Coherent structure formation,phase correlations,and finite-size effects in 2D turbulence

General conditions for the formation of long-lived coherent vortices in decaying and force-driven 2-D turbulence are investigated. It is shown by a series of numerical simulations that the emergence of closed streamlines leading to the trapped trajectories of vorticity is a necessary, but not sufficient condition for this phenomenon, so that these trapped trajectories may be considered only as seed vortices. Numerical experiments which demonstrate the relations between phase correlations, finite-size effects, and the formation of coherent vortices in 2-D decaying turbulence are presented. It is shown that there is a universal dimensionless time for the onset of intermittency in the flow which corresponds to the time of establishment of phase correlations. The stricter conditions for appearance of coherent vortices in forced turbulence in comparison with decaying turbulence are associated with phase mixing introduced by random forcing. The universal features of decaying turbulence are discussed in terms of phase portraits based on inviscid constants of motion and their decay rates.     

Monte Carlo and variance reduction methods for structural reliability analysis: A comprehensive review

Monte Carlo methods have attracted constant and even increasing attention in structural reliability analysis with a wide variety of developments seamlessly presented over decades. Along the way, a number of specialized reviews and benchmark studies have been provided from time to time, aiming at summarizing and comparing selected few approaches in detail, mainly from an implementation point of view. In contrast, the aim of the present survey is to play a comprehensive role as a methodological guidebook on Monte Carlo simulation and its related, especially variance reduction, techniques through a covering of 444 references in the relevant literature. To achieve this goal, we present an extensive review of formulations and techniques along with insightful summaries of developments of existing numerical methods, ranging from the general formulation, sub-categories and variants, to their combined uses with other simulation techniques and surrogate models, as well as the key advantages and assumptions.     

Reliability analysis on systems with self-healing and self-repairing under different environments and shock models

Inspired by the process of self-healing of biological damage, high technology materials with self-healing and self-repairing mechanisms have been developed for high reliability and long lifetime. Therefore, the reliability modeling on intelligent systems with healing performance has become a research hotspot. Based on the diversity of healing mechanisms, this paper proposes a two-phase reliability model method on self-healing and self-repairing systems. Impacts of environments, shock loads, self-healing, and self-repairing mechanisms are taken into account in this novel model. Besides, system lifetime and some reliability indexes under two shock models are derived, respectively. Moreover, Monte Carlo simulations are conducted to verify the accuracy of reliability under two models. Finally, an engineering case of metallized film capacitor is provided to illustrate the effectiveness and applicability of the proposed models by comparing numerical results and simulation results.     

Direct numerical simulations for assessment of gas-solid drag models in two-dimensional random arrays of particles

The momentum exchange between the phases plays a vital role in modelling of gas–solid flows and it is mathematically described by drag models. However, no consensus exists on which drag model gives the most accurate prediction of the drag force, and, despite the increase in available computing power, the same drag models are used in two-dimensional and three-dimensional simulations. In this study, direct numerical simulations of gas flow through multiple random configurations of static monodisperse particles are performed. The variations of solid volume fraction and particle Reynolds number are in the ranges of 0.05–0.4 and 13.7–136.9, respectively. The drag force exerted on particles is calculated and properly averaged. Based on the simulation results, thirteen drag models are compared and correction factors are introduced using the stochastic gradient descent algorithm. The correction factors provide a simple adjustment for the models to be used in 2D modelling.     

Numerical simulations of particle migration in a viscoelastic fluid subjected to shear flow

Particle migration is a relevant transport mechanism whenever suspensions flow in channels with gap size comparable to particle dimensions (e.g. microfluidic devices). Several theoretical as well as experimental studies have been performed on this topic, showing that the occurring of this phenomenon and the migration direction are related to particle size, flow rate, and the nature of the suspending liquid.In this work we perform a systematic analysis on the migration of a single particle in a sheared viscoelastic fluid through 2D finite element simulations in a Couette planar geometry. To focus on the effects of viscoelasticity alone, inertia is neglected. The suspending medium is modeled as a Giesekus fluid.An ALE particle mover is combined with a DEVSS/SUPG formulation with a log-representation of the conformation tensor giving stable and convergent results up to high flow rates. To optimize the computational effort and reduce the remeshing and projection steps, needed as soon as the mesh becomes too distorted, a ‘backprojection’ of the flow fields is performed, through which the particle in fact moves along the cross-streamline direction only, and the mesh distortion is hence drastically reduced.Our results, in agreement with recent experimental data, show a migration towards the closest walls, regardless of the fluid and geometrical parameters. The phenomenon is enhanced by the fluid elasticity, the shear thinning and strong confinements. The migration velocity trends show the existence of a master curve governing the particle dynamics in the whole channel. Three different regimes experienced by the particle are recognized, related to the particle-wall distance. The existence of a unique migration behavior and its qualitative aspects do not change by varying the fluid parameters or the particle size.     

数字通信系统中的Monte Carlo仿真

介绍数字通信系统的广泛应用和Monte Carlo算法的基本思想,重点分析数字通信系统中的差错概率和应用Monte Carlo仿真对存在噪声和干扰的数字通信系统的性能进行评估。     

QSATS: MPI-driven quantum simulations of atomic solids at zero temperature

We describe QSATS, a parallel code for performing variational path integral simulations of the quantum mechanical ground state of monatomic solids. QSATS is designed to treat Boltzmann quantum solids, in which individual atoms are permanently associated with distinguishable crystal lattice sites and undergo large-amplitude zero-point motions around these sites. We demonstrate the capabilities of QSATS by using it to compute the total energy and potential energy of hexagonal close packed solid ⁴He at the density .

Program summary

Program title:QSATSCatalogue identifier: AEJE_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEJE_v1_0.htmlProgram obtainable from: CPC Program Library, Queen?s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 7329No. of bytes in distributed program, including test data, etc.: 61 685Distribution format: tar.gzProgramming language: Fortran 77.Computer: QSATS should execute on any distributed parallel computing system that has the Message Passing Interface (MPI) [1] libraries installed.Operating system: Unix or Linux.Has the code been vectorized or parallelized?: Yes, parallelized using MPI [1].RAM: The memory requirements of QSATS depend on both the number of atoms in the crystal and the number of replicas in the variational path integral chain. For parameter sets A and C (described in the long write-up), approximately 4.5 Mbytes and 12 Mbytes, respectively, are required for data storage by QSATS (exclusive of the executable code).Classification: 7.7, 16.13.External routines: Message Passing Interface (MPI) [1]Nature of problem: QSATS simulates the quantum mechanical ground state for a monatomic crystal characterized by large-amplitude zero-point motions of individual (distinguishable) atoms around their nominal lattice sites.Solution method: QSATS employs variational path integral quantum Monte Carlo techniques to project the system?s ground state wave function out of a suitably-chosen trial wave function.Restrictions: QSATS neglects quantum statistical effects associated with the exchange of identical particles. As distributed, QSATS assumes that the potential energy function for the crystal is a pairwise additive sum of atom–atom interactions.Additional comments: An auxiliary program, ELOC, is provided that uses the output generated by QSATS to compute both the crystal?s ground state energy and the expectation value of the crystal?s potential energy. End users can modify ELOC as needed to compute the expectation value of other coordinate-space observables.Running time: QSATS requires roughly 3 hours to run a simulation using parameter set A on a cluster of 12 Xeon processors with clock speed 2.8 GHz. Roughly 15 hours are needed to run a simulation using parameter set C on the same cluster.References:

• [1] 

  For information about MPI, visit http://www.mcs.anl.gov/mpi/.

     

Probability of graphs with large spectral gap by multicanonical Monte Carlo

Graphs with large spectral gap are important in various fields such as biology, sociology and computer science. In designing such graphs, an important question is how the probability of graphs with large spectral gap behaves. A method based on multicanonical Monte Carlo is introduced to quantify the behavior of this probability, which enables us to calculate extreme tails of the distribution. The proposed method is successfully applied to random 3-regular graphs and large deviation probability is estimated.     

大型喷浆机器人的工作空间分析

对机器人工作空间的求解方法进行对比研究。针对大型喷浆机器人的具体特点 ,使用蒙特卡洛法分析其工作空间 ,取得了良好的效果。