基于ANSYS软件对SiCp/A1(ZL102)复合材料铸造渗流过程多孔介质结构的有限元处理

采用统计平均的方法描述多孔介质微观结构的影响,根据多孔介质的连续介质模型及有限元方法对多孔介质内的铝液的渗流行为进行数值模拟,给出了可视化的瞬态温度场分布,并且初步预测了不同时刻的渗流有效高度.     

恒压下树脂在双尺度多孔介质中的流动特性

基于复合材料液态模塑(LCM)工艺过程中存在半饱和区域的实验现象以及对预制体双尺度效应的逐步认识, 一些学者提出用沉浸模型来研究双尺度多孔介质的不饱和流动。通过体积均匀化方法描述了双尺度多孔介质复合材料液态模塑工艺模型的特征, 得到含有沉浸项的双尺度多孔介质的质量守恒方程, 并采用有限元法对方程进行数值求解, 通过具体算例计算了考虑双尺度效应时恒压树脂注射下不同时段的压力分布状态, 得到树脂在填充过程中流动前沿半饱和区域从出现到消失的过程, 采用不同注射压力进行模拟并比较。结果表明, 与单尺度多孔介质模型不同, 双尺度多孔介质模型更能反映实际树脂填充过程中出现的半饱和区域现象。     

多孔介质材料在低温下的传热特性实验研究

对多孔介质材料在低温下的传热特性进行了实验研究,在填充液氮以后其低温维持时间明显增加,主要原因是由于多孔材料的参与改变了传热特性;采用连续介质管束模型,用有限元分析软件对其整体温度场分布进行了数值模拟计算,计算结果和实验数据吻合。     

基于分形理论的开孔聚氨酯泡沫等效导热系数研究

开孔聚氨酯泡沫保温材料是一种典型的多孔介质。采用分形理论描述开孔聚氨酯泡沫材料的微尺度空间结构,建立了简化单元体模型,提出了计算其有效导热系数的分形模型,并导出了气相和固相热传导计算公式、热辐射等效导热系数计算公式、材料总有效导热系数计算公式。模型计算值与实验测量值比较具有较好的一致性,同时总结了多孔介质材料绝热性能的主要影响因素。该分析方法对新型绝热材料的研制和绝热性能的提高具有实用价值。     

非饱和多孔介质动力应变局部变化数值模拟

在已有研究工作基础上对非饱和多孔介质应变局部化问题进行研究,给出非饱和多孔介质的分析控制方程,其中饱和度与毛细压力关系由实验给出。采取适用于非饱和砂土的改进的广义塑性本构模型对应局部变化过程进行数值模拟,给出了试件应变局部化发展过程以及孔隙压力的变化规律。对初始饱和土中所产生非饱和剪切带进行计算的结果表明,采用非饱和模型较饱和模型将获得更为合理的结果。     

多孔介质的一种流-固耦合动态边界理论

基于Biot理论,推导了考虑渗流作用的可变形多孔介质流-固耦合问题的基本方程,建立了本问题的渗流模型,给出了所考虑问题的流体动力弥散分布概率及系数表达式,进一步建立了多孔介质中微压液体位移场模型,讨论了流体动力弥散因素对多孔介质边界的影响,建立了描述多孔介质的动态边界方程,并分别对所建立的四种边界模拟了动态结果及算例。     

多相非饱和多重孔隙介质的有效应力定律

多孔介质的有效应力定律广泛应用于流固耦合变形分析问题。该文考虑孔隙的重数、孔隙流体的相数、各向异性、非饱和、基质吸力等条件,提出了广义多相非饱和多重孔隙介质的有效应力定律。在固体相及各流体相线弹性变形的假设下,首先通过应力状态分解、边界条件叠加方法,得到了不考虑基质吸力的多相等效饱和各向异性多重孔隙介质的有效应力。考虑到非饱和多孔介质中两相界面张力引起的基质吸力,在线弹性变形基础上,叠加了基质吸力引起的变形部分,推导得到非饱和多孔介质的有效应力定律的一般形式。将所得公式根据实际需要进行简化处理,可以得到目前常用的有效应力定律的表达形式,充分说明了该文所得结论的合理性。     

热流作用的多孔介质突变面梯度分布模型

综合考虑多孔介质中热载荷及热流,建立了多孔介质自由理论力学模型,研究表明,热突变界面存在非明细的区域,而当热载荷的强度改变时,界面形态保持不变。进一步讨论了多孔介质中出现突变界面的各种梯度形态并给出了其力学意义。     

基于RMV的CaSiO_3多孔介质热质传递机理研究

基于粗宏观表征体元RMV,建立了CaSiO3多孔介质"三箱"模型,即:黑箱模型、灰箱模型和白箱模型。推导了"三箱"传热物理模型下,导热系数计算的数学模型。数值计算了101.325 k Pa、400 K的过热水蒸气在CaSiO3粗宏观表征体元中传递时的导热系数。数值模拟了孔隙率为30%和70%的CaSiO3灰箱传热模型温度场分布。结果表明,导热系数随孔隙率增大而减小,导热系数随孔隙通道分布系数和迂曲度增大而增大。研究结果对CaSiO3多孔介质及其它功能材料多孔介质热质传递机理研究具有重要的理论价值和工程应用意义。     

囊式混合介质隔振器的动力学特性试验

为了摆脱气囊的气体泄漏问题,在气囊中填充弹性单元体和液体（混合介质）来替代气体。其带来的优点是消除了泄漏现象,可减少对监测和充气设备需求,从而大幅度降低了隔振系统成本。将充填混合介质的囊式隔振器在试验台架上进行正弦扫频激励的动力学特性试验,试验结果表明：囊式混合介质隔振器的振级落差随填充单元体数量的增加而升高;频响函数曲线表现出不同于气囊隔振器的刚度特性,其刚度特性受到所填充的弹性单元体和液体的力学特性影响,随内压的变化表现特性复杂。     

Transient effect on the constriction resistance between spheres

 Systems of contacting spheres are common in engineering applications where the heat transfer analysis can be quite cumbersome due to the transient behavior and the complex geometrical arrangement. As a result, most of the previous works, in this area, have adopted the porous media approach. However, this approach requires the length scale of the representative cell to be roughly three orders of magnitude larger than the size of the spheres. Constriction resistance relations could be useful in accurately computing the temperature distribution within systems of contacting spheres, however many of the requisite relations are not available. Thus, the objective of this study is to develop these relations. In this study, the transient, three-dimensional conduction equation was solved using a finite volume scheme and a non-uniform grid. From the resulting temperature distributions, the steady-state and transient constriction resistance of one-sphere and two-sphere systems were computed and correlated. The results also showed, for the first time, the critical parameters below which the transient variations must be considered. Received 27 September 1999     

Fully coupled hydromechanical multiscale model with microdynamic effects

In this paper, a multiscale finite element framework is developed based on the first‐order homogenization method for fully coupled saturated porous media using an extension of the Hill‐Mandel theory in the presence of microdynamic effects. The multiscale method is employed for the consolidation problem of a 2‐dimensional saturated soil medium generated from the periodic arrangement of circular particles embedded in a square matrix, which is compared with the direct numerical simulation method. The effects of various issues, including the boundary conditions, size effects, particle arrangements, and the integral domain constraints for the microscale boundary value problem, are numerically investigated to illustrate the performance of a representative volume element in the proposed computational homogenization method of fully coupled saturated porous media. This study is aimed to clarify the effect of scale separation and size dependence, and to introduce characteristics of a proper representative volume element in multiscale modeling of saturated porous media.     

A two-site mean field model of discontinuous dynamic recrystallization

The paper describes a new model of discontinuous dynamic recrystallization (DDRX) which can operate in constant or variable thermomechanical conditions. The model considers the elementary physical phenomena at the grain scale such as strain hardening, recovery, grain boundary migration, and nucleation. The microstructure is represented through a set of representative grains defined by their size and dislocation density. It is linked to a constitutive law giving access to the polycrystal flow stress. Interaction between representative grains and the surrounding material is idealized using a two-site approach whereby two homogeneous equivalent media with different dislocation densities are considered. Topological information is incorporated into the model by prescribing the relative weight of these two equivalent media as a function of their volume fractions. This procedure allows accounting for the well-known necklace structures. The model is applied to the prediction of DDRX in 304 L stainless steel, with parameters identified using an inverse methodology based on a genetic algorithm. Results show good agreement with experimental data at different temperatures and strain rates, predicting recrystallization kinetics, recrystallized grain size and stress-strain curve. Parameters identified with one initial grain size lead to accurate results for another initial grain size without introducing any additional parameter.     

Flow modeling of linear and nonlinear fluids in two scale fibrous fabrics

The fundamental macroscopic material property needed to quantify the flow in a fibrous medium viewed as a porous medium is the permeability. Composite processing models require the permeability as input data to predict flow patterns and pressure fields. As permeability reflects both the magnitude and anisotropy of the fluid/fiber resistance, efficient numerical techniques are needed to solve linear and nonlinear homogenization problems online during the flow simulation. In a previous work the expressions of macroscopic permeability were derived in a double-scale porosity medium for both Newtonian and rheo-thinning resins. In the linear case only a microscopic calculation on a representative volume is required, implying as many microscopic calculations as representative microscopic volumes exist in the whole fibrous structure. In the non-linear case, and even when the porous microstructure can be described by a unique representative volume, microscopic calculation must be carried out many times because the microscale resin viscosity depends on the macroscopic velocity, which in turn depends on the permeability that results from a microscopic calculation. Thus, a nonlinear multi-scale problem results. In this paper an original and efficient offline-online procedure is proposed for the efficient solution of nonlinear flow problems in porous media.     

Flow modelling of quasi-Newtonian fluids in two-scale fibrous fabrics

Permeability is the fundamental macroscopic material property needed to quantify the flow in a fibrous medium viewed as a porous medium. Composite processing models require the permeability as input data to predict flow patterns and pressure fields. In a previous work, the expressions of macroscopic permeability were derived in a double-scale porosity medium for both Newtonian and generalized Newtonian (shear-thinning) resins. In the linear case, only a microscopic calculation on a representative volume is required, implying as many microscopic calculations as there are representative microscopic volumes in the whole fibrous structure. In the non-linear case, and even when the porous microstructure can be described by a unique representative volume, a large number of microscopic calculations must be carried out as the microscale resin viscosity depends on the macroscopic velocity, which in turn depends on the permeability that results from a microscopic calculation. An original and efficient offline-online procedure was proposed for the solution of non-linear flow problems related to generalized Newtonian fluids in porous media. In this paper, this procedure is generalized to quasi-Newtonian fluids in order to evaluate the effect of extensional viscosity on the resulting upscaled permeability. This work constitutes a natural step forward in the definition of equivalent saturated permeabilities for linear and non-linear fluids.     

A three‐scale compressible microsphere model for hyperelastic materials

This paper presents a seminumerical homogenization framework for porous hyperelastic materials that is open for any hyperelastic microresponse. The conventional analytical homogenization schemes do apply to a limited number of elementary hyperelastic constitutive models. Within this context, we propose a general numerical scheme based on the homogenization of a spherical cavity in an incompressible unit hyperelastic solid sphere, which is denoted as the mesoscopic representative volume element (mRVE). The approach is applicable to any hyperelastic micromechanical response. The deformation field in the sphere is approximated via nonaffine kinematics proposed by Hou and Abeyaratne (JMPS 40:571‐592,1992). Symmetric displacement boundary conditions driven by the principal stretches of the deformation gradient are applied on the outer boundary of the mRVE. The macroscopic quantities, eg, stress and moduli expressions, are obtained by analytically derived pointwise geometric transformations. The macroscopic expressions are then computed numerically through quadrature rules applied in the radial and surface directions of the sphere. A three‐scale compressible microsphere model is derived from the developed seminumerical homogenization framework where the micro‐meso transition is based on the nonaffine microsphere model at every point of the mRVE. The numerical scheme developed for the derivation of macroscopic homogenized stresses and moduli terms as well as the modeling capability of the three‐scale microsphere model is investigated through representative boundary value problems.     

The relaxation effects of a saturated porous media using the generalized thermoviscoelasticity theory

In this paper, a model of the generalized thermoviscoelasticity for a saturated porous media is used. Taking the rheological properties of the volume into consideration, it is developed on the basis of the thermo-hydro-elastodynamic model proposed by the authors. The formulation is applied to the generalized porous thermoviscoelasticity theories: coupling theory, Lord-Shulman theory and Green-Lindsay theory. The general characteristic equations for arbitrary coordinate system are derived using a semi-analytical approach in the Laplace domain. Exact solutions of the saturated porous thermoviscoelastic media, with a cylindrical cavity that is subjected to a time dependent thermal load, are obtained in the absence of heat sources. Numerical results are illustrated graphically employing numerical method for inverting the Laplace transform. Comparisons are made with the results predicted by these theories and the classical theory. In addition, the influence of rheological parameters on the thermoviscoelastic property is analyzed.     

Simulation of groundwater flow in unconfined aquifer using meshfree point collocation method

For appropriate management of available groundwater, the flow behavior in the porous media has to be analyzed. The complex problem of groundwater flow can be studied by solving the governing equations analytically or by using numerical methods. As the analytical solutions are available only for simple idealized cases, numerical methods such as finite difference method (FDM) and finite element method (FEM) are generally used for field problems. Meshfree (MFree) method is an alternative numerical approach to solve complex groundwater problems in simple manner. MFree method eliminates the drawback of meshing and remeshing as in FDM and FEM which can translate to substantial cost and time savings in modeling. In this paper, a model using MFree point collocation method (PCM) with multi-quadric radial basis function (MQ-RBF) is proposed for 2D groundwater flow simulation. The accuracy of the developed model is verified with available analytical solution in literature. The developed model is applied initially for a hypothetical problem and further for a field problem to compute head distribution. The PCM model results for the hypothetical problem are compared with FEM simulations while that of field problem are compared with boundary element based model results. The PCM model results are found to be satisfactory showing the applicability of the present approach.