New Versions of Image Approximations to the Ionic Solvent Induced Reaction Field

A recent article by Deng and Cai [Extending the fast multipole method for charges inside a dielectric sphere in an ionic solvent: High-order image approximations for reaction fields, J. Comput. Phys. (2007), doi: 10.1016/j.jcp.2007.09.001] introduced two fourth-order image approximations to the reaction field for a charge inside a dielectric sphere immersed in a solvent of low ionic strength. To represent such a reaction field, the image approximations employ a point charge at the classical Kelvin image point and two line charges that extend from this Kelvin image point along the radial direction to infinity, with one decaying to zero and the other growing to infinity. In this paper, alternative versions of the fourth-order image approximations are presented, using the same point charge but three different line charges, all decaying to zero along the radial direction. Similar discussions on how to approximate the line charges by discrete image charges and how to apply the resulting multiple discrete image approximations together with the fast multipole method are also included.     

Recent advances in the application of thermal infrared scanning to geological and hydrological studies

This paper describes recent advances in the development of thermal scanner technology, data processing techniques and theoretical simulation of field applications at the University of Newcastle. These developments have improved the potential of this method for practical field studies in geology and hydrology in the Australian environment. Improvements in technology have enabled the signal output of the scanner to be stabilized and calibrated by the use of internal, black-body temperature reference sources, prior to recording on magnetic tape. These improvements have opened a new dimension for the analysis and processing of thermal scanner data. Both analog and digital processing methods are used for analysis and presentation. Special presentation methods such as density slicing, contrast stretching, boundary enhancement and contouring are all used to enhance specific aspects of the data, which then assist the interpreter in gleaning additional information about his field problem. Advances have been made by the present group in theoretical studies of soil and rock temperature changes in an active thermal environment. Variations in the ground surface temperature during the diurnal cycle provide information relating to the ground thermal properties. Thus the thermal scanner can be used to study variations in the thermal properties of the ground surface over large areas. The application of thermal scanning to Australian Geological, Hydrogeological and Hydrological problems is discussed.     

Topology optimization using a reaction–diffusion equation

This paper presents a structural topology optimization method based on a reaction–diffusion equation. In our approach, the design sensitivity for the topology optimization is directly employed as the reaction term of the reaction–diffusion equation. The distribution of material properties in the design domain is interpolated as the density field which is the solution of the reaction–diffusion equation, so free generation of new holes is allowed without the use of the topological gradient method. Our proposed method is intuitive and its implementation is simple compared with optimization methods using the level set method or phase field model. The evolution of the density field is based on the implicit finite element method. As numerical examples, compliance minimization problems of cantilever beams and force maximization problems of magnetic actuators are presented to demonstrate the method’s effectiveness and utility.     

Phase field and level set methods for modeling solute precipitation and/or dissolution

The dynamics of solid–liquid interfaces controlled by solute precipitation and/or dissolution due to the chemical reaction at the interface were computed in two dimensions using a phase field models. Sharp-interface asymptotic analysis demonstrated that the phase field solutions should converge to the proper sharp-interface precipitation/dissolution limit. For the purpose of comparison, the numerical solution of the sharp-interface model for solute precipitation/dissolution was directly solved using a level set method. In general, the phase field results are found in good agreement with the level set results for all reaction rates and geometry configurations investigated. Present study supports the applications of both methods to more complicated and realistic reactive systems, including the nuclear waste release and mineral precipitation and dissolution.     

Integration of the Vlasov equation along characteristics in one and two dimensions

Methods of integration of the Vlasov equation along characteristics in one and two dimensions are discussed, in connection with the problem of the formation of a charge separation at a plasma edge. Application of these methods to the problem in which the Vlasov equation is integrated in a cylindrical geometry to calculate the electric field at a plasma edge, is presented.     

A fast level set framework for large three-dimensional topography simulations

We present fast methods to describe the surface evolution of large three-dimensional structures. Based on the sparse field level set method and the hierarchical run-length encoding level set data structure optimal figures for the computation time and for the memory consumption are achieved. Furthermore, we introduce a new multi-level-set technique, which is able to incorporate multiple material regions, and which can also handle material specific surface speeds accurately. We also describe an optimal algorithm for the visibility check for unidirectional etching. The presented techniques are demonstrated on various examples.     

Unsteady Flow Visualization by Animating Evenly-Spaced Streamlines

In recent years the work on vector field visualization has been concentrated on LIC-based methods. In this paper we propose an alternative solution for the visualization of unsteady flow fields. Our approach is based on the computation of temporal series of correlated images. While other methods are based on pathlines and try to correlate successive images at the pixel level, our approach consists in correlating instantaneous visualizations of the vector field at the streamline level. For each frame a feed forward algorithm computes a set of evenly-spaced streamlines as a function of the streamlines generated for the previous frame. This is achieved by establishing a correspondence between streamlines at successive time steps. A cyclical texture is mapped onto every streamline and textures of corresponding streamlines at different time steps are correlated together so that, during the animation, they move along the streamlines, giving the illusion that the flow is moving in the direction defined by the streamline. Our method gives full control on the image density so that we are able to produce smooth animations of arbitrary density, covering the field of representations from sparse, that is classical streamline-based images, to dense, that is texture-like images.     

Grand canonical auxiliary field Monte Carlo: a new technique for simulating open systems at high density

The computation of open many-particle systems at high densities is a major challenge since many decades due to the inherent limitations of grand canonical simulation methods based on particle exchange algorithms. In this paper we report on the statistical convergence behavior in the high density regime of a recently developed alternative called the grand canonical auxiliary field Monte Carlo method. We show on a common soft matter model widely used in polymer simulation that it possesses a more appropriate statistical behavior in the dense regime than the currently employed grand canonical Monte Carlo methods relying on particle exchange algorithms.     

Cluster Monte Carlo and numerical mean field analysis for the water liquid-liquid phase transition

Using Wolff's cluster Monte Carlo simulations and numerical minimization within a mean field approach, we study the low temperature phase diagram of water, adopting a cell model that reproduces the known properties of water in its fluid phases. Both methods allow us to study the thermodynamic behavior of water at temperatures, where other numerical approaches - both Monte Carlo and molecular dynamics - are seriously hampered by the large increase of the correlation times. The cluster algorithm also allows us to emphasize that the liquid-liquid phase transition corresponds to the percolation transition of tetrahedrally ordered water molecules.     

Right-Triangulated Irregular Networks

We describe a hierarchical data structure for representing a digital terrain (height field) which contains approximations of the terrain at different levels of detail. The approximations are based on triangulations of the underlying two-dimensional space using right-angled triangles. The methods we discuss permit a single approximation to have a varying level of approximation accuracy across the surface. Thus, for example, the area close to an observer may be represented with greater detail than areas which lie outside their field of view. We discuss the application of this hierarchical data structure to the problem of interactive terrain visualization. We point out some of the advantages of this method in terms of memory usage and speed. Received December 21, 1998; revised September 24, 1999.     

对等网信息检索的研究现状与展望

随着对等网(P2P)研完的进一步深入以及P2P网络中Peer结点和共享文件的进一步增多，如何在非集中式的P2P网络中发现所需要的文件已经成为P2P从研究走向实用的关键所在。该文首先提出了P2P挖掘的概念，然后指出P2P信息检索作为P2P挖掘中的一部分，已经成为P2P研究的一个热点。接下来提出了P2P网络的路由、搜索、挖掘的框架模型，指明了该领域研究的框架。然后分层综述了P2P信息检索的进展状况，对各种检索方法做了深入分析。并指出了它们各自的优缺点和应用局限性，最后对今后的P2P信息检索领域的发展方向进行了展望。     

Particle in cell simulation of combustion synthesis of TiC nanoparticles

A coupled continuum-discrete numerical model is presented to study the synthesis of TiC nanosized aggregates during a self-propagating combustion synthesis (SHS) process. The overall model describes the transient of the basic mechanisms governing the SHS process in a two-dimensional micrometer size system. At each time step, the continuum (micrometer scale) model computes the current temperature field according to the prescribed boundary conditions. The continuum system is discretized with a desired number of uniform computational cells. Each cell contains a convenient number of computational particles which represent the actual particles mixture. The particle-in-cell (discrete) model maps the temperature field from the (continuum) cells to the particles. Depending on the temperature reached by the cell, the titanium particles may undergo a solid-liquid transformation. If the distance between the carbon particle and the liquid titanium particles is within a certain tolerance they react and a TiC particle is formed in the cell. Accordingly, the molecular dynamic method updates the location of all particles in the cell and the amount of transformation heat accounted by the cell is entered into the source term of the (continuum) heat conduction equation. The new temperature distribution progresses depending on the cells which undergo the chemical reaction. As a demonstration of the effectiveness of the overall model some examples are shown.     

Numerical solution of the shallow water equations with a fractional step method

A fractional step technique for the numerical solution of the shallow water equations is applied to study the evolution of the potential vorticity field. The height and velocity field of the shallow water equations are discretized on a fixed Eulerian grid and time-stepped with a fractional step method recently reported in [M. Shoucri, Comput. Phys. Comm. 164 (2004) 396; M. Shoucri, A. Qaddouri, M. Tanguay, J. Côté, A Fractional Steps Method for the Numerical Solution of the Shallow Water Equations, International Workshop on Solution of Partial Differential Equations, The Fields Institute, Toronto, August 2002], where the Riemann invariants of the equations are interpolated at each time step along the characteristics using a cubic spline interpolation. The potential vorticity, which develops steep gradients and evolve into thin filaments during the evolution, is nicely calculated at every time-step from the solution of the code. The method is efficient and has lower numerical diffusion than other methods, since it evolves the equations without the iterative steps involved in the multi-dimensional interpolation problem, and without the iteration associated with the intermediate step of solving a Helmholtz equation, usually associated with other methods like the semi-Lagrangian method. The absence of iterative steps in the present technique makes it very suitable for problems in which small time steps and grid sizes are required, as for instance in the present problem where steepness of the gradients and small scale structures are the main features of the potential vorticity, and more generally for problems of regional climate modeling. The simplicity of the method makes it very suitable for parallel computer.     

融合多模态数据的药物合成反应的虚拟筛选

药物合成反应，特别是不对称反应是现代药物化学的重要组成部分。化学家们投入了巨大的人力和资源来识别各种化学反应模式，以实现高效合成和不对称催化。量子力学计算和机器学习算法在这一领域的最新研究证明了通过计算机学习现有药物合成反应数据并进行精确虚拟筛选的巨大潜力。然而，现有方法局限于单一模态的数据来源，并且由于数据少的限制，只能使用基本的机器学习方法，使它们在更广泛场景中的普遍应用受到阻碍。因此，提出两种融合多模态数据的药物合成反应的筛选模型来进行反应产率和对映选择性的虚拟筛选，并给出了一种基于Boltzmann分布进行加权的3D构象描述符，从而将分子的立体空间信息与量子力学性质结合起来。这两种多模态数据融合模型在两个代表性的有机合成反应（C-N偶联反应和N,S-缩醛反应）中进行了训练和验证，结果表明前者的R2相对于基线方法在大多数据划分上的提升超过了1个百分点，后者的平均绝对误差（MAE）相对于基线方法在大多数据划分上的下降超过了0.5个百分点。可见，在有机反应筛选的不同任务中采用基于多模态数据融合的模型都会带来好的性能。     

A time saving algorithm for the Monte Carlo method of Metropolis

A time saving algorithm for the Monte Carlo method of Metropolis is presented. The technique is tested with different potential models and number of particles. The coupling of the method with neighbor lists, linked lists, Ewald sum and reaction field techniques is also analyzed. It is shown that the proposed algorithm is particularly suitable for computationally heavy intermolecular potentials.     

Computing Local Signed Distance Fields for Large Polygonal Models

The signed distance field for a polygonal model is a useful representation that facilitates efficient computation in many visualization and geometric processing tasks. Often it is more effective to build a local distance field only within a narrow band around the surface that holds local geometric information for the model. In this paper, we present a novel technique to construct a volumetric local signed distance field of a polygonal model. To compute the local field efficiently, exactly those cells that cross the polygonal surface are found first through a new voxelization method, building a list of intersecting triangles for each boundary cell. After their neighboring cells are classified, the triangle lists are exploited to compute the local signed distance field with minimized voxel‐to‐triangle distance computations. While several efficient methods for computing the distance field, particularly those harnessing the graphics processing unit's (GPU's) processing power, have recently been proposed, we focus on a CPU‐based technique, intended to deal flexibly with large polygonal models and high‐resolution grids that are often too bulky for GPU computation.     

运动模糊图像恢复方法研究

图像恢复一直以来是图像处理领域的研究热点。本文简述了图像恢复的概念、意义及降质模型之后，对目前存在的图像恢复算法进行了分析和研究。并给出了两种典型的图像恢复算法的实验结果。最后对基于运动模糊的图像方法进行了总结及展望。     

The State of the Art in Topology‐Based Visualization of Unsteady Flow

Vector fields are a common concept for the representation of many different kinds of flow phenomena in science and engineering. Methods based on vector field topology are known for their convenience for visualizing and analysing steady flows, but a counterpart for unsteady flows is still missing. However, a lot of good and relevant work aiming at such a solution is available. We give an overview of previous research leading towards topology‐based and topology‐inspired visualization of unsteady flow, pointing out the different approaches and methodologies involved as well as their relation to each other, taking classical (i.e. steady) vector field topology as our starting point. Particularly, we focus on Lagrangian methods, space–time domain approaches, local methods and stochastic and multifield approaches. Furthermore, we illustrate our review with practical examples for the different approaches.     

Visualisation of Sensor Data from Animal Movement

A new area of biological research is identifying and grouping patterns of behaviour in wild animals by analysing data obtained through the attachment of tri-axial accelerometers. As these recording devices become smaller and less expensive their use has increased. Currently acceleration data are visualised as 2D time series plots, and analyses are based on summary statistics and the application of Fourier transforms. We develop alternate visualisations of this data so as to analyse, explore and present new patterns of animal behaviour. Our visualisations include interactive spherical scatterplots, spherical histograms, clustering methods, and feature-based state diagrams of the data. We study the application of these visualisation methods to accelerometry data from animal movement. The reaction of biologists to these visualisations is also reported.