Modeling heat and mass transfer during ground freezing subjected to high seepage velocities

Natural or induced groundwater flow may negatively influence the performance of artificial ground freezing: high water flow velocities can prevent frozen conditions from developing. Reliable models that take into consideration hydraulic mechanisms are then needed to predict the ground freezing development. For forty years, numerous thermo-hydraulic coupled numerical models have been developed. Some of these models have been validated against experimental data but only one has been tested under high water flow velocity conditions. This paper describes a coupled thermo-hydraulic numerical model completely thermodynamically consistent and designed to simulate artificial ground freezing of a saturated and non-deformable porous medium under seepage flow conditions. On some points, less restrictive assumptions than the ones usually used in the literature are considered. As for the constant-porosity assumption, its validity is verified. The model appears to be well validated against analytical solutions and a three-dimensional ground freezing experiment under high seepage flow velocity conditions. It is used to highlight key thermo-hydraulic mechanisms associated with phase change in a porous medium.     

南海区域海气热通量的变化特征分析

应用由卫星SSM/I(Special Sensor Microwave/Imager)和AVHRR(Advanced Very High Resolution Radiometer)遥感资料,使用先进的海气通量计算方法(TAGA COARE3.0),计算出南海1987年7月至2004年12月共200个月的海气界面的感热和潜热通量(0.25°×0.25°),其结果与实测结果比较发现,由卫星反演的海气热通量与实测结果非常一致.与GSSTF2的结果相比,其时空分布变化特征基本一致.由此说明,利用卫星遥感获得的热通量可以用来进行中国近海海气相互作用的研究以及作为我国气候预测研究的重要依据.由多年南海海气热通量的分析表明,南海区域热通量的变化具有显著的年变化和年际变化特征,其周期分别是0.5a、1a、准3a和6～11a.其中准3a和6～11a周期与中国旱涝的周期一致.因此,可以认为南海区域热通量的年际变化对中国的旱涝分布将起着不可低估的作用.     

Bearing capacity of strip and circular footings in sand using finite elements

Design of shallow foundations relies on bearing capacity values calculated using procedures that are based in part on solutions obtained using the method of characteristics, which assumes a soil following an associated flow rule. In this paper, we use the finite element method to determine the vertical bearing capacity of strip and circular footings resting on a sand layer. Analyses were performed using an elastic–perfectly plastic Mohr–Coulomb constitutive model. To investigate the effect of dilatancy angle on the footing bearing capacity, two series of analyses were performed, one using an associated flow rule and one using a non-associated flow rule. The study focuses on the values of the bearing capacity factors N_q and N_γ and of the shape factors s_q and s_γ for circular footings. Relationships for these factors that are valid for realistic pairs of friction angle and dilatancy angle values are also proposed.     

增强地源热泵竖直埋管地下换热器换热性能的研究

地下换热器换热性能直接影响到地源热泵系统的初投资和运行成本.地下换热器可分为单U型、双U型和1 2型三类.采用非稳态法对膨润土、水泥 砂两类回填材料进行了室内实验;在相同的边界条件下,采用扶正器和定位器施工可以保证下管,并能降低钻孔内热阻.因地制宜地选择埋管形式,采用稳定、高效的回填材料和减小热短路是提高地下换热器换热性能的必要途径.     

三维过渡等参单元在岩土工程有限元分析中的应用

土石坝等复杂土工结构物有限元三维建模中多采用精度较高的六面体单元辅以部分过渡用的退化单元,而退化单元由于形态不好,会导致有限元计算精度较差。解决该问题的途径之一是采用过渡性的等参元。总结了几种常遇到的过渡等参单元（楔形体、四面体、金字塔）的插值函数和高斯积分局部坐标和权重,并编入了有限元程序。通过比较六面体单元和金字塔单元剖分理想土石坝的有限元计算结果,说明所引入的金字塔单元是满足精度要求的。将各种过渡单元应用于实际土石坝工程的有限元计算,结果表明,使用过渡等参单元是可以在一定程度上提高计算精度的。最后讨论了二次单元在弹塑性有限元动力计算中的应用。二次单元的使用,可以改善动力计算中的超静孔压分布,提高计算精度。     

Non-linear analysis of geomechanical problems using coupled finite and infinite elements

In modeling of many geomechanics problems such as underground openings, soil-foundation structure interaction problems, and in wave propagation problems through semi-infinite soil medium the soil is represented as a region of either infinite or semi-infinite extent. Numerical modeling of such problems using conventional finite elements involves a truncation of the far field in which the infinite boundary is terminated at a finite distance. In these problems, appropriate boundary conditions are introduced to approximate the solution of the infinite or semi-infinite boundaries as closely as possible. However, the task of positioning the finite boundary in conventional finite element discretization and the definition of the boundary and its conditions is very delicate and depends on the modeller's skill and intuition. Moreover, such a choice is influenced by the size of the domain to be discretized. Consequently, the dimensions of the global matrices and the time required for solution of the problem will increase considerably and also selection of the arbitrary location of truncated boundary may lead to erroneous result. In order to over come these problems, mapped infinite elements have been developed by earlier researchers (Simoni and Schrefier, 1987). In the present work the applicability of infinite element technique is examined for different geomechanics problems. A computer program INFEMEP is developed based on the conventional finite element and mapped infinite element technique. It is then validated using selected problems such as strip footing and circular footing. CPU time taken to obtain solutions using finite element approach and infinite element approach was estimated and presented to show the capability of coupled modeling in improving the computational efficiency. Mesh configurations of different sizes were used to explore the enhancement of both computational economy and solution accuracy achieved by incorporation of infinite elements to solve elastic and elasto-plastic problems in semi-infinite/finite domain as applied to geotechnical engineering. © Rapid Science Ltd. 1998     

青藏高原夏季凝结潜热时空分布特征分析

结合青藏高原东部3″DEM和102个气象站46年(1961—2006)5—9月的降水资料,根据降水随高度分布,将站点分为三类,采用多元回归的方法,建立逐年雨季降水量估算模型,并估算和分析了46a的雨季凝结潜热。结果表明:降水量估算模型的相关性显著,基本都通过0.05的信度检验值0.69、0.82、0.90;高原东部雨季凝结潜热分布极不均匀,表现为东南多西北少的特征,凝结潜热总量变化在18.1×1020～26.8×1020J,并且有波动中增加的趋势;采用EOF分解和REOF分区,将高原东部划分为5个气候区,区域凝结潜热年际变化和第A、B、E区的凝结潜热变化有一致性,而与北坡第C、D区凝结潜热相差很大。由于高度、经度、坡度等对低海拔区降水的正调节作用,使得该区域的凝结潜热自20世纪90年代之后逐渐降低;坡向对高海拔区域降水的正调节作用,纬度对降水的负调节作用,使得该区域的凝结潜热有逐渐增加的趋势。     

Bearing capacity of interfering multiple strip footings by using lower bound finite elements limit analysis

The ultimate bearing capacity of a number of multiple strip footings, identically spaced and equally loaded to failure at the same time, is computed by using the lower bound limit analysis in combination with finite elements. The efficiency factor (ξ_γ), due to the component of soil unit weight, is computed with respect to changes in the clear spacing (S) between the footings. It is noted that the failure load for a footing in the group becomes always greater than that of a single isolated footing. The values of ξ_γ for the smooth footings are found to be always lower than the rough footings. The values of ξ_γ are found to increase continuously with a decrease in the spacing between footings. As compared to the available theoretical and experimental results reported in literature, the present analysis provides generally a little lower values of ξ_γ.     

热交换桩的作用机制及其应用

将常规桩与地源热泵技术结合起来形成了一种新型基桩--热交换桩。首先介绍了热交换桩的概念和工作机制,并对其进行了总结和分类。桩体中埋设换热器并与地表管路连接,通过换热器中的交换液体与桩体、桩周土-地基土及地下水系统进行热交换,形成封闭的地源热泵的地热交换器,使得热交换桩具有常规基桩和地源热泵预成孔直接埋设换热器的双重作用。最后分析了热交换桩目前国内外的应用现状,指出了实际应用关键技术问题及其研究的方向。分析后可以看出,热交换桩是一种经济、环保、节能的新技术,值得推广使用。     

Magnitude of latent heat in thermally loaded clays

Temperature changes are known to induce specific couplings in clay, in particular, an anomalously high thermal pressurization in undrained conditions or a thermal compaction in drained conditions, both of which are potential threats for the mechanical stability and sealing capacity of the geomaterials. Thermodynamical analysis of those peculiar thermomechanical couplings points to a potentially important latent energy, which in turn could limit the temperature change upon heating or cooling. The direct measurement of latent energy developed during a laboratory geomechanical test is challenging. Instead, proper identification of thermal hardening in conventional experiments with temperature changes provides an alternative route to estimate latent energy. In this work, existing laboratory thermomechanical tests of clays are analyzed with a rigorous thermodynamic framework to quantify the magnitude of latent energy in thermomechanically loaded clays. A thermodynamically consistent constitutive model for fully saturated clays that combines two key features, (a) the temperature dependence of the blocked energy and (b) the framework of bounding plasticity, is proposed. The performance of the model is validated by reproducing results obtained in laboratory tests for Boom and Opalinus clays. The thermomechanical loads considered to validate the model performance were then used to estimate the percentage of work that remains latent in the clayey material during plastic yielding. We find that the magnitude of latent energy is quite significant, typically a few tens of percent of the total dissipated energy, and increases significantly with temperature. Accordingly, it is expected to play an important role in the thermomechanical response of clays.     

One‐dimensional consolidation with a threshold gradient: a Stefan problem with rate‐dependent latent heat

During the process of one‐dimensional consolidation with a threshold gradient, the seepage front moves downward gradually, and the problem is indicated as a Stefan problem. The novel feature in this Stefan problem is a latent heat that varies inversely with the rate of the moving boundary. An exact solution for the external load that increases in proportion to the square root of time is constructed using the similarity transformation technique. Computational examples concerning the effect of different parameters on the motion of the seepage front are presented. The exact solution provides a worthwhile benchmark for verifying the accuracy of numerical and approximate methods. Copyright © 2013 John Wiley & Sons, Ltd.     

深基坑降水中不同的地质模型有限元应用研究

分析水-土耦合场相互作用时,将岩土视作弹塑性材料。在非线性本构关系下,考虑渗透作用,得出岩土体有限元方程,建立了基坑降水的三维水-土耦合的数学模型。对基坑降水中不同的地质模型潜水、承压水及有越流补给和实际工程条件井管、过滤管、止水帷幕分别进行了计算处理,并以武汉国际证券大厦降水工程为实例,对其降水过程进行了三维有限元计算。应用三维数值模拟方法,能很好地模拟基坑防渗帷幕、非均质和各向异性、承压-无压等一些解析法难以处理的实际工程条件和地质模型。     

节理网络有限元在倾倒斜坡稳定分析中的应用

节理网络有限元是指具有节理网络模拟功能的有限元,它不但能将野外节理统计特征直接输入到模型中,同时允许节理按一定的破坏准则产生屈服,从而较好地模拟边坡的实际结构及破坏特征。论述了倾倒变形边坡的分析方法及存在的问题,介绍了节理网络有限元的功能及参数输入,说明了其在倾倒变形边坡应用中的优势,并以黄河上游茨哈峡电站Ⅰ号倾倒体为例,计算了该倾倒边坡的稳定性及破坏趋势,结果表明,节理网络有限元是倾倒变形边坡稳定分析的一种有效、可靠的方法。     

套管式地埋管换热器热短路及换热性能

套管式地埋管换热器是深层地源热泵系统常用的换热装置。基于流体流动换热方程,建立套管式地埋管换热器与周围岩体之间的传热模型。以第一个供暖季为例,分析内管导热系数和循环水流量对换热性能的影响,并引入换热器效能对热短路现象进行评估,研究结果显示：内管导热系数越大,热短路现象越显著;热短路使内外管中循环水温差降低,管内出现热堆积,导致换热器换热功率降低;套管式地埋管换热器的换热功率随循环水流量的增大逐渐增大;内外管之间存在热短路时,出口水温随循环水流量的增大先升高后降低,随着流量增大,换热器效能增大。研究成果可为深层地源热泵系统中地埋管换热器的设计提供借鉴。     

Modeling of pile installation using contact mechanics and quadratic elements

In geotechnical engineering, numerical analysis of pile capacity is often performed in such a way that piles are modeled using only the geometry of their final position in the ground and simply loaded to failure. In these analyses, the stress changes caused by the pile installation are neglected, irrespective of the installation method. For displacement piles, which are either pushed or hammered into the ground, such an approach is a very crude simplification. To model the entire installation process of displacement piles a number of additional nonlinear effects need to be considered. As the soil adjacent to the pile is displaced significantly, small deformation theory is no longer applicable and a large deformation finite element formulation is required. In addition, the continuously changing interface between the pile and the soil has to be considered. Recently, large deformation frictional contact has been used to model the pile installation and cone penetration processes. However, one significant limitation of the analysis was the use of linear elements, which have proven to be less accurate than higher order elements for nonlinear materials such as soils.

This paper presents a large deformation frictional contact formulation which can be coupled consistently with quadratic solid elements. The formulation uses the so-called mortar-type discretisation of the contact surfaces. The performance of this contact discretisation technique is demonstrated by accurately predicting the stress transfer between the pile and the soil surfaces.     

Performance of displacement finite elements for modelling incompressible materials

It is well established that severe numerical difficulties may arise when the displacement finite element method is used to analyse the behaviour of incompressible solids and this is particularly true for axisymmetric problems. These numerical difficulties are caused by excessive kinematic constraints and are reflected by strong oscillations in the calculated stress distribution and overestimations of collapse loads. The purpose of this paper is to present new displacement finite element formulations which are particularly suitable for axisymmetric analysis of incompressible materials. A direct comparison is made of the performance of various displacement finite elements in the analysis of elastic or plastic incompressible materials under axisymmetric loading conditions. Particular attention is focused on the performance of various axisymmetric displacement elements in predicting the stress field of incompressible or nearly incompressible materials. Copyright © 2000 John Wiley & Sons, Ltd.     

Particle finite element analysis of large deformation and granular flow problems

A version of the Particle Finite Element Method applicable to geomechanics applications is presented. A simple rigid-plastic material model is adopted and the governing equations are cast in terms of a variational principle which facilitates a straightforward solution via mathematical programming techniques. In addition, frictional contact between rigid and deformable solids is accounted for using an approach previously developed for discrete element simulations. The capabilities of the scheme is demonstrated on a range of quasi-static and dynamic problems involving very large deformations.     

Application of the Fourier method to the numerical solution of moving boundary problem in heat conduction

An algorithm for the solution of a nonlinear problem of phase boundary movement and evolution of temperature distribution due to the perturbation in the basal heat flux has been discussed. The reduction of the problem to a system of nonlinear ordinary differential equations with the help of a Fourier series method leads to a stiff system. This stiffness is taken care of by the use of a modified Euler’s method. Various cases of basal heat flow variation have been considered to show the performance and stability of the technique for such a nonlinear system. The first case of step-wise function is taken to analyse the performance of the technique, and the study has been extended to other general cases of linear increase, periodic variation, and box and triangular function type variations in the heat flux. In the step-wise case the phase boundary attains a constant position rapidly if the supplied heat flux is sufficiently large. The effect of periodicity in the heat flow is clearly depicted in the phase boundary movement, where the phase boundary oscillates about the mean position at large times. The absence of any constant level in the case of linear increase in heat flux is due to a very large value of heat flux. In the cases of box car and triangular heat flux the boundary starts moving downward after the cessation of excess heat flux but does not immediately return to its original preperturbation state, instead approaches it at large times. This technique may be applied to more general cases of heat flow variation.     

橄榄石微量元素原位分析的现状及其应用

随着高精度EMPA和LA-ICP-MS分析技术的发展和矿物微量元素测试精度的提高,利用橄榄石中的微量元素示踪地幔部分熔融、地幔交代作用、岩浆早期结晶过程等地质问题成为近年来一个新兴的研究方向。一系列开拓性的研究发现也被陆续的发表,主要涉及橄榄石中Ni、Co、Al、Cr、Zn、Ti、Li、V、Sc、Mn、Ca和P等元素的示踪使用。一些卓有成效的示踪方法为:Ca、Al、Ti、Ni及Mn能够很好的用于区分橄榄石捕掳晶和斑晶;橄榄石-尖晶石地幔演化趋势线(OSMA:olivine-spinel mantle array)图解可以用于表征岩浆源区的亏损程度;玄武岩中橄榄石斑晶的Li同位素及Li含量可以很有效地指示岩浆源区是否存在地壳物质再循环及地幔交代作用;橄榄石斑晶中Ni、Ca、Mn、Cr和Al协变关系图解可以识别岩浆的辉石岩源区;利用橄榄石捕掳晶中Zr和Sc的含量差异特征可将橄榄岩中三种最主要的类型(尖晶石橄榄岩、石榴石橄榄岩以及尖晶石-石榴石橄榄岩)区分开来;一些元素的比值或组合(例如Ni/Co、Fe/Mn、V/Sc、Zr和Sc、Ca和Ti)可以指示源区交代作用、岩浆作用过程及氧化状态;基于橄榄石中Al、Cr及Ca的地质温度计可以为推算地幔热状态提供新方法;基于橄榄石分离结晶Fo-NiO演化线的原始岩浆计算模型可以较好的推算原始岩浆成分;利用橄榄石的环带及微量元素的扩散机制可以判别更多岩石成因信息,如识别交代介质、熔体类型以及地质构造背景等。基于上述最新研究的相关资料和已有成果,本文对橄榄石微量元素的地球化学示踪方法做系统性的归纳整理,并对橄榄石微量元素赋存状况、橄榄石微量元素测试方法、橄榄石微量元素的使用条件及需注意的问题等进行讨论,为读者在做相关研究时提供参考。     

非稳态热传导时层状路面体系的温度响应

利用解析层元法推导温度荷载作用下非稳态热传导时层状路面体系的温度响应解答。从热弹性理论平面应变问题的控制方程出发,借助于Laplace-Fourier积分变换,推导出单层介质及下卧半平面的精确刚度矩阵即解析层元,结合有限层法原理及边界条件,组装并求解总刚度矩阵,得到其在变换域内的解答,最后通过相应的积分逆变换得到物理域内的真实解。由于该法刚度矩阵元素中不含正指数项,计算时不会出现溢出或病态矩阵的现象。编译了相应的计算程序,所得结果与有限元模拟结果吻合较好。在此基础上,对有限深度和半平面两种假定条件下的解答进行对比分析,并分析层状路面体系中位移和温度随时间的变化趋势及沿深度的分布规律。分析表明：温度场具有一定的影响深度,超过此深度,有限深度与半平面理论解答基本一致;温度荷载的影响深度与其强度有关,强度越大,其影响深度越深。