温度对季节性冻土水分迁移的影响研究

寒区季节性冻土冻胀性质对工程实际影响很大。为了了解温度对水分迁移现象的影响,本文通过地温测试仪对野外不同深度处的土层温度进行测量,并在不同时间相应深度取土样,测其含水率,通过比较不同时间不同深度处的含水率的变化情况来分析温度变化对水分迁移现象的影响。在气温回升之前,当地表温度降低时,温度随深度的降低而升高; 随着地表温度不断降低,各深度处的温度也不断下降,温度梯度增加,各深度处地层的含水率变化大,即温度梯度的增加促进了季节性冻土区水分迁移现象的发生。     

Hysteretic Behaviour and Moisture Buffering of Hemp Concrete

Moisture transfer in hygroscopic building materials affects the indoor air quality by exchanging moisture and buffering the ambient relative humidity variations. The paper deals with experimental and numerical study on hysteretic sorption behaviour of the hemp concrete sorption process. Experimental intermediate scanning curves of hemp concrete are measured and used to compare two hysteresis models, Huang’s model and Carmeliet’s model. An original method is achieved to fit the numerical results on the experimental ones leading to the identification of the main desorption curve. The most relevant model, Huang’s model, is implemented in a heat and moisture transfer model based on Künzel formalism. The transient hydric response of hemp concrete submitted to cyclic hydric loadings is investigated and compared to experimental results issued from the literature. These investigations show the relevance to consider the hysteresis phenomenon into the model. Then, the influence of initial conditions is discussed. The results point out that transient response of hemp concrete strongly depends on the initial hydric state (initial moisture content as well as initial relative humidity).     

Numerical simulation of coupled heat and mass transfer in wood dried at high temperature

The mutual effect between heat and mass transfer is investigated for wood dried at high temperature. A numerical model of coupled heat and mass transfer under the effect of the pressure gradient is presented. Based on the macroscopic viewpoint of continuum mechanics, the mathematical model with three independent variables (temperature, moisture content and gas pressure) is constructed. Mass transfer in the pores involves a diffusional flow driven by the gradient of moisture content, convectional flow of gaseous mixture governed by the gradient of gas pressure, the Soret effect and phase change of water. Energy gain or loss due to phase change of water is taken as the heat source. Numerical methods, the finite element method and the finite difference method are used to discretize the spatial and time dimension, respectively. A direct iteration method to solve the nonlinear problem without direct evaluation of the tangential matrix is introduced. The local convergence condition based on the contraction–mapping principle is discussed. The mathematical model is applied to a 3-D wood board dried at high temperature with the Neumann boundary conditions for both temperature and moisture content, and the Dirichlet boundary conditions for gas pressure.     

Mechano-sorptive creep under compressive loading – A micromechanical model

The creep of paper is accelerated by moisture cycling, an effect known as mechano-sorptive creep. It has also been observed that the mechano-sorptive effects are larger in compression than in tension. In this paper a simplified network model for mechano-sorptive creep is presented. It is assumed that the anisotropic hygroexpansion of the fibres leads to large stresses at the fibre–fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys constitutive laws that are non-linear in stress. Geometrical fibre effects are included in the model in order to capture experimental observations of the differences between paper loaded in tension and compression. Theoretical predictions based on the developed model are compared to experimental results for paper both under tensile and compressive loading at varying moisture content. The important features in the experiments are captured by the model, i.e. the creep is accelerated by the moisture cycling and the mechano-sorptive effects are larger in compression than in tension.     

Nonisothermal moisture transport in hygroscopic building materials: modeling for the determination of moisture transport coefficients

A mathematical model for calculating the nonisothermal moisture transfer in building materials is presented in the article. The coupled heat and moisture transfer problem was modeled. Vapor content and temperature were chosen as principal driving potentials. The coupled equations were solved by an analytical method, which consists of applying the Laplace transform technique and the Transfer Function Method. A new experimental methodology for determining the temperature gradient coefficient for building materials was also proposed. Both the moisture diffusion coefficient and the temperature gradient coefficient for building material were experimentally evaluated. Using the measured moisture transport coefficients, the temperature and vapor content distribution inside building materials were predicted by the new model. The results were compared with experimental data. A good agreement was obtained.     

非饱和黏性土抗剪强度的温度效应试验研究

为了系统地了解温度对黏性土工程性质的影响,本文采用南京地区三种不同矿物成分的黏性土,制成不同含水量和干密度的试样,在5~45℃条件下,开展了抗剪强度试验,获得了三种土的非饱和重塑试样的抗剪强度与温度的关系。试验结果表明:黏性土的黏聚力随温度升高呈线性变化; 亲水矿物含量较高的黏性土抗剪强度对温度变化较敏感,黏聚力随着温度的升高而降低,表现为强度的热软化现象; 亲水矿物含量较低的黏性土,当含水量较低(w≤17%)时,表现为强度的热硬化现象,当含水量较高(w≥22%)时,表现为强度的热软化现象,且干密度越高的试样,强度的温度效应越明显。论文还分析了非饱和黏性土抗剪强度的热硬化和热软化的内在机理。     

Thermal vision,moisture content,and vegetation in the context of off-road mobile robots

This paper describes an initial investigation that shows the major impact that moisture and vegetation produce on a soil and how that effect may be measured using a thermal camera. In particular, those two variables influence how the soil compacts and, hence, the traversability of a vehicle. A broad set of experiments, under different weather conditions and with different soils, demonstrate that thermal properties derived from the thermal camera (i.e. thermal inertia) increase when moisture content of sandy soils increases. In addition to that, a relation is observed between thermal inertia and traversability (lower thermal inertia, worse traction; and vice versa). Another key behavior is noticed for vegetated soils, where a similar thermal inertia to wet sand is obtained but with only a third of moisture content. These results may be considered for maximizing traversability over sandy soils with higher thermal inertias, what eventually means higher compaction and safer routes. To the authors’ knowledge, this is the first work addressing the correlation between moisture content and vegetation, and the thermal properties of a soil using a light-weight thermal camera that can be mounted on a mobile robot.     

Origin of the echo phenomenon upon flow reversal in low molecular weight nematic liquid crystals

We analyse the transient shear stress response in tumbling nematic liquid crystals upon flow reversal within the framework of Leslie–Ericksen theory. In particular, we focus our attention on the echo phenomenon, i.e. the progressive re-emergence, upon flow reversal, of the transient oscillations observed at the flow start-up in low molecular weight (LMW) liquid crystals. We show that it is possible to interpret this phenomenon if the director distribution that develops after start-up contains a reversible and an irreversible component. In short, the formation of an echo results from the reversible component while the irreversible component attenuates its amplitude. Within this model, the relative proportion of reversible and irreversible contributions to the total director distribution determines the magnitude of the echo and its rate of decay with the increasing of elapsed time between start-up and flow reversal. The model proposed in this paper is fully analytical; in particular, we give an analytical expression for the intensity of the echo. This model is in good agreement with published experimental data for the MSHMA/5CB nematic mixture (Gu et al. J Rheol 37:985–1001, 1993; Gu and Jamieson, Macromolecules 27:337–347, 1994). We end this paper with a brief discussion contrasting the reappearance of the oscillations in liquid crystal polymers and surfactants immediately after the flow reversal, with the observation of the echo phenomenon around t?=?2t _R in LMW liquid crystals, where t _R is the time interval between the start-up and the reversal of the flow.     

Characterization and modeling of strain assisted diffusion in an epoxy adhesive layer

In this paper a coupled model for strain-assisted diffusion is derived from the basic principles of continuum mechanics and thermodynamics, and material properties characterized using diffusion experiments. The proposed methodology constitutes a significant step toward modeling the synergistic bond degradation mechanism at the bonded interface between a Fiber Reinforced Polymer (FRP) and a substrate, and for predicting debond initiation and propagation along the interface in the presence of a diffusing penetrant at the crack tip and at elevated temperatures. It is now well-known that Fick’s law is frequently inadequate for describing moisture diffusion in polymers and polymer composites. Non-Fickian or anomalous diffusion is likely to occur when a polymer is subjected to external stresses and strains, as well as elevated temperatures and humidity. In this paper, a modeling methodology based on the basic principles of continuum mechanics and thermodynamics is developed which allows characterization of the combined effects of temperature, humidity, and strain on diffusion coefficients as well as on moisture saturation level, from moisture weight gain data. For tractability, the diffusion governing equations are simplified for the special case of 1-D diffusion subjected to uniaxial strain and a uniform strain gradient. A novel test specimen that introduces a uniform strain gradient is developed, and diffusion test data for an epoxy-based primer/adhesive are presented and employed for complete characterization of material constants used in the model.     

The Use of Chemical Potential to Describe Water Transfer in Complex Media with Strong Solid–Liquid Bonding

We consider a complex medium composed of finely intertwined micro-skeletons and micro-compartments where water transfer can occur. In these media, at low moisture content, water pressure measurement is not longer possible. Mass transfer is then expressed in terms of chemical potential gradient. The assumption of local thermodynamic equilibrium, resulting in the uniformity of water chemical potential in all microstructures, is essential to define a sorption isotherm reflecting the relationship between water activity and average moisture content. In this case, it is also possible to describe water transfer by using the chemical potential gradient. Radial water transfer in wood is examined using a destructive method for calculating water flux and chemical potential gradient at the same position and at the same time. We deduce the variation of transport coefficient as a function of moisture content.     

Numerical investigation of the slope discontinuities in large deformation finite element formulations

In many multibody system applications, the system components are made of structural elements that can have different orientations, leading to slope discontinuities. In this paper, a numerical investigation of a new procedure that can be used to model structures with slope discontinuities in the finite element absolute nodal coordinate formulation (ANCF) is presented. This procedure can be applied to model slope discontinuities in the case of commutative rotations of gradient deficient elements that are used for modeling thin beam and plate structures. An important special case to which the proposed procedure can be applied is the case of all planar gradient deficient ANCF finite elements. The use of the proposed method leads to a constant orthogonal element transformation that describes an arbitrary initial configuration. As a consequence, one obtains, in the case of large commutative rotations and large deformations, a constant mass matrix for structures which have complex geometry. The procedure used in this investigation to model slope discontinuities requires the use of the concept of the intermediate finite element coordinate system. For each finite element, a new set of gradient coordinates that define, at the discontinuity node, the element deformation with respect to the intermediate element coordinate system is introduced. These new gradient coordinates are assumed to be equal for the two finite elements at the point of intersection. That is, the change of the gradients of two elements at the intersection point from their respective intermediate initial reference configuration is assumed to be the same. This procedure leads to a set of linear algebraic equations that define the orthogonal transformation matrix for the finite element. Numerical examples are presented in order to demonstrate the use of the proposed procedure for modeling slope discontinuities.     

History-dependent dimensional stability of paper

History-dependent dimensional behavior of paper has been formulated within the framework of the general linear theory of viscoelasticity and the classical lamination theory. The effect of the drying history of the papermaking process was incorporated by introducing the residual stress in the reference configuration which was taken at the final drying stage in this study. This permits us to account for complex dimensional and form changes of paper in the converting and end use processes.In order to determine the prediction performance of the computer simulation model developed, in-plane and out-of-plane dimensional responses under cyclic humidity changes were predicted on the basis of hygroviscoelastic data of paper. The simulation examples demonstrated typical irreversible dimensional responses of paper both for the in-plane dimensional change and the curvature change (curl). The computer code can easily deal with the non-uniform distribution of 1) anisotropic viscoelastic properties, 2) hygrothermal properties, and 3) moisture and temperature through the thickness.     

A finite strain kinematic hardening constitutive model based on Hencky strain: General framework, solution algorithm and application to shape memory alloys

The logarithmic or Hencky strain measure is a favored measure of strain due to its remarkable properties in large deformation problems. Compared with other strain measures, e.g., the commonly used Green-Lagrange measure, logarithmic strain is a more physical measure of strain. In this paper, we present a Hencky-based phenomenological finite strain kinematic hardening, non-associated constitutive model, developed within the framework of irreversible thermodynamics with internal variables. The derivation is based on the multiplicative decomposition of the deformation gradient into elastic and inelastic parts, and on the use of the isotropic property of the Helmholtz strain energy function. We also use the fact that the corotational rate of the Eulerian Hencky strain associated with the so-called logarithmic spin is equal to the strain rate tensor (symmetric part of the velocity gradient tensor). Satisfying the second law of thermodynamics in the Clausius-Duhem inequality form, we derive a thermodynamically-consistent constitutive model in a Lagrangian form. In comparison with the available finite strain models in which the unsymmetric Mandel stress appears in the equations, the proposed constitutive model includes only symmetric variables. Introducing a logarithmic mapping, we also present an appropriate form of the proposed constitutive equations in the time-discrete frame. We then apply the developed constitutive model to shape memory alloys and propose a well-defined, non-singular definition for model variables. In addition, we present a nucleation-completion condition in constructing the solution algorithm. We finally solve several boundary value problems to demonstrate the proposed model features as well as the numerical counterpart capabilities.     

Scalar transport in plane mixing layers

This paper describes the application of the Eulerian, single-point, single-time joint-scalar probability density function (PDF) equation for predicting the scalar transport in mixing layer with a high-speed and a low-speed stream. A finite-volume procedure is applied to obtain the velocity field with the k-ε closure being used to describe turbulent transport. The scalar field is represented through the modelled evolution equation for the scalar PDF and is solved using a Monte Carlo simulation. The PDF equation employs gradient transport modelling to represent the turbulent diffusion, and the molecular mixing term is modelled by the LMSE closure. There is no source term for chemical reaction as only an inert mixing layer is considered here. The experimental shear layer data published by Batt is used to validate the computational results despite the fact that comparisons between experiments and computational results are difficult because of the high sensitivity of the shear layer to initial conditions and free stream turbulence phenomena. However, the bimodal shape of the RMS scalar fluctuation as was measured by Batt can be reproduced with this model, whereas standard gradient diffusion calculations do not predict the dip in this profile. In this work for the first time an explanation is given for this phenomenon and the importance of a micromixing model is stressed. Also it is shown that the prediction of the PDF shape by the LMSE model is very satisfactory. Received on 27 October 1998     

Nonlocal implicit gradient-enhanced elasto-plasticity for the modelling of softening behaviour

An improved gradient-enhanced approach for softening elasto-plasticity is proposed, which in essence is fully nonlocal, i.e. an equivalent integral nonlocal format exists. The method utilises a nonlocal field variable in its constitutive framework, but in contrast to the integral models computes this nonlocal field with a gradient formulation. This formulation is considered ‘implicit’ in the sense that it strictly incorporates the higher-order gradients of the local field variable indirectly, unlike the common (explicit) gradient approaches. Furthermore, this implicit gradient formulation constitutes an additional partial differential equation (PDE) of the Helmholtz type, which is solved in a coupled fashion with the standard equilibrium condition. Such an approach is particularly advantageous since it combines the long-range interactions of an integral (nonlocal) model with the computational efficiency of a gradient formulation. Although these implicit gradient approaches have been successfully applied within damage mechanics, e.g. for quasi-brittle materials, the first attempts were deficient for plasticity. On the basis of a thorough comparison of the gradient-enhancements for plasticity and damage this paper rephrases the problem, which leads to a formulation that overcomes most reported problems. The two-dimensional finite element implementation for geometrically linear plain strain problems is presented. One- and two-dimensional numerical examples demonstrate the ability of this method to numerically model irreversible deformations, accompanied by the intense localisation of deformation and softening up to complete failure.     

多孔介质在压力梯度作用下的热质耦合数值模拟

多孔介质干燥导致热质耦合传输过程。本文基于连续介质力学的宏观尺度,对多孔介质的热、湿和气三者耦合迁移进行数值模拟,研究压力梯度对热质传输的影响。多孔介质传质机理主要为水汽和空气的对流和扩散传输、吸附水在含湿量梯度作用下的自由扩散和其在温度梯度即Soret效应驱动下的流动。采用Galerkin加权余量的有限元方法,提出了...     

Nonlinear effects in gas filtration

The existence of considerable deviations from the linear Darcy filtration law has been established for numerous systems consisting of a fluid and a porous medium. One of the manifestations of this nonlinearity is the existence of a limiting (initial) pressure gradient—the minimum value of the pressure gradient for which fluid motion occurs. (As a rule, fluid motion still takes place for subcritical values of the pressure gradient, but very slowly; on reaching the limiting value of the pressure gradient there is a marked acceleration of the filtration. The limiting-gradient concept thus provides a good approximation for velocities which are not too low.)We would also expect nonlinear effects in the motion of a Newtonian liquid or gas in a porous medium containing some amount of fluid which does not participate in the main motion. These effects can take the form of layers enclosing the particles of the porous medium and partly or completely blocking the pore channels. For sufficiently high pressure gradients rearrangement of these layers must begin, accompanied by a change of the hydrodynamic resistance of the porous medium.As a result of this restructuring it is natural to expect a disproportionately fast increase of the filtering-fluid flow rate with increase of the pressure differential; i.e., the filtration law of a Newtonian fluid in a medium containing a. layer of bound fluid having elasticity will have the form which is characteristic for pseudoplastic non-Newtonian fluids. In particular, if the initial attached-fluid content is so large that all the pore channels are blocked in the initial state, then the motion of the liquid (gas) being externally pumped begins only after the attached fluid layers are partly ruptured. Therefore, under these conditions the appearance of a limiting (initial) pressure gradient for filtration of a Newtonian fluid is possible. This can occur in filtration of a gas in argillous rocks containing connate water, since the water and the clay particles form a colloidal suspension which has some shear strength.This phenomenon may be of importance in the development of gas deposits which are associated with argillous rocks, particularly in determining the possible current and final gas yield. In fact, the most characteristic property of flows with an initial gradient is the formation of impermeable blocks; if the deposits have the usual nonuniformity the incompleteness of the gas extraction from the reservoir will manifest itself in both microscopic and macroscopi scales.In the following study we present relations describing gas filtration under such conditions and results of laboratory experiments which confirm the concepts described. Hydrodynamic estimates are also made of the possible effects of the initial gradient.The authors wish to thank I. I. Eremina for assistance in making the calculations, A. Sh. Asadov and Sh. S. Aslanov for assistance in conducting the experiments.     

GIS支持下降雨滑坡的启动机制研究与数字仿真

大气降雨对滑坡体的含水量和容重均有影响, 该影响具有一定的时间进程, 同时滑坡滑带土的含水率对内摩擦角和内聚力均有一定的影响。本文提出了滑坡启动的两种不同机制。通过降水滑体含水率滑体容重、滑带土内摩擦角、内聚力以及它们与滑坡稳定系数的定量关系及其时间效应, 建立起滑坡启动的速度、推力、方向和启动时间的预测、预报模型。