Micromechanics modeling the solute diffusivity of unsaturated granular materials

This work is devoted to modeling the evolution of the homogenized solute diffusion coefficient within unsaturated granular materials by means of micromechanics approach. On the basis of its distinct role in solute diffusion, the liquid water within unsaturated granular materials is distinguished into four types, namely intergranular layer (interconnected capillary water), isolated capillary water, wetting layer and water film. Application on two sands shows the capability of the model to accurately reproduce the experimental results. When saturation degree is higher than the residual saturation degree Sr^r, the evolution of homogenized solute diffusion coefficient with respect to the saturation degree depends significantly on the connectivity of the capillary water. Below Sr^r, depending on the connectivity of the wetting layer, the homogenized solute diffusion coefficient within unsaturated sands decreases by 2–6 orders of magnitude with respect to that in bulk liquid water. The upper bound of the solute diffusion coefficient contributed by the water films is 4–6 orders of magnitude lower than that in bulk liquid water.     

An inverse technique to determine volatile organic compounds diffusion and partition coefficients in dry building material

A mass transfer theory based model describing volatile organic compounds (VOCs) diffusion in dry building material was presented. Effects of some key parameters on the model output were theoretically studied. An inverse method was developed to estimate the diffusion coefficient (D) and partition coefficient (K) of VOC/material combination by utilizing dynamic chamber emission data. The present inverse parameter estimation problem was solved with Levenberg–Marquardt method of minimization of the ordinary least-squares norm. Sensitivity analysis showed the feasibility of simultaneous estimation of D and K. The present inverse method was first validated by a theoretical case. Measurements with different error levels were used to show their effects on the accuracy of the estimates. Results indicate that the present inverse method can be used to accurately estimate both D and K with the additional information of measured VOC concentration in chamber. Then experimental data of styrene/carpet combination obtained from standard field and laboratory emission cell emission test were used to determine D and K using the validated inverse method. Reliable results were obtained.     

The Sorption Behavior of Wood Studied by Means of an Improved Cup Method

In transport models for wood, sorption is an essential parameter. Sorption is the balancing process between the two phases of water present in wood below the fiber saturation point, namely water vapor in the lumens and bound water in the cell walls. To gain better insight into the physical background of transport processes, a special experimental test setup—the improved cup method—is presented. It allows for separation of sorption from other processes. In this test, a diffusion cup contains a thin specimen of wood, with one side facing outwards to a climate chamber and the opposite side facing inwards the cup. In contrast to the common cup method, the herein presented method uses a data logger for relative humidity and temperature placed inside the cup. The use of thin cross-cut specimens allows for explicit separation of the different processes occurring during transient moisture transport. Mass changes were determined and relative humidity inside the cups was measured for eight specimens of Norway spruce with different specimen thicknesses. Relative humidity was increased in three uniform steps in the test chamber from 4.0 up to 76.5%. The results obtained with this special test setup indicate that the sorption process is different than assumed in previous publications. This emphasizes the need of improved modeling approaches.     

Application of a n-Phase Model to the Diffusion Coefficient of Chloride in Mortar

The determination of the chloride diffusion coefficient of a concrete is needed to help the prediction of the service life of concrete structure. In this paper, we propose first a critical review of models for chloride diffusion coefficients already used in literature at different scales and then we develop an analytical model, which takes into account the characteristics of the different phases of concrete. These materials are treated as a three-phase composite, consisting of a cement continuous phase, of an aggregates dispersed phase and of an interface transition zone. Chloride diffusion coefficient using an n-layered inclusion-based micromechanical modeling is predicted. The details of calculations are summarized hereafter and experimental measurements obtained on mortars are compared with predicted results.     

车辆悬架控制臂液压衬套区间动刚度模型研究

已有文献的车辆悬架控制臂液压衬套动刚度模型仅能给出动刚度变化理想曲线,不能给出动刚度的上下变化曲线。为了解决这个问题,将流体惯性系数和流量阻尼系数定义为区间变量,运用区间不确定性理论建立了液压衬套的动刚度模型。将初始区间动刚度模型获得的仿真结果与已有的实验结果对比,吻合较好,验证了该模型的正确性。采用子区间组合法,在一定的区间精度下获得优化的区间动刚度模型,进而获得优化的动刚度上下限变化曲线。区间动刚度模型为动刚度的全面描述提供了一种方法,对文献的动刚度模型进行了改进。     

Comparison of Two Mathematical Models for 3D Groundwater Flow: Block-Centered Heads and Edge-Based Stream Functions

Traditionally, groundwater flow models, as well as oil reservoir models, are based on the block-centered finite difference method. Well-known models based on this approach are MODFLOW (groundwater) and ECLIPSE (oil and gas). Such models are well proven and robust; their underlying principles are well understood by hydrologists and petroleum reservoir engineers. Nevertheless, the desire to improve the block-centered finite difference paradigm has always been alive, for instance, to be able to apply deformed grid blocks, or to model anisotropy that is not aligned along the coordinate axes. This article introduces the edge-based stream function as a potential alternative to the paradigmatic model, not only to mitigate the above mentioned limitations, but especially for its promise to inverse modeling. Computer programs have been developed for the discrete analog equations of the stream function method and the conventional method. The two methods are tested by using synthetic forward modeling problems of uniform and radial flow. The theoretical formulation and the numerical results show that the two methods are algebraically equivalent and yield the same flux output. However, for rectangular grid blocks and anisotropy aligned along the coordinate axes, the block-centered method is shown to be computationally more efficient than the edge-based stream function method. The major advantage of the stream function method is that it is linear in the resistivities, proving it an ideal candidate for direct inverse modeling. Moreover, any arbitrary specification of stream functions yields a solution that satisfies the mass balance.     

Particle transport method for convection problems with reaction and diffusion

The paper is devoted to the further development of the particle transport method for the convection problems with diffusion and reaction. Here, the particle transport method for a convection–reaction problem is combined with an Eulerian finite‐element method for diffusion in the framework of the operator‐splitting approach. The technique possesses a special spatial adaptivity to resolve solution singularities possible due to convection and reaction terms. A monotone projection technique is used to transfer the solution of the convection–reaction subproblem from a moving set of particles onto a fixed grid to initialize the diffusion subproblem. The proposed approach exhibits good mass conservation and works with structured and unstructured meshes. The performance of the presented algorithm is tested on one‐ and two‐dimensional benchmark problems. The numerical results confirm that the method demonstrates good accuracy for the convection‐dominated as well as for convection–diffusion problems. Copyright © 2007 John Wiley & Sons, Ltd.     

Efficient identification of uncertain parameters in a large‐scale tidal model of the European continental shelf by proper orthogonal decomposition

The adjoint method can be used to identify uncertain parameters in large‐scale shallow water flow models. This requires the implementation of the adjoint model, which is a large programming effort. The work presented here is inverse modeling based on model reduction using proper orthogonal decomposition (POD). An ensemble of forward model simulations is used to determine the approximation of the covariance matrix of the model variability and the dominant eigenvectors of this matrix are used to define a model subspace. An approximate linear reduced model is obtained by projecting the original model onto this reduced subspace. Compared with the classical variational method, the adjoint of the tangent linear model is replaced by the adjoint of a linear reduced forward model. The minimization process is carried out in reduced subspace and hence reduces the computational costs. In this study, the POD‐based calibration approach has been implemented for the estimation of the depth values and the bottom friction coefficient in a large‐scale shallow sea model of the entire European continental shelf with approximately 10⁶ operational grid points. A number of calibration experiments is performed. The effectiveness of the algorithm is evaluated in terms of the accuracy of the final results as well as the computational costs required to produce these results. The results demonstrate that the POD calibration method with little computational effort and without the implementation of the adjoint code can be used to solve large‐scale inverse shallow water flow problems. Copyright © 2011 John Wiley & Sons, Ltd.     

Parameter identification of river current and diffusion by reduced Kalman filter finite element method

The objective of this study is to propose a parameter identification of a river current and diffusion coefficients by using the reduced Kalman filter finite element method, which has been previously presented and now extended by the authors. For comparison, the estimation computations of velocity, water elevation, and chemical oxygen demand (COD) concentration are carried out on the basis of nonlinear shallow water flow and compared with the observations carried out at the Teganuma river in Japan. A marked discrepancy in COD concentration is found between the computed and observed results. The correlation factor between the computed and observed results is 0.51. To reduce the discrepancy, the authors believe that the diffusion coefficients should be identified. Assuming that the diffusion coefficient is constant in the entire domain and over the entire total duration, satisfactory results were not obtained. Thus, the computational domain is divided into four subdomains according to the water depth. Assuming that the diffusion coefficients are constant in each subdomain, the identification is carried out. Relatively good, albeit unsatisfactory, results are obtained. The discrepancy between the computed and observed COD concentration has special features. In some time zones, the computed results are larger whereas in other time zones, they are smaller than the observed results. To compensate this discrepancy, we assumed that the diffusion coefficient is a function of COD concentration. The correlation factor is improved to be 0.73. The identified diffusion coefficients are time functions that change cyclically with a period of 24 h. This fact suggests that biological phenomenas occurred in the river. Copyright © 2011 John Wiley & Sons, Ltd.     

流固耦合破坏分析的多分辨率PD-SPH方法

流固耦合破坏是一类涉及结构变形与破坏以及复杂自由表面现象的强非线性力学问题.结合近场动力学(peridynamics, PD)与光滑粒子流体动力学(smoothed particle hydrodynamics, SPH)各自的优势并考虑其计算效率问题,提出一种适用于分析流-固耦合破坏问题的多分辨率PD-SPH混合方法.分别采用SPH和PD方法以不同的空间和时间分辨率对流体和结构进行离散与求解,利用具有与流体粒子相同光滑长度的虚粒子处理流-固界面,以高精度满足界面边界条件.通过两个经典算例:液柱静压力下弹性板的变形和溃坝流体冲击弹性闸门的变形问题,表明提出的多分辨率PD-SPH方法兼具较高的计算精度和计算效率;对含裂缝的Koyna重力坝水力劈裂问题进行模拟计算,所得裂缝扩展路径与文献结果吻合,说明该方法适用于涉及结构破坏的流固耦合问题仿真.最后尝试采用该方法进行流体冲击作用下含裂纹混凝土板崩塌过程数值仿真,准确描述混凝土板的断裂破坏和全过程中的流体运动.多分辨率PD-SPH混合方法或可为流-固耦合作用下的结构损伤破坏仿真提供一种新选择.     

基于非线性状态空间辨识的气动弹性模型降阶

高维、非线性气动弹性系统的模型降阶是当前气动弹性力学与控制领域的研究热点之一.然而国内外现有的非线性模型降阶方法仍存在辨识算法复杂、精度有待提高等问题.本研究提出了一种基于非线性状态空间辨识的跨音速气动弹性模型降阶方法. 首先,该方法基于非定常空气动力的单位脉冲响应数据,采用特征系统实现算法对非线性状态空间模型的线性动力学部分进行系统辨识. 其次,引入状态和控制输入的非线性函数, 采用优化算法对非线性函数的系数矩阵进行优化,进而得到考虑非线性效应的空气动力降阶模型.为了验证该降阶模型在预测跨音速气动弹性力学行为的精确性,本文以三维机翼为研究对象,分别从基于非线性降阶模型的气动力辨识、跨声速颤振边界计算和极限环振荡预测三方面进行了算例验证,并与现有的模型降阶方法进行了对比, 进一步说明本文所提出方法的有效性.研究结果表明, 该降阶模型对上述三类问题的计算精度与直接流-固耦合方法相吻合,可用于高效预测飞行器跨声速气动弹性力学行为.      

基于弹性力学第一性原理的数据驱动力学建模

提出了一种基于弹性力学第一性原理的数据驱动力学建模方法,其能够从基于弹性力学方程的数值计算结果建立简洁且能准确捕捉变形机制的力学模型。基于有限元计算得到的高精度数据和无监督数据驱动控制方程识别方法Seq-SVF,从梁的载荷和位移数据中自动识别出了Timoshenko梁形式的弯曲控制微分方程,得到了三种不同加载条件下剪切影响系数关于结构尺寸和力学参数的函数表达式。揭示了经典模型适用的加载条件,同时还给出了一种未发现的新模型。通过将基于弹性力学的第一性原理计算与数据驱动范式相结合,克服了传统建模方法的局限性和对人类经验的强依赖性,为建立简洁的力学模型提供了一种新途径。     

Forward and Inverse Bio-Geochemical Modeling of Microbially Induced Calcite Precipitation in Half-Meter Column Experiments

Microbially induced calcite precipitation (MICP) offers an alternative solution to a wide range of civil engineering problems. Laboratory tests have shown that MICP can immobilize trace metals and radionuclides through co-precipitation with calcium carbonate. MICP has also been shown to improve the undrained shear response of soils and offers potential benefits over current ground improvement techniques that may pose environmental risks and suffer from low “certainty of execution.” Our objective is to identify an effective means of achieving uniform distribution of precipitate in a one-dimensional porous medium. Our approach involves column experiments and numerical modeling of MICP in both forward and inverse senses, using a simplified reaction network, with the bacterial strain Sporoscarcina pasteurii. It was found that the stop-flow injection of a urea- and calcium-rich solution produces a more uniform calcite distribution as compared to a continuous injection method, even when both methods involve flow in opposite direction to that used for bacterial cell emplacement. Inverse modeling was conducted by coupling the reactive transport code TOUGHREACT to UCODE for estimating chemical reaction rate parameters with a good match to the experimental data. It was found, however, that the choice of parameters and data was not sufficient to determine a unique solution, and our findings suggest that additional time and space-varying analytical data of aqueous species would improve the accuracy of numerical modeling of MICP.     

Kinetics of the interaction of a liquid diffusing into another one as studied by reflection spectroscopy (ATR)

An approach is offered for determining the reaction rate constant (k) between two liquid substances, the one penetrating into the other. The procedure is based on the experimental measurement of the diffusion coefficient (D). As model reaction the isotopic exchange process in the ketone octanone-2 molecule is chosen, whose active hydrogen atoms undergo deuteration by the strong base trioctyl-methyl-ammonium deuteroxide (TOMAOD). The diffusion coefficient of the penetrating TOMA-OD, when this reaction takes place, and the rate constant of the latter are determined by an attenuated total reflection (ATR) spectroscopic method [1] on the grounds of appropriate mathematical modeling [2]. The application of this simple and comparatively rapid approach results in thek-value of 1.04 × 10^?2 sec^?1 for the monomolecular interaction mentioned above. The reasons for such an assumption arise from the only initial process stage treatment, where the reactant (TOMA-OD) particles enter the substrate (the ketone) surrounded by an excess of its own molecules. This further allows an analytical solution of the resulting diffusion problem.     

Investigation of the accuracy of models of light-induced gas diffusion by the direct statistical modeling method

A variant of the direct statistical modeling method is proposed as a means of calculating the diffusion velocity in a dense buffer gas; it is then used to investigate the accuracy of the strong collision model for the atomic diffusion regimes most frequently encountered in experiments; the accuracy of the approximation, which uses a constant collision frequency, is investigated by comparing the diffusion velocities obtained for different particle interaction potentials.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 149–155, September–October, 1990.The author wishes to thank M. N. Kogan for his interest in the work and useful discussions, A. A. Abramov and M. S. Ivanov for advice on the calculation method, and S. V. Ivanov for useful discussions.     

Application of equation regulation method to the solution of inverse problems in multiphase materials

In this paper, we propose a new method of determining local material properties of multiphase composites given the experimentally measured displacements and known traction boundary conditions. In the proposed method, an “observation” term is added to the original differential equation, and the modified equation is solved in terms of a regulation parameter. We call this approach the equation regulation (ER) method. By appropriately adjusting the value of the regulation parameter based on the noise level in the input data, we get faster convergence and improved stability than prevailing methods of solving the inverse problem in elliptic ordinary differential equations. Several numerical examples to the solution of this non-linear problem with continuous and discontinuous coefficient functions are given to show the accuracy and reliability of the proposed method.     

Theoretical Analysis of Anion Exclusion and Diffusive Transport Through Platy-Clay Soils

Diffusive transport through geosynthetic clay liners and engineered compacted clay landfill liners is the primary mechanism for mass transport from well-engineered modern landfills. For this reason, accurate estimates of diffusion coefficients for clay soils are essential for the design of engineered liner systems. A long-standing theoretical problem is the role of anion exclusion on the estimation of diffusion coefficients for ionic solutes migrating through charged porous media. This paper describes the steady-state solution of a fully coupled set of transport equations modeling ion movement through a permanently charged platy-clay soil. The microscale analysis takes into account the actual diffusion coefficient for each ion species, ion-pairing (as required by electroneutrality of the solution), as well as anion exclusion and cation inclusion ,arising from the permanent charge on clay particles. To render the problem tractable, the theoretical analysis focuses on an extremely small two-dimensional unit cell in an ideal, saturated, two-phase porous medium. The analysis presented here is limited to a particular geometrical example, but this example is sufficiently general for characteristic behaviours of systems of this kind to be identified. Most importantly, new insight is gained into the mechanism of ion migration through a charged platy-clay soil. The numerical results obtained from this study show that the identification of macroscopic transport quantities such as effective diffusion coefficients and membrane potentials from diffusion cell tests using standard diffusion theory only hold for a specific system. While ion exclusion behaviours are often referred to in the literature, as far as the authors are aware there has been no previous detailed microscale analysis of their role in steady-state diffusion through a charged platy-clay soil.     

Simulation of viscous water column collapse using adapting hierarchical grids

An adaptive hierarchical grid‐based method for predicting complex free surface flows is used to simulate collapse of a water column. Adapting quadtree grids are combined with a high‐resolution interface‐capturing approach and pressure‐based coupling of the Navier–Stokes equations. The Navier–Stokes flow solution scheme is verified for simulation of flow in a lid‐driven cavity at Re=1000. Two approaches to the coupling of the Navier–Stokes equations are investigated as are alternative face velocity and hanging node interpolations. Collapse of a water column as well as collapse of a water column and its subsequent interaction with an obstacle are simulated. The calculations are made on uniform and adapting quadtree grids, and the accuracy of the quadtree calculations is shown to be the same as those made on the equivalent uniform grids. Results are in excellent agreement with experimental and other numerical data. A sharp interface is maintained at the free surface. The new adapting quadtree‐based method achieves a considerable saving in the size of the computational grid and CPU time in comparison with calculations made on equivalent uniform grids. Copyright © 2005 John Wiley & Sons, Ltd.     

PRECISE INTEGRAL ALGORITHM BASED SOLUTION FOR TRANSIENT INVERSE HEAT CONDUCTION PROBLEMS WITH MULTI-VARIABLES

IntroductionIHCPs (InverseHeatConductionProblems)arecloselyassociatedwithmanyengineeringaspects,andwelldocumentedintheliteratures,coveringtheidentificationsofthermalparameters[1,2 ],boundaryshapes[3],boundaryconditions[4 ]andsource_relatedterms[5 ,6 ]etc .Howeveritseemsthatonlylittleworkisdirectlyconcernedwithmulti_variablesidentificationsbyauthors’knowledge.Tsengetal.presentedanapproachtodeterminingtwokindsofvariables[7],butonlygavefewnumericalexamplestodeterminethemsimultaneously .Thesol…     

Robust disturbance rejection method for uncertain system with disturbances of unknown frequencies

In this paper, we discuss a robust disturbance rejection method for dealing with disturbances of unknown frequencies. Unlike many other approaches, the method proposed here does not require the disturbance frequencies of the separate harmonics to be estimated. The current approach is based on disturbance reduction and disturbance suppression. This novel disturbance reduction controller consists of an inverse of the nominal model with an input deduction and a high gain integral term. The proposed controller can reduce both periodic and nonperiodic unknown disturbances with uncertainties in both stable and unstable systems. In addition, undesired responses caused by residual disturbances and residual modeling uncertainties are suppressed by combining the novel disturbance reduction controller with a sliding mode controller. The simulation results demonstrate that the proposed disturbance rejection method performs well under different disturbance inputs including random signals.