetized plasmas. We provide numerical examples of both explicit and implicit time integrators and discuss their advantages and disadvantages. Supported by the U.S. DOE under contract No. DE-SC0008382.
Dickenson, Joshua A; Sansalone, John J
2009-11-01
Modeling the separation of dilute particulate matter (PM) has been a topic of interest since the introduction of unit operations for clarification of rainfall-runoff. One consistent yet controversial issue is the representation of PM and PM separation mechanisms for treatment. While Newton's Law and surface overflow rate were utilized, many historical models represented PM as a lumped gravimetric index largely out of economy and lack of particle analysis methods. As a result such models did not provide information about particle fate in or through a unit operation. In this study, PM discrete phase modeling (DPM) and computational fluid dynamics (CFD) are applied to model PM fate as a function of particle size and flow rate in two common types of hydrodynamic separator (HS) units. The study examines the discretization requirements (as a discretization number, DN) and errors for particle size distributions (PSDs) that range from the common heterodisperse to a monodisperse PSD. PSDs are categorized based on granulometric indices. Results focus on ensuring modeling accuracy while examining the role of size dispersivity and overall PM fineness on DN requirements. The fate of common heterodisperse PSDs is accurately predicted for a DN of 16, whereas a single particle size index, commonly the d(50m), is limited to monodisperse PSDs in order to achieve similar accuracy.
Korsunsky, Alexander M; Hofmann, Felix; Song, Xu; Eve, Sophie; Collins, Steve P
2010-09-01
Materials characterization at the nano-scale is motivated by the desire to resolve the structural aspects and deformation behavior at length scales relevant to those mechanisms that define the novel and unusual properties of nano-structured materials. A range of novel techniques has recently become accessible with the help of synchrotron X-ray beams that can be focused down to spot sizes of less than a few microns on the sample. The unique combination of tunability (energy selection), parallelism and brightness of synchrotron X-ray beams allows their use for high resolution diffraction (determination of crystal structure and transformations, analysis of dislocation sub-structures, orientation and texture analysis, strain mapping); small angle X-ray scattering (analysis of nano-scale voids and defects; orientation analysis) and imaging (radiography and tomography). After a brief review of the state-of-the-art capabilities for monochromatic and white beam synchrotron diffraction, we consider the usefulness of these techniques for the task of bridging the gap between experiment and modeling. Namely, we discuss how the experiments can be configured to provide information relevant to the validation and improvement of modeling approaches, and also how the results of various simulations can be post-processed to improve the possibility of (more or less) direct comparison with experiments. Using the example of some recent experiments carried out on beamline 116 at Diamond Light Source near Oxford, we discuss how such experimental results can be interpreted in view and in conjunction with numerical deformation models, particularly those incorporating dislocation effects, e.g., finite-element based pseudo-continuum strain gradient formulations, and discrete dislocation simulations. Post-processing of FE and discrete dislocation simulations is described, illustrating the kind of information that can be extracted from comparisons between modeling and experimental data.
Evolution of geometrically necessary dislocation density from computational dislocation dynamics
NASA Astrophysics Data System (ADS)
Guruprasad, P. J.; Benzerga, A. A.
2009-07-01
This paper presents a method for calculating GND densities in dislocation dynamics simulations. Evolution of suitably defined averages of GND density as well as maps showing the spatial nonuniform distribution of GNDs are analyzed under uniaxial loading. Focus is laid on the resolution dependence of the very notion of GND density, its dependence upon physical dimensions of plastically deformed specimens and its sensitivity to initial conditions. Acknowledgments Support from the National Science Foundation (CMMI-0748187) is gratefully acknowledged.
Akasheh, F.; Zbib, H. M.; Hirth, J. P.; Hoagland, R. G.; Misra, A.
2007-08-01
Plastic deformation in nanoscale multilayered structures is thought to proceed by the successive propagation of single dislocation loops at the interfaces. Based on this view, we simulate the effect of predeposited interfacial dislocation on the stress (channeling stress) needed to propagate a new loop parallel to existing loops. Single interfacial dislocations as well as finite parallel arrays are considered in the computation. When the gliding dislocation and the predeposited interfacial array have collinear Burgers vectors, the channeling stress increases monotonically as the density of dislocations in the array increases. In the case when their Burgers vectors are inclined at 60 deg. , a regime of perfect plasticity is observed which can be traced back to an instability in the flow stress arising from the interaction between the glide dislocation and a single interfacial dislocation dipole. This interaction leads to a tendency for dislocations of alternating Burgers vectors to propagate during deformation leading to nonuniform arrays. Inclusion of these parallel interactions in the analysis improves the strength predictions as compared with the measured strength of a Cu-Ni multilayered system in the regime where isolated glide dislocation motion controls flow, but does not help to explain the observed strength saturation when the individual layer thickness is in the few nanometer range.
Hydrogen diffusion in the elastic fields of dislocations in iron
Sivak, A. B. Sivak, P. A.; Romanov, V. A.; Chernov, V. M.
2016-12-15
The effect of dislocation stress fields on the sink efficiency thereof is studied for hydrogen interstitial atoms at temperatures of 293 and 600 K and at a dislocation density of 3 × 10{sup 14} m{sup –2} in bcc iron crystal. Rectilinear full screw and edge dislocations in basic slip systems 〈111〉(110), 〈111〉(112), 〈100〉(100), and 〈100〉(110) are considered. Diffusion of defects is simulated by means of the object kinetic Monte Carlo method. The energy of interaction between defects and dislocations is calculated using the anisotropic theory of elasticity. The elastic fields of dislocations result in a less than 25% change of the sink efficiency as compared to the noninteracting linear sink efficiency at a room temperature. The elastic fields of edge dislocations increase the dislocation sink efficiency, whereas the elastic fields of screw dislocations either decrease this parameter (in the case of dislocations with the Burgers vector being 1/2〈111〉) or do not affect it (in the case of dislocations with the Burgers vector being 〈100〉). At temperatures above 600 K, the dislocations affect the behavior of hydrogen in bcc iron mainly owing to a high binding energy between the hydrogen atom and dislocation cores.
Hydrogen diffusion in the elastic fields of dislocations in iron
NASA Astrophysics Data System (ADS)
Sivak, A. B.; Sivak, P. A.; Romanov, V. A.; Chernov, V. M.
2016-12-01
The effect of dislocation stress fields on the sink efficiency thereof is studied for hydrogen interstitial atoms at temperatures of 293 and 600 K and at a dislocation density of 3 × 1014 m-2 in bcc iron crystal. Rectilinear full screw and edge dislocations in basic slip systems <111>{110}, <111>{112}, <100>{100}, and <100>{110} are considered. Diffusion of defects is simulated by means of the object kinetic Monte Carlo method. The energy of interaction between defects and dislocations is calculated using the anisotropic theory of elasticity. The elastic fields of dislocations result in a less than 25% change of the sink efficiency as compared to the noninteracting linear sink efficiency at a room temperature. The elastic fields of edge dislocations increase the dislocation sink efficiency, whereas the elastic fields of screw dislocations either decrease this parameter (in the case of dislocations with the Burgers vector being 1/2<111>) or do not affect it (in the case of dislocations with the Burgers vector being <100>). At temperatures above 600 K, the dislocations affect the behavior of hydrogen in bcc iron mainly owing to a high binding energy between the hydrogen atom and dislocation cores.
NASA Astrophysics Data System (ADS)
Katzarov, Ivaylo H.; Pashov, Dimitar L.; Paxton, Anthony T.
2017-08-01
We demonstrate a kinetic Monte Carlo simulation tool, based on published data using first-principles quantum mechanics, applied to answer the question: under which conditions of stress, temperature, and nominal hydrogen concentration does the presence of hydrogen in iron increase or decrease the screw dislocation velocity? Furthermore, we examine the conditions under which hydrogen-induced shear localization is likely to occur. Our simulations yield quantitative data on dislocation velocity and the ranges of hydrogen concentration within which a large gradient of velocity as a function of concentration is expected to be observed and thereby contribute to a self-perpetuating localization of plasticity—a phenomenon that has been linked to hydrogen-induced fracture and fatigue failure in ultrahigh strength steel. We predict the effect of hydrogen in generating debris made up of edge dipoles trailing in the wake of gliding screw dislocations and their role in pinning. We also simulate the competing effects of softening by enhanced kink-pair generation and hardening by solute pinning. Our simulations act as a bridge between first-principles quantum mechanics and discrete dislocation dynamics, and at the same time offer the prospect of a fully physics-based dislocation dynamics method.
Ji, S.; Hanes, D.M.; Shen, H.H.
2009-01-01
In this study, we report a direct comparison between a physical test and a computer simulation of rapidly sheared granular materials. An annular shear cell experiment was conducted. All parameters were kept the same between the physical and the computational systems to the extent possible. Artificially softened particles were used in the simulation to reduce the computational time to a manageable level. Sensitivity study on the particle stiffness ensured such artificial modification was acceptable. In the experiment, a range of normal stress was applied to a given amount of particles sheared in an annular trough with a range of controlled shear speed. Two types of particles, glass and Delrin, were used in the experiment. Qualitatively, the required torque to shear the materials under different rotational speed compared well with those in the physical experiments for both the glass and the Delrin particles. However, the quantitative discrepancies between the measured and simulated shear stresses were nearly a factor of two. Boundary conditions, particle size distribution, particle damping and friction, including a sliding and rolling, contact force model, were examined to determine their effects on the computational results. It was found that of the above, the rolling friction between particles had the most significant effect on the macro stress level. This study shows that discrete element simulation is a viable method for engineering design for granular material systems. Particle level information is needed to properly conduct these simulations. However, not all particle level information is equally important in the study regime. Rolling friction, which is not commonly considered in many discrete element models, appears to play an important role. ?? 2009 Elsevier Ltd.
Feng, Rui; Xenos, Michalis; Girdhar, Gaurav; Kang, Wei; Davenport, James W; Deng, Yuefan; Bluestein, Danny
2012-01-01
Flow and stresses induced by blood flow acting on the blood cellular constituents can be represented to a certain extent by a continuum mechanics approach down to the order of the μm level. However, the molecular effects of, e.g., adhesion/aggregation bonds of blood clotting can be on the order of nm. The coupling of the disparate length and timescales between such molecular levels and macroscopic transport represents a major computational challenge. To address this challenge, a multiscale numerical approach based on discrete particle dynamics (DPD) methodology derived from molecular dynamics (MD) principles is proposed. The feasibility of the approach was firstly tested for its ability to simulate viscous flow conditions. Simulations were conducted in low Reynolds numbers flows (Re = 25-33) through constricted tubes representing blood vessels with various degrees of stenosis. Multiple discrete particles interacting with each other were simulated, with 1.24-1.36 million particles representing the flow domain and 0.4 million particles representing the vessel wall. The computation was carried out on the massive parallel supercomputer NY BlueGene/L employing NAMD-a parallel MD package for high performance computing (HPC). Typical recirculation zones were formed distal to the stenoses. The velocity profiles and recirculation zones were in excellent agreement with computational fluid dynamics (CFD) 3D Navier-Stokes viscous fluid flow simulations and with classic numerical and experimental results by YC Fung in constricted tubes. This feasibility analysis demonstrates the potential of a methodology that widely departs from a continuum approach to simulate multiscale phenomena such as flow induced blood clotting.
Pure Intrathoracic Scapular Dislocation.
Demirkiran, Nihat Demirhan; Kar, Adem
2016-01-01
Scapular dislocation, also known as locked scapula or scapulothoracic dislocation, is a rare entity that can be identified as extrathoracic or intrathoracic dislocation, depending on the penetration of the scapula into the thoracic cavity. The 3 reported cases of intrathoracic scapular dislocations in the literature are associated with a preexisting condition, such as sternoclavicular separation, prior rib fracture, thoracotomy for a lung transplant procedure, or surgical resection of superior ribs during breast or pulmonary tumor excisions. There are also 3 published cases of intrathoracic scapular impaction, involving comminuted scapular fractures with intrathoracic impaction of the inferior fragment through intercostal space. We report an intrathoracic scapular dislocation that was not associated with fracture of the scapula or predisposing factors. To our knowledge, this is the first case of pure intrathoracic dislocation.
NASA Astrophysics Data System (ADS)
Das, Sambit; Gavini, Vikram
2017-07-01
We use a real-space formulation of orbital-free DFT to study the core energetics and core structure of an isolated screw dislocation in Aluminum. Using a direct energetics based approach, we estimate the core size of a perfect screw dislocation to be ≈ 7 |b|, which is considerably larger than previous estimates of 1-3 |b| based on displacement fields. The perfect screw upon structural relaxation dissociates into two Shockley partials with partial separation distances of 8.2 Å and 6.6 Å measured from the screw and edge component differential displacement plots, respectively. Similar to a previous electronic structure study on edge dislocation, we find that the core energy of the relaxed screw dislocation is not a constant, but strongly dependent on macroscopic deformations. Next, we use the edge and screw dislocation core energetics data with physically reasonable assumptions to develop a continuum energetics model for an aggregate of dislocations that accounts for the core energy dependence on macroscopic deformations. Further, we use this energetics model in a discrete dislocation network, and from the variations of the core energy with respect to the nodal positions of the network, we obtain the nodal core force which can directly be incorporated into discrete dislocation dynamics frameworks. We analyze and classify the nodal core force into three different contributions based on their decay behavior. Two of these contributions to the core force, both arising from the core energy dependence on macroscopic deformations, are not accounted for in currently used discrete dislocation dynamics models which assume the core energy to be a constant excepting for its dependence on the dislocation line orientation. Using case studies involving simple dislocation structures, we demonstrate that the contribution to the core force from the core energy dependence on macroscopic deformations can be significant in comparison to the elastic Peach-Koehler force even up to
Incorporating Discrete Irregular Fracture Zone Networks into 3D Paleohydrogeologic Simulations
NASA Astrophysics Data System (ADS)
Normani, S. D.
2015-12-01
Dual continuum computational models which include both porous media and discrete fracture zones are valuable tools in assessing groundwater migration and pathways in fractured rock systems. Fracture generation models can produce stochastic realizations of fracture networks which honor geological structures and fracture propagation behaviors. Surface lineament traces can be propagated to depth based on fracture zone statistics to produce representations of geological structures in rock. The generated discrete, complex and irregular fracture zone networks, represented as a triangulated mesh, are embedded using orthogonal quadrilateral elements within a three-dimensional hexahedral finite element mesh. A detailed coupled density-dependent paleohydrogeologic groundwater analysis of a hypothetical 104 km2 portion of the Canadian Shield has been conducted using the discrete-fracture dual continuum finite element model FRAC3DVS to investigate the characterization of large-scale fracture zone networks on groundwater and tracer movement during a 120,000 year paleoclimate cycle. Permeability reduction due to permafrost was also applied. Time series data for the depth of permafrost, along with ice thickness and lake depth, were provided by the University of Toronto (UofT) Glacial Systems Model. The crystalline rock between fracture zones was assigned properties characteristic of those reported for the Canadian Shield. Total dissolved solids concentrations of 300 g/L are encountered at depth. Surface water features and a Digital Elevation Model (DEM) were used in a GIS framework to define the watershed boundaries at surface water divides and to populate the finite element mesh. This work will illustrate the long-term evolution and stability of the geosphere and groundwater systems to external perturbations caused by glaciation through the use of performance measures such as Mean Life Expectancy and the migration of a unit tracer to depth over a paleoclimate cycle.
Impact of Interaction Laws and Particle Modeling in Discrete Element Simulations
NASA Astrophysics Data System (ADS)
Cao, Hong-Phong; Renouf, Mathieu; Dubois, Frédéric
2009-06-01
To describe the evolution of divided media, Discrete Elements Methods (DEMs) appear as one of the most appropriate tools. Medium evolution is directly related to assumptions about local contact area, body deformations and contact interactions. In some circumstance such assumptions have a strong influence on the macroscopic behaviour of the media and consequently become questionable. Using the Contact Dynamics framework, the paper presents how classical assumptions could be extended to avoid numerical effects. A reflection is proposed taking into account both physical and numerical aspects. Static and dynamic configuration have been used to illustrate the paper purposes.
Second-order discrete Kalman filtering equations for control-structure interaction simulations
NASA Technical Reports Server (NTRS)
Park, K. C.; Belvin, W. Keith; Alvin, Kenneth F.
1991-01-01
A general form for the first-order representation of the continuous, second-order linear structural dynamics equations is introduced in order to derive a corresponding form of first-order Kalman filtering equations (KFE). Time integration of the resulting first-order KFE is carried out via a set of linear multistep integration formulas. It is shown that a judicious combined selection of computational paths and the undetermined matrices introduced in the general form of the first-order linear structural systems leads to a class of second-order discrete KFE involving only symmetric, N x N solution matrix.
Irreducible posterolateral elbow dislocation.
Atkinson, Cameron T; Pappas, Nick D; Lee, Donald H
2014-02-01
Elbow dislocations are a high-energy traumatic event resulting in loss of congruence of a stable joint. The majority of elbow dislocations can be reduced by closed means and treated conservatively. We present a case of an irreducible elbow dislocation with reduction blocked by the radial head buttonholed through the lateral ligamentous complex. We performed open reduction with release followed by repair of the lateral ligamentous complex. Clinicians need to understand this unique variant of an elbow dislocation to appropriately treat this operative injury.
Dislocated shoulder - aftercare
... aftercare; Shoulder subluxation - aftercare; Shoulder reduction - aftercare; Glenohumeral joint dislocation ... that connect bone to bone) of the shoulder joint. All of these tissues help keep your arm ...
Bilateral Anterior Shoulder Dislocation
Siu, Yuk Chuen; Lui, Tun Hing
2014-01-01
Introduction: Unilateral anterior shoulder dislocation is one of the most common problems encountered in orthopedic practice. However, simultaneous bilateral anterior dislocation of the shoulders is quite rare. Case Presentation: We report a case of a 75-year-old woman presented with simultaneous bilateral anterior shoulder dislocation following a trauma, complicated with a traction injury to the posterior cord of the brachial plexus. Conclusions: Bilateral anterior shoulder dislocation is very rare. The excessive traction force during closed reduction may lead to nerve palsy. Clear documentation of neurovascular status and adequate imaging before and after a reduction should be performed. PMID:25685749
Scale transition using dislocation dynamics and the nudged elastic band method
NASA Astrophysics Data System (ADS)
Sobie, Cameron; Capolungo, Laurent; McDowell, David L.; Martinez, Enrique
2017-08-01
Microstructural features such as precipitates or irradiation-induced defects impede dislocation motion and directly influence macroscopic mechanical properties such as yield point and ductility. Dislocation-defect interactions involve both atomic scale and long range elastic interactions. Thermally assisted dislocation bypass of obstacles occurs when thermal fluctuations and driving stresses contribute sufficient energy to overcome the energy barrier. The Nudged Elastic Band (NEB) method is typically used in the context of atomistic simulations to quantify the activation barriers for a given reaction. In this work, the NEB method is generalized to coarse-grain continuum representations of evolving microstructure states beyond the discrete particle descriptions of first principles and atomistics. This method enables the calculation of activation energies for a 1 / 2 [ 111 ] (1 1 bar 0) glide dislocation bypassing a [001] self-interstitial atom loop of size in the range of 4-10 nm with a spacing larger than 150 nm in α-iron for a range of applied stresses and interaction geometries. Further, the study is complemented by a comparison between atomistic and continuum based prediction of barriers.
Aggarwal, S.; Ryland, S.; Peck, R.
1980-06-19
This report outlines a methodology to study the effects of disruptive events on nuclear waste material in stable geologic sites. The methodology is based upon developing a discrete events model that can be simulated on the computer. This methodology allows a natural development of simulation models that use computer resources in an efficient manner. Accurate modeling in this area depends in large part upon accurate modeling of ion transport behavior in the storage media. Unfortunately, developments in this area are not at a stage where there is any consensus on proper models for such transport. Consequently, our work is directed primarily towards showing how disruptive events can be properly incorporated in such a model, rather than as a predictive tool at this stage. When and if proper geologic parameters can be determined, then it would be possible to use this as a predictive model. Assumptions and their bases are discussed, and the mathematical and computer model are described.
NASA Astrophysics Data System (ADS)
Langthjem, M. A.; Nakano, M.
2005-11-01
An axisymmetric numerical simulation approach to the hole-tone self-sustained oscillation problem is developed, based on the discrete vortex method for the incompressible flow field, and a representation of flow noise sources on an acoustically compact impingement plate by Curle's equation. The shear layer of the jet is represented by 'free' discrete vortex rings, and the jet nozzle and the end plate by bound vortex rings. A vortex ring is released from the nozzle at each time step in the simulation. The newly released vortex rings are disturbed by acoustic feedback. It is found that the basic feedback cycle works hydrodynamically. The effect of the acoustic feedback is to suppress the broadband noise and reinforce the characteristic frequency and its higher harmonics. An experimental investigation is also described. A hot wire probe was used to measure velocity fluctuations in the shear layer, and a microphone to measure acoustic pressure fluctuations. Comparisons between simulated and experimental results show quantitative agreement with respect to both frequency and amplitude of the shear layer velocity fluctuations. As to acoustic pressure fluctuations, there is quantitative agreement w.r.t. frequencies, and reasonable qualitative agreement w.r.t. peaks of the characteristic frequency and its higher harmonics. Both simulated and measured frequencies f follow the criterion L/uc+L/c0=n/f where L is the gap length between nozzle exit and end plate, uc is the shear layer convection velocity, c0 is the speed of sound, and n is a mode number (n={1}/{2},1,{3}/{2},…). The experimental results however display a complicated pattern of mode jumps, which the numerical method cannot capture.
Probing the character of ultra-fast dislocations
Ruestes, C. J.; Bringa, E. M.; Rudd, R. E.; Remington, B. A.; Remington, T. P.; Meyers, M. A.
2015-01-01
Plasticity is often controlled by dislocation motion, which was first measured for low pressure, low strain rate conditions decades ago. However, many applications require knowledge of dislocation motion at high stress conditions where the data are sparse, and come from indirect measurements dominated by the effect of dislocation density rather than velocity. Here we make predictions based on atomistic simulations that form the basis for a new approach to measure dislocation velocities directly at extreme conditions using three steps: create prismatic dislocation loops in a near-surface region using nanoindentation, drive the dislocations with a shockwave, and use electron microscopy to determine how far the dislocations moved and thus their velocity at extreme stress and strain rate conditions. We report on atomistic simulations of tantalum that make detailed predictions of dislocation flow, and find that the approach is feasible and can uncover an exciting range of phenomena, such as transonic dislocations and a novel form of loop stretching. The simulated configuration enables a new class of experiments to probe average dislocation velocity at very high applied shear stress. PMID:26592764
Probing the character of ultra-fast dislocations
Rudd, R. E.; Ruestes, C. J.; Bringa, E. M.; Remington, B. A.; Remington, T. P.; Meyers, M. A.
2015-11-23
Plasticity is often controlled by dislocation motion, which was first measured for low pressure, low strain rate conditions decades ago. However, many applications require knowledge of dislocation motion at high stress conditions where the data are sparse, and come from indirect measurements dominated by the effect of dislocation density rather than velocity. Here we make predictions based on atomistic simulations that form the basis for a new approach to measure dislocation velocities directly at extreme conditions using three steps: create prismatic dislocation loops in a near-surface region using nanoindentation, drive the dislocations with a shockwave, and use electron microscopy to determine how far the dislocations moved and thus their velocity at extreme stress and strain rate conditions. We report on atomistic simulations of tantalum that make detailed predictions of dislocation flow, and find that the approach is feasible and can uncover an exciting range of phenomena, such as transonic dislocations and a novel form of loop stretching. Furthermore, the simulated configuration enables a new class of experiments to probe average dislocation velocity at very high applied shear stress.
Probing the character of ultra-fast dislocations
NASA Astrophysics Data System (ADS)
Ruestes, C. J.; Bringa, E. M.; Rudd, R. E.; Remington, B. A.; Remington, T. P.; Meyers, M. A.
2015-11-01
Plasticity is often controlled by dislocation motion, which was first measured for low pressure, low strain rate conditions decades ago. However, many applications require knowledge of dislocation motion at high stress conditions where the data are sparse, and come from indirect measurements dominated by the effect of dislocation density rather than velocity. Here we make predictions based on atomistic simulations that form the basis for a new approach to measure dislocation velocities directly at extreme conditions using three steps: create prismatic dislocation loops in a near-surface region using nanoindentation, drive the dislocations with a shockwave, and use electron microscopy to determine how far the dislocations moved and thus their velocity at extreme stress and strain rate conditions. We report on atomistic simulations of tantalum that make detailed predictions of dislocation flow, and find that the approach is feasible and can uncover an exciting range of phenomena, such as transonic dislocations and a novel form of loop stretching. The simulated configuration enables a new class of experiments to probe average dislocation velocity at very high applied shear stress.
Probing the character of ultra-fast dislocations
Rudd, R. E.; Ruestes, C. J.; Bringa, E. M.; ...
2015-11-23
Plasticity is often controlled by dislocation motion, which was first measured for low pressure, low strain rate conditions decades ago. However, many applications require knowledge of dislocation motion at high stress conditions where the data are sparse, and come from indirect measurements dominated by the effect of dislocation density rather than velocity. Here we make predictions based on atomistic simulations that form the basis for a new approach to measure dislocation velocities directly at extreme conditions using three steps: create prismatic dislocation loops in a near-surface region using nanoindentation, drive the dislocations with a shockwave, and use electron microscopy tomore » determine how far the dislocations moved and thus their velocity at extreme stress and strain rate conditions. We report on atomistic simulations of tantalum that make detailed predictions of dislocation flow, and find that the approach is feasible and can uncover an exciting range of phenomena, such as transonic dislocations and a novel form of loop stretching. Furthermore, the simulated configuration enables a new class of experiments to probe average dislocation velocity at very high applied shear stress.« less
Dislocation dynamics. I. A proposed methodology for deformation micromechanics
Amodeo, R.J.; Ghoniem, N.M. Nuclear Engineering Department, University of California, Los Angeles, Los Angeles, California 90024 )
1990-04-01
A new methodology in computational micromechanics, dislocation dynamics (DD), is introduced. Dislocation dynamics is developed for examining the dynamic behavior of dislocation distributions in solid materials. Under conditions of externally applied stress, dislocations exhibit glide with a velocity proportional to a power of the applied stress {sigma}{sup {ital m}} and climb motion with a velocity that is a function of the applied stress and temperature. These motions result from long-range force fields, comprising both externally applied stress and long-range interactions between individual dislocations. Short-range reactions are represented as discrete events. The DD methodology is to be differentiated from particle methods in statistical mechanics (e.g., molecular dynamics and the Monte Carlo method) in two respects. First, DD is developed to study the dynamical behavior of defects'' in the solid. Generally, the density of defects is less than that of the particles that make up the solid. Second, the small number of dislocations allows for a complete dynamical representation of the evolution of dislocations in the material medium without the requirement of statistical averaging. The purpose of the DD methodology is to bridge the gap between experimentally observed phenomena and theoretical descriptions of dislocation aggregates, particularly the evolution of self-organized dislocation structures under temperature, stress, and irradiation conditions.
Metallurgy: Starting and stopping dislocations
NASA Astrophysics Data System (ADS)
Minor, Andrew M.
2015-09-01
A comparison of dislocation dynamics in two hexagonal close-packed metals has revealed that dislocation movement can vary substantially in materials with the same crystal structure, associated with how the dislocations relax when stationary.
A Study on Modeling Approaches in Discrete Event Simulation Using Design Patterns
2007-12-01
Port Security ( FPPS ) 16. PRICE CODE 17. SECURITY CLASSIFICATION OF REPORT Unclassified 18. SECURITY CLASSIFICATION OF THIS PAGE...79 Figure 52 Event-graph Logic of the FPPS Simulation Application.................................81 Figure 53 FPPS Simulation...module. For the random-utility package, a discussion on empirical analysis is conducted. Finally, this chapter looks how the FPPS application has been
Modeling Airport Ground Operations using Discrete Event Simulation (DES) and X3D Visualization
2008-03-01
studies, because it offers a number of features as for example: 12 1. Open source 2. Character animation support (CAL3D) 3. Game engine with...Simulation, DES, Simkit, Diskit, Viskit, Savage, XML, Distributed Interactive Simulation, DIS, Blender , X3D Edit 16. PRICE CODE 17. SECURITY...10 5. Blender Authoring Tool
Comas, Mercè; Castells, Xavier; Hoffmeister, Lorena; Román, Rubén; Cots, Francesc; Mar, Javier; Gutiérrez-Moreno, Santiago; Espallargues, Mireia
2008-12-01
To outline the methods used to build a discrete-event simulation model for use in decision-making in the context of waiting list management strategies for cataract surgery by comparing a waiting list prioritization system with the routinely used first-in, first-out (FIFO) discipline. The setting was the Spanish health system. The model reproduced the process of cataract, from incidence of need of surgery (meeting indication criteria), through demand, inclusion on a waiting list, and surgery. "Nonexpressed Need" represented the population that, even with need, would not be included on a waiting list. Parameters were estimated from administrative data and research databases. The impact of introducing a prioritization system on the waiting list compared with the FIFO system was assessed. For all patients entering the waiting list, the main outcome variable was waiting time weighted by priority score. A sensitivity analysis with different scenarios of mean waiting time was used to compare the two alternatives. The prioritization system shortened waiting time (weighted by priority score) by 1.55 months (95% CI: 1.47 to 1.62) compared with the FIFO system. This difference was statistically significant for all scenarios (which were defined from a waiting time of 4 months to 24 months under the FIFO system). A tendency to greater time savings in scenarios with longer waiting times was observed. Discrete-event simulation is useful in decision-making when assessing health services. Introducing a waiting list prioritization system produced greater benefit than allocating surgery by waiting time only. Use of the simulation model would allow the impact of proposed policies to reduce waiting lists or assign resources more efficiently to be tested.
Traumatic vertical atlantoaxial dislocation.
Payer, M; Wetzel, S; Kelekis, A; Jenny, B
2005-08-01
We present a case of traumatic vertical atlantoaxial dislocation of 16 millimetres with a fatal outcome. We hypothesize that this extremely rare traumatic vertical atlantoaxial dislocation results from insufficiency of the C1/C2 facet capsules after rupture of the tectorial membrane and the alar ligaments.
Traumatic proximal tibiofibular dislocation.
Burgos, J; Alvarez-Montero, R; Gonzalez-Herranz, P; Rapariz, J M
1997-01-01
Proximal tibiofibular dislocation is an exceptional lesion. Rarer still is its presentation in childhood. We describe the clinical case of a 6-year-old boy, the victim of a road accident. He had a tibiofibular dislocation associated with a metaphyseal fracture of the tibia.
Huang, Hai; Plummer, Mitchell; Podgorney, Robert
2013-02-01
Advancement of EGS requires improved prediction of fracture development and growth during reservoir stimulation and long-term operation. This, in turn, requires better understanding of the dynamics of the strongly coupled thermo-hydro-mechanical (THM) processes within fractured rocks. We have developed a physically based rock deformation and fracture propagation simulator by using a quasi-static discrete element model (DEM) to model mechanical rock deformation and fracture propagation induced by thermal stress and fluid pressure changes. We also developed a network model to simulate fluid flow and heat transport in both fractures and porous rock. In this paper, we describe results of simulations in which the DEM model and network flow & heat transport model are coupled together to provide realistic simulation of the changes of apertures and permeability of fractures and fracture networks induced by thermal cooling and fluid pressure changes within fractures. Various processes, such as Stokes flow in low velocity pores, convection-dominated heat transport in fractures, heat exchange between fluid-filled fractures and solid rock, heat conduction through low-permeability matrices and associated mechanical deformations are all incorporated into the coupled model. The effects of confining stresses, developing thermal stress and injection pressure on the permeability evolution of fracture and fracture networks are systematically investigated. Results are summarized in terms of implications for the development and evolution of fracture distribution during hydrofracturing and thermal stimulation for EGS.
NASA Astrophysics Data System (ADS)
Morgan, Julia K.
2015-05-01
Particle-based numerical simulations of cohesive contractional wedges can yield important perspectives on the formation and evolution of fold and thrust belts, offering particular insights into the mechanical evolution of the systems. Results of several discrete element method simulations are presented here, demonstrating the stress and strain evolution of systems with different initial cohesive strengths. Particle assemblages consolidated under gravity, and bonded to impart cohesion, are pushed from the left at a constant velocity above a weak, unbonded décollement surface. Internal thrusting causes horizontal shortening and vertical thickening, forming wedge geometries. The mean wedge taper is similar for all simulations, consistent with their similar residual and basal sliding friction values. In all examples presented here, both forethrusts and back thrusts occur, but forethrusts accommodate most of the shortening. Fault spacing and offset increase with increasing cohesion. Significant tectonic volume strain also occurs, with the greatest incremental volume strain occurring just outboard of the deformation front. This diffuse shortening serves to strengthen the unfaulted domain in front of the deformed wedge, preconditioning these materials for brittle (dilative) failure. The reach of this volumetric strain and extent of décollement slip increase with cohesive strength, defining the extent of stress transmission. Stress paths for elements tracked through the simulations demonstrate systematic variations in shear stress in response to episodes of both décollement slip and thrust fault activity, providing a direct explanation for stress fluctuations during convergence.
Kittipittayakorn, Cholada
2016-01-01
Many hospitals are currently paying more attention to patient satisfaction since it is an important service quality index. Many Asian countries' healthcare systems have a mixed-type registration, accepting both walk-in patients and scheduled patients. This complex registration system causes a long patient waiting time in outpatient clinics. Different approaches have been proposed to reduce the waiting time. This study uses the integration of discrete event simulation (DES) and agent-based simulation (ABS) to improve patient waiting time and is the first attempt to apply this approach to solve this key problem faced by orthopedic departments. From the data collected, patient behaviors are modeled and incorporated into a massive agent-based simulation. The proposed approach is an aid for analyzing and modifying orthopedic department processes, allows us to consider far more details, and provides more reliable results. After applying the proposed approach, the total waiting time of the orthopedic department fell from 1246.39 minutes to 847.21 minutes. Thus, using the correct simulation model significantly reduces patient waiting time in an orthopedic department. PMID:27195606
Khalid, Ruzelan; M. Nawawi, Mohd Kamal; Kawsar, Luthful A.; Ghani, Noraida A.; Kamil, Anton A.; Mustafa, Adli
2013-01-01
M/G/C/C state dependent queuing networks consider service rates as a function of the number of residing entities (e.g., pedestrians, vehicles, and products). However, modeling such dynamic rates is not supported in modern Discrete Simulation System (DES) software. We designed an approach to cater this limitation and used it to construct the M/G/C/C state-dependent queuing model in Arena software. Using the model, we have evaluated and analyzed the impacts of various arrival rates to the throughput, the blocking probability, the expected service time and the expected number of entities in a complex network topology. Results indicated that there is a range of arrival rates for each network where the simulation results fluctuate drastically across replications and this causes the simulation results and analytical results exhibit discrepancies. Detail results that show how tally the simulation results and the analytical results in both abstract and graphical forms and some scientific justifications for these have been documented and discussed. PMID:23560037
NASA Astrophysics Data System (ADS)
Sakaguchi, Ryohei; Akiyama, Toru; Nakamura, Kohji; Ito, Tomonori
2016-05-01
The compositional inhomogeneity of group III elements around threading dislocations in III-nitride semiconductors are theoretically investigated using empirical interatomic potentials and Monte Carlo simulations. We find that the calculated atomic arrangements around threading dislocations in Al0.3Ga0.7N and In0.2Ga0.8N depend on the lattice strain around dislocation cores. Consequently, compositional inhomogeneity arises around edge dislocation cores to release the strain induced by dislocation cores. In contrast, the compositional inhomogeneity in screw dislocation is negligible owing to relatively small strain induced by dislocation cores compared with edge dislocation. These results indicate that the strain relief around dislocation cores is decisive in determining the atomic arrangements and resultant compositional inhomogeneity around threading dislocations in III-nitride semiconductor alloys.
NASA Astrophysics Data System (ADS)
Gao, F. Q.; Kang, H. P.
2016-04-01
When rock failure is unavoidable, the designer of engineering structures must know and account for the residual strength of the rock mass. This is particularly relevant in underground coal mine openings. Pre-existing discontinuities play an important role in the mechanical behavior of rock masses and thus it is important to understand the effects of such pre-existing discontinuities on the residual strength. For this purpose, the present study demonstrates a numerical analysis using a discrete element method simulation. The numerical results indicate that fracture intensity has no significant influence on the residual strength of jointed rock masses, independent of confining conditions. As confining pressures increase, both peak and residual strengths increase, with residual strength increasing at a faster rate. The finding was further demonstrated by analyzing documented laboratory compressive test data from a variety of rocks along with field data from coal pillars. A comprehensive interpretation of the finding was conducted using a cohesion-weakening-friction-strengthening (CWFS) model. The effect of rock bolts on rock mass strength was also evaluated by using a discrete element method model which suggested that rock bolts can significantly increases residual strength but have limited effect on increasing the peak strength of rock masses.
Modelling an outpatient unit in a clinical health centre using discrete event simulation
NASA Astrophysics Data System (ADS)
Sharif, Nurul Atikah Mohd; Aziz, Azizah; Ahmad, Norazura; Nawawi, Mohd Kamal Mohd
2016-10-01
This paper describes a project paper of a simulation modelling course. It presents the potential of computer simulation in modelling the current performance of an outpatient department of a clinical health centre in a rural area. The model was run using Arena student version 14.5. From the 60 replication length run, the obtained result shows that the patient's waiting time is 26.4 minutes, which is lesser than the established standard waiting time of 30 minutes.
The IDES framework: A case study in development of a parallel discrete-event simulation system
Nicol, D.M.; Johnson, M.M.; Yoshimura, A.S.
1997-12-31
This tutorial describes considerations in the design and development of the IDES parallel simulation system. IDES is a Java-based parallel/distributed simulation system designed to support the study of complex large-scale enterprise systems. Using the IDES system as an example, the authors discuss how anticipated model and system constraints molded the design decisions with respect to modeling, synchronization, and communication strategies.
Poblete, Simón; Wysocki, Adam; Gompper, Gerhard; Winkler, Roland G
2014-09-01
We investigate the hydrodynamic properties of a spherical colloid model, which is composed of a shell of point particles by hybrid mesoscale simulations, which combine molecular dynamics simulations for the sphere with the multiparticle collision dynamics approach for the fluid. Results are presented for the center-of-mass and angular velocity correlation functions. The simulation results are compared with theoretical results for a rigid colloid obtained as a solution of the Stokes equation with no-slip boundary conditions. Similarly, analytical results of a point-particle model are presented, which account for the finite size of the simulated system. The simulation results agree well with both approaches on appropriative time scales; specifically, the long-time correlations are quantitatively reproduced. Moreover, a procedure is proposed to obtain the infinite-system-size diffusion coefficient based on a combination of simulation results and analytical predictions. In addition, we present the velocity field in the vicinity of the colloid and demonstrate its close agreement with the theoretical prediction. Our studies show that a point-particle model of a sphere is very well suited to describe the hydrodynamic properties of spherical colloids, with a significantly reduced numerical effort.
NASA Astrophysics Data System (ADS)
Chang, Kuo-Jen; Taboada, Alfredo
2009-09-01
We present Contact Dynamics discrete element simulations of the earthquake-triggered Jiufengershan avalanche, which mobilized a 60 m thick, 1.5 km long sedimentary layer, dipping ˜22°SE toward a valley. The dynamic behavior of the avalanche is simulated under different assumptions about rock behavior, water table height, and boundary shear strength. Additionally, seismic shaking is introduced using strong motion records from nearby stations. We assume that seismic shaking generates shearing and frictional heating along the surface of rupture, which, in turn, may induce dynamic weakening and avalanche triggering; a simple "slip-weakening" criterion was adopted to simulate shear strength drop along the rupture surface. We investigate the mechanical processes occurring during triggering and propagation of an avalanche mobilizing shallowly dipping layers. Incipient deformation forms a pop-up structure at the toe of the dip slope. As the avalanche propagates, the pop-up deforms into an overturned fold, which overrides the surface of separation along a décollement. Simultaneously, uphill layers slide at high velocity (125 km/h) and are folded and disrupted as they reach the toe of the dip slope. The avalanche foot forms a wedge that is pushed forward as deformed rocks accrete at its rear. We simulated five cross sections across the Jiufengershan avalanche, which differ in the geometry of the surface of separation. Topographic and simulated surface profiles are similar. The friction coefficient at the surface of separation determined from back analysis is abnormally low (μSS = 0.2), possibly due to lubrication by liquefied soils. The granular deposits of simulated earthquake- and rain-triggered avalanches are similar.
Direct Simulation of Multiple Scattering by Discrete Random Media Illuminated by Gaussian Beams
NASA Technical Reports Server (NTRS)
Mackowski, Daniel W.; Mishchenko, Michael I.
2011-01-01
The conventional orientation-averaging procedure developed in the framework of the superposition T-matrix approach is generalized to include the case of illumination by a Gaussian beam (GB). The resulting computer code is parallelized and used to perform extensive numerically exact calculations of electromagnetic scattering by volumes of discrete random medium consisting of monodisperse spherical particles. The size parameters of the scattering volumes are 40, 50, and 60, while their packing density is fixed at 5%. We demonstrate that all scattering patterns observed in the far-field zone of a random multisphere target and their evolution with decreasing width of the incident GB can be interpreted in terms of idealized theoretical concepts such as forward-scattering interference, coherent backscattering (CB), and diffuse multiple scattering. It is shown that the increasing violation of electromagnetic reciprocity with decreasing GB width suppresses and eventually eradicates all observable manifestations of CB. This result supplements the previous demonstration of the effects of broken reciprocity in the case of magneto-optically active particles subjected to an external magnetic field.
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
2017-04-01
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.
NASA Astrophysics Data System (ADS)
Spieler, Diana; Schwarze, Robert; Schütze, Niels
2017-04-01
In the past a variety of different modeling approaches has been developed in catchment hydrology. Even though there is no argument on the relevant processes taking place, there is no unified theory on how best to represent them computationally. Thus a vast number of models has been developed, varying from lumped models to physically based models. Most of them have a more or less fixed model structure and follow the "one fits all" paradigm. However, a more flexible approach could improve model realism by designing catchment specific model structures based on data availability. This study focuses on applying the flexible hydrological modelling framework RAVEN (Craig et al., 2013), to systematically test several conceptual model structures on the 19 km2 Große Ohe Catchment in the Bavarian Forest (Germany). By combining RAVEN with the DREAM algorithm (Vrugt et al., 2009), the relationship between catchment characteristics, model structure, parameter uncertainty and data availability are analyzed. The model structure is progressively developed based on the available data of the well observed forested catchment area. In a second step, the impact of the catchment discretization is analyzed by testing different spatial resolutions of topographic input data.
Direct simulation of multiple scattering by discrete random media illuminated by Gaussian beams
Mackowski, Daniel W.; Mishchenko, Michael I.
2011-01-15
The conventional orientation-averaging procedure developed in the framework of the superposition T-matrix approach is generalized to include the case of illumination by a Gaussian beam (GB). The resulting computer code is parallelized and used to perform extensive numerically exact calculations of electromagnetic scattering by volumes of discrete random medium consisting of monodisperse spherical particles. The size parameters of the scattering volumes are 40, 50, and 60, while their packing density is fixed at 5%. We demonstrate that all scattering patterns observed in the far-field zone of a random multisphere target and their evolution with decreasing width of the incident GB can be interpreted in terms of idealized theoretical concepts such as forward-scattering interference, coherent backscattering (CB), and diffuse multiple scattering. It is shown that the increasing violation of electromagnetic reciprocity with decreasing GB width suppresses and eventually eradicates all observable manifestations of CB. This result supplements the previous demonstration of the effects of broken reciprocity in the case of magneto-optically active particles subjected to an external magnetic field.
Direct Simulation of Multiple Scattering by Discrete Random Media Illuminated by Gaussian Beams
NASA Technical Reports Server (NTRS)
Mackowski, Daniel W.; Mishchenko, Michael I.
2011-01-01
The conventional orientation-averaging procedure developed in the framework of the superposition T-matrix approach is generalized to include the case of illumination by a Gaussian beam (GB). The resulting computer code is parallelized and used to perform extensive numerically exact calculations of electromagnetic scattering by volumes of discrete random medium consisting of monodisperse spherical particles. The size parameters of the scattering volumes are 40, 50, and 60, while their packing density is fixed at 5%. We demonstrate that all scattering patterns observed in the far-field zone of a random multisphere target and their evolution with decreasing width of the incident GB can be interpreted in terms of idealized theoretical concepts such as forward-scattering interference, coherent backscattering (CB), and diffuse multiple scattering. It is shown that the increasing violation of electromagnetic reciprocity with decreasing GB width suppresses and eventually eradicates all observable manifestations of CB. This result supplements the previous demonstration of the effects of broken reciprocity in the case of magneto-optically active particles subjected to an external magnetic field.
Understanding the mechanisms of sickle cell disease by simulations with a discrete particle model
NASA Astrophysics Data System (ADS)
Hui, Katrina; Lin, Guang; Pan, Wenxiao
2013-01-01
Sickle cell disease (SCD) is an inherited blood disorder characterized by rigid, sickle-shaped red blood cells (RBCs). Because of their rigidity and shape, sickle cells can get stuck in smaller blood vessels, causing blockages and depriving oxygen to tissues. This study develops and applies mathematical models to better understand the mechanism of SCD. Two-dimensional models of RBCs and blood vessels have been constructed by representing them as discrete particles interacting with different forces. The nonlinear, elastic property of healthy RBCs could be adequately reproduced using a cosine angle bending force and a worm-like chain spring force. With the ability to deform, RBCs can squeeze through narrow blood vessels. In modeling sickle cells as rigid bodies and applying repelling and friction forces from the blood vessel, this study shows that geometrical factors (dimensions of the sickle cell and blood vessels) as well as rigidity and adhesiveness of the sickle cell all play an important role in determining how, and if, sickle cells become trapped within narrow blood capillaries. With lack of data to validate the model, this study primarily provides a sensitivity analysis of factors influencing sickle cell occlusion and identified critical data to support future modeling.
NASA Technical Reports Server (NTRS)
Leonard, Daniel; Parsons, Jeremy W.; Cates, Grant
2014-01-01
In May 2013, NASA's GSDO Program requested a study to develop a discrete event simulation (DES) model that analyzes the launch campaign process of the Space Launch System (SLS) from an integrated commodities perspective. The scope of the study includes launch countdown and scrub turnaround and focuses on four core launch commodities: hydrogen, oxygen, nitrogen, and helium. Previously, the commodities were only analyzed individually and deterministically for their launch support capability, but this study was the first to integrate them to examine the impact of their interactions on a launch campaign as well as the effects of process variability on commodity availability. The study produced a validated DES model with Rockwell Arena that showed that Kennedy Space Center's ground systems were capable of supporting a 48-hour scrub turnaround for the SLS. The model will be maintained and updated to provide commodity consumption analysis of future ground system and SLS configurations.
Pan, Chong; Zhang, Dali; Kon, Audrey Wan Mei; Wai, Charity Sue Lea; Ang, Woo Boon
2015-06-01
Continuous improvement in process efficiency for specialist outpatient clinic (SOC) systems is increasingly being demanded due to the growth of the patient population in Singapore. In this paper, we propose a discrete event simulation (DES) model to represent the patient and information flow in an ophthalmic SOC system in the Singapore National Eye Centre (SNEC). Different improvement strategies to reduce the turnaround time for patients in the SOC were proposed and evaluated with the aid of the DES model and the Design of Experiment (DOE). Two strategies for better patient appointment scheduling and one strategy for dilation-free examination are estimated to have a significant impact on turnaround time for patients. One of the improvement strategies has been implemented in the actual SOC system in the SNEC with promising improvement reported.
Karnon, Jonathan
2003-10-01
Markov models have traditionally been used to evaluate the cost-effectiveness of competing health care technologies that require the description of patient pathways over extended time horizons. Discrete event simulation (DES) is a more flexible, but more complicated decision modelling technique, that can also be used to model extended time horizons. Through the application of a Markov process and a DES model to an economic evaluation comparing alternative adjuvant therapies for early breast cancer, this paper compares the respective processes and outputs of these alternative modelling techniques. DES displays increased flexibility in two broad areas, though the outputs from the two modelling techniques were similar. These results indicate that the use of DES may be beneficial only when the available data demonstrates particular characteristics.
Dislocation structure of the magnesium nanocrystal in uniaxial loading
NASA Astrophysics Data System (ADS)
Vlasova, A. M.; Nikonov, A. Yu.; Zhuravlev, A. K.; Kesarev, A. G.
2016-11-01
We report on molecular-dynamics (MD) simulations of compression loading of nanocrystalline magnesium modeled by the embedded-atom method (EAM) potential. It is shown that plastic deformation is by basal slip and (102) twinning. The formation of stable configurations of dislocation grids is observed. Some dislocation reactions are suggested to explain the occurrence of grids. The structure of the dislocation core is shown with the Burgers vector 1 /18 [0 4 ¯43 ] .