准饱和多孔介质中圈闭气体对渗透系数的影响

含圈闭气体的地下水流称为准饱和流,准饱和流中的圈闭气体对含水层渗透系数有重大影响。通过柱试验开展了粉砂、细砂、中砂和粗砂4种介质圈闭气体饱和度与准饱和渗透系数关系的研究。结果表明:圈闭气体饱和度明显受介质的粒径影响,在细粒介质中圈闭气体饱和度明显较大;4种介质圈闭气体饱和度在0~15%范围内,准饱和渗透系数与完全饱和相比减少了32.82%~56.38%,且准饱和渗透系数与圈闭气体饱和度之间可表达为一个负线性相关的经验公式;该公式与Faybishenko公式等效,但形式简单,参数较少,使用方便;准饱和渗透系数的变化规律可概化为圈闭气体占据了原有的有效孔隙,造成原有效孔隙度减少,从而使渗透系数减小。利用该理论,Kozeny-Carman方程能较准确地描述准饱和渗透系数的变化规律,而基于哈根-泊肃叶方程的渗透系数公式则存在较大误差,不适用于描述准饱和渗透系数;试验结果证明了室内测定饱和渗透系数时排除圈闭气体的必要性。     

Field experiments yield new insights into gas exchange and excess air formation in natural porous media

Gas exchange between seepage water and soil air within the unsaturated and quasi-saturated zones is fundamentally different from gas exchange between water and gas across a free boundary layer, e.g., in lakes or rivers. In addition to the atmospheric equilibrium fraction, most groundwater samples contain an excess of dissolved atmospheric gases which is called “excess air”. Excess air in groundwater is not only of crucial importance for the interpretation of gaseous environmental tracer data, but also for other aspects of groundwater hydrology, e.g., for oxygen availability in bio-remediation and in connection with changes in transport dynamics caused by the presence of entrapped air bubbles. Whereas atmospheric solubility equilibrium is controlled mainly by local soil temperature, the excess air component is characterized by the (hydrostatic) pressure acting on entrapped air bubbles within the quasi-saturated zone. Here we present the results of preliminary field experiments in which we investigated gas exchange and excess air formation in natural porous media. The experimental data suggest that the formation of excess air depends significantly on soil properties and on infiltration mechanisms. Excess air was produced by the partial dissolution of entrapped air bubbles during a sprinkling experiment in fine-grained sediments, whereas similar experiments conducted in coarse sand and gravel did not lead to the formation of excess air in the infiltrating water. Furthermore, the experiments revealed that the noble gas temperatures determined from noble gases dissolved in seepage water at different depths are identical to the corresponding in situ soil temperatures. This finding is important for all applications of noble gases as a paleotemperature indicator in groundwater since these applications are always based on the assumption that the noble gas temperature is identical to the (past) soil temperature.     

准饱和多孔介质中地下水驱替速率、圈闭气体饱和度和准饱和渗透系数的关系

圈闭有不连续、非流动和非混溶气体的含水层,被称为准饱和含水层。圈闭气体的存在会降低含水层的渗透性能,但准饱和渗透系数、圈闭气体饱和度和驱替速率之间的关系还没有完全被理解。本文通过室内砂性介质的驱替试验开展了圈闭气体饱和度、准饱和渗透系数和驱替速率三者相互关系的研究。研究表明,虽然总体上圈闭气体饱和度随驱替速率的增大而减小,但是在高驱替速率和低驱替速率分别出现了圈闭气体饱和度不随驱替速率变化的等值区间,该“台阶”型曲线可以采用与van Genuchten曲线类似的数学模型来表征,而砂性介质的准饱和渗透系数与圈闭气体饱和度之间呈简单的线性负相关关系。这两个经验模型说明可以用驱替速率定量估算准饱和渗透系数,该关系为利用驱替速率估算准饱和渗透系数提供了理论依据。上述准饱和水力学性质为深入理解准饱和地下水水流提供了依据。另外,驱替速率与圈闭气体饱和度的关系还表明,实验室测量完全饱和渗透系数时,应以大于初始毛细上升速率的速率进行饱和,以避免介质中的空气被水圈闭。     

Experimental and numerical investigations on zigzag porous screen breakwater

Pile supported porous breakwaters are commonly adopted for shielding small-scale harbors, such as marinas, fishing harbors, and recreational harbors from external waves. To improve efficiency of such breakwaters, attempts have been made to provide a filter screen containing porous material. Possible alternative to the existing pile supported screen breakwaters is the zigzag breakwater. This article details on the numerical and experimental investigations carried out on the performance of zigzag porous screen breakwater. Results related to the wave amplification in front of the structure, wave reflection, and transmission, forces exerted by the waves on the structure are discussed. The studies clearly indicate that the transmission coefficient of the order of 0.5 can be achieved for H _i/gT ² > 0.008 for 50% submergence, whereas the reflection coefficient can be effectively reduced to 0.2. Reduction in wave force is of the order of 50% which is primarily due to reduction in reflection coefficient. Substantial reduction in wave force contributes directly to reduction in the cost of construction of the breakwater, a definite advantage over other similar breakwaters.     

Experimental study on the velocity-dependent dispersion of the solute transport in different porous media

The hydrodynamic dispersion is an important factor influencing the reactive solute transport in the porous media, and many previous studies assumed that it linearly varied with the average velocity of the groundwater flow. Actually, such linear relationship has been challenged by more and more experimental observations, even in homogeneous media. In this study, we aim to investigate the relationship between hydrodynamics dispersion and the flow velocity in different types of porous media through a laboratory-controlled experiment. The results indicate that(1) the dispersion coefficient should not be a linear function of the flow velocity when the relationship between the flow velocity and the hydraulic gradient can be described by Darcy's law satisfactorily;(2) Power function works well in describing the dispersion coefficient changing with the flow velocity for different types of porous media, and the power value is between 1.0-2.0 for different particle sizes.     

Experimental study on the velocity-dependent dispersion of the solute transport in different porous media

The hydrodynamic dispersion is an important factor influencing the reactive solute transport in the porous media, and many previous studies assumed that it linearly varied with the average velocity of the groundwater flow. Actually, such linear relationship has been challenged by more and more experimental observations, even in homogeneous media. In this study, we aim to investigate the relationship between hydrodynamics dispersion and the flow velocity in different types of porous media through a laboratory-controlled experiment. The results indicate that (1) the dispersion coefficient should not be a linear function of the flow velocity when the relationship between the flow velocity and the hydraulic gradient can be described by Darcy’s law satisfactorily; (2) Power function works well in describing the dispersion coefficient changing with the flow velocity for different types of porous media, and the power value is between 1.0-2.0 for different particle sizes.     

Solid–liquid–air coupling in multiphase porous media

This paper addresses various issues concerning the modelling of solid–liquid–air coupling in multiphase porous media with an application to unsaturated soils. General considerations based on thermodynamics permit the derivation and discussion of the general form of field equations; two cases are considered: a three phase porous material with solid, liquid and gas, and a two phase porous material with solid, liquid and empty space. Emphasis is placed on the presentation of differences in the formulation and on the role of the gas phase. The finite element method is used for the discrete approximation of the partial differential equations governing the problem. The two formulations are then analysed with respect to a documented drainage experiment carried out by the authors. The merits and shortcomings of the two approaches are shown. Copyright © 2003 John Wiley & Sons, Ltd.     

多孔介质油水两相k~s~p关系数学模型的实验研究

数学模型是研究相对渗透率与饱和度关系曲线的重要方法。采用自行开发设计的人工平面多孔介质模型,测定了相对渗透率与饱和度的关系曲线。多孔介质选择粒径为0.5~1mm、1~2mm的标准砂,纯净的水为湿润相,用3号苏丹红染色的93#汽油为非湿润相,组成多孔介质油水两相流动系统。采用Van Genuchten and Mualeum(VGM)和Brooks-Corey-Burdine(BCB)两种数学模型计算相对渗透率与饱和度的关系曲线,通过比较两种数学模型计算结果之间和模型计算结果与实测结果的差异以及模型的应用、多相渗流系统自身特征,得出VGM、BCB两种数学模型计算结果符合实际情况,VGM模型应用过程更为简便,但VGM模型具有一定适用条件;在砂性多孔介质中,BCB模型计算相对渗透率与饱和度关系曲线更准确。     

结合蒸汽和空气注入修复多孔介质中DNAPL污染物的多目标多相流模拟优化

基于数值方法模拟对比分析仅注入蒸汽与同时注入蒸汽和空气两种情形下原位曝气方法修复多孔介质中DNAPL污染物(以TCE为例)的修复过程。结果显示蒸汽的注入速率和焓值对蒸汽锋面在含水层中的运移速率影响显著;同时注入蒸汽和空气能够有效抑制只注入蒸汽时出现的DNAPL再冷凝现象,从而提高修复效率。基于模拟-优化方法对同时注入蒸汽和空气修复的理想范例建立了多目标多相流优化模型,采用一种自适应通用多目标优化算法AMALGAM求解Pareto锋面。优化结果表明基于混合优化框架的AMALGAM优化算法能有效综合利用多种独立进化算法的优点,自适应地根据独立进化算法的优劣生成子代,保证子代种群最优,收敛速度较NSGA算法更快。     

基于水气二相流原理确定陶土头进气值、发泡点

张力计为测定土壤水势的一种仪器,其陶土头的进气值与发泡点对其性能影响的报道较少,测定其进气值与发泡点的方法也未见报道。根据水气二相流原理,本文提出了确定其进气值与发泡点的方法并用试验进行了验证。试验结果表明:(1)进气值易受饱和历史影响,测定具有不可重复性,而发泡点测试的重现性较好;(2)进气值与发泡值之间存在一定的相关性,可利用发泡值的1/4来估计张力计的适用范围,简化了测试过程;(3)吸附渗透水的概念模型和变径的毛细管模型可以解释陶土头的排水过程。     

多孔介质两相系统毛细压力与饱和度关系试验研究

两相系统毛细压力-饱和度(h~S)关系曲线的确定是多孔介质多相流动研究的基础。采用简易试验装置对理想和实际介质中水-气和油-水两相系统中的h~S关系曲线进行了测定。试验结果表明,对于相同两相系统,多孔介质孔隙度愈小,同一毛细压力对应的饱和度相应愈大;对于不同两相系统,理想介质的关系曲线在一定毛细压力以下平缓,较大毛细压力时陡直,实际介质关系曲线走势相对较陡。分析结果表明,水-气和油-水两相系统的实测数据符合Parker等提出的基于van Genuchten(1980)关系式的折算理论;应用折算理论,可以在同一多孔介质某一两相系统h~S关系已知的情况下较好地估计同一孔隙度条件下其它两相系统的h~S关系曲线。     

多孔介质水油两相系统相对渗透率与饱和度关系试验研究

相对渗透率和饱和度定量关系是多孔介质多相流动系统中重要的动力学参数关系。使用设计的试验装置及试验方法测定了一维砂柱中油水两相动态流动系统的相对渗透率及饱和度数据,由试验结果分析表明:所测得的相对渗透率与实际情况吻合较好;细砂的强亲水性对不同先湿条件下的油、水相相对渗透率及饱和度有较大影响,并造成了不同先湿条件下流体间驱替机制的差异;相饱和度是影响相对渗透率的主要因素,孔隙空间中两种流体的分布方式和流体的饱和历史也影响各相相对渗透率。对试验结果用VGM模型(Parker-Lenhard模型)进行拟合所得结果较好;在水先湿条件下,将van公式拟合毛细压力-饱和度数据所得拟合参数用于VGM模型预测相对渗透率-饱和度曲线,所得结果与VGM模型直接拟合所得结果有差异,但两者所得结果均较好。     

Crack nucleation in saturated porous media

Crack nucleation has been the subject of important contributions in the last two last decades. However, it seems that few attention has been granted to the case of saturated porous media. This is the question addressed in the present paper which is devoted to nucleation in traction mode. From a physical point of view, nucleation is a sudden phenomenon, so that the material response is both adiabatic and undrained. In the spirit of the variational approach, the nucleated crack is viewed as the final state of a region of space in which the material undergoes a full damage process. In traction mode, the opening of a saturated crack in undrained condition induces a drop of fluid pressure. In case of low fluid compressibility, the presence of the fluid delays the brittle failure usually associated with nucleation, as long as the fluid pressure remains above the saturation vapor pressure. Nucleation is therefore possible only if a partial vaporization of the fluid takes place.     

Modeling microbial processes in porous media

 The incorporation of microbial processes into reactive transport models has generally proceeded along two separate lines of investigation: (1) transport of bacteria as inert colloids in porous media, and (2) the biodegradation of dissolved contaminants by a stationary phase of bacteria. Research over the last decade has indicated that these processes are closely linked. This linkage may occur when a change in metabolic activity alters the attachment/detachment rates of bacteria to surfaces, either promoting or retarding bacterial transport in a groundwater-contaminant plume. Changes in metabolic activity, in turn, are controlled by the time of exposure of the microbes to electron acceptors/donor and other components affecting activity. Similarly, metabolic activity can affect the reversibility of attachment, depending on the residence time of active microbes. Thus, improvements in quantitative analysis of active subsurface biota necessitate direct linkages between substrate availability, metabolic activity, growth, and attachment/detachment rates. This linkage requires both a detailed understanding of the biological processes and robust quantitative representations of these processes that can be tested experimentally. This paper presents an overview of current approaches used to represent physicochemical and biological processes in porous media, along with new conceptual approaches that link metabolic activity with partitioning of the microorganism between the aqueous and solid phases. Received, January 1999 · Revised, June 1999, July 1999 · Accepted, October 1999     

Weak discontinuity in porous media: an enriched EFG method for fully coupled layered porous media

In this paper, a series of multimaterial benchmark problems in saturated and partially saturated two‐phase and three‐phase deforming porous media are addressed. To solve the process of fluid flow in partially saturated porous media, a fully coupled three‐phase formulation is developed on the basis of available experimental relations for updating saturation and permeabilities during the analysis. The well‐known element free Galerkin mesh‐free method is adopted. The partition of unity property of MLS shape functions allows for the field variables to be extrinsically enriched by appropriate functions that introduce existing discontinuities in the solution field. Enrichment of the main unknowns including solid displacement, water phase pressure, and gas phase pressure are accounted for, and a suitable enrichment strategy for different discontinuity types are discussed. In the case of weak discontinuity, the enrichment technique previously used by Krongauz and Belytschko [Int. J. Numer. Meth. Engng., 1998; 41:1215–1233] is selected. As these functions possess discontinuity in their first derivatives, they can be used for modeling material interfaces, generating only minor oscillations in derivative fields (strain and pressure gradients for multiphase porous media), as opposed to unenriched and constrained mesh‐free methods. Different problems of multimaterial poro‐elasticity including fully saturated, partially saturated one, and two‐phase flows under the assumption of fully coupled extended formulation of Biot are examined. As a further development, problems involved with both material interface and impermeable discontinuities, where no fluid exchange is permitted across the discontinuity, are considered and numerically discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Modeling the impact of microbial activity on redox dynamics in porous media

The present study investigates the interaction between microbial growth and activity and the redox dynamics in natural porous media. The impact the transport regime has on this interaction is also addressed. Expressions for microbial growth are incorporated into a geochemical reaction network linking redox reaction rates to the activity of the microorganisms. A flexible simulation environment, the Biogeochemical Reaction Network Simulator (BRNS) is used for this purpose. Two reactive transport applications relevant to fields of contaminant hydrology and early diagenesis are simulated with the BRNS. Model results are evaluated based on a comparison with comprehensive datasets on the biodegradation of lactate in a sand column experiment and on the distribution of redox-sensitive chemical species in marine sediments of the Skagerrak, Denmark. It is shown that, despite quite different transport regimes, the geomicrobiological model performs equally well in the reproduction of measured chemical species distribution for both applications. This result emphasizes the broad applicability of the proposed approach. Our simulations support that the competitive behavior between various microbial groups is a process controlling the development of redox stratified environments. Furthermore, it is also shown that the transport regime is a key controlling factor for the degree of spatial correlation between microbial biomass distributions and redox reaction rates. Although all our simulations yield a pronounced stratification of the redox processes in the system, the biomass distribution is related to the associated reaction rates only in case of the advection controlled column experiment. In the early diagenetic application, mixing due to bioturbation is the dominant transport process for particulate matter, hence leading to fairly homogeneous distribution of bacterial biomasses which are unrelated to the spatial distribution of redox reaction rates. This homogeneous biomass distribution combined with the 1G carbon degradation model approach might explain why the steady state concentration profiles in such systems can be reproduced by diagenetic models without explicit representation of microbial growth.     

悬浮微小颗粒对多孔介质渗流特性影响的实验研究

在清洁水渗流实验的基础上,开展了恒水头条件下不同浓度的含悬浮颗粒流体在多种填充粒径的多孔介质内的渗流实验。结果表明:悬浮颗粒在多孔介质段的沉积,并不一定使多孔介质局部相对渗透系数(k_t/k_0)降低,相反在一定情况下会出现局部相对渗透系数增大的情况。较小悬浮颗粒浓度下,悬浮颗粒粒径越小,曲线波峰出现越迟,相对渗透系数增加幅度越大;而较大进口浓度则抑制(k_t/k_0)曲线波峰的生成,直至相对渗透系数单调递减。通过包括前人实验数据在内的悬浮颗粒浓度C_0与颗粒直径比值d_p/D_p(悬浮颗粒直径/多孔介质颗粒直径)有无相对渗透系数增加现象的对照图,进一步揭示了含悬浮颗粒流体在多孔介质内的运移与沉积规律。     

Noble gas recharge temperatures and the excess air component

The calculation of a groundwater recharge temperature based on the dissolved concentrations of Ne, Ar, Kr and Xe requires a correction for noble gas supersaturation due to excess air entrainment. This entrainment is commonly attributed to the recharge process or to air contamination at the wellhead during sample collection. With the exception of some local studies, most work has concentrated on interpretation of the recharge temperature or quantification of the radiogenic content for palaeoclimatic and dating purposes. The magnitude and source of the excess air is not directly relevant to these studies and so is often ignored. In this work, excess air Ne and other data have been calculated from new and published noble gas data sets for several groundwater systems. For younger groundwaters which have been recharged under one broad climatic regime, the amount of air entrainment increases according to lithology in the order granites, sandstones and limestones respectively. A negative correlation between precipitation and excess air entrainment is identified in at least one aquifer, and some of the mechanisms which may influence the entrainment process are discussed.     

多孔介质中气泡尺寸对流动阻力的影响

多孔介质中的泡沫能够封堵某些孔道,降低气体流动性。由于直接观察多孔介质中的泡沫流动比较困难,所以关于泡沫流动性与气泡尺寸之间的关系研究较少。利用Navier-Stokes方程与守恒的水平集方法耦合模拟孔隙介质中气泡尺寸对流动阻力的影响,结果证实,气泡尺寸是影响流体阻力的重要因素。当气泡半径小于孔喉半径时,气泡不受孔道约束,气泡流动过程中流动阻力较稳定,此时含气泡流体流动阻力与纯液态流体流动阻力相等,所以小于孔喉尺寸的气泡对孔道无封堵作用。当气泡半径大于孔喉半径时,孔道影响气泡变形,其流动阻力存在波动性,最大流动阻力随气泡体积变大呈线性增加。当气泡体积增加到使最大流动阻力达极大值时,继续增大气泡体积,最大流动阻力随气泡体积增大而线性降低。最大流动阻力随气泡体积增大而线性增大与减小变化具有周期性,周期为单位孔体体积。     

Dynamic capillary effects in heterogeneous porous media

In standard multi-phase flow models on porous media, a capillary pressure saturation relationship developed under static conditions is assumed. Recent experiments have shown that this static relationship cannot explain dynamic effects as seen for example in outflow experiments. In this paper, we use a static capillary pressure model and a dynamic capillary pressure model based on the concept of Hassanizadeh and Gray and examine the behavior with respect to material interfaces. We introduce a new numerical scheme for the one-dimensional case using a Lagrange multiplier approach and develop a suitable interface condition. The behavior at the interface is discussed and verified by various numerical simulations.