井内混合效应对弥散尺度依赖性影响的实验研究

弥散度是刻画孔隙介质中溶质运移和扩散的重要参数,对于污染物的预测和修复至关重要,但野外示踪试验往往会选择忽略真实存在的井内混合效应。通过室内砂槽实验方法,模拟具有水平分层结构的含水层,该含水层主要由3种介质充填而成。采用埋藏传感器和井中布设传感器2种监测方式,对比在有/无混合效应情况下,穿透曲线的形态差异,进而探究井内混合效应对弥散尺度依赖性的影响情况。实验结果表明,井内混合效应会使穿透曲线呈现阶梯式增长,并伴有显著的拖尾现象;当使用对流弥散方程进行计算时,混合效应会导致弥散度被高估;观测到的弥散度与真实弥散度的差异会随着注入井和观测井间距离的增加而增大;此外,2种观测方式（埋藏/井内）均能发现弥散的尺度依赖性,且井内混合效应显著增强了弥散尺度效应,该实验结果可为污染物运移的评价和预测提供参考。      

Mortar coupling and upscaling of pore-scale models

Pore-scale models are becoming increasingly useful as predictive tools for modeling flow and transport in porous media. These models can accurately represent the 3D pore-structure of real media. Currently first-principles modeling methods are being employed for obtaining qualitative and quantitative behavior. Generally, artificial, simple boundary conditions are imposed on a model that is used as a stand-alone tool for extracting macroscopic parameters. However, realistic boundary conditions, reflecting flow and transport in surrounding media, may be necessary for behavior that occurs over larger length scales or including pore-scale models in a multiscale setting. Here, pore-scale network models are coupled to adjacent media (additional pore-scale or continuum-scale models) using mortars. Mortars are 2D finite-element spaces employed to couple independent subdomains by enforcing continuity of pressure and flux at shared boundary interfaces. While mortars have been used in the past to couple subdomains of different models, physics, and meshes, they are extended here for the first time to pore-scale models. The approach is demonstrated by modeling single-phase flow in coupled pore-scale models, but the methodology can be utilized to model dynamic processes and perform multiscale modeling in 3D continuum simulators for flow and transport.     

Laboratory validation of lattice Boltzmann method for modeling pore-scale flow in granular materials

Characteristics of fluid flow through various engineering structures, such as granular filters and asphalt pavements, influence their design life. Numerical simulation of fluid flow is useful for evaluating the hydraulic characteristics of these materials. Among various techniques, the lattice Boltzmann (LB) method is widely accepted due to the ease of implementing boundary conditions and the numerical stability in a wide variety of flow conditions. It has proven to be extremely efficient in the simulation of fluid flow through the complex geometries of granular materials. In this study, two-dimensional and three-dimensional LB models were developed to represent pore-scale monophasic Newtonian incompressible fluid flow in granular materials. Three-dimensional geometries of compacted aggregates and asphalt specimens were generated from X-ray Computed Tomography technique and used as input for the LB model. The accuracy of the models was verified by comparing the results with analytical solutions of simple geometries and hydraulic conductivity measurements on the compacted aggregates and hot mix asphalt specimens. The results of LB simulations were in excellent agreement with those obtained from analytical calculations and laboratory measurements.     

A new algorithm for representing transport in porous media in one dimension, including convection, dispersion, and interaction with the immobile phase with first-order kinetics

The model uses, in one-dimensional flow, the random-walk method on particles and integrates them into a discretized representation of space which eliminates the individual management of each particle. The method of computing allows a simulation of mass transfer in adsorbing media by dissociating the roles of convection, dispersion, and the exchange occurring between the mobile and immobile phases. This gives the parameters that have to be fitted, such as the dispersivity or the exchange rate, a meaning which is closer to their physical reality than that defined by global models (e.g., apparent dispersivity without considering exchange between phases). The model was tested first on analytical solutions and also on data from laboratory experiments on a double porosity chalk column and showed that, with the same limited set of parameters, it could fit concentration/time restitutions at different distances from the injection point. Because of its structure, the algorithm can easily be modified so as to simulate distributed injections and transfers in a regime of variable flow rates.     

纵向弥散作用与渗透介质非均质性定量关系的模拟研究

在地下水污染模拟预报中,弥散参数是很难确定的一个模型参数。因实验室小尺度弥散规律一般不能用于大尺度弥散过程,而野外示踪试验却耗资大、周期长,限制了其实用性。文中利用随机数值模拟手段、基于随机理论的蒙特卡罗方法及序贯高斯模拟技术来生成渗透系数随机场,并研究渗透系数对数场的方差、相关长度以及变异函数类型在不同尺度上对纵向弥散度的影响,进而建立纵向弥散度与随机分布渗透系数场的方差和相关长度的统计定量关系,并与Gelhar理论计算结果进行比较。数值模拟结果表明,经过一定迁移距离后纵向弥散度与随机分布渗透系数对数场的方差和相关长度具有良好的线性统计关系,与Gelhar理论公式表达的关系类型类似。但对于较大的方差,纵向弥散度模拟结果明显大于Gelhar理论计算值,而对于较大相关长度在迁移距离不很大时,纵向弥散度模拟结果明显小于Gelhar理论计算值。本研究可为野外大尺度地下水污染预报模型中水动力弥散参数的确定提供方法借鉴。     

Effects of sinuosity factor on hydrodynamic parameters estimation in karst systems: a dye tracer experiment from the Beyyayla sinkhole (Eskişehir,Turkey)

The sinuosity factor (SF) is a critical value in karst systems in terms of estimating their hydrodynamic parameters including groundwater velocity, coefficient of dispersion, etc., through dye tracer experiments. SF has been used in a number of different dye tracer experiments in karstic systems to estimate a representative flow path. While knowing SF is crucially important in the estimation of hydrodynamic parameters, its calculation is associated with significant uncertainty due to the complexity of subsurface karstic features. And yet, only a few studies have discussed its uncertainties, which might lead some errors in estimation of hydrodynamic parameters from dye tracer experiment. In this study, dye tracer experiments were conducted in two consecutive years (2003 and 2004) representing low and high flow conditions in the Beyyayla sinkhole (Eski?ehir, Turkey) where the flow path is well known. Uranine was used in experiments as a tracer and QTRACER computer program was used to determine the hydrodynamic properties of the Beyyayla karst system as well as to gain insights into the effects of SF from dye tracer experiments on estimated parameters. The results showed that the breakthrough curve follows a unimodal and a bimodal distribution in low and high flow conditions, respectively. These different distributions stem from the water transport mechanisms, where velocities were calculated as 58.2 and 93.6 m h^?1 during low and high flow conditions observed in a spring emerging from the south side of the studied system. The results also show that the coefficient of dispersion, Reynolds number, and Peclet number increased and longitudinal dispersivity decreased with the higher flow rate. Furthermore, the estimated parameters did not vary with either the flow conditions or the tracer transit time, but they have shown some variations with SF. When SF was increased by 50 %, a change in these parameters was obtained in the range of 50–125 %.     

A pore-scale numerical model for non-Darcy fluid flow through rough-walled fractures

This paper aims at developing a pore-scale numerical model for non-Darcy fluid flow through rough-walled fractures. A simple general relationship between the local hydraulic conductivity and the flow velocity is proposed. A new governing equation for non-Darcy fluid flow through rough-walled fractures is then derived by introducing this relationship into the Reynolds equation. Based on the non-linear finite element method, a self-developed code is used to simulate the non-Darcy fluid flow through fractures. It is found that the macroscopic results obtained by the numerical simulation agree well with the experimental results. Furthermore, some interesting experimental observations can be reproduced.     

西宁市贵德县河滨公园林场第四系含水层弥散试验研究

地下水污染问题日益严重,研究溶质运移的弥散理论开始应用于实际问题。建立地下水溶质运移模型,对地下水中污染物的运移及发展趋势进行准确预测,是对地下水进行保护、对地下水污染进行控制的基础。而弥散参数的确定则是地下水溶质运移模型建立的关键环节之一,直接影响着模型预测结果的精度和准确性。 对西宁市贵德县地下水污染的水质运移规律进行分析,在贵德县河滨公园林场采用径向收敛流水动力弥散理论方法进行了第四系含水层现场弥散试验,计算了试验场地潜水含水层的弥散度,获得纵向弥散度(a_L)为0.843~0.998 cm,横向弥散度(a_T)经验推断值为0.17~0.20 cm,为进一步建立该地区的地下水溶质运移模型、预测地下水污染的发展趋势和评价该地区地下水环境质量提供了数据参考。     

Upscaling polydispersed particle transport in porous media using pore network model

Understanding particle transport in porous media is critical in the sustainability of many geotechnical and geoenvironmental infrastructure. To date, the determination of the first-order rate coefficients in the advection–dispersion equation for simulating attachment and detachment of particles in saturated porous media typically has been relied on the result of laboratory-scale experiments. However, to determine attachment and detachment coefficients under varied hydraulic and geochemical variables, this method requires a large experimental matrix because each test provides only one set of attachment and detachment coefficients. The work performed in this study developed a framework to upscale the results obtained in pore-scale modeling to the continuum scale through the use of a pore network model. The developed pore network model incorporated variables of mean particle size, the standard deviation of particle size distribution, and interparticle forces between particles and sand grains. The obtained retention profiles using the pore network model were converted into attachment coefficients in the advection–dispersion equation for long-term and large-scale simulation. Additionally, by tracking individual particles during and after the simulation, the pore network model introduced in this study can also be employed for modeling the clogging phenomenon, as well as fundamental investigation of the impact of particle size distribution on particle retention in the sand medium.

     

CFD simulation of blasting dust for the design of physical barriers

A computational fluid dynamics (CFD) model has been developed to simulate the dispersion of dust generated in blasting located in limestone quarries. This is a complex phenomenon that has been studied through the use of several digital video recordings of blasts and dust concentration field measurements by ‘light scattering’ dust collectors. In addition, the subsequent simulation of the dispersion of the dust cloud by means of multiphase CFD has also been studied. CFD calculations were carried out using software Ansys CFX 10.0, through transitory models with Lagrangian particle models crossing an Eulerian air continuous phase. This paper presents results obtained by model simulations where physical barriers are set close to the blasting, with the aim of decreasing the dust cloud dispersal and the associated environmental impact.     

Simulation of nitrate transport and wastewater seepage in groundwater flow system

Groundwater quantity and quality modeling is one approach for optimal use of available water resources in arid and semiarid regions. This study was conducted to simulate flow treatment and nitrate transport on Shahrekord aquifer using three-dimensional solute transport model and geographical information system. Hydraulic conductivity, specific yield and recharge values in flow simulation process and effect molecular diffusion coefficient, longitudinal dispersivity and distribution coefficient in quality model were calibrated. 120 water samples during July 2007 to June 2008 were collected monthly from 10 wells and measurements of nitrate were carried out. The results show that the developed model is successfully used to simulate flow path and nitrate transport in saturated porous media. The highest values of nitrate occur along Bahram–Abad village and the surroundings. The groundwater quality in the area represents a complex system, which is affected by different factors of pollution, such as urban wastewater and leaching of agricultural lands.     

土柱变密度溶质运移试验推求水平及垂直方向弥散度研究

基于溶质运移对流弥散理论的变密度海水入侵模型广泛用于海水入侵研究,而水动力弥散系数是影响模型模拟效果的关键性参量之一。利用传统土柱溶质运移试验结合旁侧抽水,采用数值反演法成功获取了水平及垂直两方向弥散度。相对于传统方法,在不增加试验复杂度的前提下,同时推求了不同方向的弥散度,提高了试验效率,节约了试验成本,可广泛用于测定水动力弥散系数等参数。     

Some Mathematical and Computational Problems in Reactive Flow

We present an overview of several applications of reactive flow and transport in which interesting mathematical issues and computational challenges arise. The topics include front propagation through porous media in the presence of dispersion; numerical simulation of large-scale reactive transport problems; the reliability of processes that involve in-situ generation of viscosifying agents; methods for describing pore-scale geometry in granular media; and the information content of reactive tracer experiments. The intent of this survey is to heighten awareness of underlying mathmetical aspects and, it is hoped, to stimulate new contributions in these areas.     

河流纵向分散系数研究

研究了天然河流纵向分散系数理论公式及其参数的确定问题.借助于抛物型断面型态方程确定了河道垂线水深沿河宽的分布及流速分布对断面平均流速的偏离u‘的横向分布,给出了横向混合系数计算方法.在此基础上通过对Fischer的三重积分的直接求解,建立了新的天然河流纵向分散系数计算公式.这一新建立的纵向分散系数计算公式与原有的有代表性的经验公式以及26条美国河流上实测的59组资料进行了比较,比较结果表明,本文建立的纵向分散系数计算公式能给出与实测纵向分散系数最接近的预测值.与现有的其它纵向分散系数计算公式相比,本文建立的天然河流纵向分散系数公式理论上更加合理,机理上更加清楚,并且具有最小的预测误差.     

Simulation of chloride transport in an aggregated soil using three conceptual models

Breakthrough curves (BTCs) of chloride displaced through columns of loessial soil aggregates of different sizes were measured under saturated steady flow conditions. The data were simulated using three conceptual models. Model I (CDE) assumed that all soil water was mobile and physical equilibrium existed in the system. Model II (two-region model) partitioned the soil water into mobile and immobile regions, and convective diffusive solute transport was limited to the mobile water region. Model III (two-flow region model) also divided the soil water into two regions based on their flow velocities, but both of the regions had a non-zero flow rate. Transfer of the chloride solute between the two soil water regions was assumed to occur at a rate proportional to the difference in solute concentration. The two unknown parameters in model I, three in model II, and four in model III were estimated by fitting the experimental data. The three models could well describe all the BTCs measured for columns packed with all the aggregate sizes at the low pore water velocity (0.68 cm/h); however, the values of the fitted parameters varied greatly. The Peclet numbers derived from both the two-region (model II) and two-flow region (model III) models behaved similarly and increased with increases in aggregate size. But the Peclet numbers derived from the convection dispersion equation (model I) were about two orders of magnitude greater than those derived from the other two models. The mobile water fraction obtained for the two-flow region model decreased with increases of aggregate size. The mass transfer coefficient decreased with an increase in pore water velocity due to the shorter residence time of the chloride solute in the soil columns.     

Calculating the effective permeability of sandstone with multiscale lattice Boltzmann/finite element simulations

The lattice Boltzmann (LB) method is an efficient technique for simulating fluid flow through individual pores of complex porous media. The ease with which the LB method handles complex boundary conditions, combined with the algorithm’s inherent parallelism, makes it an elegant approach to solving flow problems at the sub-continuum scale. However, the realities of current computational resources can limit the size and resolution of these simulations. A major research focus is developing methodologies for upscaling microscale techniques for use in macroscale problems of engineering interest. In this paper, we propose a hybrid, multiscale framework for simulating diffusion through porous media. We use the finite element (FE) method to solve the continuum boundary-value problem at the macroscale. Each finite element is treated as a sub-cell and assigned permeabilities calculated from subcontinuum simulations using the LB method. This framework allows us to efficiently find a macroscale solution while still maintaining information about microscale heterogeneities. As input to these simulations, we use synchrotron-computed 3D microtomographic images of a sandstone, with sample resolution of 3.34 μm. We discuss the predictive ability of these simulations, as well as implementation issues. We also quantify the lower limit of the continuum (Darcy) scale, as well as identify the optimal representative elementary volume for the hybrid LB–FE simulations.     

裂隙网络管道模型弥散试验

为了探求不同裂隙几何参数对裂隙网络溶质运移的影响,基于离散裂隙网络思想和优势流、沟槽流理论,建立裂隙管道网络概念模型,搭建不同管径、不同连通方式的管道网络试验装置,进行渗流和溶质运移实验。运用应用广泛的模拟软件CHEMFLO-2000建立等效多孔介质模型,拟合不同几何参数下等效弥散度,定量刻画不同管道网络几何参数对溶质运移的影响,讨论了不同管径、连通方式等与等效弥散度之间的关系。通过进一步分析得出:在连通方式相同的情况下,不同管径的管道网络等效弥散度存在差异,但是差异不大;溶质在小管径中的穿透时间短于大管径,穿透曲线缓和程度相差不大;管道网络连通方式越复杂,其等效弥散度越大、对溶质运移的影响越大、穿透曲线越缓和、路径越长,等效弥散度越大;用这种等效弥散度的方法表征管道网络对溶质运移的影响,与多孔介质弥散度具有相似性;管道数目、管道面数目与等效弥散度成正相关关系,且等效弥散度随尺度的增大而增加。     

井内混合效应与尺度效应对注入井附近溶质径向弥散过程的影响

径向弥散是指溶质在径向流场下的迁移规律,被广泛用于描述含水层修复领域中污染物的迁移过程.然而,在现有描述径向弥散的模型中,往往忽略了井内混合效应对溶质径向弥散的影响.建立新的注入井附近溶质径向运移动力学模型,同时考虑井内混合效应与弥散度的尺度效应.采用Laplace变换推导该模型的半解析解,利用Stehfest数值逆变换获取溶质在实数空间的解.通过与不考虑混合效应的模型对比研究混合效应对溶质径向弥散的影响,并利用室内渗流槽中的溶质径向弥散实验数据验证模型的合理性与适用性.结果表明:混合效应和尺度效应对注水井附近溶质径向弥散有显著影响.具体地讲,井内的混合效应越显著,在井壁处及含水层中的穿透曲线越低,溶质浓度达到峰值所需时间越长,与不考虑混合效应模型的差异越明显;随尺度效应的增强,溶质提前穿透且扩散范围变大,溶质浓度达到峰值所需时间越长;与前人的模型相比,本研究模型能更好地模拟注水井附近的溶质径向弥散问题.      

Two‐scale modeling of solute dispersion in unsaturated double‐porosity media: Homogenization and experimental validation

A two‐scale modeling of solute transport in double‐porosity (DP) media under unsaturated water flow conditions is presented. The macroscopic model was developed by applying the asymptotic homogenization method. It is based on theoretical and empirical considerations dealing with the orders of magnitude of characteristic quantities involved in the process. For this purpose a physical model that mimics the behavior of DP medium was built. The resulting two‐equation model relies on a coupling exchange term between micro‐ and macro‐porosity subdomains associated with local non‐equilibrium solute concentrations. The model was numerically implemented (Comsol Multiphysics^®) to simulate the macroscopic one‐dimensional physical process taking place into the porous medium of 3D periodic microstructure. A series of dispersion experiments of NaCl solution under unsaturated steady‐state flow conditions were performed. The experimental results were used first to calibrate the dispersion coefficient of the model, and second to validate it through two other independent experiments. The excellent agreement between the numerical simulations and the measurements of the time evolution of the non‐symmetrical breakthrough curves provides a proof of predictive capacity of the developed model. Copyright © 2010 John Wiley & Sons, Ltd.     

论大陆岩石圈形成过程中的克拉通化阶段

流体控制着地球系统中质量和能量的传输。流体岩石反应过程在岩石圈中是十分普遍的现象。在流体流动体系中 ,流体在岩石裂隙中通过平流、扩散、水动力弥散和动力学弥散等方式进行输运并与岩石发生同位素交换反应。文章首先对封闭体系、“封闭”体系、开放体系以及缓冲体系中的同位素交换模型进行了简单讨论 ,然后在讨论氧同位素动力学交换机制的基础上 ,建立了平衡交换模型和动力学交换模型。结合流体流动的归一化速率、弥散系数和流体岩石的反应速率等变量 ,对流体岩石交换反应的流体运动方程中的Peclet数和Damk¨ohler数进行了讨论 ,并详细解释了它们在流体岩石交换反应过程中的物理意义和地质用途。