The P1 approximation is a computationally efficient model for thermal radiation.Here,we present a P1 formulation in the context of the combined computational fluid dynamics and discrete element method(CFD-DEM),includi...The P1 approximation is a computationally efficient model for thermal radiation.Here,we present a P1 formulation in the context of the combined computational fluid dynamics and discrete element method(CFD-DEM),including closures for dependent scattering and coarse-graining.Using available analytical and semi-analytical solutions,we find agreement for steady-state and transient quantities in sizedisperse systems.Heat flux is identified as the most sensitive quantity to predict,displaying unphysical spatial oscillations.These oscillations are due to a temperature slip at the locations of abrupt change in solid fraction.We propose two techniques that mitigate this effect:smoothing of the radiative properties,and pseudo-scattering.Furthermore,using up to a million times enlarged particles,we demonstrate practically limitless compatibility with coarse-graining.Finally,we compare predictions made with our code to experimental data for a pebble bed under vacuum conditions,and in presence of nitrogen.We find that a carefully calibrated simulation can replicate trends observed in experiments,with relative temperature error of less than 10%.展开更多
Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) is a commonly used numerical method to model gas-solid flow in fluidised beds and other multiphase systems. A significant limitation of CFD-D...Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) is a commonly used numerical method to model gas-solid flow in fluidised beds and other multiphase systems. A significant limitation of CFD-DEM is the feasibility of the realistic simulation of large numbers of particles. Coarse-graining (CG) approaches, through which groups of multiple individual particles are represented by single, larger particles, can substantially reduce the total number of particles while maintaining similar system dynamics. As these three CG models have not previously been compared, there remains some debate, however, about the best practice in the application of CG in CFD-DEM simulations. In this paper, we evaluate the performance of three typical CG methods based on simulations of a bubbling fluidised bed. This is achieved through the use of a numerical validation framework, which makes full use of the high-resolution 3D positron emission particle tracking (PEPT) measurements to rigorously validate the outputs of CFD-DEM simulations conducted using various different coarse-graining models, and various different degrees of coarse-graining. The particle flow behaviours in terms of the particle occupancy field, velocity field, circulation time, and bubble size and velocity, are comprehensively analysed. It is shown that the CG simulation starts to fail when the size ratio between the bed chamber and the particles decreases to approximately 20. It is also observed, somewhat surprisingly, that the specific CG approach applied to interparticle contact parameters does not have a substantial effect on the simulation results for the bubbling bed simulations across a wide range of CG factors.展开更多
This paper proposes a novel method for the parameter optimization of complex networks established through coarsening and phase space reconstruction.Firstly,we identify the change-points of the time series based on the...This paper proposes a novel method for the parameter optimization of complex networks established through coarsening and phase space reconstruction.Firstly,we identify the change-points of the time series based on the cumulative sum(CUSUM)control chart method.Then,we optimize the coarse-graining parameters and phase space embedding dimension based on the evolution analysis of the global topology index(betweenness)at the mutation point.Finally,we conduct a simulation analysis based on real-time data of Chinese copper spot prices.The results show that the delay of the copper spot prices in Chinese spot market is 1 day,and the optimal embedding dimension of the phase space reconstruction is 3.The acceptance level of the investors towards the small fluctuations in copper spot prices is 0.2 times than the average level of price fluctuations,which means that an average price fluctuation of 0.2 times is the optimal coarse-graining parameter.展开更多
A general operational protocol which provides permanent macroscopic coherence of the response of any stable complex system put in an ever-changing environment is proposed. It turns out that the coherent response consi...A general operational protocol which provides permanent macroscopic coherence of the response of any stable complex system put in an ever-changing environment is proposed. It turns out that the coherent response consists of two parts: 1) a specific discrete pattern, called by the author homeostatic one, whose characteristics are robust to the statistics of the environment;2) the rest part of the response forms a stationary homogeneous process whose coarse-grained structure obeys universal distribution which turns out to be scale-invariant. It is demonstrated that, for relatively short time series, a measurement, viewed as a solitary operation of coarse-graining, superimposed on the universal distribution results in a rich variety of behaviors ranging from periodic-like to stochastic-like, to a sequences of irregular fractal-like objects and sequences of random-like events. The relevance of the Central Limit theorem applies to the latter case. Yet, its application is still an approximation which holds for relatively short time series and for specific low resolution of the measurement equipment. It is proven that the asymptotic behavior in each and every of the above cases is provided by the recently proven decomposition theorem.展开更多
Collagen is a basic biopolymer usually found in animal bodies, but its mechanical property and behavior are not sufficiently understood so as to apply to effective regenerative medicine and so on. Since the collagen m...Collagen is a basic biopolymer usually found in animal bodies, but its mechanical property and behavior are not sufficiently understood so as to apply to effective regenerative medicine and so on. Since the collagen material is composed of many hierarchical structures from atomistic level to tissue or organ level, we need to well understand fundamental and atomistic mechanism of the collagen in mechanical response. First, we approach at exactly atomistic level by using all-atom modeling of tropocollagen (TC) molecule, which is a basic structural unit of the collagen. We perform molecular dynamics (MD) simulations concerning tensile loading of a single TC model. The main nature of elastic (often superelastic) behavior and the dependency on temperature and size are discussed. Then, to aim at coarse-graining of atomic configuration into some bundle structure of TC molecules (TC fibril), as a model of higher collagen structure, we construct a kind of mesoscopic model by adopting a simulation framework of beads-spring model which is ordinarily used in polymer simulation. Tensile or compression simulation to the fibril model reveals that the dependency of yield or buckling limit on the number of TCs in the model. Also, we compare the models with various molecular orientations in winding process of initial spiral of TC. The results are analyzed geometrically and it shows that characteristic orientational change of molecules increases or decreases depending on the direction and magnitude of longitudinal strain.展开更多
We calculate current correlation functions （CCFs） of dissipative particle dy- namics （DPD） and compare them with results of molecular dynamics （MD） and solutions of linearized hydrodynamic equations. In particul...We calculate current correlation functions （CCFs） of dissipative particle dy- namics （DPD） and compare them with results of molecular dynamics （MD） and solutions of linearized hydrodynamic equations. In particular, we consider three versions of DPD, the empirical/classical DPD, coarse-grained （CG） DPD with radial-direction interactions only and full （radial, transversal, and rotational） interactions between particles. To fa- cilitate quantitative discussions, we consider specifically a star-polymer melt system at a moderate density. For bonded molecules, it is straightforward to define the CG variables and to further derive CG force fields for DPD within the framework of the Mori-Zwanzig formalism. For both transversal and longitudinal current correlation functions （TCCFs and LCCFs）, we observe that results of MD, DPD, and hydrodynamic solutions agree with each other at the continuum limit. Below the continuum limit to certain length scales, results of MD deviate significantly from hydrodynamic solutions, whereas results of both empirical and CG DPD resemble those of MD. This indicates that the DPD method with Markovian force laws possibly has a larger applicability than the continuum description of a Newtonian fluid. This is worth being explored further to represent gen- eralized hydrodynamics.展开更多
With the rapid development of big data, the scale of realistic networks is increasing continually. In order to reduce the network scale, some coarse-graining methods are proposed to transform large-scale networks into...With the rapid development of big data, the scale of realistic networks is increasing continually. In order to reduce the network scale, some coarse-graining methods are proposed to transform large-scale networks into mesoscale networks. In this paper, a new coarse-graining method based on hierarchical clustering (HCCG) on complex networks is proposed. The network nodes are grouped by using the hierarchical clustering method, then updating the weights of edges between clusters extract the coarse-grained networks. A large number of simulation experiments on several typical complex networks show that the HCCG method can effectively reduce the network scale, meanwhile maintaining the synchronizability of the original network well. Furthermore, this method is more suitable for these networks with obvious clustering structure, and we can choose freely the size of the coarse-grained networks in the proposed method.展开更多
This paper presents a new methodology for coarse-grained atomistic simulation of inelastic material behavior including phase transformations in ceramics and dislocation mediated plasticity in metals. The methodology c...This paper presents a new methodology for coarse-grained atomistic simulation of inelastic material behavior including phase transformations in ceramics and dislocation mediated plasticity in metals. The methodology combines an atomistic formulation of balance equations and a modified finite element method. With significantly fewer degrees of freedom than those of a fully atomistic model and without additional constitutive rules but the interatomic force field, the new coarse-grained （CG） method is shown to be feasible in predicting the nonlinear constitutive re- sponses of materials and also reproducing atomic-scale phenomena such as phase transformations （diamond --, 13-Sn） in silicon and dislocation nucleation and migration, formation of dislocation loops and stacking faults ribbons in single crystal nickel. Direct comparisons between CG and the corresponding full molecular dynamics （MD） simulations show that the present methodology is efficient and promising in modeling and simulation of inelastic material behavior without losing the essential atomistic features. The potential applications and the limitations of the CG method are also discussed.展开更多
基金funded through Marie SKEODOWSKA-CURIE Innovative Training Network MATHEGRAM,the People Programme(Marie SKLODOWSKA-CURIE Actions)of the European Union's Horizon 2020 Programme H2020 under REA grant agreement No.813202.
文摘The P1 approximation is a computationally efficient model for thermal radiation.Here,we present a P1 formulation in the context of the combined computational fluid dynamics and discrete element method(CFD-DEM),including closures for dependent scattering and coarse-graining.Using available analytical and semi-analytical solutions,we find agreement for steady-state and transient quantities in sizedisperse systems.Heat flux is identified as the most sensitive quantity to predict,displaying unphysical spatial oscillations.These oscillations are due to a temperature slip at the locations of abrupt change in solid fraction.We propose two techniques that mitigate this effect:smoothing of the radiative properties,and pseudo-scattering.Furthermore,using up to a million times enlarged particles,we demonstrate practically limitless compatibility with coarse-graining.Finally,we compare predictions made with our code to experimental data for a pebble bed under vacuum conditions,and in presence of nitrogen.We find that a carefully calibrated simulation can replicate trends observed in experiments,with relative temperature error of less than 10%.
文摘Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) is a commonly used numerical method to model gas-solid flow in fluidised beds and other multiphase systems. A significant limitation of CFD-DEM is the feasibility of the realistic simulation of large numbers of particles. Coarse-graining (CG) approaches, through which groups of multiple individual particles are represented by single, larger particles, can substantially reduce the total number of particles while maintaining similar system dynamics. As these three CG models have not previously been compared, there remains some debate, however, about the best practice in the application of CG in CFD-DEM simulations. In this paper, we evaluate the performance of three typical CG methods based on simulations of a bubbling fluidised bed. This is achieved through the use of a numerical validation framework, which makes full use of the high-resolution 3D positron emission particle tracking (PEPT) measurements to rigorously validate the outputs of CFD-DEM simulations conducted using various different coarse-graining models, and various different degrees of coarse-graining. The particle flow behaviours in terms of the particle occupancy field, velocity field, circulation time, and bubble size and velocity, are comprehensively analysed. It is shown that the CG simulation starts to fail when the size ratio between the bed chamber and the particles decreases to approximately 20. It is also observed, somewhat surprisingly, that the specific CG approach applied to interparticle contact parameters does not have a substantial effect on the simulation results for the bubbling bed simulations across a wide range of CG factors.
基金supported by the Science and Technology Foundation of State Grid Corporation of China(SGCC)(J2022116)。
文摘This paper proposes a novel method for the parameter optimization of complex networks established through coarsening and phase space reconstruction.Firstly,we identify the change-points of the time series based on the cumulative sum(CUSUM)control chart method.Then,we optimize the coarse-graining parameters and phase space embedding dimension based on the evolution analysis of the global topology index(betweenness)at the mutation point.Finally,we conduct a simulation analysis based on real-time data of Chinese copper spot prices.The results show that the delay of the copper spot prices in Chinese spot market is 1 day,and the optimal embedding dimension of the phase space reconstruction is 3.The acceptance level of the investors towards the small fluctuations in copper spot prices is 0.2 times than the average level of price fluctuations,which means that an average price fluctuation of 0.2 times is the optimal coarse-graining parameter.
文摘A general operational protocol which provides permanent macroscopic coherence of the response of any stable complex system put in an ever-changing environment is proposed. It turns out that the coherent response consists of two parts: 1) a specific discrete pattern, called by the author homeostatic one, whose characteristics are robust to the statistics of the environment;2) the rest part of the response forms a stationary homogeneous process whose coarse-grained structure obeys universal distribution which turns out to be scale-invariant. It is demonstrated that, for relatively short time series, a measurement, viewed as a solitary operation of coarse-graining, superimposed on the universal distribution results in a rich variety of behaviors ranging from periodic-like to stochastic-like, to a sequences of irregular fractal-like objects and sequences of random-like events. The relevance of the Central Limit theorem applies to the latter case. Yet, its application is still an approximation which holds for relatively short time series and for specific low resolution of the measurement equipment. It is proven that the asymptotic behavior in each and every of the above cases is provided by the recently proven decomposition theorem.
文摘Collagen is a basic biopolymer usually found in animal bodies, but its mechanical property and behavior are not sufficiently understood so as to apply to effective regenerative medicine and so on. Since the collagen material is composed of many hierarchical structures from atomistic level to tissue or organ level, we need to well understand fundamental and atomistic mechanism of the collagen in mechanical response. First, we approach at exactly atomistic level by using all-atom modeling of tropocollagen (TC) molecule, which is a basic structural unit of the collagen. We perform molecular dynamics (MD) simulations concerning tensile loading of a single TC model. The main nature of elastic (often superelastic) behavior and the dependency on temperature and size are discussed. Then, to aim at coarse-graining of atomic configuration into some bundle structure of TC molecules (TC fibril), as a model of higher collagen structure, we construct a kind of mesoscopic model by adopting a simulation framework of beads-spring model which is ordinarily used in polymer simulation. Tensile or compression simulation to the fibril model reveals that the dependency of yield or buckling limit on the number of TCs in the model. Also, we compare the models with various molecular orientations in winding process of initial spiral of TC. The results are analyzed geometrically and it shows that characteristic orientational change of molecules increases or decreases depending on the direction and magnitude of longitudinal strain.
基金funding support of the U.S.Army Research Laboratory with Cooperative Agreement No.W911NF-12-2-0023
文摘We calculate current correlation functions （CCFs） of dissipative particle dy- namics （DPD） and compare them with results of molecular dynamics （MD） and solutions of linearized hydrodynamic equations. In particular, we consider three versions of DPD, the empirical/classical DPD, coarse-grained （CG） DPD with radial-direction interactions only and full （radial, transversal, and rotational） interactions between particles. To fa- cilitate quantitative discussions, we consider specifically a star-polymer melt system at a moderate density. For bonded molecules, it is straightforward to define the CG variables and to further derive CG force fields for DPD within the framework of the Mori-Zwanzig formalism. For both transversal and longitudinal current correlation functions （TCCFs and LCCFs）, we observe that results of MD, DPD, and hydrodynamic solutions agree with each other at the continuum limit. Below the continuum limit to certain length scales, results of MD deviate significantly from hydrodynamic solutions, whereas results of both empirical and CG DPD resemble those of MD. This indicates that the DPD method with Markovian force laws possibly has a larger applicability than the continuum description of a Newtonian fluid. This is worth being explored further to represent gen- eralized hydrodynamics.
文摘With the rapid development of big data, the scale of realistic networks is increasing continually. In order to reduce the network scale, some coarse-graining methods are proposed to transform large-scale networks into mesoscale networks. In this paper, a new coarse-graining method based on hierarchical clustering (HCCG) on complex networks is proposed. The network nodes are grouped by using the hierarchical clustering method, then updating the weights of edges between clusters extract the coarse-grained networks. A large number of simulation experiments on several typical complex networks show that the HCCG method can effectively reduce the network scale, meanwhile maintaining the synchronizability of the original network well. Furthermore, this method is more suitable for these networks with obvious clustering structure, and we can choose freely the size of the coarse-grained networks in the proposed method.
基金supported by the National Science Foundation under award numbers CMMI-0855795 and 1129976DARPA under award number N66001-10-1-4018+1 种基金Department of Energy under award number DOE/DE-SC0006539supported in part by the National Science Foundation through Teragrid resources provided by TACC
文摘This paper presents a new methodology for coarse-grained atomistic simulation of inelastic material behavior including phase transformations in ceramics and dislocation mediated plasticity in metals. The methodology combines an atomistic formulation of balance equations and a modified finite element method. With significantly fewer degrees of freedom than those of a fully atomistic model and without additional constitutive rules but the interatomic force field, the new coarse-grained （CG） method is shown to be feasible in predicting the nonlinear constitutive re- sponses of materials and also reproducing atomic-scale phenomena such as phase transformations （diamond --, 13-Sn） in silicon and dislocation nucleation and migration, formation of dislocation loops and stacking faults ribbons in single crystal nickel. Direct comparisons between CG and the corresponding full molecular dynamics （MD） simulations show that the present methodology is efficient and promising in modeling and simulation of inelastic material behavior without losing the essential atomistic features. The potential applications and the limitations of the CG method are also discussed.
