粘性金属中的正激波稳定性问题

G.H.Miller等把高压金属中的粘性激波作为强间断面处理,解析推论出：在大粘性系数条件下小扰动激波是不稳定的,物质粘性是导致失稳的因素。本文中针对平面正激波,认为高压金属中的粘性激波的物理量是连续变化的,利用线性稳定性理论,用数值解推论出：在有粘性条件下小扰动激波都是稳定的,物质粘性是致稳的因素。指出G.H.Miller等获得错误结论的原因在于：从无粘流动解推出的小扰动边界条件导致粘性激波小扰动增长。给出实验确定的小扰动速度梯度的边界条件,这样既可以把粘性正激波作为强间断面处理,也能够保证粘性正激波的稳定性。     

Generalized Riemann problem for gas dynamic combustion

The generalized Riemann problem for gas dynamic combustion in a neighborhood of the origin t > 0 in the (x, t) plane is considered. Under the modified entropy conditions, the unique solutions are constructed, which are the limits of the selfsimilar Zeldovich-von Neumann-Dring (ZND) combustion model. The results show that, for some cases, there are intrinsical differences between the structures of the perturbed Riemann solutions and the corresponding Riemann solutions. Especially, a strong detonation in the...     

Eigenvalue integral relation and stability condition for a mode

In this paper, we point out that for a static system the eigenvalue integral relation (iσ)²I+^iσΦ+Σ=0 derived from an eigenequation only gives the sufficiency of condition gZ>0 for the stability of a certain mode, but it can not provide the necessity to the condition in general. Three theorems presented in this paper show that under some conditions, Π>0 makes the mode stable, while Π<0 makes the mode unstable. The project supported by the National Natural Science Foundation of China.     

[mü]MAD, the adhesion based dynamic micro-manipulator

In this paper, a micro-manipulation method based on adhesion forces and dynamic effects is proposed. A prototype manipulator, called [mü]MAD, has been constructed and successfully experimented. Moreover, the advanced capabilities of [mü]MAD, especially two new interesting applications, are presented: sorting of micro-objects and mechanical characterizations.     

Molecular dynamics simulation of peeling a DNA molecule on substrate

Molecular dynamics (MD) simulations are performed to study adhesion and peeling of a short fragment of single strand DNA (ssDNA) molecule from a graphite surface. The critical peel-off force is found to depend on both the peeling angle and the elasticity of ssDNA. For the short ssDNA strand under investigation, we show that the simulation results can be explained by a continuum model of an adhesive elastic band on substrate. The analysis suggests that it is often the peak value, rather than the mean value, of adhesion energy which determines the peeling of a nanoscale material.The project supported by the Distinguished Young Scholar Fund of NSFC (10225209) and key project from the Chinese Academy of Sciences (KJCX2-SW-L2)     

基于Cosserat理论的广义协调元法

首次将广义协调元理论与Cosserat理论相结合,构造了一个基于Cosserat理论的平面四节点广义协调等参元。依据常应力与线性应力下的广义协调条件,推导了广义协调位移,进而得到有限元列式。利用广义协调元,一方面克服了协调元过于刚硬的缺点,另一方面消除了非协调元不一定收敛的弱点。分析了带有圆孔的应力集中问题,可以看出,...     

纳米铁热力学性质的分子动力学计算

应用分子动力学方法计算了包括纳米铁晶体的熵、Helmholtz自由能等一系列热力学性质,发现纳米晶体的熵比单晶体的熵值高得多,且这些差别随温度增高而迅速加大,这些结果可以用纳米晶体的特殊结构来解释。纳米晶体中大量不规则原子的存在是熵值增大的根本原因。因而,只要改变纳米晶体的尺寸（事实上也是改变了其中不规则原子的数目）将会改变它的熵值及其它的热力学量,从微观结构与热力学量的关系来说,它将有助于改善纳米晶体的性能。     

Molecular dynamics investigation of patterning via cold welding

We perform a series of molecular dynamics simulations of a subtractive cold-welding patterning process. The effects of film thickness and work of adhesion between the thin film and substrate are examined. For small works of adhesion, the film elastically debonds from the substrate before the onset of plastic deformation inside the film during stamp retraction. A simple model is proposed to describe the debonding and deformation of the film. The model provides an analytical framework that describes the playoff between adhesion, yield strength, and film thickness in determining the debond length of the film induced by stamp retraction.     

Contact Simulation for Many Particles Considering Adhesion

Abstract

This article deals with the calculation and administration of contact between many moving bodies in the planar case. The main issue is the contact determination between bodies of either round or polygonal shape. The modeling is done by means of molecular dynamics to investigate the motion of many bodies very efficiently. Besides elastic normal forces, which prevent penetration of the bodies, damping forces and adhesion is described and considered for the computations. Our interest is directed toward collision detection. By means of sorting algorithms, neighboring bodies are found easily and, therefore, systems consisting of a large number of bodies can be determined efficiently.     

Investigation on the Velocity-Dependent Adhesion Hysteresis via Molecular Dynamics Simulation

Adhesion plays an important role in miniaturized devices and technologies,which depends not only on indentation depth but also on the history of contact making and breaking,giving rise to adhesion hysteresis.In the present work,adhesion hysteresis has been investigated via molecular dynamics simulations on approaching and retracting a rigid tip to and from a substrate.The results show that hysteresis in the force-displacement curve that depends on approaching and retraction velocities arises under both elastic and plastic deformation.The underlying mechanisms have been analyzed.The implications of the results in friction have been discussed briefly.     

薄板有限元广义混合法及克服病态问题研究

以薄板理论的广义混合变分原理为基础,建立了一种适用于薄板弯曲分析的有限元广义混合模式,给出了较实用的选择分裂因子的方法,算例表明有限元广义混合法比常规位移模式的精度高,同时还能克服常规有限元中的某些病态问题.本文还讨论了该法克服常规有限元中某些病态问题的机理.     

Nonextensive statistical mechanics: A brief introduction

Boltzmann-Gibbs statistical mechanics is based on the entropy . It enables a successful thermal approach to ubiquitous systems, such as those involving short-range interactions, markovian processes, and, generally speaking, those systems whose dynamical occupancy of phase space tends to be ergodic. For systems whose microscopic dynamics is more complex, it is natural to expect that the dynamical occupancy of phase space will have a less trivial structure, for example a (multi)fractal or hierarchical geometry. The question naturally arises whether it is possible to study such systems with concepts and methods similar to those of standard statistical mechanics. The answer appears to be yes for ubiquitous systems, but the concept of entropy needs to be adequately generalized. Some classes of such systems can be satisfactorily approached with the entropy (with , and ). This theory is sometimes referred in the literature as nonextensive statistical mechanics. We provide here a brief introduction to the formalism, its dynamical foundations, and some illustrative applications. In addition to these, we illustrate with a few examples the concept of stability (or experimental robustness) introduced by B. Lesche in 1982 and recently revisited by S. Abe.Received: 27 May 2003, Accepted: 27 August 2003, Published online: 11 February 2004PACS: 05.10.-a, 05.20.Gc, 05.20.-y, 05.45.-a, 05.70.Ln, 05.90. + m, 05.60.CdCorrespondence to: C. Tsallis     

Vertical two-phase flow in porous media

New concepts are introduced to describe single-component two-phase flow under gravity. The phases can flow simultaneously in opposite directions (counterflow), but information travels either up or down, depending on the sign of the wavespeedC. Wavespeed, saturation and other quantities are defined on a two-sheeted surface over the mass-energy flow plane, the sheets overlapping in the counterflow region. A saturation shock is represented as an instantaneous displacement along a line of constant volume fluxJ _Q in the flow plane. Most shocks are of the wetting type, that is, they leave the environment more saturated after their passage. When flow is horizontal all shocks are wetting, but it is a feature of vertical two-phase flow that for sufficiently small mass and energy flows there also exist drying shocks associated with lower final saturations.     

树形带球铰的多刚体系统动力学方程的建立方法

本文提出了一种建立树形带球铰多刚体系统动力学方程的新方法,运用约束系统动力学研究成果,提出了广义刚体的概念以代替多刚体系统的子系统,并借助于广义刚体的不断扩充,求得了多刚体系统动力学方程建立的递推方法,该方法简单直观,几何概念清楚,并允许多刚体系统的扩充,且便于进行计算机符号处理。     

一种厚板薄板通用的新型广义协调元

在适用于中厚板的八结点平板弯曲单元基础上，通过引入剪应变与位移的广义协调条件，建立起一种新型广义协调元，不仅保留了原单元适用于中厚板的特点，同时对薄板也给出了较精确的解，是一种厚板和薄板通用的新型广义协调元。     

电磁弹性动力学初边值问题12类变量广义变分原理

基于Yao建立的电磁弹性固体广义变分原理,运用关于非传统Hamilton型广义变分原理的方法,建立了电磁弹性动力学初边值问题的12类变量广义变分原理,可反映该问题的全部特征,其独立变分变量为该问题的全部变量,即位移、速度、动量、应变、应力、电位移、磁感应强度、电场强度、磁场强度、电标量势、磁标量势和磁矢量势.本文建立的...     

Entropy Flux Relation for Viscoelastic Bodies

Thermodynamic restrictions of elastic materials in general are well-known based on the Clausius–Duhem inequality by employing the simple Coleman–Noll procedure. One of the basic assumptions in this entropy inequality is that the entropy flux is defined as the heat flux divided by the absolute temperature. To avoid this unnecessary and possibly too restrictive assumption, the general entropy inequality has been proposed and its thermodynamic consequences exploited following the Müller–Liu procedure in which supply-free bodies are considered and Lagrange Multipliers are introduced. In this new thermodynamic theory, the entropy flux and heat flux relation identical to the above assumption has not been proved for elastic bodies in general. For isotropic elastic bodies, it was proved by Müller in 1971, using explicit isotropic representations for constitutive functions. Unfortunately, the procedure contains a flaw which was later pointed out, but can not be easily resolved. Although it was shown later that it can be proved by Müller–Liu procedure, it has not been available in the literature. In this paper, we shall establish this result, providing the missing details in the previous proof. The analysis will be carried out for isotropic viscoelastic materials and the case of elastic materials follows as a special case.