Nucleation and hysteresis in Ising model: classical theory versus computer simulation

We have studied the nucleation in the nearest neighbour ferromagnetic Ising model, in different (d) dimensions, by extensive Monte-Carlo simulation using the heat-bath dynamics. The nucleation time () has been studied as a function of the magnetic field (h) for various system sizes in different dimensions (d=2,3,4). The logarithm of the nucleation time is found to be proportional to the power (-(d-1)) of the magnetic field (h) in d dimensions. The size dependent crossover from coalescence to nucleation regime is observed in all dimensions. The distribution of metastable lifetimes are studied in both regions. The numerical results are compared and found to be consistent with the classical theoretical predictions. In two dimensions, we have also studied the dynamical response to a sinusoidally oscillating magnetic field. The reversal time is studied as a function of the inverse of the coercive field. The applicability of the classical nucleation theory to study the hysteresis and coercivity has been discussed. Received: 21 January 1998 / Accepted: 17 March 1998     

High-resolution frequency estimation technique for recovering phase distribution in interferometers

An integral approach to phase measurement is presented. First, the use of a high-resolution technique for the pixelwise detection of phase steps is proposed. Next, the robustness of the algorithm that is developed is improved by incorporation of a denoising procedure during spectral estimation. The pixelwise knowledge of phase steps is then applied to the Vandermonde system of equations for retrieval of phase values at each pixel point. Conceptually, our proposal involves the design of an annihilating filter that has zeros at the frequencies associated with the polynomial that describes the fringe intensity. The parametric estimation of this annihilating filter yields the desired spectral information embedded in the signal, which in our case represents the phase steps. The proposed method offers the advantage of extracting the interference phase of nonsinusoidal waveforms in the presence of miscalibration error of the piezoelectric transducer. In addition, in contrast to previously reported methods, this method does not require the application of selective phase steps between data frames for nonsinusoidal waveforms.     

Path-length distribution of ions reflected from a solid: Theory and computer simulation

Theoretical methods and Monte Carlo procedure are used to study path-length distributions of ions reflected from a solid. The theoretical analysis is based on the solution of the integral Chandrasekhar equation for the Laplace transform of the distribution function. A family of curves is obtained for path-length distributions at several ion energies and mass ratios of ions and target atoms. A computer code for simulation is based on the approximation of pair collisions and a gas model of solid. The simulated results are compared with the theoretical results and published data.     

Van hove self-correlation function of a hard-disk fluid: Enskog theory and computer simulation

The van Hove self-correlation function in a hard-disk fluid is analyzed using the Lorentz-Enskog kinetic equation and the kinetic model method of solution. Numerical convergence of the model solutions is demonstrated and accurate model results are used to interpret molecular dynamics simulation data at finite wave numbers. It is found that at about 60% of freezing density the error in the Enskog theory can be mainly attributed to an underestimate of the effective self-diffusion coefficient, but at 90% freezing density a theory which treats correlated collisions is needed to describe the width behavior of the singleparticle density fluctuation spectrum.     

Excess properties of Lennard-Jones binary mixtures from computer simulation and theory

Monte Carlo simulation and theory are used to calculate the excess thermodynamic properties of binary mixtures of spherical Lennard-Jones molecules. We study the excess functions of three binary mixtures characterized by the following size and dispersive energy ratios: (1) (σ₂₂/σ₁₁)³ = 2 and ?₂₂/?₁₁ = 2; (2) (σ₂₂/σ₁₁)³ = 1 and ?₂₂/?₁₁ = 1/2 and (3) (σ₂₂/σ₁₁)³ = 1/2 and ?₂₂/?₁₁ = 2. In all cases, the unlike size parameter, σ₁₂, is kept constant and equal to the value given by the Lorentz combining rule (σ₁₂ = (σ₁₁ + σ₂₂)/2). However, different unlike dispersive energy parameter values are considered through the following combining rules: (a) ?₁₂ = (?₁₁?₂₂)^1/2 (Berthelot rule); (b) ?₁₂ = ?₁₁ (association); and (c) ?₁₂ = ?₂₂ (solvation). The pressure and temperature dependence of the excess volume and excess enthalpy is studied using the NpT Monte Carlo simulation technique for all the systems considered. Additionally, the simplest conformal solution theory is used to check the adequacy of this approach in predicting the excess properties in a wide range of thermodynamic conditions and variety of binary mixtures. In particular, we have applied the van der Waals one-fluid theory to describe Lennard-Jones binary mixtures through the use of the Johnson et al. [1993, Molec. Phys., 78, 591] Helmholtz free energy. Agreement between simulation results and theoretical predictions is excellent in all cases and thermodynamic conditions considered. This work confirms the applicability of the van der Waals one-fluid theory in predicting excess thermodynamic properties of mixtures of spherical molecules. Furthermore, since binary mixtures of spherical Lennard-Jones molecules constitute the reference fluid to be used in perturbation theories for complex fluids, such as the statistical association fluid theory (SAFT), this work shows clearly the applicability of the conformal solution theory within the framework of SAFT for predicting excess functions.     

Radiofrequency electrode vibration-induced shear wave imaging for tissue modulus estimation: a simulation study

Quasi-static electrode displacement elastography, used for in-vivo imaging of radiofrequency ablation-induced lesions in abdominal organs such as the liver and kidney, is extended in this paper to dynamic vibrational perturbations of the ablation electrode. Propagation of the resulting shear waves into adjoining regions of tissue can be tracked and the shear wave velocity used to quantify the shear (and thereby Young's) modulus of tissue. The algorithm used utilizes the time-to-peak displacement data (obtained from finite element analyses) to calculate the speed of shear wave propagation in the material. The simulation results presented illustrate the feasibility of estimating the Young's modulus of tissue and is promising for characterizing the stiffness of radiofrequency-ablated thermal lesions and surrounding normal tissue.     

双间隙耦合腔电子电导的理论与计算仿真

在速调管双间隙耦合谐振腔中,电子与每个间隙的电场相互作用进行能量交换,在两个间隙上体现出不同的电子负载效应.传统的电子电导计算模型,只能从整体上而无法在每个间隙上考虑这个效应.基于空间电荷波理论,建立了双间隙耦合腔中单个间隙电子电导的理论模型,推导出相应的计算公式.利用三维粒子模拟工具进行了仿真研究,理论计算与仿真结果相符.与传统的电子电导模型相比,该理论模型能反映出双间隙耦合腔中每个间隙的电子负载效应.利用该模型能更加深入和准确地进行间隙注波互作用的研究以及耦合腔中模式稳定性的分析. 关键词： 双间隙耦合腔 空间电荷波理论 电子电导 粒子模拟     

Acoustic tomography of temperature distribution in a medium heated by a focused ultrasound beam

A new method is proposed for measuring temperature in the focal region of a high-intensity ultrasonic emitter on the assumption of axially symmetric temperature field. This method is based on solving the integral equation relating the temperature of the medium to the delay times of the probe ultrasound pulse intersecting the heated region in the transverse direction at different distances from the beam axis. The accuracy of the algorithm for calculating temperature with allowance for the finite experimental data set is analyzed. The calculation results are compared with the experimental data. Original Russian Text ? S.A. Tsysar’, O.A. Sapozhnikov, V.G. Andreev, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 4, pp. 558–561.     

Determination of temperature and emissivity of opaque heated bodies via thermal radiation spectrum: simulation of measurements in spectral window

Computer simulation of emitted radiation intensity spectrum of tantalum object was carried out. Simulation measurements occurred in a narrow spectral window, which moved along the spectrum with the given step. In this way, spectral ranges at which the dependence of the emissivity (or its logarithm) on the wavelength is the simplest and fairly accurate, in particular linear, were sought for. In the case of successful search for the specified spectral range, the required temperature was determined in the spectral window with the least squares technique. If the emissivity (or its logarithm) is linearly dependent on the wavelength then the alternative estimation of the true temperature is possible. In this case, the required temperature is determined via the change of the convexity of the spectral emissivity dependence at selection of its numerical value from the values lower than the true temperature value to the values higher than the true temperature value. This approach is simple, does not require solution of the system of equations and can significantly narrow temperature range to which the true temperature belongs.     

LEADS-DC: A computer code for intense dc beam nonlinear transport simulation

An intense dc beam nonlinear transport code has been developed. The code is written in Visual FORTRAN 6.6 and has ∼13000 lines. The particle distribution in the transverse cross section is uniform or Gaussian. The space charge forces are calculated by the PIC (particle in cell) scheme, and the effects of the applied fields on the particle motion are calculated with the Lie algebraic method through the third order approximation. Obviously, the solutions to the equations of particle motion are self-consistent. The results obtained from the theoretical analysis have been put in the computer code. Many optical beam elements are contained in the code. So, the code can simulate the intense dc particle motions in the beam transport lines, high voltage dc accelerators and ion implanters.

     

回旋速调管放大器时域非线性理论与模拟

研究了一种回旋速调管放大器时域非线性理论模型.该模型由广义电报方程来表示回旋速调管内的电磁场,采用引导中心近似的电子运动方程来推动粒子,由粒子更新得到的电流密度为源激励电磁场.基于上述理论模型,从回旋速调管电子注横向速度满足高斯分布出发,建立了速度分散的分布模型,编写了相应的时域非线性注波互作用模拟程序,对回旋速调管放大器的注波互作用进行了深入的分析和研究,并应用粒子模拟软件与自洽非线性模拟程序进行对比验证,两者结果基本一致.     

Three-dimensional site percolation problem and effective-medium theory: A computer study

A site percolation approach to classical transport in disordered two-phase materials is presented. A Monte Carlo computer experiment gives the bulk conductivity of a 30 x 30 x 30 site simple cubic resistor network consisting of two kinds of unit resistor with different conductance. A modified effective-medium theory predicts very accurately the bulk conductivity of the network. This theory is found to agree well with available data for the thermal conductivity of real two-phase materials: glass particle-silicon rubber; glass fiber-plastics; air-saturated porous sandstone; and air-saturated fire brick.     

On the material frame indifference controversy: Some results from group theory and computer simulation.

Group theory and computer simulation are used to show that molecular and atomic scale time cross correlation functions have different symmetry properties in different frames of reference. Constitutive equations, written in any frame of reference, are approximations to classical molecular dynamics, and must therefore reproduce the results from group theory applied to ensemble averages such as time correlation functions, and corroborated by computer simulation. This may help to resolve some of the contemporary literature claims regarding material frame indifference and the implementation of “co- rotating” frames.     

Investigation of concentration profiles and growth rate of InAs LPE by computer simulation technique

Summary The diffusive-transport model has been used to study the mechanism of LPE growth of InAs. Computer simulation technique has been followed to calculate the concentration profiles of solute at successive equally spaced places in front of the growing interface under normal conditions of LPE at successive time intervals with applying appropriate boundary conditions. Using the data simulated for concentration profiles, the growth rate of InAs has been calculated. This model also allows to investigate the amount of InAs etched or grown as a function of time. The results are discussed in detail.