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级配颗粒堆积体密度估算方法研究
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摘要
颗粒堆积密度是反映颗粒介质内部构造的一个基本指标,是土木工程中的一项必不可少的设计参数,也是研究许多颗粒力学问题的切入点。鉴于颗粒堆积密度的重要性,本文结合水利围堰中密度参数确定问题,基于PFC3D数值平台,开展了颗粒材料堆积密度的数值试验,研究影响颗粒堆积密度的相关因素,建立了堆积密度计算公式。同时进行了物理模型试验和离心机模型试验验证计算结果的可靠性。研究内容与研究成果如下:
     1.建立了颗粒堆积密度的计算公式。基于颗粒的“填隙”和“替换”机制,从单粒径颗粒、二元颗粒的堆积密度,推广到级配颗粒的密度计算模式。通过数值试验研究计算公式中参数的影响,针对容器中颗粒堆积以及无侧向边界限制的颗粒堆积,分别建立了一整套估算级配颗粒堆积体密度的方法及计算公式。
     2.基于Matlab的图像处理功能,进行颗粒形状信息提取和颗粒形状重建。从颗粒的三视图图像中提取圆度信息并对其进行加权平均,得到加权圆度作为反映颗粒三维形状特征的指标。在Matlab中创建具有指定加权圆度的三维凸包,对凸包进行体素化和骨架化处理,提取颗粒的三维空间信息,从而在PFC3D中重建具有特定形状的clump块体,为形状颗粒的堆积试验提供基础。
     3.研究了颗粒材料特性对颗粒堆积密度的影响。针对颗粒的摩擦系数、颗粒形状、颗粒级配等颗粒材料特性,设计了相关的数值试验,得到各因素与颗粒堆积密度的影响关系。讨论了颗粒试样宏观摩擦角与颗粒间摩擦系数之间的联系,提出了反映颗粒级配情况的粒度分布宽度的计算方法。
     4.研究了静水条件下颗粒堆积密度的变化规律。颗粒在水中的堆积受到重力、浮力、阻力等作用,也受到颗粒与颗粒之间相互干扰的影响,这些外力作用和影响最终反映在堆积密度的变化上。本文设计了级配颗粒在流体中的堆积试验,得出了在静水中颗粒堆积密度与级配之间的关系。
     5.研究了颗粒堆积深度与密度的关系。基于数值试验结果,根据Janssen压力公式拟合得到随深度变化的孔隙比计算公式。在PFC3D中进行了不同摩擦系数的颗粒侧限压缩试验,将试验成果作为上覆荷载作用下颗粒孔隙比变化的依据,对建立的孔隙比随深度变化的公式进行扩充和完善。
     6.进行了物理模型试验以及离心机模型试验,对建立的估算公式的有效性进行判断。在不同高度的圆柱筒容器中进行玻璃球和风化砂的抛填试验,对级配风化砂进行不同加速度的离心机试验,模拟不同深度的风化砂自重压密效果。将估算值与模型试验值进行对比,两者具有较好的一致性。
Particle packing density is one of the most basic parameters which reflect the internalstructure of granules medium. It is one of the key design parameters in civil engineering andis a point of penetration of many particle mechanics problems. In view of the importance ofstudying particle packing density, numerical test is carried out in PFC3D numerical platformbased on the deep water cofferdam project. Relevant factors of particle packing density arestudied and computational formula of particle packing density is established in thisdissertation. Meanwhile, physical model test and centrifugal model test are also carried out tocheck the result of computation. The research contents and results are as follows.
     1The computational formula of particle packing density.is set up. Based on the mechanism ofparticle "filling" and particle "replacing", the estimation model of particle density is extendedfrom mono-size particle and binary particle to multiple size particles. The influence of theparameters in computational formula is studied by the numerical test. A whole set of methodand computational formula estimating particle packing density is set up respectively for thedifferent situations
     2. Particle shape information is extracted and its shape is reconstructed based on the imageprocessing in Matlab.Weighted roundness of the particle is defined as shape indices. Thereconstruction process can be state as follows: fetching roundness information from threeviews of particles which are shooted by digital camera firstly, then calculate the averageroundness is weighted by the areas.3D convex hulls which have certain weighted roundnessare created in Matlab and then extract three-dimensional information by means ofvoxelization and skeletonizing. The clumps are created according to the three-dimensionalspace coordinate information in PFC3D.
     3. The influence of particles characteristics on packing densityis studied. Knowing thatpacking density are influenced by factors such as friction coefficient between particlesparticle shape and gradation of particles, correlative numerical test are carried out and therelationship between the packing density and factors is clear. The relationship between macro-frictional angle of granular media and micro-friction coefficient between particles isdiscussed and defined the calculative process of width of size distributions.
     4. The packing density in the condition of fluid-structure interaction studied. The particlesyielded force such as gravity, buoyancy, resistance and disturbing force between particles inthe fluid. Packing density will be modified due to the external force. Numerical tests ofgradation particles were designed to study the influence of still water on packing density.
     5. The influence of packing depth and vertical press on the packing density is studied. Fittingformula of void ratio varied with depth and vertical press is established according to Janssenformulas and numerical experimental results. Meantime, the confined compression numericaltest is carried out by PFC3D and its results can be the basis of variation of void ratio with theimpact on cover load. Also the fitting formula whose void ratio varied with depth and verticalpress is improved.
     6. Physical model and centrifuge model testing are designed to verify the validity of packingdensity estimation model. Glass marble and weathered sand are poured to cylinder in differentheight and weathered sand with graduation are packed in centrifugal machine in the conditionof different acceleration to simulate self-weight compression. The calculated results agreewell with those of the physical model testing and centrifuge model test.
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