用于模拟颗粒增强复台材料破坏过程的梁-颗粒细观模型的实验验证

本文的主要目的是验证梁－颗粒细观模型在模拟混凝土和砂岩类颗粒增强复合材料连续破坏过程的有效性。文中首先介绍了梁－颗粒细观数值模型的基本原理，然后给出了由细钢丝粘结成的正方体试件的单轴抗压实验结果，最后用梁－颗粒细观模型对物理实验进行了数值模拟。研究结果表明，物理实验和数值模拟所得到的试件破坏模式和荷载－位移曲线，两者基本一致。从而初步证明了梁－颗粒细观数值模型是模拟颗粒增强复合材料破坏过程，以及解释其损伤机理的一个有效工具。     

用于模拟颗粒增强复合材料破坏过程的梁—颗粒细观模型的实验验证

本文的主要目的是验证梁－颗粒细观模型在模拟混凝土和砂岩类颗粒增强复合材料连续破坏过程的有效性，文中首先介绍了梁－颗粒细数值模型的基本原理，然后给出了由细钢丝粘结成的正方体试验的单轴抗压实验结果，最后用梁－颗粒细观模型对物理实验进行了数值模拟，研究结果表明，物理实验和数值模拟所得到的试件破坏模式和荷载－位移曲线，两者基本一致，从而初步证明了梁－颗粒细观数值模型及模拟颗粒增强复合材料破坏过程，以及解释     

动能弹侵彻素混凝土板的数值模拟

用二维梁-颗粒模型BPM2D(Beam-Particle Model in Two Dimensions)模拟了刚性弹体侵彻和贯穿素混凝土板的过程．梁-颗粒模型BPM2D是在离散元法基础上,结合有限元法开发的二维数值计算模型．在模型中用三种类型梁单元形成混凝土数值试样．每种类型梁单元的力学性质均按韦布尔(Weibull)分布随机赋值以模拟混凝土细观结构的非均匀性,同时梁单元的强度随应变率不同而变化．利用此模型分析了弹体侵彻下混凝土板的破坏过程,并给出不同弹体初始速度条件下各时刻混凝土和弹体内部速度场．通过将计算结果与实验数据及LS—DYNA程序模拟结果相比较,表明梁-颗粒模型可有效应用于计算和模拟脆性材料动态破坏问题．     

冲击载荷下脆性空心颗粒破碎机理

为考察脆性空心颗粒在冲击载荷作用下的应变率效应和破碎行为的细观机理,以粉煤灰漂珠为研究对象,基于低速冲击实验和有限元数值模拟,对比了典型空心颗粒材料在不同加载速率下的力学响应特性和细观压溃行为,阐释了材料宏观应变率效应产生的细观机理,获得以下结果。（1）在0.001～300 s?1应变率范围,漂珠颗粒的破碎率和Hardin破碎势平均提升了约21%和10%～30%,材料比吸能提升了50%～125%,比吸能的额外增加主要与动态颗粒滑移产生的摩擦耗能相关。颗粒平均尺寸较大的试样体现出更强的应变率效应。（2）初始压溃阶段的应力应变响应特征的数值模拟结果与实验结果较吻合,低速冲击下动态二次压溃现象产生的细观机理为动态颗粒滑移和压紧行为对加载速率的依赖性。(3) 数值模拟表明,冲击加载下产生相同应变时颗粒的损伤程度和范围大于准静态加载,这与实验所得破碎势随应变率增加的结果一致。对比低速冲击实验的相对破碎势分析和细观数值模拟结果可知,脆性颗粒堆积材料在动态冲击下表现出的宏观应变率效应主要归因于颗粒压溃行为的率敏感性和动态加载下颗粒破碎能量利用率的降低。     

梁-颗粒模型在混凝土侵彻问题中的应用

用二维梁-颗粒模型BPM2D（beam-particle model in two dimensions）模拟了混凝土侵彻问题。在模型中用3种类型梁单元形成混凝土数值试样。每种类型梁单元的力学性质均按韦伯分布随机赋值来模拟混凝土细观结构的非均匀性。利用此模型对在钢弹侵彻下混凝土圆板的破坏过程进行了数值模拟。通过将计算结果与实验数据及LS-DYNA程序模拟结果比较,表明梁-颗粒模型可有效地应用于计算和模拟材料从连续介质向非连续介质转变的动态破坏问题。     

混凝土拉伸断裂的细观数值分析

根据混凝土试件拉伸和三点弯曲的物理模型,用梁-颗粒模型BPM 2D(B eam-Particle M ode l)模拟了混凝土拉伸和三点弯曲试件微裂纹的萌生、扩展直至试件宏观破坏的全过程。在梁-颗粒模型中用三种类型梁单元形成混凝土细观数值模型,每种类型梁单元的力学性质均按韦伯(W e ibu ll)分布随机赋值以模拟混凝土细观结构的非均匀性。数值模拟结果给出了混凝土拉伸应力-应变曲线和三点弯曲载荷-位移曲线,以及混凝土试件破坏过程最大应力分布图和裂纹扩展图。数值模拟结果显示混凝土破坏过程实际上就是微裂纹萌生、扩展、贯通,直到宏观裂纹产生导致混凝土失稳断裂的过程。通过对数值模拟结果的分析,揭示出混凝土在拉伸条件下裂纹尖端的拉应力集中是裂纹扩展的动力,混凝土组成材料力学性质的非均匀性是造成裂纹扩展路径曲折的重要原因。     

利用连接尺度方法的颗粒材料破碎数值分析

基于已有的颗粒材料连接尺度方法(BSM)[1-2],发展了在细尺度上采用离散颗粒集合体模型与离散单元法(DEM)并引入了颗粒破碎模型,而在粗尺度上采用Cosserat连续体模型与有限单元法(FEM)的BSM。仅在有限局部区域内采用DEM从细观层次关注颗粒材料破碎现象,而在全域采用储存空间和花费时间较少的FEM,同时在粗细两个尺度采用不同的时间步长。讨论了颗粒材料发生破碎时,颗粒材料结构的承载能力与微结构的演变。数值算例结果说明了所提出可模拟破碎的BSM的可用性和优越性,以及颗粒破碎对颗粒材料微观力学行为的影响。     

岩石圆盘径向压缩破坏的Voronoi子块体单元DDA方法模拟

非连续变形分析(DDA)方法是计算离散可变形块体系统力学行为的数值计算方法，可通过子块体单元DDA方法模拟岩石的开裂破坏。考虑到Voronoi多边形颗粒与细观尺度下岩石矿物晶粒形态的相似性，提出一种基于随机圆的Voronoi颗粒单元模型生成方法；并通过完整及带预制裂纹岩石圆盘径向压缩破坏的模拟，验证岩石破裂问题Voronoi子块体单元DDA模拟方法的适用性。结果表明，当子块体单元数较小时，圆盘表现出更高的整体强度；随着子块体单元数的增大，起裂处位置更接近真实，开裂破坏路径更清晰；子块体单元数较大时不同倾角预制裂纹圆盘破坏的模拟结果与实验结果高度吻合，并能有效反映圆盘中心加工小孔对开裂破坏路径的影响。使用Voronoi子块体单元DDA方法能够有效模拟岩石的开裂破坏过程，为进一步开展基于Voronoi颗粒单元模型的岩石开裂破坏模拟创造了条件。     

散体系统冲击破碎的动力学分析

针对球形粒子组成的散体系统,基于离散单元法,将球形粒子离散成弹簧-球单元系统,给出了离散单元的运动方程,建立了离散单元之间的弹性力和接触力的计算模型,并用Mohr -Coulomb型破坏准则判断粒子的破碎。运用上述方法,对圆筒内由脆性材料组成的散体系统在冲击载荷下的挤压破碎过程进行了数值模拟;计算过程中,跟踪散体系统中每个粒子在不同时刻的破碎情况;分析了散体系统冲击破碎过程数值模拟结果的主要影响因素。结果显示:数值模拟过程中需综合考虑计算精度和计算时间之间的平衡;相同的计算条件下,颗粒的初始堆积方式不同,计算得到的散体系统的破碎程度不同。     

砂土地基锚板基础抗拔承载力PFC数值分析

在模型试验的基础上,采用PFC离散单元法对条形锚板基础在中密砂土地基中的抗拔性能进行了数值分析。数值模拟采用簇颗粒单元来模拟砂粒的不规则形状,颗粒级配根据模型试验福建标准砂的级配按照相似级配法生成,细观参数根据数值双轴试验确定。水平锚板数值模拟结果与模型试验结果基本一致。与模型试验结果相比,颗粒流数值模拟能得到颗粒间接触力链的分布及其演化规律,能从细观角度来探明宏观抗拔承载力特性的演化机理。在此基础上对倾斜锚板上拔过程进行了模拟,分析了锚板前后砂粒的运动趋势以及接触力链的演化规律,并与已有承载力结果进行了对比分析。     

Periodic Solutions of an Autonomous Reaction-Diffusion System—A Model System

We describe the dynamics of an autonomous system of two reaction-diffusion equations which can be looked at as a model system for more general reaction-diffusion systems. In our system all solutions tend to zero or to (finitely many) periodic orbits which can be fully described—including their stability properties. Furthermore, we construct invariant sets for the period map and show how a new invariant called torsion number is related to our model system.     

On the viscosity and thermal conduction of fluids with multivalued internal energy

This work is concerned with an extension of the classical compressible Euler model of fluid dynamics in which the fluid internal energy is a measure-valued quantity. This model can be derived from the hydrodynamic limit of a kinetic model involving a specific class of collision operators. In the present paper, we investigate diffusive corrections of this fluid dynamical model derived from a Chapman–Enskog expansion of the kinetic model, in the case where the collision time depends on the particle energy in the fluid frame. We show that the closure relations for the stress tensor and heat flux vector differ from their expression in the usual Navier–Stokes model. We argue why such a feature could be used as a tool towards an understanding of fluid turbulence from kinetic theory.     

The Optimal Shape of Riblets in the Viscous Sublayer

Our aim is to find the optimal shape of periodically distributed microstructures on surfaces of swimming bodies in order to reduce their drag. The model describes the flow in the viscous sublayer of the boundary layer of a turbulent flow. The microscopic optimization problem is reduced applying homogenization. In the reduced so-called macroscopic optimization problem we minimize the Navier constant subject to the boundary layer equations which are solved in a very small part of the original domain. Under the assumptions that the microstructures can be represented as smooth functions the sensitivity can be determined analytically. The optimization problem is then solved by a sensitivity based method (steepest descent with optimal step size) and the state equations are solved in each iteration with an external software. Our reduced model is validated by comparing the results from the homogenized model with those obtained by simulating the whole rough channel. An improved shape is found and a drag reduction up to 10% can be shown.     

The Role of Biofilm Growth in Bacteria-Facilitated Contaminant Transport in Porous Media

Groundwater contaminants adhered to colloid surfaces may migrate to greater distances than predicted by using the conventional advective-dispersive transport equation. Introduction of exogenous bacteria in a bioremediation operation or mobilization of indigenous bacteria in groundwater aquifers can enhance the transport of contaminants in groundwater by reducing the retardation effects. Because of their colloidal size and favorable surface conditions, bacteria can be efficient contaminant carriers. In cases where contaminants have low mobility because of their high partition with aquifer solids, facilitated contaminant transport by mobile bacteria can create high contaminant fluxes. In this paper, we developed a methodology to describe the bacteria-facilitated contaminant transport in a subsurface environment using the biofilm theory. The model is based on mass balance equations for bacteria and contaminant. The contaminant is utilized as a substrate for bacterial growth. Bacteria are attached to solid surfaces as a biofilm. We investigate the role of the contaminant adsorption on both biofilm and mobile bacteria on groundwater contaminant transport. Also, the effect of bacterial injection on the contaminant transport is evaluated in the presence of indigenous bacteria in porous media. The model was solved numerically and validated by experimental data reported in the literature. Sensitivity analyses were conducted to deduce the effect of critical model parameters. Results show that biofilm grows rapidly near the top of the column where the bacteria and contaminant are injected, and is detached by increasing fluid shear stress and re-attach downstream. The adsorption of contaminant on bacterial surfaces reduces contaminant mobility remarkably in the presence of a biofilm. The contaminant concentration decreases significantly along the biofilm when contaminant partition into bacteria. Bacterial injection and migration in subsurface environments can be important in bioremediation operations regardless of the presence of indigenous bacteria.     

On twinning and domain switching in ferroelectric Pb(Zr1−xTix)O3—part I: twins and domain walls

We study the electromechanical behavior of lead zirconate titanate ferroelectric ceramics (PZT), by means of a three-dimensional continuum model for deformable ferroelectric bodies in their polar phase characterized by spontaneous polarization and strain. Spontaneous polarization and strain organize into a domain structure which minimizes electrostatic and elastic energies and which can be modified by the application of electromechanical loads. Such process, which is called “domain switching”, is associated with electrical and mechanical hysteresis and can be studied as a minimization problem for a functional which reminds the micromagnetic energy of deformable ferromagnetics. In this paper, which is the first of two, we deal with the electromechanical model and related constitutive assumptions, as well as with the analysis of domain structure in PZT. In particular, following the discover of a new monoclinic phase in PZT carried by Noheda and co-workers, we analyze twinning between spontaneous strain at the various phase boundaries and show that both non-generic, non-conventional twins and finely-twinned laminates are possible, and also that the presence of a monoclinic phase may explain PZT exceptional properties.     

Inversion of Masing models via continuous Iwan systems

It is shown that for any material or structural model expressible as a Masing model, there exists a unique parallel-series (displacement-based) Iwan system that characterizes that model as a function of displacement history. This poses advantages both in terms of more convenient force evaluation in arbitrary deformation histories as well as in terms of model inversion. Characterization as an Iwan system is demonstrated through the inversion of the Ramberg-Osgood model, a force(stress)-based material model that is not explicitly invertible. An implication of the inversion process is that direct, rigorous comparisons of different Masing models, regardless of the ability to invert their constitutive relationship, can be achieved through the comparison of their associated Iwan distribution densities.     

Numerical simulation of Gerotor pumps considering rotor micro-motions

Gerotor units are widely used in low-pressure (up to 30 bar) fluid power applications, injection as well as lubrication systems, due to their compact package and low cost. Their performance in terms of volumetric efficiency, flow pulsations, internal pressure peaks or localized cavitation depends on many parameters, such as the rotors’ profiles and the manufacturing tolerances. This paper proposes a multi-domain simulation approach for the numerical analysis of the performance of Gerotor units. Characterized by simulation swiftness, the model can be used for virtual prototyping of units considering the actual geometry of the rotors, their geometrical tolerances and the properties of the working fluid. The approach is based on the coupling of different models: a numerical geometric model evaluates the instantaneous volumes and flow areas inside the unit; a lumped parameter fluid dynamic model describes the displacing process of the tooth space volumes; finally, a mechanical model evaluates the internal micro-motions of the rotors axes according to their tolerances. In this way, the model determines the actual loading of the rotors, considering also the actual location of the points of contact. After presenting the approach, the paper describes the potentials of the proposed method with reference to a particular Gerotor pump design. Specific experiments were performed within this research to permit a detailed model validation, and comparisons in terms of significant steady-state as well as transient pressure and flow features are presented. The approach used in the current paper can be considered valuable when studying the impact of real-life technological clearances on the fluid-dynamic performance of the pump.     

相变温控导热增强的多孔金属梯度优化设计

高孔隙率金属多孔材料比表面积大、导热性能好且掺混能力强,是理想的相变换热导热增强体材料。增材制造能够精准制备几何高度复杂的微结构,为多孔金属任意梯度设计提供可能。为实现更高的相变换热性能,建立了多孔金属相变温控导热增强的梯度优化设计模型。该优化模型以孔隙率分布为设计变量,以多孔金属用量为约束,以关键位置的温度最低为设计目标,基于考虑相变过程的多孔介质两方程模型为分析方法,通过遗传算法对优化模型进行求解。通过与实验结果的对比,验证了分析方法的有效性。两个具体算例证实了梯度设计能够大幅度提高多孔金属介质导热增强的相变温控性能。     

Theoretical framework and experimental procedure for modelling mesoscopic volume fraction stochastic fluctuations in fiber reinforced composites

Many engineering materials exhibit fluctuations and uncertainties on their macroscopic mechanical properties. This randomness results from random fluctuations observed at a lower scale, especially at the meso-scale where microstructural uncertainties generally occur. In the present paper, we first propose a complete theoretical stochastic framework (that is, a relevant probabilistic model as well as a non-intrusive stochastic solver) in which the volume fraction at the microscale is modelled as a random field whose statistical reduction is performed using a Karhunen–Loeve expansion. Then, an experimental procedure dedicated to the identification of the parameters involved in the probabilistic model is presented and relies on a non-destructive ultrasonic method. The combination of the experimental results with a micromechanical analysis provides realizations of the volume fraction random field. In particular, it is shown that the volume fraction can be modelled by a homogeneous random field whose spatial correlation lengths are determined and may provide conditions on the size of the meso-volumes to be considered.     

Development of an efficient numerical model for hail impact simulation based on experimental data obtained from pressure sensitive film

Aircraft are subjects to a number of unpredictable loadings that can seriously affect their performance. In the spirit of ever increasing the safety of passengers, hail impact has been studied. This paper shows the progress that has been made using pressure sensitive film to measure the hail impact event. Moreover, the smooth particle hydrodynamics (SPH) method in LS-DYNA is used to create a numerical model in order to validate the numerical hail model so that it can be used in future advanced simulations of hail impact on components of aircraft. Results show that the SPH method can be effectively used to create a numerical hail model and that pressure sensitive film is a simple and inexpensive tool to capture the experimental data.