Shear instabilities in granular flows.

Unstable waves have been long studied in fluid shear layers. These waves affect transport in the atmosphere and oceans, in addition to slipstream stability behind ships, aeroplanes and heat-transfer devices. Corresponding instabilities in granular flows have not been previously documented, despite the importance of these flows in geophysical and industrial systems. Here we report that breaking waves can form at the interface between two streams of identical grains flowing on an inclined plane downstream of a splitter plate. Changes in either the shear rate or the angle of incline cause such waves to appear abruptly. We analyse a granular flow model that agrees qualitatively with our experimental data; the model suggests that the waves result from competition between shear and extensional strains in the flowing granular bed. We propose a dimensionless shear number that governs the transition between steady and wavy flows.     

Signatures of granular microstructure in dense shear flows

Granular materials and ordinary fluids react differently to shear stresses. Rather than deforming uniformly, materials such as dry sand or cohesionless powders develop shear bands--narrow zones of large relative particle motion, with essentially rigid adjacent regions. Because shear bands mark areas of flow, material failure and energy dissipation, they are important in many industrial, civil engineering and geophysical processes. They are also relevant to lubricating fluids confined to ultrathin molecular layers. However, detailed three-dimensional information on motion within a shear band, including the degree of particle rotation and interparticle slip, is lacking. Similarly, very little is known about how the microstructure of individual grains affects movement in densely packed material. Here we combine magnetic resonance imaging, X-ray tomography and high-speed-video particle tracking to obtain the local steady-state particle velocity, rotation and packing density for shear flow in a three-dimensional Couette geometry. We find that key characteristics of the granular microstructure determine the shape of the velocity profile.     

颗粒材料热力学理论在简单剪切流中的应用

颗粒物质由大量离散的粗大固体颗粒聚集而成,在外界作用下可能发生准静态变形、变形局部化和破坏,直至流动,其中采用固体力学和流体力学对准静态变形和流动开展的研究较为系统,而变形局部化和破坏的研究非常薄弱．分析内部结构演化规律,建立颗粒体系非平衡态热力学,统一描述颗粒体系力学行为是重要的基础研究课题,Granular Solid Hydrodynamics（GSH）理论为该研究提供了基础．本文完善了该理论涉及的颗粒温度表达式,确定了相应参数数值;与动理学理论和离散元模拟一起对恒定体积简单剪切流进行了分析,对比了三种方法得到的正应力与颗粒体积分数的关系,发现由于基本假设的限制,动理学理论只能适用于稀疏颗粒流,而完善后的颗粒体系热力学理论比动理学涵盖范围更广,可以从稀疏体系到密集体系．这样该理论可以连贯地描述颗粒体系准静态变形、变形局部化和破坏,直至流动的力学行为．     

黏性流中的链环涡管演化与螺旋度分析

本文通过直接数值模拟研究了链环涡管在不可压缩黏性流中的演化过程.在初始时刻的链环涡管由两个变形的涡环组合而成,其螺旋度为依赖于涡轴参数方程的解析表达式,进而可利用该初始流场进行涡管演化研究与螺旋度分析.发现当初始链环涡管的涡量方向具有相同手性时,链环涡管和环面纽结涡管的演化具有类似的涡动力学过程;而当它们具有相反手性时,涡管间的强涡量梯度会使两涡环在短时间内产生剧烈的涡重联,从而导致涡环由快速的尺度级串过程达到类湍流状态.     

类比学习机制的研究

类比学习是机器学习中的一种重要方法。文中重点讨论了实现这种学习方法所需解决的主要问题及类比学习的研究进展和状况,并对6种具有代表性的类比学习方法和系统(转换类比学习、派生类比学习、属性类比学习、因果关系类比学习、联想类比学习和基于事例的学习)的基本原理、学习机制等进行了深入的研究和分析。     

三角翼周围非定常流动的三维涡方法数值模拟

为了研究三角翼周围不可压缩流动,利用离散涡元模拟了旋涡的产生、聚集和输送过程.用Lagrange法描述离散涡元的移动,涡元的移动速度通过广义Biot-Savart公式结合快速多级子展开方法计算,粘性扩散通过粒子强度交换(PSE)法模拟.数值模拟了80°后掠角的细长三角翼在低Reynolds数不同攻角下前缘涡的发展和非对称破裂特性,通过分析滚转力矩在攻角增大时的变化,得出自激滚转振动的起振攻角为23°.     

驻涡燃烧室内涡系分布研究

为清晰了解驻涡燃烧室内流场的涡系分布,对中心驻体为67%燃烧室当量宽度的驻涡燃烧室流场,在驻涡腔无喷射和有喷射两种状况下分别进行冷态数值模拟,详细研究流场内旋涡分布情况.结果表明:流道前部存在4个影响很小、沿流动方向运动、分布于驻体与端壁角区的螺旋状旋涡;流道后部存在1个由2个旋涡构成、面积较大的回流区;流道中部的驻涡区旋涡情况复杂,在驻涡腔无喷射情况下,主流与驻涡区之间悬挂驻留2个稳定的驻涡,驻涡腔内存在不稳定且数目不确定的旋涡;当驻涡腔前驻体有喷射存在时,驻涡腔内形成4个均匀且稳定的旋涡.     

类比在物理教学中的应用

结合物理教学实践和相应的教学理论,针对学生的一些物理概念、物理规律难以理解的现象,在教学中应用类比方法,使学生能够较好地掌握这些物理知识.应用类比方法可以把抽象的新知识融入到学生已有的知识体系中,不失为一种促进学生知识迁移、提高学生素质和培养能力的有效途径.     

新的螺旋桨尾涡近似方法

基于解析尾涡模式,提出了一种新的尾涡近似方法．尾涡分为近区和远区．在近区,尾涡在选定扇形截面上解析地求出,在其他处通过非线性插值求得;利用了解析尾涡的特点同时减少了计算量．根据流体连续性方程确定尾涡的收缩半径,避免了梢涡半径的不收敛性,为重负荷螺旋桨设计和非设计工况下螺旋桨性能计算提供了一种有效的尾涡近似计算方法．     

A one-dimensional chain state of vortex matter.

Magnetic flux penetrates isotropic type II superconductors in flux-quantized vortices, which arrange themselves into a lattice structure that is independent of the direction of the applied field. In extremely anisotropic high-transition-temperature (high-Tc) superconductors, a lattice of stacks of circular 'pancake' vortices forms when a magnetic field is applied perpendicular to the copper oxide layers, while an orthogonal elongated lattice of elliptical Josephson vortices forms when the applied field is parallel to the layers. Here we report that when a tilted magnetic field is applied to single crystals of Bi2Sr2CaCu2O8+delta, these lattices can interact to form a new state of vortex matter in which all stacks of pancake vortices intersect the Josephson vortices. The sublattice of Josephson vortices can therefore be used to manipulate the sublattice of pancake vortices. This result explains the suppression of irreversible magnetization by in-plane fields as seen in Bi2Sr2CaCu2O8+delta crystals, a hitherto mysterious observation. The ability to manipulate sublattices could be important for flux-logic devices, where a 'bit' might be represented by a pancake vortex stack, and the problem of vortex positioning is overcome through sublattice interactions. This also enables the development of flux transducers and amplifiers, considerably broadening the scope for applications of anisotropic high-Tc superconductors.     

水工建筑物进水口前立轴旋涡的研究

结合平面螺旋流模型，通过简化N－S方程，分析了立轴旋涡运动的数学模型，并描绘了立轴旋涡的空间形态。研究了立轴旋涡运动状态的影响因素，特别是衡量立轴旋涡涡旋强度重要指标之一的速度环量。此外，结合立轴旋涡产生的条件，分析了进水口的设计原则。对于现有工程中存在的立轴旋涡，提出了运用防涡梁或垂直消涡格栅等结构物消除旋涡的方法。     

类比方法在生物进化论研究中的作用

类比是一种在科学研究中有重要作用的逻辑思维方法。本文就类比方法在证明生物进化、生命起源、生物进化、人类起源以及生物进化原因研究中的作用予以了论述。     

查尔斯·泰勒论自由：一种处境化自由

分析介绍了泰勒的自由观,认为面对当代自由主义者对消极自由的过分推崇,泰勒试图在思想史的回复中重新审视消极自由和积极自由,泰勒认为代表消极自由的离根哲学的道德外观并没有错,错在佐证其道德外观的道德根源的丧失,而蕴含积极自由的浪漫主义则启示我们给予自由的外观以深厚的道德内涵,但它容易导向道德相对主义。泰勒最终主张的是消极自由和积极自由的调和,而其提出的＂处境化自由＂正是这种调和的表达,＂处境化自由＂即是在特定的生活实践或历史传统中的自由。     

一种基于Taylor级数的齐次扩容精细算法

借助齐次扩容技巧及Taylor级数,设计了求解非齐次线性定常系统的一种新的精细算法－基于Taylor级数的齐次扩容精细算法（HHPD－T）。该算法有效地解决了HPD－F算法中涉及矩阵求逆的问题,因而计算量小,同时具有设计合理、易于推广、易于实现等特点。两个典型算例表明,应用HHPD－T求解,数值结果更为理想。     

Double-quantum vortex in superfluid 3He-A

Linear defects are generic in continuous media. In quantum systems they appear as topological line defects which are associated with a circulating persistent current. In relativistic quantum field theories they are known as cosmic strings, in superconductors as quantized flux lines, and in superfluids and low-density Bose-Einstein condensates as quantized vortex lines. A conventional quantized vortex line consists of a central core around which the phase of the order parameter winds by 27(pi)n, while within the core the order parameter vanishes or is depleted from the bulk value. Usually vortices are singly quantized (that is, have n = 1). But it has been theoretically predicted that, in superfluid 3He-A, vortex lines are possible that have n = 2 and continuous structure, so that the orientation of the multicomponent order parameter changes smoothly throughout the vortex while the amplitude remains constant. Here we report direct proof, based on high-resolution nuclear magnetic resonance measurements, that the most common vortex line in 3He-A has n = 2. One vortex line after another is observed to form in a regular periodic process, similar to a phase-slip in the Josephson effect.     

实有理函数展为Taylor级数的一般方法

在求实函数的f(x)=(Ax+B/(x2+px+q)k)(p、q、A、B∈R,k∈N,p2-4q＜O)Taylor级数展开法的基础上,给出实有理函数展开为Taylor级数的一个普适方法。     

单位圆内无穷级Taylor级数

定义了关于单位圆内Taylor级数的型函数和型函数的级,研究了单位圆内无穷级Taylor级数,得到了其关于型函数U(1/1-r)的级与系数之间的几种关系.     

一种基于信息粒度的高属性维稀疏数据聚类算法

目前的高属性维稀疏数据算法大多面向二态数据,而且没有聚类结果的评价方法,给应用带来很大局限。针对这些问题,给出一种基于信息粒度的高属性维聚类算法。首先通过设计面向数据稀疏特征的半模糊聚类算法对数据进行离散化,并基于此给出稀疏相似度和初始等价关系的定义,然后设计可变精度的二次聚类模型对初始聚类结果进行修正,使算法具有较强抗噪声能力,最后结合应用领域定义一种新的聚类质量的评价模型。实验证明,算法具有更广应用性,可提供多粒度分析结果,准确度更高,得到的聚类结果能真实反映数据的特征。