纳米颗粒形状对螺旋管强化传热的影响

采用欧拉-拉格朗日两相法研究了螺旋管内不同形状纳米流体的传热性能。分析了纳米颗粒形状对螺旋管内传热强度、压降、热力性能的影响。研究结果显示:不同颗粒形状对螺旋管内纳米流体传热强度和压降的影响趋势一致,即传热强度和压降由高到低的顺序为血小板形、圆柱形、刀片形、球形、砖形;在颗粒球形度一定时,粒径越小,传热强度越大,压降也越大;血小板形纳米流体的热力性能优于其他颗粒形状,其在Re为1 750时热力性能系数达到最大值1.11。     

块石颗粒形状对碎石土渗流特性影响研究

碎石土是由不同形状的块石颗粒组成,块石颗粒形状不仅会影响碎石土内部的细观结构,也 会影响其渗流特性．基于碎石土细观结构模型,将不同形状的块石颗粒简化为圆形、正方形、正六边 形和正八边形,分析块石形状对碎石土的渗流特性影响．发现不同形状块石组成的碎石土中水的渗 流量不同,随着孔隙率增大,块石形状对其渗流特性的影响越明显．常水头渗流试验表明：不同形状 块石,渗透系数差异明显,其中规则块石（近似正方体）渗透系数最大,泥岩颗粒（近似多面体）渗透 系数最小．数值模拟试验结果与常水头试验结果二者的规律吻合,说明数值模拟试验结果可靠．     

复杂形状颗粒DEM模拟及其对宏观力学响应影响研究

借助有限元网格划分技术,提出了一种复杂形状颗粒的DEM模拟方法,实现了包括凹多边形在内的复杂形状颗粒的模拟,通过参数控制使得生成的颗粒形状更加真实.生成6组不同形状的颗粒集合体,采用双轴压缩试验研究了颗粒形状对力学响应的影响,并解释了细观影响机理.计算结果表明,在最密实状态下,颗粒间的孔隙填充效率与颗粒边数有关,颗粒边数越少,孔隙率越小;不同形状颗粒集合体在双轴压缩过程中,加载初期的变形模量相近,偏应力和体积变形规律相似,峰值摩擦角与形状系数呈线性关系;颗粒边数越少,加载过程中颗粒转动越剧烈.规则颗粒边数为5~10时,颗粒集合体的峰值强度和体积变形差异较小,这主要是受颗粒旋转的影响:颗粒绕形心转动时,颗粒边数越少,局部剪胀越明显,所需外界功越大;当颗粒绕接触点旋转时,偶数边颗粒所需外界功和导致的体积剪胀比奇数边颗粒试样大.随着颗粒集合体凹凸度的增大,颗粒间的排列更加密实,咬合作用更强,但颗粒形状随机多样性与颗粒间的咬合作用并无单调关系,当λ=0.2时,颗粒集合体的峰值强度和体积应变最大.     

电化学镀铝基体局域应力对镀层形貌影响研究

通过电化学方法在带有局域应力的铜表面电镀形成一层铝膜.实验表明沉积在铜表面的铝颗粒形状与局域应力缓释有直接的联系。局域应力越大，形成颗粒越大，形状呈针状。随着局域应力的降低，颗粒度明显降低，颗粒形状逐渐向球形过渡。结果表明基体的局域应力可以改变沉积颗粒的形状，促进沉积颗粒的形核与增长.     

组合Clump颗粒加筋砂土三轴剪切试验离散元模拟分析

针对常规离散元数值模拟中采用的球形颗粒难以再现颗粒间嵌锁作用的问题,基于不同形状的Clump颗粒,利用离散元软件PFC3D开展了多组三轴剪切数值模拟试验.通过改变球形度和加筋层数研究了颗粒形状和加筋层数对砂土剪切特性的影响,并初步探讨了筋土界面作用机理.结果表明:随着颗粒球形度的增加,试样峰值抗剪强度与残余强度逐渐减小...     

矸石块体三维形状参数自动获取方法与量化分析

为了实现对不规则矸石块体三维形状参数的自动获取以及三维形状指标的量化分析,研究块体形状对破碎矸石充填材料承载压缩力学特性的影响规律,设计了CT扫描试验并利用三维重构技术得到了不规则矸石块体的数字化三维模型,提出了基于最小包容长方体准则与遗传算法的不规则矸石块体三维形状参数自动获取方法,建立了以针度、扁平度、球形度为形状指标的不规则矸石块体三维形状特征量化表征方法,并通过具体案例对所提出方法的可行性与优越性进行了分析.结果表明:不同形状指标对矸石块体形状差异的敏感度不同,敏感度从高到低依次为针度、扁平度、球形度;矸石块体的平均针度约为1.5,平均扁平度约为0.7,平均球形度约为0.8,说明不规则矸石块体具有一定的狭长性和片状性.以10^#矸石块体为例,基于本文方法所得矸石块体针度,扁平度分别为1.634,0.849,接近于传统人工测算针度平均值1.638、扁平度平均值0.839,准确度均处于95%的置信范围内.     

点火形状对激光推力器推进性能的影响

在单脉冲激光能量为70 J作用下,用数值计算方法,对点火形状沿抛物形喷管对称轴的变化及其对激光推力器推进性能的影响进行了研究。研究表明,对于内表面母线方程y^2=20x,开口直径为0.08 m的喷管,当点火位置距离喷管内壁顶点0.02 m,点火区体积不变时,椭球或圆柱点火形状的变化对冲量耦合系数的影响很小;点火形状沿水平方向越细长,推力的变化幅度越小。通过分析点火形状对推进性能的影响,为设计聚焦系统提供了参考依据。     

颗粒形态对粗粒土渗透系数影响的数值模拟研究

土体渗透系数是关系工程稳定性、安全性的关键因素。土体是一种散粒体材料,已有研究多关注颗粒级配、密实度、颗粒大小对渗透系数的影响。为研究颗粒形态这一关键微观特征对土体渗透系数的影响,采用孔隙流体渗流过程数值模拟方法,研究了球形颗粒和多面体颗粒试样的渗透系数。结果表明,颗粒形状越复杂,孔隙形状的复杂程度越高,渗透系数因而越低。多面体颗粒试样的渗透系数明显小于球形颗粒试样,两者比值在35%到50%之间。     

基于FDEM的岩石颗粒破碎后碎片形状的统计分析

针对碎片尺寸和形状会影响岩石强度和变形的问题,采用三维扫描技术获得真实岩石颗粒的表面点云数据,然后通过数字图像处理技术重构数字颗粒,利用连续-离散耦合方法（FDEM）模拟单个颗粒在平板压缩下的断裂破碎. 识别颗粒破碎后所产生的碎片,并进行碎片形状的表征和量化,分析碎片形状与颗粒初始形状、碎片尺寸的关系. 为了准确描述裂纹尖端的应力梯度和损伤演化,进行颗粒有限元网格密度的敏感性分析,结果表明,断裂过程区至少需要5、6个界面单元以减弱网格尺寸的影响. 本研究关注颗粒破碎后所产生碎片的整体形态,忽略断裂引起的碎片表面局部起伏和粗糙变化. 尽管所研究颗粒的初始形状存在较大差异,仍发现颗粒破碎后所产生碎片的形状指标分布具有一些共性特征. 碎片的圆度、扁平率、Domokos因子和凸度对颗粒初始形状的敏感性逐渐增强,并且除了圆度外,其他形状指标分布与碎片尺寸之间并未发现显著的相关性. 不同粒径组碎片的圆度分布表明,较大的碎片棱角更明显.     

再生粗骨料颗粒整形的必要性

传统观点认为碎石比卵石好,并认为混凝土再生粗骨料经过整形处理后,去掉了棱角使粒形趋于圆形是不利的．笔者通过分别配制卵石混凝土和碎石混凝土进行对比,指出卵石因粒形比碎石好,卵石混凝土的性能优于碎石混凝土;通过分别配制颗粒整形再生粗骨料混凝土和简单破碎再生粗骨料混凝土进行对比,指出经过整形处理后的再生粗骨料,粒形和界面都得到了改善,所配制的混凝土性能得到显著提高．碎石混凝土强度高于卵石混凝土强度的观点是不全面的,有必要对再生粗骨料进行整形处理．     

Constitutive Laws for Visco-plastic Porous Medium Shaped by Regularly Distributed Circular Particles

A numerical study is presented, using a homogenization technique to consider the plain strain problem of visco-plastic porous medium shaped by regularly distributed circular particles. Based on a rigid plastic material, the paper derives the macroscopic constitutive laws for homogenous equivalent medium. By changing the shape parameter of circular particles, the effect of pore shape on macroscopic constitutive laws is explored. Yield surfaces with different pore shapes are obtained. About voids, a two-scale conception is introduced, which regards main void as macroscopic scale and secondary cavities as microscopic scale. The macroscopic potential involving main and secondary voids is achieved. The proposed macroscopic constitutive law taking microscopic features as influence factors is helpful for exploring the macroscopic mechanical properties of porous medium when numerical simulation is required.     

Modeling and Software Development of the Interfacial Transition Zone of Ellipsoidal Aggregate in Cement-Based Composites

The interfacial transition zone(ITZ) between the aggregates and the bulk paste is the weakest zone of ordinary concrete, which largely determines its mechanical and transporting properties. However, a complete understanding and a quantitative modeling of ITZ are still lacking. Consequently, an integrated modeling and experimental study were conducted. First, the theoretical calculation model of the ITZ volume fraction about the rotary ellipsoidal aggregate particles was established based on the nearest surface function formula. Its calculation programs were written based on Visual Basic 6.0 language and achieved visualization and functionalization. Then, the influencing factors of ITZ volume fraction of the ellipsoidal aggregate particles and the overlapping degree between the ITZ were systematically analyzed. Finally, the calculation models of ITZ volume fraction on actual ellipsoidal aggregate were given, based on cobblestones or pebbles particles with naturally ellipsoidal shape. The results indicate that the calculation model proposed is highly reliable.     

Influence of water content and shear rate on the mechanical behavior of soil-rock mixtures

Soil-rock mixtures (S-RMs) are widely distributed in the nature. The mesoscopic deformation and failure mechanisms as well as the macro-mechanical behaviors of the S-RMs depend largely upon the rate of deformation, water content and particle sizes. In this research, a series of large-scale direct shear tests with different water contents and different grain-size distributions were conducted to study the influence of the aforementioned factors on the mechanical properties of the S-RMs. Due to the effect of the rock blocks’ breakage in the S-RMs, the relationship between the shear strength and the vertical stress of S-RM follows a power law instead of a linear one. It is found that there exists a threshold value for the vertical stress during the shearing process, below which the soil strength is mainly determined by the inter-locking of particles and the re-arrangement of meso-structure, and otherwise large-sized rock blocks are gradually broken into smaller fragments, resulting in a decrease in the soil strength. The shear rate can also significantly influence the degree of particle breakage and the meso-structural rearrangement of the S-RMs, namely, under low shear rate, the particles of the samples are fully broken resulting in enhanced macro-strength. As a result, the lower the shear rate, the higher the macroscopic strength. So under unsaturated conditions, the water content will affect the strength of the S-RMs by reducing the strength of rock blocks. As the water content increases, the soil strength decreases gradually, and assumes a moderate value when the water content reaches 8%. At the same water content, the soil strength increases with the sizes of large rock blocks. For the occlusion, breakage and structure re-arrangement of the oversized rock blocks inside S-RM, which have a huge influence on the mechanical characteristics of the samples.     

土石混合体边坡稳定性的三维颗粒离散元分析

为探究土石混合体边坡的稳定性和破坏机理,基于土工离心模型试验基本原理,结合已开发的不规则块石和土石混合体三维离散元建模方法建立土石混合体边坡细观结构的三维离散元模型.引入颗粒流强度折减法并提出基于能量演化的方法来判别边坡失稳破坏.利用提出的土石混合体边坡三维离散元建模方法和稳定性分析方法,分析不同块石含量、块石形状、空间结构、空间形态等因素对土石混合体边坡稳定性和破坏模式的影响,并揭示这些因素影响的细观机理.结果表明:随含石量增加,土石混合体边坡的安全系数逐渐增大;土石混合体边坡中呈现出多滑动面现象,且滑面较为曲折,具有明显的绕石特征,坡脚部位的部分大块石使得剪出口的位置发生明显的改变;随块石形状由球体、卵石到碎石,土石混合体边坡的安全系数逐渐增大;上覆堆积体相对较厚且坡度不是很大的二元结构边坡或一元结构边坡的破坏模式主要是堆积体内部发生的弧形滑动破坏,上覆堆积体厚度不大且基岩面相对平整的二元结构边坡的破坏模式主要是沿基-覆界面的整体平移滑动破坏;随土石混合体边坡空间形态由条带型、敞口型到锁口型,其安全系数逐渐提高,其中锁口型边坡具有显著的支撑拱效应,且含石量越大这种效应越明显.     

改性高水材料尺寸与形状效应研究

高水材料广泛应用于地下采空区充填支护,其形状和尺寸对力学性质影响显著。为了研究不同形状和尺寸改性高水材料的力学性质,借助四川大学ETM力学试验系统,通过对不同尺寸和形状下的改性高水材料的破坏过程、抗压强度和变形特性等性质的测试,系统研究了改性高水材料的尺寸和形状效应。结果表明：改性高水材料是一种弹塑性材料,其变形破坏过程可以分为弹性阶段、屈服阶段、塑形阶段和破坏阶段四个过程;改性高水材料的尺寸仅影响其应力—应变曲线峰值强度,而形状对应力—应变曲线的峰值强度和形状均有较大影响;在一定范围内,随着材料尺寸的增加,圆柱体和立方体试件的抗压强度、弹性模量和变形模量均在一定范围内逐渐减小;在相同条件下,圆柱体试件的抗压强度、弹性模量大于立方体试件,变形模量小于立方体试件。     

岩心三维模型形状分布刻画（研究生论坛）

针对形状分布算法和统计特征函数不能有效刻画岩心三维模型的问题,提出了一种岩心形状分布算法。根据边界点对连线与目标相的穿透关系,将形状分布D2距离直方图分类为内嵌、外接、联合三种状态,分别刻画孔隙骨架结构,并利用分类发生频率刻画孔隙骨架分布情况,提取岩心形态特征描述符。在统计足量随机边界点对之后,生成概率分布函数。最后,给出了量化比较岩心三维模型形态相似性的方法。对多组岩样进行了实验,结果表明该算法能够有效提取岩心形态特征,比较各岩心之间的形态相似性,具有比原有形状分布和统计特征函数更为准确的刻画能力。     

The flowability of ground powders and the comminuting mechanism of mills

The ground powders with the same particle size distribulion and the sanw mean particle diameter were prepared by five different types of mills. The flowability index (FI) and the particle shape indices, namely. Wadell‘s working sphericity ψw and circularity ψc , of five kinds of test powders were nwasured. The effect of the comminuting mechanisms on the flowability of ground powders was investigated, and the relationship between the flowability of ground powders and the partticle shape indices was analyzed. The experimental restdts show that the ground turn,tiers obtained by collision have irregular particle shapes and smooth surfaces, showing a high flowability. On the other hand, though the particle obtained by grinding is close to a spherical particle, but it has a rugged suttee, and shows a bad flowability. Furthermore, the flowability index is more corrdated with the circularity titan the working sphericity is. This means that the surface roughness is more effective, in determining the flowability of powders than the roundtwss is.     

深井巷道围岩应力及变形规律的数值分析

采用非线性有限元法，对深井巷道围岩应力及变形状态进行了数值计算．分析了巷道埋深、围岩性质和断面形状对围岩变形的影响．结果表明：当巷道埋探小于某一临界深度时，巷道周边围岩移近量随巷道埋深增加呈线性增加；当超过临界深度时，巷道围岩移近量呈现指数规律增加趋势．围岩变形破坏的临界深度取决于围岩的力学性质及巷道断面形状等因素．