通道宽度对二维粗糙边界斜面颗粒流的影响

在此之前已经报道了二维斜面颗粒流在通道中的分布规律以及二维斜面粗糙边界附近的颗粒 流量密度（ξ=ρ·ν）分布.本文则主要研究通道宽度W对边界附近颗粒流量密度(ξ=ρ· ν)分布的影响.结果表明，颗粒流量密度随通道宽度的变化（ξ W）存在一临界通道宽度W c.在本实验条件下临界通道宽度Wc=70d.当通道宽度小于临界宽度Wc时 ，通道中距边界20d—30d区间内的相对颗粒流量密度随斜面倾斜角的变化可描述为ξ∝( sinθ)α，α是与通道宽度W有关的参数，其数值在032至085之间. 关键词： 二维颗粒流 颗粒物质 颗粒流量密度     

振动驱动颗粒气体体系的局域态本构关系的实验验证

对准二维、水平边界振动驱动的颗粒气体体系的流体力学 参量进行了局域态本构关系的实验研究. 实验观测结果与经典动力学理论预测进行了比较.由于颗粒气体空间分布的不均匀性, 颗粒体系的整体本构关系不成立, 有必要对局域态进行分析. 局域态本构关系是指颗粒系统的局域温度、局域压强和局域数密度之间的关系. 通过颗粒速度的方向变化, 可以得到颗粒的碰撞点. 因此在计算压力张量的对角线项时, 除了动力学部分之外, 我们计入了颗粒碰撞的影响, 得到了一个约为常数的压力张量迹, 即颗粒压强的空间分布, 与流体力学理论预测以及分子动力学模拟结果相符合; 但是颗粒温度和数密度的空间分布, 在振动的正反两个方向的分量出现差异, 并且温度、压强和数密度之间的局域本构关系, 无论在低密度或高密度区域, 实验与理论预测在定性上一致, 但定量上都有较大差别. 因此经典流体力学理论在描述这样的体系时需加以修正. 关键词： 颗粒气体 态方程 流体力学     

二维斜面粗糙边界附近颗粒流量密度分布

以往的实验研究中，已经知道粗糙边界条件下，二维颗粒流的颗粒流量是以通道中心横向对称分布的，颗粒流横向分布既受通道宽度的影响，同时也受通道斜面倾斜角的影响，而且颗粒在通道两侧的分布明显少于通道中间. 主要研究粗糙边界附近颗粒分布随通道宽度以及通道斜面倾斜角的变化规律. 在稀疏流状态以及保持边界墙体的粗糙度不变的条件下，对应不同通道宽度，粗糙边界附近10d范围内的颗粒流量密度(ξ=ρv)随斜面倾斜角的增大而减小，颗粒流量密度随通道宽度的变化存在一临界宽度Wc；在通道宽度小于Wc时，粗糙墙体附近10d范围内颗粒流量密度ξ随sinθ呈指数衰减，通道宽度大于Wc时，颗粒流量密度ξ_sinθ曲线随θ增大几乎呈线性减小. 关键词： 二维颗粒流 颗粒物质 颗粒流量密度     

低颗粒浓度气固两相均匀剪切流

基于颗粒动理学方法，研究了低颗粒浓度气固均匀剪切流流动，建立颗粒压力张量表达式。理论结果表明压力分布呈现各向异性，并且是颗粒弹性恢复系数的函数。计算说明该理论预测的压力分布与Ｊｅｎｋｉｎｓ＆Ｒｉｃｈｍａｎ的分析结果是一致的。     

颗粒堆的体积分数与制备流量关系的实验研究

通过实验研究发现,颗粒堆的体积分数随制备颗粒堆的颗粒流流量指数衰减,减小或增加流量到一定程度时,体积分数都达到饱和;出料口直径与颗粒粒径的比值小到一临界值时,随着流量的减小体积分数增加急剧变缓;颗粒粒径小到一临界值时,随着流量的增加,体积分数的减小急剧变缓.结合颗粒物质的强耗散性、空气作用、瓶颈效应和碰撞理论解释了实验现象,从连续性原理出发推出的颗粒堆体积分数随制备流量变化的函数与实验数据的拟合公式相同.     

边界条件对二维斜面颗粒流颗粒分布的影响

实验研究了粗糙边界条件、不同通道宽度以及不同斜面倾斜角下二维颗粒流的颗粒分布 ，发现颗粒流在稀疏流状态下尽管在现象上表现出类似流体的性质，但由于颗粒流系统的能 量耗散,所以颗粒流的速度和密度分布和牛顿流体完全不同，颗粒流在通道中的分布既受通 道宽度的影响同时也受重力场的影响，颗粒在通道中以通道中轴线呈对称分布，通道两侧的 颗粒密度明显高于通道中间，由于通道中颗粒受剪切力的作用，导致颗粒在通道中的密度分 布的变化，通道边界粗糙度明显影响着颗粒流横向分布. 关键词： 二维颗粒流 颗粒物质 稀疏流     

4.0 GPa压力下纯橄岩弹性纵波速度和衰减的实验研究

 应用超声波透射法和超声波频谱振幅比法、在4.0 GPa压力条件下、测量了弹性纵波通过纯橄岩的波速（v_P）和品质因子值（Q_P，用于表征衰减）随压力的变化，并分析了纯橄岩内部结构的变化对波速和衰减的影响。在实验压力范围内（≤4.0 GPa），随压力升高，纯橄岩的纵波速度逐渐增大：从7.6 km/s（0.4 GPa）逐渐增大至8.5 km/s（4.0 GPa），升高了11.8%，低压时的增大幅度大于高压时的增大幅度。纯橄岩的品质因子值呈两段式线性变化：从低压区间到高压区间品质因子值的增大幅度明显变小（0.4~2.4 GPa压力范围内Q_P增大了358.5%，2.4~4.0 GPa压力范围内Q_P仅升高了7.6%）。纯橄岩的品质因子从54（0.4 GPa）升高至266.4（4.0 GPa），增大幅度达393.3%。在相对低压（0.4~2.4 GPa）条件下，纵波通过纯橄岩的速度增大、衰减降低主要是由于样品内的孔隙和微裂纹大量闭合，岩石密度增大，纵波在通过样品内孔隙和裂缝时损失的机械能降低，因此纵波通过纯橄岩的能量增大（即衰减降低），波速升高；当围压较高（2.4~4.0 GPa）时，纯橄岩内大部分裂纹已经闭合，而且矿物颗粒边界接触紧密。岩石内部的裂纹和颗粒之间由于摩擦滑动而损失的能量也变少，所以在高压时纯橄岩的波速和衰减的变化幅度变缓，但波速仍呈线性增大，而品质因子值（衰减）随压力升高几乎趋于不变。     

Couette颗粒系统中静态应力和侧压力系数的非线性弹性理论分析

经典弹性理论能否适用于静态颗粒物质是颗粒物理的一个基本问题.报道了非线性颗粒弹性方程对环柱几何(或称Couette几何)状样品中应力分布的计算结果.由于这些结果大都可在今后用实验直接测量,因而可将它们用于进一步判断这些弹性方程对颗粒材料的适用性以及澄清这个基本问题.另外,这些结果还可用于分析计算工程中的侧压力系数. 关键词： 颗粒物质 侧压力系数 非线性弹性理论     

粮仓效应的圆盘堆积模型

为研究圆筒中颗粒堆内部的应力分布,将颗粒堆简化为规则的二维圆盘堆积模型.对圆盘堆积中力的传递进行分析后给出了圆盘堆积中力的传递规律,利用计算机模拟了圆盘对容器底部的压力,发现底部压力随圆盘层数增加会达到饱和,并将二维圆盘堆积的规律推广到三维颗粒堆,从理论上推导出容器底部平均压强p与颗粒填充高度Z的关系.     

竖直振动颗粒床中的倍周期运动

实验研究了竖直振动颗粒床中颗粒对容器底部的压力随振动强度的变化情况.发现压力随振动加速度的增加经历倍周期分岔，典型的分岔序列为：2P，4P，混沌，3P，6P，混沌，4P，8P，混沌.观察表明，伴随倍周期分岔现象，在颗粒床底部出现颗粒的聚集态.聚集态内颗粒密堆积在一起并作整体的上下运动.采用完全非弹性蹦球模型分析了颗粒对容器底的冲击力，并给出了倍周期分岔现象的一种解释. 关键词： 颗粒物质 混沌 倍周期分岔 非弹性碰撞     

Force distribution in an inhomogeneous sandpile

The force perturbation field in a two-dimensional pile of frictionless gravity-loaded discs or spheres arising from lattice distortions is derived to first order. The starting point is the model proposed by Liffman et al. (Powder Technology (1992) pp. 255-267) and Hong (Phys. Rev. E 47, 760-762 (1993)) in which discs of uniform size are arranged on a regular lattice: this predicts a uniform normal stress distribution at the base of the pile. The analysis is applied to two problems: (i) deformable (rather than rigid) grains that undergo Hertzian deformation at the points of contact; (ii) a pile containing a gradient in particle size from the centre to the free surfaces. The former results in the classical pressure dip at the centre; the latter also produces a dip if the larger particles are at the centre. Received 29 January 1998 and Received in final form 7 September 1998     

Strength of free-standing chemically vapour-deposited diamond measured by a range of techniques

The strength of free-standing chemically vapour-deposited diamond was determined using a bursting-disc test, a three-point bend test and a four-point bend test. The bursting-disc technique has advantages for determining the strength of ceramic materials since the maximum tensile stress is at the centre of the disc. The much smaller stresses near the sample edges are rarely responsible for failure and so edge effects are negligible. However, the bursting-disc test is often impractical for ascertaining the strength of chemically vapour-deposited diamond since the cost of samples is high. A three-point bend test is often employed instead, since it may be performed on smaller samples. However, the area stressed in the three-point bend test is smaller, and so there is less chance of stressing a large flaw, resulting in higher strength values. A further disadvantage is that specimen edges are stressed and, unless the specimen sides are polished, as well as its faces, misleading results may be obtained (although this is not the case for chemically vapour-deposited diamond). A four-point bend test is often used because it stresses a larger volume of material than the three-point bend test and so the results are more reliable. Furthermore, the diamond used in a four-point bend test can be of similar size to those used in a three-point bend test and so the expenditure is comparable. However, ensuring that the four-point bend test is applied correctly is technically more difficult. Strength data are reported for both the growth and the nucleation sides, for difference nucleation densities and for as-received and polished samples.     

Optimization of mechanical strength of titania fibers fabricated by direct drawing

Nanostructured polycrystalline titania (TiO₂) microfibers were produced by direct drawing from visco-elastic alkoxide precursors. The fiber crystallinity and grain size were shown to depend on post-treatment calcination temperature. Tensile tests with individual fibers showed strong sensitivity of the elastic modulus and the tensile strength to microstructural details of the fibers. The elastic modulus of as-fabricated fibers increased about 10 times after calcination at 700 ^°C, while the strain at failure remained almost the same at ～1.4 %. The highest tensile strength of more than 800 MPa was exhibited by nanoscale grained fibers with a bimodal grain size distribution consisting of rutile grains embedded into an anatase matrix. This structure is believed to have reduced the critical defect size, and thus increased the tensile strength. The resultant fibers showed properties that were appropriate for reinforcement of different matrixes.     

Dislocation structure of plastic relaxation waves in polycrystals and alloys under intense shock wave loading

The influence of the structure factors (sizes of grains and precipitates) on the dislocation structure formed in polycrystals and alloys behind the shock wave front (elastic precursor) has been theoretically discussed in terms of the dislocation kinetic relationships and kinetic equation for the dislocation density. The critical conditions of the transition from the cellular dislocation structure to a uniform dislocation distribution have been formulated. These conditions are used to determine the dependences of the critical pressure, above which the dislocation distribution becomes uniform, on the grain size and precipitate volume density.     

A phase-field model for elastically anisotropic polycrystalline binary solid solutions

A phase-field model for modeling the diffusional processes in an elastically anisotropic polycrystalline binary solid solution is described. The elastic interactions due to coherency elastic strain are incorporated by solving the mechanical equilibrium equation using an iterative-perturbation scheme taking into account elastic modulus inhomogeneity stemming from different grain orientations. We studied the precipitate interactions among precipitates across a grain boundary and grain boundary segregation–precipitate interactions. It was shown that the local pressure field from one coherent precipitate influences the shape of precipitates in other grains. The local pressure distribution due to primary coherent precipitates near the grain boundary leads to inhomogeneous solute distribution along the grain boundary, resulting in non-uniform distribution of secondary nuclei at the grain boundary.     

Benchmarking vdW‐DF first‐principles predictions against Coupled Electron–Ion Monte Carlo for high‐pressure liquid hydrogen

We report first‐principles results for the nuclear structure and optical responses of high‐pressure liquid hydrogen along two isotherms in the region of molecular dissociation. We employ density functional theory with the vdW‐DF approximation (vdW) and benchmark the results against existing predictions from Coupled Electron–Ion Monte Carlo (CEIMC). At fixed density and temperature, we find that the pressure obtained from vdW is higher than that from CEIMC by about 10 GPa in the molecular insulating phase and about 20 GPa in the dissociated metallic phase. Molecules are found to be over‐stabilized using vdW, with a slightly shorter bond length and with a stronger resistance to compression. As a consequence, pressure dissociation along isotherms using vdW is more progressive than that computed with CEIMC. Below the critical point, the liquid–liquid phase transition is observed with both theories in the same density region, but the one predicted by vdW has a smaller density discontinuity, i.e. a smaller first‐order character. The optical conductivity computed using Kubo–Greenwood formulation is rather similar for the two systems and reflects the slightly more pronounced molecular character of vdW.     

Evolution of dislocation patterns in a tricrystal model subjected to cyclic loading

When a polycrystalline aggregate is deformed beyond the elastic range, dislocations pile up at grain boundaries and make some patterned structures within the grains. If the external load is reversed, most dislocations at grain boundaries and in the patterned structure are supposed to disappear or change their arrangements, but the details are not yet well known. In this study, we examined such changes in dislocation structures by a crystal plasticity analysis. Models for the polycrystalline aggregates consist of three grains and the generation and degeneration of the geometrically necessary dislocations during cyclic loading are examined in detail. The results show that there are some groups of dislocations that do not dissipate but evolve upon reverse loading.     

The equation of state of a system of hard spherocylinders

Using the Monte Carlo method, we have computed the equation of state of a system of hard spherocylinders (cylinders with a hemisphere at each end), of length-to-breadth ratio equal to 3, in the isotropic liquid phase. We obtain a pressure slightly smaller than that predicted by the scaled-particle theory (SPT). The SPT predicts a liquid to nematic transition when the density is increased; we have observed that the isotropic liquid phase is stable up to densities significantly higher than the SPT transition density. Using the free-volume theory, we have also determined the behaviour of the pressure at very high densities, for any value of the length-to-breadth ratio γ. Moreover, we have shown that the packing fraction (number density times the volume of one spherocylinder) corresponding to the beginning of the fusion of the solid is an increasing function of γ.     

高压下RhB的相变、弹性性质、电子结构及硬度的第一性原理计算

采用密度泛函理论中的赝势平面波方法系统地研究了高压下RhB的结构相变、弹性性质、电子结构和硬度.分析表明,RhB在25.3 GPa时从anti-NiAs结构相变到FeB结构,这两种结构的弹性常数、体弹模量、剪切模量、杨氏模量和弹性各向异性因子的外压力效应明显.电子态密度的计算结果显示,这两种结构是金属性的,且费米能级附近的峰随着压强的增大向两侧移动,赝能隙变宽,轨道杂化增强,共价性增强,非局域化更加明显.此外,硬度计算结果显示,anti-NiAs-RhB的金属性比较弱,有着较高的硬度,属于硬质材料.