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Homogenization Techniques (homogenization + techniques)
Selected AbstractsDiffusion of strongly sorbed solutes in soil: a dual-porosity model allowing for slow access to sorption sites and time-dependent sorption reactionsEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2010M. Ptashnyk We use homogenization techniques to derive a dual (or double) porosity model of solute diffusion and reaction in soil, allowing for slow access to sorption sites within micro-aggregates and time-dependent sorption reactions. We give a means for determining the conditions in which micro-scale concentration gradients affect macro-scale gradients and fluxes. We present equations for a unit volume of soil represented as a series of uniformly-spaced, porous spherical particles, containing and surrounded by solution through which solutes diffuse. The methods we use can, in principle, be applied to more complex geometries. We compare the model's predictions with those of the equivalent single porosity model for commonly used boundary conditions. We show that failure to allow for slow access to reaction sites can lead to seriously erroneous results. Slow access has the effect of decreasing the sorption of solute into soil from a source or desorption from soil to a sink. As a result of slow access, the diffusion coefficients of strongly-sorbed solutes measured at the macro-scale will be time-dependent and will depend on the method of measurement. We also show that slow access is more often likely to limit macro-scale diffusion than rates of slow chemical reactions per se. In principle, the unimportance of slow reactions except at periods longer than several weeks of diffusion simplifies modelling because, if slow access is correctly allowed for, sorption can be described with equilibrium relations with an understanding of speciation and rapid sorption-desorption reactions. [source] Micromechanical models of the strength of a sandstoneINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 3 2010L. Dormieux Abstract In this paper, the strength of a sandstone is determined from a micromechanical approach. The microstructure of the rock is described as a porous polycrystal. A von Mises criterion is used for the strength of the solid grains. The grains are surrounded by Mohr,Coulomb interfaces describing the cement phase. The macroscopic strength of the polycrystal is determined by means of nonlinear homogenization techniques. The failure mechanism of the grains is assumed to be ductile. It is combined with a failure mechanism of the interfaces, which is successively assumed to be ductile and brittle. The theoretical predictions are then compared with the experimental data. Copyright © 2009 John Wiley & Sons, Ltd. [source] On the capillary stress tensor in wet granular materialsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 10 2009L. Scholtès Abstract This paper presents a micromechanical study of unsaturated granular media in the pendular regime, based on numerical experiments using the discrete element method, compared with a microstructural elastoplastic model. Water effects are taken into account by adding capillary menisci at contacts and their consequences in terms of force and water volume are studied. Simulations of triaxial compression tests are used to investigate both macro and micro-effects of a partial saturation. The results provided by the two methods appear to be in good agreement, reproducing the major trends of a partially saturated granular assembly, such as the increase in the shear strength and the hardening with suction. Moreover, a capillary stress tensor is exhibited from capillary forces by using homogenization techniques. Both macroscopic and microscopic considerations emphasize an induced anisotropy of the capillary stress tensor in relation with the pore fluid distribution inside the material. Insofar as the tensorial nature of this fluid fabric implies shear effects on the solid phase associated with suction, a comparison has been made with the standard equivalent pore pressure assumption. It is shown that water effects induce microstructural phenomena that cannot be considered at the macro level, particularly when dealing with material history. Thus, the study points out that unsaturated soil stress definitions should include, besides the macroscopic stresses such as the total stress, the microscopic interparticle stresses such as the ones resulting from capillary forces, in order to interpret more precisely the implications of the pore fluid on the mechanical behaviour of granular materials. Copyright © 2009 John Wiley & Sons, Ltd. [source] Multiscale modeling of impact on heterogeneous viscoelastic solids containing evolving microcracksINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 4 2010Flavio V. Souza Abstract Multiscale computational techniques play a major role in solving problems related to viscoelastic composites due to the complexities inherent to these materials. In this paper, a numerical procedure for multiscale modeling of impact on heterogeneous viscoelastic solids containing evolving microcracks is proposed in which the (global scale) homogenized viscoelastic incremental constitutive equations have the same form as the local-scale viscoelastic incremental constitutive equations, but the homogenized tangent constitutive tensor and the homogenized incremental history-dependent stress tensor at the global scale depend on the amount of damage accumulated at the local scale. Furthermore, the developed technique allows the computation of the full anisotropic incremental constitutive tensor of viscoelastic solids containing evolving cracks (and other kinds of heterogeneities) by solving the micromechanical problem only once at each material point and each time step. The procedure is basically developed by relating the local-scale displacement field to the global-scale strain tensor and using first-order homogenization techniques. The finite element formulation is developed and some example problems are presented in order to verify the approach and demonstrate the model capabilities. Copyright © 2009 John Wiley & Sons, Ltd. [source] Voxel-based meshing and unit-cell analysis of textile compositesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 7 2003Hyung Joo Kim Abstract Unit-cell homogenization techniques are frequently used together with the finite element method to compute effective mechanical properties for a wide range of different composites and heterogeneous materials systems. For systems with very complicated material arrangements, mesh generation can be a considerable obstacle to usage of these techniques. In this work, pixel-based (2D) and voxel-based (3D) meshing concepts borrowed from image processing are thus developed and employed to construct the finite element models used in computing the micro-scale stress and strain fields in the composite. The potential advantage of these techniques is that generation of unit-cell models can be automated, thus requiring far less human time than traditional finite element models. Essential ideas and algorithms for implementation of proposed techniques are presented. In addition, a new error estimator based on sensitivity of virtual strain energy to mesh refinement is presented and applied. The computational costs and rate of convergence for the proposed methods are presented for three different mesh-refinement algorithms: uniform refinement; selective refinement based on material boundary resolution; and adaptive refinement based on error estimation. Copyright © 2003 John Wiley & Sons, Ltd. [source] Evaluation of different RNA extraction methods for small quantities of plant tissue: Combined effects of reagent type and homogenization procedure on RNA quality-integrity and yieldPHYSIOLOGIA PLANTARUM, Issue 1 2006Mary Portillo Highly sensitive techniques for transcriptome analysis, such as microarrays, complementary DNA-amplified fragment length polymorphisms (cDNA-AFLPs), and others currently used in functional genomics require a high RNA quality and integrity, as well as reproducibility among extractions of replicates from the same tissue. There are, however, few technical papers comparing different homogenization techniques and reagents to extract RNA from small quantities of plant tissue. We extracted RNA from tomato seedlings with the three different commercial reagents TRIZOL LS®, TRIZOL®, and TRI Reagent® in combination with pulverization, homogenization-maceration in a mortar, and homogenization with mild vibration plus glass beads, and evaluated total RNA integrity-quality and yield. Pulverization under liquid nitrogen combined with TRIZOL LS® as extraction reagent and homogenization-maceration in mortar with TRI Reagent®, are the procedures that rendered higher RNA yield, integrity and quality, as well as reproducibility among independent RNA extractions. In contrast, short mild vibration pulses (4500 r.p.m. for 5 s) mixed with glass beads, rendered low extraction efficiency and caused, in most cases, partial RNA degradation. [source] | ||||||||||