International Journal of Modern Physics C

The natural convection problem in a square cavity filled with heterogeneously porous medium is solved by lattice Boltzmann method. The temperature distribution is fully coupled with the fluid velocity through relaxation time. The present calculated results are in good agreement with available published data. It is found that the porosity of porous media near the walls has significant influence on the heat transfer, and the porosity of middle porous medium has little influence on the natural convection. It is of particular interest for thermal management in electronic packages, since it can reduce the space of air.

On directed Barabási–Albert networks with two and seven neighbours selected by each added site, the Ising model does not seem to show a spontaneous magnetisation. Instead, the decay time for flipping of the magnetisation follows an Arrhenius law for Metropolis and Glauber algorithms, but for Wolff cluster flipping the magnetisation decays exponentially with time. On these networks the magnetisation behaviour of the Ising model, with Glauber, HeatBath, Metropolis, Wolf or Swendsen–Wang algorithm competing against Kawasaki dynamics, is studied by Monte Carlo simulations. We show that the model exhibits the phenomenon of self-organisation (= stationary equilibrium) defined in Ref. 8 when Kawasaki dynamics is not dominant in its competition with Glauber, HeatBath and Swendsen–Wang algorithms. Only for Wolff cluster flipping the magnetisation, this phenomenon occurs after an exponentially decay of magnetisation with time. The Metropolis results are independent of competition. We also study the same process of competition described above but with Kawasaki dynamics at the same temperature as the other algorithms. The obtained results are similar for Wolff cluster flipping, Metropolis and Swendsen–Wang algorithms but different for HeatBath.

The surface structure, surface energy, and surface vacancy formation energy for B and N vacancy of the cubic boron nitride (c-BN)(110) surface have been investigated by performing Hartree-Fock and DFT calculations. Results are compared with available literature values. The vacancy formation energies [unrelaxed and relaxed (E_f)] are reported for the first time for c-BN(110).

Coupled first order IVPs are frequently used in many parts of engineering and sciences. In this article, we presented a code including three computer programs which are joint with the Matlab software to solve and plot the solutions of the first order coupled stiff or non-stiff IVPs. Some engineering and scientific problems related to IVPs are given and fuel depletion (production of the ²³⁹Pu isotope) in a Pressurized Water Nuclear Reactor (PWR) are computed by the present code.

We compare the efficiency of two prominent techniques for simulation of complex systems: parallel tempering and Wang–Landau sampling. We show that both methods are of comparable efficiency but are optimized for different platforms. Parallel tempering should be chosen on multi-processor system while Wang–Landau sampling is easier to implement on a single-processor computer.

The thermodynamic and structural properties of Tyrosine-Glycine-Leusine-Phenylalanine (YGLF, in a one letter code) and Lysine-Valine-Leusine-Proline-Valine-Proline-Glutamine (KVLPVPQ) peptide sequences were studied by three-dimensional molecular modeling in vacuum and solution. All the three-dimensional conformations of each peptide sequences were obtained by multicanonical simulations with using ECEPP/2 force field and each simulation started from completely random initial conformation. Solvation contributions are included by a term that is proportional to solvent-accessible surface areas of peptides. In the present study, we calculated the average values of total energy, specific heat, fourth-order cumulant and end-to-end distance for two peptide sequences of milk protein as a function of temperature. With using major advantage of this simulation technique, Ramachandran plots were prepared and analysed to predict the relative occurrence probabilities of β-turn, γ-turn and helical structures. Although structural predictions of these sequences indicate both the presence of high level of γ-turns and low level of β-turns in vacuum and solvent, it was observed that these probabilities in vacuum were higher than the ones in solvent model.

The standard spatial formulation of the iterated Prisoner's Dilemma is ahistoric (memoryless): i.e., only the results generated in the last round are taken into account in deciding the next choice. Historic memory can be implemented by featuring players by a summary of their previous winnigs and choices. Here we study the effect of limited trailing memory: only the last three iterations are recorded. The effects of full and discounted memory are assessed. It is concluded that this short-type memory stimulates cooperation.

A new cellular automaton model for pedestrian dynamics based on motor schema is presented. Each pedestrian is treated as an intelligent mobile robot, and motor schemas including move-to-goal, avoid-away and avoid-around drive pedestrians to interact with their environment. We investigate the phenomenon of many pedestrians with different move velocities escaping from a room. The results show that the pedestrian with high velocity have predominance in competitive evacuation, if we only consider repulsion from or avoiding around other pedestrians, and interaction with each other leads to disordered evacuation, i.e., decreased evacuation efficiency. Extensions of the model using learning algorithms for controlling pedestrians, i.e., reinforcement learning, neural network and genetic algorithms, etc. are noted.

We present a new explicit hybrid two step method for the solution of second order initial value problem. It costs only nine function evaluations per step and attains eighth algebraic order so it is the cheapest in the literature. Its coefficients are chosen to reduce amplification and phase errors. Thus the method is well suited for facing problems with oscillatory solutions. After implementing a MATLAB program, we proceed with numerical tests that justify our effort.

In 1994, S. Pride offered a system of equations describing interdependent propagation of acoustic and electromagnetic (EM) fields in porous media saturated by a fluid electrolyte (electrokinetic effect). We proved that the displacement vectors of frame and of pore fluid can be represented as a linear combinations of gradients of two scalar potentials and rotors of two vector potentials provided the coefficients of Pride's equations are independent of spatial coordinates. Each of these potentials satisfies Helmholtz equation with appropriate complex velocity. This representation underlies the computer code created by us for exploring borehole acoustics logging problems in axial-symmetric radially layered medium. Noteworthy, layers can be of any of the following types: uniform porous medium saturated by fluid electrolyte, uniform nonporous elastic medium, compressible nonviscous fluid or compressible viscous fluid. Results of numerical experiments are presented and discussed. The most important of them is that the back influence on acoustic disturbance of EM field excited by this disturbance is negligibly small in usually used frequency band and for realistic formation parameters. The mechanism of generation and propagation of different types of EM waves during acoustic logging is discussed as well.

Possibilities for performing stochastic simulations on the analog and fully parallelized Cellular Neural Network UniversalMachine (CNN-UM) are investigated. By using a chaotic cellular automaton perturbed with the natural noise of the CNN-UM chip, a realistic binary random number generator is built. As a specific example for Monte Carlo type simulations, we use this random number generator and a CNN template to study the classical site-percolation problem on the ACE16K chip. The study reveals that the analog and parallel architecture of the CNN-UM is very appropriate for stochastic simulations on lattice models. The natural trend for increasing the number of cells and local memories on the CNN-UM chip will definitely favor in the near future the CNN-UM architecture for such problems.

We present a model of evolution of the age structured population based on the Monte Carlo method. We have assumed that the health status of an individual is described by variance of its fluctuations. Each expressed deleterious mutation increases the fluctuations. Additionally, the fluctuations of the environment are superimposed on the fluctuations of individuals in the population. An individual dies if the combination of both stochastic processes trespass the limit (level of homeostasis) set as the model parameter.

The genes are switched on chronologically, what leads to accumulating defective genes expressed during the late periods of life in the genetic pool of the population. That results in the specific age structured population, in accordance with the predictions of Medawar's hypothesis of ageing and the results of the Penna model simulations. A decrease of the variation of the environmental noise increases the average expected lifespan of individuals.