In situ multi-satellite detection of coherent vortices as a manifestation of Alfvénic turbulence

Turbulence in fluids and plasmas is a ubiquitous phenomenon driven by a variety of sources-currents, sheared flows, gradients in density and temperature, and so on. Turbulence involves fluctuations of physical properties on many different scales, which interact nonlinearly to produce self-organized structures in the form of vortices. Vortex motion in fluids and magnetized plasmas is typically governed by nonlinear equations, examples of which include the Navier-Stokes equation, the Charney-Hasegawa-Mima equations and their numerous generalizations. These nonlinear equations admit solutions in the form of different types of vortices that are frequently observed in a variety of contexts: in atmospheres, in oceans and planetary systems, in the heliosphere, in the Earth's ionosphere and magnetosphere, and in laboratory plasma experiments. Here we report the discovery by the Cluster satellites of a distinct class of vortex motion-short-scale drift-kinetic Alfvén (DKA) vortices-in the Earth's magnetospheric cusp region. As is the case for the larger Kelvin-Helmholtz vortices observed previously, these dynamic structures should provide a channel for transporting plasma particles and energy through the magnetospheric boundary layers.     

Logarithmic rate dependence of force networks in sheared granular materials

Many models of slow, dense granular flows assume that the internal stresses are independent of the shearing rate. In contrast, logarithmic rate dependence is found in solid-on-solid friction, geological settings and elsewhere. Here we investigate the rate dependence of stress in a slowly sheared two-dimensional system of photoelastic disks, in which we are able to determine forces on the granular scale. We find that the mean (time-averaged) stress displays a logarithmic dependence on the shear rate for plastic (irreversible) deformations. However, there is no perceivable dependence on the driving rate for elastic (reversible) deformations, such as those that occur under moderate repetitive compression. Increasing the shearing rate leads to an increase in the strength of the force network and stress fluctuations. Qualitatively, this behaviour resembles the changes associated with an increase in density. Increases in the shearing rate also lead to qualitative changes in the distributions of stress build-up and relaxation events. If shearing is suddenly stopped, stress relaxations occur with a logarithmic functional form over long timescales. This slow collective relaxation of the stress network provides a mechanism for rate-dependent strengthening.     

The jamming route to the glass state in weakly perturbed granular media

It has been suggested that a common conceptual framework known as 'jamming' (refs 1 and 2) may be used to classify a wide variety of physical systems; these include granular media, colloidal suspensions and glass-forming liquids, all of which display a critical slowdown in their dynamics before a sudden transition to an amorphous rigid state. Decreasing the relevant control parameter (such as temperature, drive or inverse density) may cause geometrical constraints to build up progressively and thus restrict the accessible part of the system's phase space. In glass-forming liquids (thermal molecular systems), jamming is provided by the classical vitrification process of supercooling, characterized by a rapidly increasing and apparently diverging viscosity at sufficiently low temperatures. In driven (athermal) macroscopic systems, a similar slowdown has been predicted to occur, notably in sheared foam or vibrated granular media. Here we report experimental evidence for dynamic behaviour, qualitatively analogous to supercooling, in a driven granular system of macroscopic millimetre-size particles. The granular medium is perturbed by isolated tapping or continuous vibration, with the perturbation intensity serving as a control parameter. We observe the random deflection of an immersed torsion oscillator that moves each time the grains rearrange, like a 'thermometer' sensing the granular noise. We caution that our granular analogy to supercooling is based on similarities in the dynamical behaviour, rather than quantitative theory.     

Analysis and Design of Cylindrical Magnetorheological Fluid Brake

Magnetorheological (MR) fluids consist of stable suspensions of magnetic particles in a carrying fluid such as water or silicone oils. The magnetorheological response of MR fluids results from the polarization induced in suspended particles by application of an external magnetic field. The interaction between the induced dipoles causes the particles to form columnar structure, parallel to the applied field. These chain-like structures restrict the motion of fluids, thereby increasing the viscosity and yield...     

论热力学过程的可逆性与自发性

讨论了热力学过程的可逆性与自发性，指出过程的可逆性与自发性之间一致性和不一致性的情况，说明了所谓的可逆性（可逆与不可逆）决定于完成过程的方式，而其自发性（包括自发和反自发）则取决于体系的始终态的能量态势．     

液-固两相流的运动描述与数值模拟

提出一种数值方案,模拟颗粒状固体在液体中沿一条水平管道的流动.粒子的运动和液体的速度场,是通过一个应用程序交替计算而确定的,即对所有单个粒子的运动应用牛顿定律得到盘状颗粒物的轨迹,求解Navier-Stokes方程得到液速场.利用本文模型及数值模拟方案,可以在计算机屏幕上直观地显示出每一个粒子的流动.     

对电磁感应现象可逆性的讨论

本文从空间是否存在导体两种情况去讨论电磁感应现象的可逆性.结论是:当导体存在时,除不闭合导体中感生电场已达稳定情况外,都是不可逆的;当不存在导体时,电磁感应现象则是可逆的     

脉冲电场作用下Hela细胞穿孔率研究

以宫颈癌Hela细胞为实验对象,利用自制脉冲电源和台盼蓝染色法计数,针对不同的电脉冲参数作用于Hela细胞上,研究了细胞可逆和不可逆电穿孔的场强阈值范围,重点研究了脉冲个数、脉冲宽度和电场强度对细胞不可逆穿孔率的影响,并选择了优化的参数组合。实验发现,在固定脉冲宽度50 μs和20个脉冲个数不变的情况下,Hela细胞出...     

能量传递和转换的动力学分解关系

利用基本强度量乘以与其共轭的基本广延量的平衡方程,导出了能量传递和转换的普遍化动力学分解关系。该普遍化分解关系式表示出了任意形式的能量与其他形式能量之间的传递和转换关系,可以由此导出各种形式能量的动力学方程。利用这个普遍化方程导出了在工程领域常见的动能、化学能、压能和热能传递和转换的动力学分解关系。利用这些分解关系式给出了系统总能量的变化方程。这些动力学关系式清楚地反映出,在满足能量守恒的条件下,系统内部不同形式能量之间存在的可逆或不可逆的转换关系。能量的动力学分解关系有助于加深对能量传递和转换的理解,可以为合理地利用能源提供理论指导。     

Crystal symmetry and the reversibility of martensitic transformations

Martensitic transformations are diffusionless, solid-to-solid phase transitions, and have been observed in metals, alloys, ceramics and proteins. They are characterized by a rapid change of crystal structure, accompanied by the development of a rich microstructure. Martensitic transformations can be irreversible, as seen in steels upon quenching, or they can be reversible, such as those observed in shape-memory alloys. In the latter case, the microstructures formed on cooling are easily manipulated by loads and disappear upon reheating. Here, using mathematical theory and numerical simulation, we explain these sharp differences in behaviour on the basis of the change in crystal symmetry during the transition. We find that a necessary condition for reversibility is that the symmetry groups of the parent and product phases be included in a common finite symmetry group. In these cases, the energy barrier to lattice-invariant shear is generically higher than that pertaining to the phase change and, consequently, transformations of this type can occur with virtually no plasticity. Irreversibility is inevitable in all other martensitic transformations, where the energy barrier to plastic deformation (via lattice-invariant shears, as in twinning or slip) is no higher than the barrier to the phase change itself. Various experimental observations confirm the importance of the symmetry of the stable states in determining the macroscopic reversibility of martensitic transformations.     

An effective gravitational temperature for sedimentation

The slow sedimentation of suspensions of solid particles in a fluid results in complex phenomena that are poorly understood. For a low volume fraction (phi) of particles, long-range hydrodynamic interactions result in surprising spatial correlations in the velocity fluctuations; these are reminiscent of turbulence, even though the Reynolds number is very low. At higher values of phi, the behaviour of sedimentation remains unclear; the upward back-flow of fluid becomes increasingly important, while collisions and crowding further complicate inter-particle interactions. Concepts from equilibrium statistical mechanics could in principle be used to describe the fluctuations and thereby provide a unified picture of sedimentation, but one essential ingredient--an effective temperature that provides a mechanism for thermalization--is missing. Here we show that the gravitational energy of fluctuations in particle number can act as an effective temperature. Moreover, we demonstrate that the high-phi behaviour is in fact identical to that at low phi, provided that the suspension viscosity and sedimentation velocity are scaled appropriately, and that the effects of particle packing are included.     

Pressure Gradient Force, Saffman Lift, and Magnus Lift on the Fiber-like Particle in Fluid

Flher-like particle suspensions are common in both na-ture and industry, but there is little work reported on it.The forces acting on the fiber - like particle in fluid arestudied in this paper, and the Magnus lift, Saffman lift,pressure gradient force, and then the dynamics modelhave been received. The numerical study of the simpleshear flow past the cylinders shows that the particles 'motion is controlled by the vortex.     

Ionic colloidal crystals of oppositely charged particles

Colloidal suspensions are widely used to study processes such as melting, freezing and glass transitions. This is because they display the same phase behaviour as atoms or molecules, with the nano- to micrometre size of the colloidal particles making it possible to observe them directly in real space. Another attractive feature is that different types of colloidal interactions, such as long-range repulsive, short-range attractive, hard-sphere-like and dipolar, can be realized and give rise to equilibrium phases. However, spherically symmetric, long-range attractions (that is, ionic interactions) have so far always resulted in irreversible colloidal aggregation. Here we show that the electrostatic interaction between oppositely charged particles can be tuned such that large ionic colloidal crystals form readily, with our theory and simulations confirming the stability of these structures. We find that in contrast to atomic systems, the stoichiometry of our colloidal crystals is not dictated by charge neutrality; this allows us to obtain a remarkable diversity of new binary structures. An external electric field melts the crystals, confirming that the constituent particles are indeed oppositely charged. Colloidal model systems can thus be used to study the phase behaviour of ionic species. We also expect that our approach to controlling opposite-charge interactions will facilitate the production of binary crystals of micrometre-sized particles, which could find use as advanced materials for photonic applications.     

A Taylor vortex analogy in granular flows

Fluids sheared between concentric rotating cylinders undergo a series of three-dimensional instabilities. Since Taylor's archetypal 1923 study, these have proved pivotal to understanding how fluid flows become unstable and eventually undergo transitions to chaotic or turbulent states. In contrast, predicting the dynamics of granular systems--from nano-sized particles to debris flows--is far less reliable. Under shear these materials resemble fluids, but solid-like responses, non-equilibrium structures and segregation patterns develop unexpectedly. As a result, the analysis of geophysical events and the performance of largely empirical particle technologies might suffer. Here, using gas fluidization to overcome jamming, we show experimentally that granular materials develop vortices consistent with the primary Taylor instability in fluids. However, the vortices observed in our fluidized granular bed are unlike those in fluids in that they are accompanied by novel mixing-segregation transitions. The vortices seem to alleviate increased strain by spawning new vortices, directly modifying the scale of kinetic interactions. Our observations provide insights into the mechanisms of shear transmission by particles and their consequent convective mixing.     

SLK涡流选粉机重力分级区内颗粒的运动特性

 选粉机内颗粒的重力分级对后续离心分级有重要影响。为研究颗粒在选粉机重力分级区内的运动特性,在对颗粒进行受力分析的基础上,分别建立y向和x 向颗粒运动方程,通过理论分析探索了颗粒在y 向和x 向的运动特性。运用计算流体力学理论,考察重力分级区内气流速度在y 向和x 向的变化规律,研究不同粒径颗粒的运动轨迹。结果表明,重力分级区内气流速度在y向上递增,颗粒在y 向做速度不断增大的变加速运动;气流速度在x 向上由返料锥近壁端向远壁端递减,颗粒在x 向做速度不断减小的变减速运动。研究结果可为选粉机的设计提供理论参考和数据基础。     

气尘两相旋流中尘粒运动特性及分离效率研究

根据气尘两相旋流内尘粒运动方程,从机理上分析并确定了这种旅流中平衡尘粒的粒径,非平衡尘粒的运动特性时间和尘粒分离之间的关系,另外,还将其与有关文献中的实验进行了比较,且吻合较好。     

气体放电中等离子体流体模型的级数解

数值模拟技术已成为低温等离子体研究的一种有效方法,然而由于耦合性极强的电子、正负离子方程组＂刚性问题＂的存在,使模拟计算受到很大限制,这是等离子体模拟所面临的困难.本文针对气体放电中多离子等离子体流体模型,考虑正负离子、电子间的相互碰撞作用及正负离子、电子的产生消耗和迁移扩散问题,对一般初始边界问题,基于分离变量法,导出了一维形式的正、负离子、电子密度随时间空间变化的级数解,获得了气体放电过程中正、负离子、电子密度时空分布一维形式的解析表示.由于研究的出发点是基于一般形式的带电粒子及电势边界条件,并未涉及具体的电极上电压施加方式,因此本方法不仅适用于直流放电过程而且还适应于交流放电过程.     

基于非解析计算流体力学和离散单元法的大颗粒在流场中的高效率运动模拟

为实现占据多个流体网格的大颗粒在流场中运动的仿真,基于计算流体力学和离散单元法耦合(computational fluid dy namics-discrete element mothod,CFD-DEM),提出了一种新的数值方法.使用黏结颗粒模型将大颗粒近似表示为多个小球形颗粒黏结而成,基于非解析CFD-DEM方法计算流体对每个小球颗粒的作用力,将所有小球颗粒运动参数的平均值用于描述整个黏结颗粒的运动状态.通过黏性流体中球形大颗粒的沉降运动模拟,比较仿真结果与相关实验数据,结果表明:该方法不仅能准确模拟球形大颗粒的沉降运动,而且与浸没边界法相比计算效率更高.与传统的解析CFD-DEM方法相比,此方法还可以方便且准确地模拟三维情况下非球形大颗粒在流场中的运动.