基于三维离散元堆积碎石土细-宏观力学参数相关性研究

以室内三轴压缩试验为基础,运用PFC3D离散元软件生成含有不同形状碎石块的1∶1堆积碎石土数值模型,标定得到相应的细观参数(相对误差控制在10%以内)。改变细观参数,定量探讨其与宏观力学参数(粘聚力和内摩擦角)关系,得到细观参数与粘聚力、内摩擦角的细-宏观参数关系式,并对其可靠性进行了验证。研究发现,(1)粘聚力与接触粘结力、颗粒摩擦系数呈线性正相关;同时受粘结力比值M=σc/τc的影响,当M1时,粘聚力随着M的增加而线性增加;M≥4时,粘聚力趋于稳定。(2)内摩擦角与颗粒摩擦系数呈对数关系,与接触粘结力呈抛物线关系。(3)细-宏观参数关系式是在含水率为7%室内试验结果基础上研究得出,将关系式运用到含水率为9%和11%的室内试验结果进行标定验证:依据含水率为9%和11%室内试验得到的c和φ值,运用细-宏观参数关系式计算得到细观参数,经验证其数值-室内试验结果相对误差在10%以内,说明通过细-宏观力学关系式得到的细观参数符合精度要求,关系式可靠。为后续土石混合体数值试验研究者进行细观参数标定工作提供了参考依据。     

离散颗粒流动堆积行为离散元模拟及实验研究

工程应用中存在许多颗粒流动堆积问题.首先设计了一系列测量方法,通过大量的实验和统计分析,得到了颗粒的多种物理参数.并以工程中高炉炉顶称量料罐为背景,采用离散元方法模拟不同物理条件下离散颗粒的流动堆积行为,得出料罐内颗粒系统中颗粒之间力的分布不均匀,而且强力链分布主要与料罐的左下壁方向平行.同时,设计并制作了具有多参数调节的离散颗粒料罐实验模型,进行了相应的物理实验,实测结果与数值模拟吻合良好.     

转动驱动圆角立方体颗粒有序堆积的离散元模拟

宏观颗粒体系的有序堆积研究可以为热系统中微观粒子的自组装提供研究模型,也有助于工业生产中提高颗粒材料的填充率.实验发现圆筒中的圆角立方体颗粒受容器往复旋转剪切作用会实现有序堆积.为探究旋转圆筒中立方体颗粒有序堆积过程的内部结构演化过程和动力学机理,采用超二次曲面方程构造了圆角立方体颗粒,基于离散元方法对旋转剪切作用下圆...     

振动系统实模态参数灵敏度分析

1、引言国内外先后发展并加以应用的各种进行振动系统固有特性灵敏度分析的方法一般都是通过整个结构上各个部分的试探修改进行灵敏度数值计算,由此寻找出灵敏度较大的一些点。对于大型或复杂结构,这种大范围的搜索计算仍需很大的计算量,或者难以对整个结构进行完整的灵敏度分析,从而导致遗漏最灵敏点,影响灵敏度分析结果的可靠性,甚至使得分析工作失去意义. 本文根据实模态参数的矩阵摄动分析算式。对机械系统的无阻尼固有振频和固有振型随结构的质量和刚度变化的灵敏度分析,找出了结构修改的最灵敏点,得出了一系列有实     

砂土类材料离散元细观参数的自动识别

准确的细观参数是采用离散元进行合理模拟的前提,基于最优化理论,本文提出了一种砂土类材料离散元细观参数的自动标定方法:采用PFC离散元软件模拟砂土的双轴压缩试验,以使PFC数值模拟的应力–应变曲线与某标准砂土室内双轴压缩实验结果的误差最小为优化目标,采用模式搜索法实现砂土类材料离散元细观参数的自动标定。本文提出的离散元细观参数自动标定方法具有识别效率快,精度高的优点。     

黏性土宏细观参数相关性研究

运用颗粒流程序中的bond原理结合Mohr-Coulomb破坏准则对黏性土双轴试验进行了数值模拟,探讨了黏性土宏观强度指标与其对应的细观参数之间的关系.分析结果表明,黏性土内摩擦角与颗粒摩擦角呈线性关系,黏性土黏聚力-颗粒接触强度两者拟合曲线的斜率与颗粒间摩擦系数呈幂函数关系,且两者的相关性都很好.通过该宏细观关系,得出上海第②层褐黄色粉质黏土细观参数,对其进行了双轴试验的颗粒流模拟,计算得到的应力应变曲线与实测结果一致.研究成果对于黏性土细观参数的取值及颗粒流模拟具有指导意义.     

土的工程力学性质的颗粒流模拟

基于颗粒流理论,引入不同的颗粒接触连接本构模型,分别建立了砂土和粘性土的颗粒流模型.通过颗粒流数值模型试验,对砂土和粘性土的室内平面应变试验及其剪切带形成和发展进行了数值模拟,分别对比了不同围压下颗粒流试样与室内试验的应力应变关系曲线,基本再现了砂土和粘性土试样应力-应变关系.通过砂土和粘性土PFC试样剪切带模拟表明,当围压较小时试样内部颗粒位移量小而且分布范围较广,当围压增大时,试样内部颗粒位移量也增大,而且发生较大位移颗粒的分布范围趋于集中,同时随着围压的增大试样内部形成明显的剪切带.无论砂土还是粘性土的PFC试样,随着围压的增加剪切带的形状趋于集中,而且剪切带宽度在减小.在围压很小时,试样内形成大的破坏区域,在围压较大时出现明显的线破坏区.这些规律基本与室内试验结果相似.     

惯性平台台体结构实验模态参数识别及灵敏度分析

惯性平台台体的动态特性直接决定着惯性仪表的工作精度和可靠性，模态分析是研究机械系统动态特性的主要方法之一。在概述了实验模态分析理论的基础上，建立了某型号平台台体结构的实验模型，对其进行了实验模态分析。通过对实验结果与有限元计算结果比较，验证了有限元结果较为准确；同时针对结构存在的问题，通过灵敏度分析对结构的动力修改提出了改进意见。     

颗粒材料内部滑动变形局部化的离散元模拟

颗粒材料在加载过程中表现出非常复杂的变形性质.在微观尺度上,单个颗粒的运动、单个孔隙胞元内的变形都是非均匀的,但也不是完全随机、没有规律可循的,而是呈现一定结构性的分布,如微带、剪切带等.本文用基于孔隙胞元的离散元方法对颗粒体进行双轴加载数值试验,模拟了以滑动变形表征的变形局部化现象.数值结果发现,对应加载过程中的不同...     

单自由度非线性随机参数振动系统的可靠性灵敏度分析

较好地解决了单自由度非线性随机参数振动系统的可靠性灵敏度问题.应用四阶矩技术和Edgeworth级数把未知的响应和状态函数的概率分布展开成标准的正态分布的表达式,进而确定了系统的可靠度和可靠性灵敏度,并放松了随机参数的分布概型和激励类型.     

Finite element and sensitivity analysis of thermally induced flow instabilities

This paper presents a finite element algorithm for the simulation of thermo‐hydrodynamic instabilities causing manufacturing defects in injection molding of plastic and metal powder. Mold‐filling parameters determine the flow pattern during filling, which in turn influences the quality of the final part. Insufficiently, well‐controlled operating conditions may generate inhomogeneities, empty spaces or unusable parts. An understanding of the flow behavior will enable manufacturers to reduce or even eliminate defects and improve their competitiveness. This work presents a rigorous study using numerical simulation and sensitivity analysis. The problem is modeled by the Navier–Stokes equations, the energy equation and a generalized Newtonian viscosity model. The solution algorithm is applied to a simple flow in a symmetrical gate geometry. This problem exhibits both symmetrical and non‐symmetrical solutions depending on the values taken by flow parameters. Under particular combinations of operating conditions, the flow was stable and symmetric, while some other combinations leading to large thermally induced viscosity gradients produce unstable and asymmetric flow. Based on the numerical results, a stability chart of the flow was established, identifying the boundaries between regions of stable and unstable flow in terms of the Graetz number (ratio of thermal conduction time to the convection time scale) and B, a dimensionless ratio indicating the sensitivity of viscosity to temperature changes. Sensitivities with respect to flow parameters are then computed using the continuous sensitivity equations method. We demonstrate that sensitivities are able to detect the transition between the stable and unstable flow regimes and correctly indicate how parameters should change in order to increase the stability of the flow. Copyright © 2009 John Wiley & Sons, Ltd.     

参数灵敏度分析的神经网络方法及其工程应用

在系统分析中,参数灵敏度分析不仅为判断各系统参数的重要性大小提供了依据,量化的灵敏度指标也是后续参数估计的前提。然而,在多效实际系统中,系统参数与系统状态间的显式函数关系不易得到,导致一阶灵敏度指标无法直接求取。简化的单因素分析方法亦存在模型粗糙、精度不高的缺点。本文研究采用人工神经网络的高精度泛化映射,通过少量样本的训练,建立复杂系统中多个系统参数与系统状态间的近似映射关系,继而推导得到统一的灵敏度计算列式。简单结构的神经网络方法和解析方法的对比计算显示了方法的有效性和可靠性。最后,应用该法对某斜拉桥结构的荷载参数和刚度参数进行了考查,得到一般性结论。     

Shape sensitivity analysis of elastic structures by multi-domain boundary element method

Based on the direct differentiation of boundary element equations to design variables, an algorithm of the boundary element method which is suitable for the shape sensitivity analysis of serial multi-domain elastic structures is built. By the algorithm, it is not necessary to form and solve the equations for the structure as a whole. Each domain can be solved separately. Two computational examples are presented to demonstrate the accuracy and versatility of the algorithm.This work has been partially supported by National Natural Science Foundation of China     

A multi-length scale sensitivity analysis for the control of texture-dependent properties in deformation processing

Material property evolution during processing is governed by the evolution of the underlying microstructure. We present an efficient technique for tailoring texture development and thus, optimizing properties in forming processes involving polycrystalline materials. The deformation process simulator allows simulation of texture formation using a continuum representation of the orientation distribution function. An efficient multi-scale sensitivity analysis technique is then introduced that allows computation of the sensitivity of microstructure field variables such as slip resistances and texture with respect to perturbations in macro-scale forming parameters such as forging rates, die shapes and preform shapes. These sensitivities are used within a gradient-based optimization framework for computational design of material property distribution during metal forming processes. Effectiveness of the developed computational scheme is demonstrated through computationally intensive examples that address control of properties such as Young’s modulus, strength and magnetic hysteresis loss in finished products.     

改进的SPH边界处理方法与土体大变形模拟

沙土滑坡往往会造成重大的人身财产损失,研究这类土体大变形问题对防灾工程具有指导意义。光滑粒子流体动力学SPH（Smoothed Particle Hydrodynamics）方法是一种拉格朗日型无网格粒子法,十分适用于模拟大变形问题。在SPH方法中,合适的边界处理方法一直是个难点,传统的边界虚粒子法或排斥力法较难模拟复杂边界。本文引入了一种能处理任意形状边界的方法——统一半解析壁面边界条件处理方法USAW（unified semi-analytical wall boundary conditions）,通过在控制方程中引入修正因子并保留边界面积分项来弥补边界缺失。为了更准确模拟问题域边界,提出无质量边界粒子的新概念。利用该方法成功模拟了土体滑坡算例,验证了方法的可靠性,并避免了边界零粒子层问题。通过数值模拟,分析了内摩擦角和黏聚力等土体物性参数对滑坡过程的影响。最后,应用该方法研究了滑坡冲击楔形体时的压力响应。     

砖石古塔模型振动台试验的离散元模拟分析

砖石古塔块体间黏结强度较低,受地震作用易产生裂缝后发生破坏,为研究砖石古塔在地震作用下开裂及裂缝发展机制,以玄奘塔1/8缩尺模型结构为对象,建立离散元模型,计算了地震波激励下结构的加速度及位移反应,与振动台试验结果进行对比,分析了塔体开裂破坏全过程。结果表明,数值计算所得结构的加速度及位移反应与振动台试验结果一致,当地震烈度较低时,两者顶层位移变化曲线基本一致;烈度提高后,塔体开裂导致结构动力响应的计算值与试验结果出现差异,但两者的变化规律相同。在地震波激励下,塔体首层先开裂,随地震烈度的提高延伸至中部塔层,块体第2层出现阶梯状错动,模型损伤过程与试验裂缝发展状况基本吻合。研究结果为砖石古塔地震损伤及破坏过程分析提供了参考。     

Design sensitivity and finite element analysis of free surface flows with application to optimal design of casting rigging systems

A novel, fully-analytical design sensitivity formulation for transient, turbulent, free surface flows is derived and implemented in the context of finite element analysis. The time-averaged, turbulent form of the Navier–Stokes equations are solved using a mixing length model, in conjunction with the volume of fluid (VOF) method to model the free surface movement. The design derivatives of these governing equations are computed and solved to find the analytical sensitivities of the fluid position, velocity and pressure fields with respect to shape design variables. The computational efficiency produced by evaluating the sensitivities analytically is demonstrated. The design of the runner and gating system of a simple block casting is presented as an example application for using sensitivity information in design. The analytical sensitivity routine is coupled to a numerical optimizer to yield an automated method for optimal design of the casting rigging system. The results produce runner shapes which eliminate mold-gas aspiration. © 1998 John Wiley & Sons, Ltd.     

Shape sensitivity analysis for non-differentiable performance measures in multiscale crack propagation simulation using regression hybrid method

In this paper, we present a regression hybrid method that calculates shape sensitivity coe?cients for multiscale crack propagation problems with performance measures that are non-differentiable in numerical implementation. These measures are crack propagation speed (or crack speed) defined at atomistic level obtained by solving coupled atomistic/continuum structures using the bridging scale method (BSM). The major contributions of this paper are: first, by analyzing the characteristics of the performance measures of crack speed in design space, this paper verifies for the first time that these measures are theoretically continuous and differentiable with respect to design variables, and as a result, the sensitivity coe?cients exist in theory; second, to overcome the non-differentiability of the performance measures in numerical computation due to the finite size of integration time step, this paper proposes a regression hybrid method that calculates the shape sensitivity coe?cients of crack speed through polynomial regression analysis based on the sensitivity of atomic responses, which is calculated through analytical shape design sensitivity analysis (DSA). And finally, the proposed method supports for 3D crack propagation problems with periodic boundary condition in one direction. A nano-beam example is used to demonstrate numerically the feasibility, accuracy, and e?ciency of the proposed method.