International Journal of Modern Physics C

We study a random fiber bundle model with tips of the fibers placed on a graph having co-ordination number 3. These fibers follow local load sharing with uniformly distributed threshold strengths of the fibers. We have studied the critical behavior of the model numerically using a finite size scaling method and the mean field critical behavior is established. The avalanche size distribution is also found to exhibit a mean field nature in the asymptotic limit.

In this paper, experiments were carried out to validate the motor schema-based cellular automaton model for pedestrian dynamics presented by Weng et al. [Int. J. Mod. Phys. C17, 853 (2006)]. Unidirectional pedestrian flow with different walk velocities is studied by means of simulations and experiments. The lower walk velocity was provided by the young people with special equipment, which made them walk slowly, to imitate the elderly people. Travel time of each pedestrian from start line to goal line with different initial distribution patterns and obstacles in the way was measured to compare with simulation results. The comparison results indicate that the experimental observations can be well reproduced with the motor schema-based cellular automaton model. And the simulation results of the mean travel time agree well with the experimental data.

A new method for community identification is proposed which is founded on the analysis of successive neighborhoods, reached through hierarchical growth from a starting vertex, and on the definition of communities as a subgraph whose number of inner connections is larger than outer connections. In order to determine the precision and speed of the method, it is compared with one of the most popular community identification approaches, namely Girvan and Newman's algorithm. Although the hierarchical growth method is not as precise as Girvan and Newman's method, it is potentially faster than most community finding algorithms.

A multigrid (MG) numerical solution procedure for a mathematical model for epitaxial growth in metalorganic chemical vapor deposition reactors is developed. The model is governed by the full compressible Navier–Stokes equations for two-dimensional (plane and axisymmetric) laminar flows extended by transport equations for the chemical species, the energy equations and the equation of state, together with boundary conditions providing information on the reactor geometry and experimental conditions. The MG numerical solution is implemented in a finite element procedure that uses the same linear finite elements to discretize both convection and diffusion fluxes by adding to the model a stabilizing term, resulting in easy implementation of the procedure. A deposition process for a model of GaAs growth is calculated with our procedure showing consistency with experimental data.

There has been considerable interest in non-thermal discharges over the past decade due to the increased number of industrial applications. The properties of discharges in electronegative gases are most frequently used for technological applications. For the improvement of performance in these applications, it is necessary to understand discharge dynamics experimentally and numerically. In this paper, a Monte Carlo simulation is carried out in sulfur hexafluoride (SF₆) in uniform electric fields. The streamer propagation, electron, positive and negative ion distributions and space charge fields are studied in detail as time increases.

A novel lattice Boltzmann model is proposed for groundwater isotropic flows (LABGIF). It solves the isotropic groundwater equation. This is generally suitable for both steady and unsteady groundwater problems. Only a 5-speed square lattice is required for 2D and 7-speed cubic for 3D cases in the model. The Dirichlet and Neumann conditions associated with the common boundary conditions encountered in the groundwater modeling are treated with a simple procedure. The stability conditions for the model are described. The model is verified by being applied to steady and unsteady groundwater flows. There are excellent agreements between numerical predictions and exact/analytical solutions. This indicates that the scheme is simple, accurate, and robust.

The transient deformation of a liquid-filled elastic capsule, simulating a red blood cell, was studied in simple shear flow. The simulation is based on a hybrid method which introduces the immersed boundary concept in the framework of the multi-block lattice Boltzmann model. The method was validated by the study on deformation of an initially circular capsule with Hooke's membrane. Also studied were capsules with Skalak membrane of initially elliptical and biconcave shapes, which are more representative of a red blood cell. Membrane tank treading motion is observed. As the ratio between membrane dilation modulus and shear modulus increases, the capsule shows asymptotic behavior. For an initially elliptical capsule, it is found that the steady shape is independent of initial inclination angle. For an initially biconcave capsule, the tank treading frequency from two-dimensional modeling is comparable to that of real cells. Another interesting finding is that the tank treading velocity has not attained steady state when the capsule shape becomes steady; and at this state there is the internal vortex pair. The treading velocity continues to decrease and reaches a steady value when the internal vortex pair has developed into a single vortex.

A new four-dimensional continuous autonomous hyperchaotic system is considered. It possesses two parameters, and each equation of it has one quadratic cross product term. Some basic properties of it are studied. The dynamic behaviors of it are analyzed by the Lyapunov exponent (LE) spectrum, bifurcation diagrams, phase portraits, and Poincaré sections. The system has larger hyperchaotic region. When it is hyperchaotic, the two positive LE are both large and they are both larger than 1 if the system parameters are taken appropriately.

In this article, we propose a network spreading model for HIV epidemics, wherein each individual is represented by a node of the transmission network and the edges are the connections between individuals along which the infection may spread. The sexual activity of each individual, measured by its degree, is not homogeneous but obeys a power-law distribution. Due to the heterogeneity of activity, the infection can persistently exist at a very low prevalence, which has been observed in the real data but cannot be illuminated by previous models with homogeneous mixing hypothesis. The model displays a clear picture of hierarchical spread: In the early stage the infection is adhered to these high-risk persons, and then, diffuses toward low-risk population. Furthermore, we find that to reduce the risky behaviors is much more effective in the fight against HIV/AIDS rather than the antiretroviral drug therapies. The prediction results show that the development of epidemics can be roughly categorized into three patterns for different countries, and the pattern of a given country is mainly determined by the average sex-activity and transmission probability per sexual partner. In most cases, the effect of HIV epidemics on demographic structure is very small. However, for some extremely countries, like Botswana, the number of sex-active people can be depressed to nearly a half by AIDS.

We extended the Biham–Middleton–Levine model to incorporate the origin and destination effect of drivers trips on the traffic in cities. The destination sites are randomly chosen from some origin-destination distances probability distribution "ODDPD." We use three different distributions: exponential, uniform, and power-law. We found that the traffic dynamics of the model are greatly influenced by the ODDPD. Moreover, it is found that we can adjust the ODDPD to enhance the road capacity of the city and to minimize the arrival times of drivers.

A quantitative method is suggested, where meanings of words, and grammatic rules about these, of a vocabulary are represented by real numbers. People meet randomly, and average their vocabularies if they are equal; otherwise they either copy from higher hierarchy or stay idle. The presence of teachers broadcasting the same (but arbitrarily chosen) vocabulary leads the language formations to converge more quickly.

We introduce kernel smoothing method to extract the global trend of a time series and remove short time scales variations and fluctuations from it. A multifractal detrended fluctuation analysis (MF-DFA) shows that the multifractality nature of TEPIX returns time series is due to both fatness of the probability density function of returns and long range correlations between them. MF-DFA results help us to understand how genetic algorithm and kernel smoothing methods act. Then we utilize a recently developed genetic algorithm for carrying out successful forecasts of the trend in financial time series and deriving a functional form of Tehran price index (TEPIX) that best approximates the time variability of it. The final model is mainly dominated by a linear relationship with the most recent past value, while contributions from nonlinear terms to the total forecasting performance are rather small.