Assessment of an equivalent porous medium for coupled stress and fluid flow in fractured rock

The equivalent porous medium (EPM) for coupled stress and fluid flow in fractured rock was assessed. The assessment was focused on the contributions of the seepage-induced force to the system's equilibrium and deformation as well as fluid flux through the fractures, with the special emphasis on the inter-relations among these three aspects. Seepage-induced average stress was calculated for the EPM. And it was proved, with respect to the fluid flow system (FFS) proposed by Long et al. [Long JCS, Remer JC, Wilson CR, Witherspoon PA. Porous medium equivalents for networks of discontinuous fractures. Water Resour Res 1982;18:645–658], that the equivalence of the equilibrium contribution of the seepage-induced force is unconditionally satisfied in the EPM. The equivalence of fluid flux and seepage-induced deformation was evaluated using numerical techniques based on the FFS, in which the seepage-induced deformation was considered as ‘tensile’ deformation under fluid pressures. The numerical simulation results of the two cases suggest that the EPM established in terms of fluid flux based on the FFS guarantees equivalence of coupled stress and fluid flow. Also, the numerical simulation results indicate that the percolation theory can not be applied to a system with fractures of remarkably different size and hydraulic conductivity.     

Quadruply coupled linear free vibrations of thin-walled beams with a generic open section

The coupled vibration of thin-walled beams with a generic open section induced by the boundary conditions is investigated using the finite element method. If the axial displacement of the pin end is restrained at another point rather than the centroid of the asymmetric cross section, the axial vibration, two bending vibrations, and torsional vibration may be all coupled. The element developed here has two nodes with seven degrees of freedom per node. The shear center axis is chosen to be the reference axis and the element nodes are chosen to be located at the shear centers of the end cross sections of the beam element. Different sets of element nodal degrees of freedom corresponding to different pin ends are considered here. The relation between element matrices referred to different sets of element nodal degrees of freedom is derived.

Numerical examples are presented to demonstrate the accuracy of the proposed method and to investigate the effects of different pin ends on the coupled vibrations of the thin-wall beam.     

汽油燃烧烟气的热解吸/气相色谱/质谱分析

采用热解吸进样和气相色谱/质谱联用技术(GC/MS),研究汽油燃烧产生烟气的特征组分以及不同种类载体对汽油燃烧产生的烟气特征组分的影响。实验选择93#汽油原样及其在两种典型常见载体上燃烧产生的烟气作为研究对象,在获得各样品的总离子流色谱图后,利用标准质谱库进行检索确定汽油燃烧烟气的特征组分。结果表明,汽油燃烧烟气中的最常见组分是苯、3-辛稀、邻苯二甲酸-2-甲基-庚酯、己二酸庚酯、邻苯二甲酸丁酯,其中苯和3-辛烯是汽油燃烧烟气的特征组分。热解吸进样和气相色谱/质谱联用技术对火灾现场烟气可以有效地进行鉴定,同时这种方法因为省去了萃取等实验环节而变得更简单。     

瓦斯抽采过程中的煤层透气性动态演化规律与数值模拟

 为了研究煤层瓦斯抽采过程中的煤体渗透性变化规律,基于Kozeny-Carman方程,利用表面物理化学与含瓦斯煤的有效应力理论,建立考虑有效应力变化、瓦斯解吸和煤基质收缩效应的煤层渗透率动态变化模型,并结合数值模拟分析煤层瓦斯抽采过程中煤体透气性动态演化规律。研究结果表明：(1) 所建立的煤层渗透率动态演化模型能较好地描述煤层瓦斯抽采过程中的煤体透气性动态演化规律。(2) 煤体渗透率与煤体孔隙压力之间呈现出“V”字型变化趋势,低瓦斯压力阶段煤基质收缩效应占主导地位,煤层渗透率随瓦斯压力降低而增大;高瓦斯压力阶段有效应力作用占主导地位,煤层渗透率随瓦斯压力降低而减小。(3) 从煤层内部逐渐接近抽采钻孔过程中,煤层瓦斯压力较高时,煤体渗透率先减小后增加;煤层瓦斯压力较低时,煤体渗透率不断增大。研究结果可以为我国煤矿瓦斯治理和煤层瓦斯抽采提供理论支撑,具有指导性意义。     

装载机后车架主梁焊接变形的计算和预测

在对金属结构的焊接过程进行理论分析之后，本介绍了焊接变形计算的计算机程序。然后，针对９３６Ｅ装载机后车架主梁，建立了有限元模型，按不同的焊接工艺次序进行了计算分析，计算结果与实测结果吻合较好，为控制焊接变形、制定结构的施焊工艺方案，提供了可靠的根据依据。     

A modeling approach for analysis of coupled multiphase fluid flow, heat transfer, and deformation in fractured porous rock

This paper presents the methodology in which two computer codes—TOUGH2 and FLAC3D—are linked and jointly executed for coupled thermal–hydrologic–mechanical (THM) analysis of multiphase fluid flow, heat transfer, and deformation in fractured and porous rock. TOUGH2 is a well-established code for geohydrological analysis with multiphase, multicomponent fluid flow and heat transport, while FLAC3D is a widely used commercial code that is designed for rock and soil mechanics with thermomechanical and hydromechanical interactions. In this study, the codes are sequentially executed and linked through external coupling modules: one that dictates changes in effective stress as a function of multi-phase pore pressure and thermal expansion, and one that corrects porosity, permeability, and capillary pressure for changes in stress. The capability of a linked TOUGH-FLAC simulator is demonstrated on two complex coupled problems related to injection and storage of carbon dioxide in aquifers and to disposal of nuclear waste in unsaturated fractured porous media.     

煤系地层隧道大变形分析及处治措施

某大断面浅埋隧道穿越煤系地层,采用双侧壁导坑法施工,但初支变形大,对初支变形大的原因进行了分析,并对此提出了处治措施。对已施工段的监测数据分析表明,该处治措施可有效的控制围岩的变形。     

The energy flow analysis in stiffened plates of marine structures

The transmission of vibrational energy flow in stiffened plates is analyzed by the structural intensity method. Three typical cases for ship and marine structures are selected as examples: a plate with one longitudinal stiffener under lateral area pressure excitation, a longitudinally stiffened plate under point force excitation, and a cross-stiffened plate under in-plane pressure excitation. The relationships between structural intensity and structural mode shapes and the effects of stiffeners on the changes of energy flow in plate are discussed. Finally, the potential application of structural intensity technique towards the design for stiffened plates of marine structures is explored and presented.     

基于DTM阈值分割法的孔裂隙煤岩体瓦斯渗流数值模拟

 在岩溶发育地区开展大型工程多采用钻孔灌注桩基础,而桩底溶洞的存在可能造成塌孔、埋钻、漏浆等事故,给在建工程和周围建筑造成重大影响。由于钻孔灌注桩需要泥浆护壁,现行的物探方法很难在钻孔灌注桩的泥浆内对桩底溶洞进行有效勘查。本研究结合声呐应力波传播特点,提出了一种钻孔灌注桩桩底溶洞声呐探测方法,并研发了桩底溶洞声呐探测仪(JL-SONAR)和信号分析软件(PBCA)。JL-SONAR探测仪实现了现场声呐信号的发射与采集,PBCA软件完成了探测数据的分析与整理。该技术充分利用了钻孔灌注桩的泥浆条件,可以在成孔过程中跟踪探测桩底10 m内溶洞发育情况,具有成本低、速度快、精度高等优点。并通过两个具体工程案例验证了声呐探测技术在实际工程中的应用。本文的研究对岩溶地区尤其是液态环境下的溶洞勘查提出了一种新的解决方法。     

Three-dimensional nonlinear analysis for the coupled problem of the heat transfer of the surrounding rock and the heat convection between the air and the surrounding rock in cold-region tunnel

According to the basic theories of heat transfer, geocryology and fluid mechanics, taking the coupled problem of the heat transfer of the rock surrounding the tunnel and the heat convective between the air in the tunnel and the rock surrounding the tunnel into account, three-dimensional calculating model of the coupled problem are presented. The finite element formulae of this problem are obtained by Galerkin’s method, and the computer program of the finite element is compiled. Using the program, three-dimensional nonlinear analyses for the coupled problem of the heat transfer of the rock surrounding the tunnel and the heat convective between the air in the tunnel and the rock surrounding Fenghuo mountain tunnel on the Qinghai–Tibet Railway are made. The agreement between the calculated results and the in-situ observed data is seen to be very good. The calculated results illustrate that the freezing–thawing situation of the rock surrounding the tunnel can correctly be predicted even if the air temperature distribution along the tunnel is unknown. In thus way, the large cost of in-situ observation for the air temperature in the tunnel can be saved.     

Laboratory test and modelling of gas pressure under geomembrane subjected to the rise of groundwater in plain reservoirs

To investigate the development of pore pressures in the air-confined system with a rise of groundwater under a geomembrane, a laboratory test was conducted. Then, a numerical model with the consideration of the water-gas-solid three-phase system was developed based on the hydro-mechanical coupling theory for unsaturated soils. The model was validated by the experimental result and then a numerical study was conducted on the Datun plain reservoir. The result confirms the fact that the rise of groundwater is one of primary driving force for gas pressure under the geomembrane. When there is no surcharge over the geomembrane, air bulge would be produced. As the groundwater under the geomembrane replenished by water infiltration from groundwater around the reservoir, the gas pressure increases from the edge to the center of the reservoir and reaches the same final value with time. The increment of the pore gas pressure is significant during the increase of reservoir water, especially for the soils with larger compression strain. Thus, the coupled deformation is necessary to be considered for the regulating reservoir that undergoes the fluctuation of reservoir water frequently.     

非饱和土的变形与渗流耦合的一维解析分析及参数研究

一维的渗流和变形耦合的解析解通过Fourier积分变化获得。指数函数可用来表达水力传导率和孔隙水压力的关系,及土水特征曲线。此解析解考虑了任意初始的孔隙水压力分布和流量边界条件。解析分析表明,渗流和变形耦合对非饱和土中水的入渗产生重要的影响。在渗流的早期阶段,耦合与非耦合情况下的差异随着时间变化显著;随着趋近于稳定状态,这差异逐渐消失。进一步分析了耦合解的各个参数的影响。降雨强度与渗透系数的比值(q/ks)对耦合的渗流和变形产生较大的影响。耦合与非耦合之间的差异与去饱和系数(desaturation coefficient)有关。     

煤与瓦斯突出过程的细观机制研究

 煤与瓦斯突出过程是地下煤矿开采中的一个很复杂的动力灾害问题。在综合收集整理国内外煤与瓦斯突出模拟研究基础上,以颗粒法为基础,建立数值模型和模拟过程,对煤与瓦斯突出过程中相关的微裂纹和位移演化、应力和速度场演化、瓦斯压力和颗粒分层刚度比等细观机制进行模拟和研究。研究结果表明：采用颗粒法模拟所得出的煤与瓦斯突出过程是一个相对很快的动力过程,发生的时间极短,煤岩体损伤主要以拉裂纹为主,剪切裂纹主要集中在瓦斯突出前端,而拉裂纹则深入到煤岩体中的更深部位;同时研究认为瓦斯压力对煤岩体损伤有很大影响,当瓦斯压力相对较小时,剪切裂纹主要在分布侵入体前端,而拉裂纹则沿着侵入煤岩体贯入一定的深度。当瓦斯压力相对较大时,剪切裂纹与拉裂纹贯入深度基本一致,且在煤岩体的损伤中,剪切裂纹比例随之增大;最后对模型中的分层刚度比与裂纹分布形态和应力分布进行了研究,当分层刚度比不相同时,其裂纹传播速度和形态都不相同。这些研究对于认识瓦斯突出机制和瓦斯突出防治提供了理论基础。     

CFD simulation of gas and non-Newtonian fluid two-phase flow in anaerobic digesters

This paper presents an Eulerian multiphase flow model that characterizes gas mixing in anaerobic digesters. In the model development, liquid manure is assumed to be water or a non-Newtonian fluid that is dependent on total solids (TS) concentration. To establish the appropriate models for different TS levels, twelve turbulence models are evaluated by comparing the frictional pressure drops of gas and non-Newtonian fluid two-phase flow in a horizontal pipe obtained from computational fluid dynamics (CFD) with those from a correlation analysis. The commercial CFD software, Fluent12.0, is employed to simulate the multiphase flow in the digesters. The simulation results in a small-sized digester are validated against the experimental data from literature. Comparison of two gas mixing designs in a medium-sized digester demonstrates that mixing intensity is insensitive to the TS in confined gas mixing, whereas there are significant decreases with increases of TS in unconfined gas mixing. Moreover, comparison of three mixing methods indicates that gas mixing is more efficient than mixing by pumped circulation while it is less efficient than mechanical mixing.     

冲击煤层煤柱变形与地面建筑物保护关系研究

保护煤柱合理宽度(或停采位置)是确保地面重要设施长期稳定的关键。以山东某矿地面引水渠("南水北调"工程组成部分)下不规则下山保护煤柱宽度设计为工程背景,首先,通过分析煤柱变形可能引起地面建(构)筑物的破坏方式,提出了控制长期高应力作用下冲击煤层煤柱变形的原则:1走向方向煤柱不发生冲击失稳破坏;2走向方向煤柱不发生煤体长时强度降低而导致的失稳破坏;3倾斜方向煤柱保持均匀变形,从而使地面不发生明显拉伸破坏。其次,依据煤柱上方覆岩空间结构形式,建立了煤柱应力估算模型,提出了保持煤柱长期稳定及地面建筑安全的煤柱设计方法。最后,应用研究成果,分析了工程案例的煤柱应力、围岩稳定性和变形特征,并对该引水渠工程和井下开采设计进行了安全评估。该研究成果对类似开采条件下的保护煤柱宽度确定具有指导意义。     

Simulation of pipe progression in a levee foundation with coupled seepage and pipe flow domains

A very large percentage of piping cases have been brought about by internal erosion, which is the primary cause of dam failures. This study developed a numerical model to simulate the pipe progression in a levee foundation by analyzing the inception and transportation of erodible particles from the soil fabric. An approach that considers the turbulent flow in an erodible pipe and the seepage flow in the remaining area of a levee foundation is employed to capture the main hydraulic characteristics of piping. The mechanical analysis of individual erodible particles is considered to quantify the critical condition for particle inception in an erodible pipe. In addition, physical piping model tests are numerically simulated to examine the proposed approach. The simulation demonstrates that the flow in a pipe can progress backward from downstream to upstream when the upstream water head reaches a critical value. Furthermore, the function mechanism of a cut-off wall can be explained by this model. The results have revealed that this model can reproduce the experimental data, such as the critical water head and the progression time, which are obtained from the physical model. The relationship between the depth of a suspended cut-off wall and the critical water head is obtained; this relationship facilitates the practical design of the critical depth of a cut-off wall for a given water head.     

不同围压加载方式下土石混合体变形破坏机制颗粒流模拟研究

土石混合体是一种非连续、非均质的混合多相介质材料,在轴向荷载作用下,由于内部变形的不均匀性,其变形破坏受围压加载方式影响显著。考虑刚性和柔性两种围压加载方式,采用颗粒流程序(PFC2D)开展了不同含石量和块石空间分布下土石混合体的双轴试验研究,从宏细观上探究了土石混合体在不同加载方式下的变形破坏规律。数值模拟研究表明:在不同围压加载方式下,受侧向变形约束不同的影响,土石混合体在峰后表现出了不同的力学特性,且在刚性加载下,其峰值偏应力较柔性加载下的高;同时,土石混合体在不同围压加载方式下局部化剪切带的形成演化过程也是不同的,其破坏型式不仅取决于围压加载方式,同时也决于块石含量和空间分布,在刚性加载下,多表现为复杂的多叉型破坏,在柔性加载下,多表现为非对称单叉型鼓胀破坏,而且随着含石量增加,破坏型式由简单的单叉型向复杂的多叉型转变,即使在相同含石量下,块石空间分布不同,破坏型式也不一样;土石混合体在不同加载方式下表现出了不同峰后力学行为的根本原因是由于内部土石颗粒间接触力的传递演化规律的不同所致,而且其应力应变关系是内部颗粒间切向接触力的外在宏观表现。     

Numerical investigation of coal and gas outbursts in underground collieries

Coal and gas outbursts are a complex catastrophic unstable phenomenon that involve the ejection of large volumes of coal, and are often accompanied by gas, such as methane, carbon dioxide or a mixture of the two. Coal and gas outbursts are prevalent in deep and gassy mines where face advance rates are rapid, and where gas drainage is either poor or absent. The occurrence of progressively larger coal and gas outbursts, and the potential for the catastrophic collapse of coal pillars, is of increasing importance as mining is extended deeper in seams rich in methane and other hydrocarbons. A unique coupled gas flow and solid deformation numerical model, viz., RFPA^2D-GasFlow, has been developed and is applied to simulate the evolutionary process of such catastrophic coal failures in underground collieries. The finite element model, which incorporates the physics of gas flow in the coal seam, the physics of coal deformation and instantaneous failure, and the cross-couplings between them, is proposed. The model also incorporates small-scale variability in deformation modulus and strength of the coal and surrounding rock. The variability in modulus and strength is distributed via a fine-scale resolution model according to the Weibull distribution, where the distribution parameter determines the level of heterogeneity. This numerical model is applied to simulate the whole process of coal and gas outbursts, including stress concentration, coal fracturing, gas pressure-driven expansion, and outburst. The instantaneous outburst process and associated stress fields, gas pressure gradients and displacement vectors are presented step by step. The numerical simulations indicate that the instantaneous outburst is a complex phenomenon involving interactions between gas pressure, stress and the physico-mechanical properties of the coal, and it can occur under a variety of conditions. Successful numerical simulation of the whole coal and gas outburst process provides the basis for identifying the outburst mechanisms, parameterizing the causative processes, and to defining potential precursors of failure.     

Self-sealing of fractures in indurated claystones measured by water and gas flow

Self-sealing of fractures in the indurated Callovo-Oxfordian (COX) and Opalinus (OPA) claystones, which are considered as host rocks for disposal of radioactive waste, was investigated on artificially fractured samples. The samples were extracted from four lithological facies relatively rich in clay mineral, carbonate and quartz, respectively. The self-sealing of fractures was measured by fracture closure, water permeability variation, gas penetration, and recovery of gas-induced pathways. Most of the fractured samples exhibited a dramatic reduction in water permeability to low levels that is close to that of intact rock, depending on their mineralogical composition, fracture intensity, confining stress, and load duration. The self-sealing capacity of the clay-rich samples is higher than that of the carbonate-rich and sandy ones. Significant effects of sample size and fracture intensity were identified. The sealed fractures become gas-tight for certain injection pressures. However, the measured gas breakthrough pressures are still lower than the confining stresses. The gas-induced pathways can recover when contacting water. These important findings imply that fractures in such indurated claystones can effectively recover to hinder water transport but allow gas release under relatively low pressures without compromising the rock integrity.