复杂场景流固耦合的高效模拟

大规模复杂流体场景的高效交互模拟技术在灾难仿真、虚拟现实和影视特效制作等领域都有重要的应用价值.针对现有基于物理的流体算法大多只适于尺度较小的场景,难以有效地模拟复杂流体大场景中流固耦合时的固体破碎效果的问题,提出一种复杂场景的固液耦合高效模拟方法.首先提出一种基于流体隐式粒子法和散度为零的SPH法相混合的计算框架,在确保流体物理属性的同时充分利用细粒度的隐式粒子来丰富流体运动细节,提高场景模拟的真实感;然后采用一种多维度的流固耦合分合计算策略来进一步提高流固耦合效率;为实现流体冲击下的固体破碎效果,采用一种物理与几何相混合的破碎方法:以基于断裂力学中的应变能密度模型来确定固体破碎时碎片的分布,采用基于几何的质心Voronoi方法快速生成碎片,最终实现百万量级粒子参与的复杂流体场景的交互模拟,以及高速流体冲击下的固体破碎效果的高效模拟.     

虚拟手术流血模拟的GPU加速实现

目的 流血效果是虚拟手术模拟器视觉效果的重要组成部分,血流与固体交互的庞大计算量使取得实时的流血模拟效果具有很大的挑战性。提出一种基于图形处理单元(GPU)加速的虚拟手术流血效果模拟方法。方法 该方法以Müller等人提出的光滑粒子动力学(SPH)作为基础,采用温度项使粒子具有不同速度模拟血流形成的血槽,同时基于构建均匀空间网格的思想,利用通用并行计算架构(CUDA)多线程并行加速技术完成粒子控制方程的求解和血流与固体交互的计算,从而取得实时的效果。结果 实验结果表明,本文方法能够满足虚拟手术中切割表面流血和血液在器官中流动的模拟需求,在粒子个数为9000时仅需20 ms,对比于纯CPU的实现取得20.15倍的加速比,实现了大量粒子下的实时流血模拟。 结论 本文方法具有较好的灵活性和实时性的特点,可以应用于虚拟手术仿真系统之中。     

液固交互过程中固体破碎现象的实时模拟

提出一种适用于液固交互过程中的固体破碎模拟方法.针对传统液固交互边界处理算法复杂,难以实时模拟固体破碎的缺陷,本文基于SPH算法,提出对液体与固体使用统一的粒子表示,将预测校正方案应用于处理液固交互过程中固体之间的碰撞问题,从而将固体破碎过程简化为固体碎片边界粒子生成的过程,并使用形状匹配算法对固体碎片进行形状约束.实验结果证明,采用文中方法能够实时模拟出液固交互过程中,细节丰富的固体破碎现象,效果真实.     

流-固交互及可变形体破裂的真实感模拟

为了模拟流体与动态环境的相互作用,提出一种流-固交互及可变形体破裂的真实感建模与绘制的算法.该算法使用光滑粒子流体动力学(SPH)与有限单元法(FEM)分别对流体与变形固体进行建模;再根据流-固交互作用的特点,给出一种快速分离液体表面粒子与固体表面网格的交互方法,并采用虚节点的流-固耦合模型模拟了液-固相互作用力.文中算法可用于多个流-固交互破裂的现象,如水管崩裂、水冲堤坝等.     

真实感固流交互动画的统一物质点法模拟

固流交互模拟是基于物理的流体模拟技术的重要研究内容,其中如何解决非穿透 和滑移接触是难点。为此,提出一种真实感固流交互动画的统一物质点法模拟方法。首先,给 出一种基于物质点法的快速微可压缩流体模拟方法,并在统一背景欧拉网格上对固体和流体动 量方程进行求解;其次,检测固流接触区域并在其上构建局部多重背景网格,给出一种动量守 恒保持的速度修正方法对固体和流体各自网格结点进行速度修正,从而实现固流交互的非穿透 和滑移接触效果模拟。实验结果表明,该方法可以模拟稳定、真实的固流交互动画,适用于计 算机图形学和虚拟现实领域中的真实感模拟应用。     

大规模流体场景的真实感与实时模拟

目的 基于物理的流体动画模拟是计算机图形学领域中的研究热点,针对实际应用中仍难以实现大规模流体场景的真实感与实时模拟,提出了基于shallow water方程的物理模拟方法。方法 首先,给出shallow water方程的稳定欧拉数值求解方法,解决模拟过程中存在的毛刺、陡坡水滴斑点等数值求解的不稳定性问题;其次,提出刚体和粒子系统与流体高度场的稳定耦合模型,实现双向固流耦合和流体表面细节的真实感模拟;最后,设计高度场的多精度网格算法以及粒子的隔点采样方法,加速大规模流体的物理模拟计算。结果 实验结果表明,本文方法解决了传统欧拉方法求解shallow water方程的流体模拟过程中存在的不稳定和计算复杂等问题,在300×300网格分辨率和2.2×10⁴粒子数的规模下,达到了20帧/s的实时模拟速度。结论 本文算法具有良好的高效性和稳定性,适用于电子游戏和视景仿真等实时应用领域中的大规模流体场景的真实感模拟。     

基于完全拉格朗日粒子方法的物理动画模拟

自然界中不同类型物体和行为的真实感模拟是物理动画的重要内容,但实现基于统一数值方法的模拟仍是研究难点。基于光滑粒子动力学方法和连续介质力学物理模型,提出一种基于完全拉格朗日粒子方法的物理动画模拟算法,模拟刚性物体、弹性物体和不可压缩流体的动力学行为。此外,结合固体表面粒子的自适应采样方案和相对贡献计算方案,提出一种稳定的固流交互算法。实验结果表明,该方法适合于虚拟环境中不同类型物体运动和交互的统一建模和模拟。     

微可压缩SPH流体的稳定性固体边界处理算法

固流耦合,即流体的固体边界处理一直是基于物理的流体模拟技术的研究重点.为解决SPH流体模拟中固流耦合存在的交界面处流体粒子衰减和穿透问题,提出一种固体采样边界粒子与动量守恒保持的位置-速度修正方案相结合的固流耦合方法.首先在预处理阶段对快速格子形状匹配(fast lattice shape matching,FLSM)模型表示的固体边界进行表面和内部边界粒子采样;然后在运行过程中计算流体粒子密度和受力时考虑边界固体粒子的相对贡献;最后利用动量守恒保持的位置-速度修正方案对流体粒子进行位置和速度的修正.为了提高计算速度以满足交互式应用需求,把每个迭代步长内的计算完全并行化后加载到GPU上进行加速处理.实验结果表明,该算法实现了微可压缩SPH流体与刚体以及弹性体的双向耦合,并可以高效、稳定地模拟固流耦合中的非穿透、液滴飞溅、溶解等复杂现象.     

SPH扩展方法实现液体与固体交互实时模拟

为解决模拟液体和固体交互时速度缓慢与效果不真实的问题,对传统SPH(Smoothed Particle Hydrodynamics)方法进行扩展.首先,将液体和固体均视为粒子的集合,使用SPH扩展方法模拟液体的流动,固体粒子作为液体粒子的一部分与液体交互,同时保持固体自身的形状,减少了计算量;其次,为了加快搜索液固交互时相互作用粒子的速度,解决SPH中搜索最近相邻粒子速度较慢的问题,提出链表搜索方法搜索相互作用的粒子;最后,运用Marching Cubes体绘制算法重建液体表面,实现液固交互的真实模拟.实验表明:5000粒子规模以下、光滑半径为0.01时,液固交互的绘制帧率达到14-74帧/秒;同时,液体的飞溅及漩涡表现更加真实.     

多样化实时刚体破碎模拟

目的 现有刚体破碎仿真模拟中,基于物理受力分析的方法往往难以应用在对实时性要求较高的场景中;而基于非物理方法的破碎模拟,破碎效果大多缺乏多样性。为了使得破碎模拟同时满足实时性和破碎效果多样化,提出了一种多样化实时刚体破碎模拟方法。方法 进行破碎模拟时首先由选定的种子点生成类型得到种子点集,采用Sweep Plane算法生成Voronoi图后基于Voronoi图信息对模型进行空间剖分;然后选择破碎时行为模拟方式,对物体破碎时所受外力进行模拟;最后对破碎时碰撞检测及碰撞后碎片的运动过程进行模拟并渲染显示。结果 通过组合不同的种子点生成类型和破碎时行为模拟方式,得到了多样化的刚体破碎效果。对单个刚体进行破碎模拟时,碎片数目不超过200个时帧率可以达到75帧/s,满足实时性的需求;对多个可破碎目标同时存在的复杂场景,破碎仿真模拟的平均帧率可以达到50帧/s,同样满足实时性要求;与现有方法对比的结果也验证了本文方法在计算效率和破碎效果多样性两方面达到了较好的平衡。结论 本文方法在满足实时性要求的同时,丰富了刚体破碎的效果,不同种子点生成类型和破碎时行为模拟方式的组合可以实现破碎效果多样性。     

European Community policy and the market

Abstract This paper starts with some reflections on the policy considerations and priorities which are shaping European Commission (EC) research programmes. Then it attempts to position the current projects which seek to capitalise on information and communications technologies for learning in relation to these priorities and the apparent realities of the marketplace. It concludes that while there are grounds to be optimistic about the contribution EC programmes can make to the efficiency and standard of education and training, they are still too technology driven.     

一种自适应子融合集成多分类器方法

融合集成方法已经广泛应用在模式识别领域,然而一些基分类器实时性能稳定性较差,导致多分类器融合性能差,针对上述问题本文提出了一种新的基于多分类器的子融合集成分类器系统。该方法考虑在度量层融合层次之上通过对各类基多分类器进行动态选择,票数最多的类别作为融合系统中对特征向量识别的类别,构成一种新的自适应子融合集成分类器方法。实验表明,该方法比传统的分类器以及分类融合方法识别准确率明显更高,具有更好的鲁棒性。     

Explanation and prediction: an architecture for default and abductive reasoning

Although there are many arguments that logic is an appropriate tool for artificial intelligence, there has been a perceived problem with the monotonicity of classical logic. This paper elaborates on the idea that reasoning should be viewed as theory formation where logic tells us the consequences of our assumptions. The two activities of predicting what is expected to be true and explaining observations are considered in a simple theory formation framework. Properties of each activity are discussed, along with a number of proposals as to what should be predicted or accepted as reasonable explanations. An architecture is proposed to combine explanation and prediction into one coherent framework. Algorithms used to implement the system as well as examples from a running implementation are given.     

Three Process Perspectives: Organizations, Teams, and People

This paper provides the author's personal views and perspectives on software process improvement. Starting with his first work on technology assessment in IBM over 20 years ago, Watts Humphrey describes the process improvement work he has been directly involved in. This includes the development of the early process assessment methods, the original design of the CMM, and the introduction of the Personal Software Process (PSP)^SM and Team Software Process (TSP){^SM}. In addition to describing the original motivation for this work, the author also reviews many of the problems he and his associates encountered and why they solved them the way they did. He also comments on the outstanding issues and likely directions for future work. Finally, this work has built on the experiences and contributions of many people. Mr. Humphrey only describes work that he was personally involved in and he names many of the key contributors. However, so many people have been involved in this work that a full list of the important participants would be impractical.     

基于复小波噪声方差显著修正的SAR图像去噪

提出了一种基于复小波域统计建模与噪声方差估计显著性修正相结合的合成孔径雷达(Synthetic Aperture Radar,SAR)图像斑点噪声滤波方法。该方法首先通过对数变换将乘性噪声模型转化为加性噪声模型,然后对变换后的图像进行双树复小波变换(Dualtree Complex Wavelet Transform,DCWT),并对复数小波系数的统计分布进行建模。在此先验分布的基础上,通过运用贝叶斯估计方法从含噪系数中恢复原始系数,达到滤除噪声的目的。实验结果表明该方法在去除噪声的同时保留了图像的细节信息,取得了很好的降噪效果。     

How do children do mathematics with LOGO?

Abstract  This paper considers some results of a study designed to investigate the kinds of mathematical activity undertaken by children (aged between 8 and 11) as they learned to program in LOGO. A model of learning modes is proposed, which attempts to describe the ways in which children used and acquired understanding of the programming/mathematical concepts involved. The remainder of the paper is concerned with discussing the validity and limitations of the model, and its implications for further research and curriculum development.     

Editorial: Special issue on recent advances in hybrid control theory and applications

正The demands of a rapidly advancing technology for faster and more accurate controllers have always had a strong influence on the progress of automatic control theory.In recent years control problems have been arising with increasing frequency in widely different areas,which cannot be addressed using conventional control techniques.The principal reason for this is the fact that a highly competitive economy is forcing systems to operate in regimes where     

Information for Authors

正Aim The Journals of Zhejiang University-SCIENCE(A/B/C)areedited by the international board of distinguished Chinese andforeign scientists,and are aimed to present the latest devel-opments and achievements in scientific research in China andoverseas to the world’s scientific circles,especially to stimulateand promote academic exchange between Chinese and for-eign scientists everywhere.     

Enhancing a leaf radiative transfer model to estimate concentrations and in vivo specific absorption coefficients of total carotenoids and chlorophylls a and b from single-needle reflectance and transmittance

The relative concentrations of different pigments within a leaf have significant physiological and spectral consequences. Photosynthesis, light use efficiency, mass and energy exchange, and stress response are dependent on relationships among an ensemble of pigments. This ensemble also determines the visible characteristics of a leaf, which can be measured remotely and used to quantify leaf biochemistry and structure. But current remote sensing approaches are limited in their ability to resolve individual pigments. This paper focuses on the incorporation of three pigments—chlorophyll a, chlorophyll b, and total carotenoids—into the LIBERTY leaf radiative transfer model to better understand relationships between leaf biochemical, biophysical, and spectral properties.Pinus ponderosa and Pinus jeffreyi needles were collected from three sites in the California Sierra Nevada. Hemispheric single-leaf visible reflectance and transmittance and concentrations of chlorophylls a and b and total carotenoids of fresh needles were measured. These data were input to the enhanced LIBERTY model to estimate optical and biochemical properties of pine needles. The enhanced model successfully estimated reflectance (RMSE = 0.0255, BIAS = 0.00477, RMS%E = 16.7%), had variable success estimating transmittance (RMSE = 0.0442, BIAS = 0.0294, RMS%E = 181%), and generated very good estimates of carotenoid concentrations (RMSE = 2.48 µg/cm², BIAS = 0.143 µg/cm², RMS%E = 20.4%), good estimates of chlorophyll a concentrations (RMSE = 10.7 µg/cm², BIAS = − 0.992 µg/cm², RMS%E = 21.1%), and fair estimates of chlorophyll b concentrations (RMSE = 7.49 µg/cm², BIAS = − 2.12 µg/cm², RMS%E = 43.7%). Overall root mean squared errors of reflectance, transmittance, and pigment concentration estimates were lower for the three-pigment model than for the single-pigment model. The algorithm to estimate three in vivo specific absorption coefficients is robust, although estimated values are distorted by inconsistencies in model biophysics. The capacity to invert the model from single-leaf reflectance and transmittance was added to the model so it could be coupled with vegetation canopy models to estimate canopy biochemistry from remotely sensed data.     

Mr. Twinn’s bombes

This article discusses the history and design of the special versions of the bombe key-finding machines used by Britain’s Government Code & Cypher School (GC&CS) during World War II to attack the Enigma traffic of the Abwehr (the German military intelligence service). These special bombes were based on the design of their more numerous counterparts used against the traffic of the German armed services, but differed from them in important ways that highlight the adaptability of the British bombe design, and the power and flexibility of the diagonal board. Also discussed are the changes in the Abwehr indicating system that drove the development of these machines, the ingenious ways in which they were used, and some related developments involving the bombes used by the U.S. Navy’s cryptanalytic unit (OP-20-G).