水泥基材料接触溶蚀的三维有限元数值模拟

数值模拟方法是研究水泥基材料接触溶蚀问题的有效手段,但三维有限元数值模拟问题一直还未得到有效解决,为此,笔者开展了如下研究:①基于质量守恒定律,推导钙离子传输控制方程,并结合固液平衡关系曲线、孔隙率演化模型与有效扩散系数演化模型建立三维钙溶蚀数学模型;引入离子交换边界条件考虑接触边界浓度变化,为钙溶出量计算提供依据;②根据Galerkin有限元方法导出有限元计算格式,实现模型数值求解;通过算例验证,检验三维接触溶蚀模型及程序的正确性。结果表明:该三维模型能够用于模拟水泥基材料发生的多维溶蚀;针对圆柱体试件开展三维溶蚀数值试验,得到了各溶蚀特征量的时空分布规律;在三维状态下,接触溶蚀发展呈现出由表及里,各处溶蚀速率由快到慢,径向溶蚀速率大于轴向溶蚀速率的总体特征。研究结果为水泥基材料的溶蚀影响分析提供了理论基础及方法。     

环境水侵蚀下水泥净浆钙溶蚀的模拟与验证

钙溶蚀是导致水环境中混凝土等水泥基材料耐久性退化的重要原因之一。为获得软水环境下水泥净浆的钙溶蚀过程, 首先, 基于Fick定律及质量守恒定律, 利用钙溶蚀过程中材料骨架内固体钙含量和孔溶液中钙离子浓度之间的化学平衡关系及Newton边界条件, 建立软水环境下水泥净浆的钙溶蚀模型, 并通过有限差分法, 对该模型进行数值求解; 其次, 进行不同水灰比的水泥净浆试件在6M NH₄Cl溶液中的加速钙溶蚀试验, 测定该溶液中各水泥净浆试件在不同溶蚀时间的钙硅比与孔隙率, 并将所建立模型的计算结果与实测结果进行对比分析, 验证模型的合理性; 最后, 利用验证后的钙溶蚀模型, 数值分析了环境水侵蚀下水泥净浆薄板孔溶液中钙离子浓度、固体钙含量及孔隙率的时空分布规律。结果表明, 模型的计算结果与试验测试结果基本一致; 溶蚀前期, 试件中固体钙含量下降速度和孔隙率增加速率均较大, 溶蚀后期, 试件固体钙溶蚀速率和孔隙率的增加速率逐渐减小。     

氢氧化钙与聚丙烯酰胺对高含水率泥浆固结的影响

为加快高含水率软土排水固结,运用氢氧化钙和阴离子型聚丙烯酰胺(APAM)处理上海某施工现场钻孔灌注桩的高含水率泥浆,并进行试验研究。通过一维固结试验揭示不同添加量下废浆孔隙比、固结系数和渗透系数随荷载的变化规律,确定氢氧化钙和APAM的最优添加量;并开展最优添加量下废浆的真空预压模型试验。结果表明:①一维固结试验中,氢氧化钙和APAM改良后废浆的固结系数和渗透系数均有较大提高,加快了废浆的排水固结速率;②同时,氢氧化钙和质量分数0.2%的APAM溶液的最优添加量约为0.8%和11.2%,超过最优添加量,废浆的固结系数和渗透系数均有所减小;③真空预压模型试验中,经氢氧化钙改良废浆固结系数明显提高,其平均径向固结系数分别为添加APAM溶液废浆与原状废浆的5.1,7.2倍,添加APAM溶液的废浆平均径向固结系数仅为原状废浆的1.4倍。因此氢氧化钙改良联合真空预压法能有效提高废浆排水固结速率。     

不同矿渣掺量水泥基材料表面接触性溶蚀试验

水泥基材料中的可溶性组分可在去离子水的作用下发生溶解,使得材料细观结构等发生改变,进而影响其耐久性。通过室内试验,研究不同矿渣掺量的水泥基材料在去离子水作用下接触性溶蚀特征。通过拟合,得出各周期内电导率值及最大值,并预测不同时刻各类试件侵蚀溶液的矿化度;再通过建立电导率和Ca~(2+)浓度关系,预测未来周期内Ca~(2+)浓度溶出量。结合电导率、侵蚀液的矿化度及Ca~(2+)浓度变化的数学方程,揭示了在水固比较低条件下,矿渣含量越高的水泥基试件,其抗表面接触性溶蚀越强的规律。     

侵蚀性水作用下混凝土的钙溶蚀模型

为了评估侵蚀性水环境下混凝土的钙溶蚀规律,依据软水和硝酸环境下混凝土钙溶蚀机理,建立了侵蚀性水环境作用下混凝土二维钙溶蚀模型。模型考虑了混凝土中孔隙的曲折度和阻塞率,以及混凝土界面过渡区(ITZ)对孔隙率的影响。利用COMSOL软件对理论模型进行求解,并将数值分析结果与试验值进行对比,验证了理论模型的可靠性。对混凝土二维钙溶蚀规律的分析结果表明:混凝土在软水及硝酸环境中的钙溶蚀深度都与时间的平方根成正比。     

Statistical evaluation of calcium carbonate equilibrium in natural water

A simple statistical model for the Langelier Saturation Index has been developed for all drinking water intakes supplying Cracow. The model has been tested by comparison with the solution of the set of equations governing calcium carbonate equilibria. A high degree of agreement with the results of calculations was achieved, including not only the ionic strength of water solutions but also the so‐called ion‐pair effects. The model is summarised by simple formulae, which can be used for manual calculation of the Langelier Saturation Index.     

Modelling the competition of planktonic and sessile aerobic heterotrophs for complementary nutrients in biofilm reactor.

A comprehensive, simplified microbial biofilm model was developed to evaluate the impact of bioreactor operating parameters on changes in microbial population abundance. Biofilm simulations were conducted using three special cases: fully penetrated, internal mass transfer resistance and external mass transfer resistance. The results of model simulations showed that for certain operating conditions, competition for growth limiting nutrients generated oscillations in the abundance of planktonic and sessile microbial populations. These oscillations resulted in the violation of the competitive exclusion principle where the number of microbial populations was greater than the number of growth limiting nutrients. However, the operating conditions which impacted microbial community diversity were different for the three special cases. Comparing the results of model simulations for dispersed-growth, biofilms and bioflocs showed that oscillations and microbial community diversity were a function of competition as well as other key features of the ecosystem. The significance of the current study is that it is the first to examine competition as a mechanism for controlling microbial community diversity in biofilm reactors.     

A general management model to optimize lake liming operations

One of the most common ways to combat the ecosystem effects of acid rain is liming, which can be performed in many ways, such as lake liming, wetland liming, full‐scale drainage area liming, and by dosers in tributaries. This study presents results from extensive liming operations in many Swedish lakes. A practically useful management model for planning and predicting the outcome of liming is presented. The model is based on a validated, general dynamic model for radiocaesium, which has also been used with very good results for radiostrontium, different metals and phosphorus. In this work, the model is applied for calcium from liming. Lake liming can be regarded as a tracer experiment to test the dynamic model. A few of the lakes have been used for the calibrations of the model, and then the model has been validated (blind‐tested) using 68 lakes for pH and 17 lakes for calcium concentrations. Results were very good or good in 97% of the cases for pH and 94% of the cases for calcium. Controls in the two lakes where the dynamic model did not give good pH predictions revealed that these two lakes were fertilized and limed a few years before this study started. If those conditions are accounted for, the model would give good results in these cases also. The model also includes submodels to calculate initial pH, and how this is related to catchment area and lake morphometric conditions, as well as target pH and how this is related to initial pH, and the large‐scale, long‐time influences of deposition of sulphur and other acidifying compounds from the burning of fossil fuels. In order to make the model practically useful, only variables readily accessible from maps and standard monitoring programmes have been accepted as driving variables (such as lake area and mean depth). This work also presents critical model tests (sensitivity and uncertainty analyses using Monte Carlo techniques). These tests have demonstrated that the critical part determining the predictive success of the model is related to the final calculation step where the calculated calcium concentration is transformed into a change in lake pH. It has also been demonstrated that more detailed characterizations of the catchment areas by means of drainage area zonation or Geographical Information System techniques will not increase the predictive accuracy of initial pH values, and that most lakes in the northern part of Sweden were excessively limed to pH values higher, or much higher, than the ‘natural’ or target pH. This is a waste of resources as there are many lakes, especially in the southern part of the country, which need to be limed.     

纳米SiO2和碳酸钙晶须制备水泥基材料性能试验

采用纳米SiO₂和碳酸钙晶须制备水泥基材料，利用SEM、XRD和TG-DSC等技术手段对水泥基材料的水化产物、微观结构和热稳定性等进行有效表征，并试验研究了双掺0%、1%、2%、3%、5%、10%的碳酸钙晶须和1%纳米SiO₂保温水泥砂浆的力学性能和导热性能。研究结果表明：纳米-毫米两种尺度材料掺入水泥浆内部后，纳米SiO₂与水泥水化产物Ca(OH)₂晶体发生二次水化反应，生成C-S-H凝胶体，有效地填充水泥基体孔隙、细化水泥基内部孔径尺寸，碳酸钙晶须具备纤维和微粒双重作用，可以在水泥基中产生纤维的桥联效应，两者材料结合起来，可在水泥基内部形成密实网状絮凝结构；纳米SiO₂和碳酸钙晶须掺入后可以提高砂浆的强度，3%碳酸钙晶须和1%纳米SiO₂配制的保温水泥砂浆抗压和抗折强度分别为25.6 MPa和6.19 MPa,导热系数为0.456 7 W/(m·K),强度和导热性能兼顾。     

磁处理的防垢除垢试验和机理探讨

通过中试研究表明,磁处理能使循环冷却水的水质达标,炭钢A3和炭达96.5%和97.4%.对磁处理防垢除垢的机理进行探讨,认为磁处理防垢除垢的实质是同晶异构现象生成同晶异构体,使碳酸钙生成能量较高的胶体碳酸钙、球霰石和霰石,不易结垢,并通过水溶液传递能量,老垢脱落,并用此机理解释了磁处理的"记忆"效应.     

不同水灰比条件下碳酸钙晶须对水泥净浆收缩性能的影响

为探究碳酸钙晶须对水泥净浆收缩性能的影响,分别测试了3种水灰比试样在不同龄期的干燥收缩、化学收缩和自收缩,并采用X射线衍射(XRD)和压汞法(MIP)分别分析了自收缩试样的物相和孔隙结构变化特性。结果表明:碳酸钙晶须在水泥基体中具有吸附和转移自由水的特性；在不同水灰比条件下,碳酸钙晶须对干燥收缩的影响不同,且影响程度与其掺量呈正相关关系；化学收缩随碳酸钙晶须掺量呈先升后降的变化趋势；碳酸钙晶须对自收缩具有明显的改善作用,尤其是早期自收缩,且水灰比的提高使碳酸钙晶须对早期自收缩的改善效果更加显著；碳酸钙晶须细化了孔隙结构,且未生成新的水化产物。     

TEMPORAL AND SPATIAL DISCRETIZATION ON QUASI-3-D GROUNDWATER FINITE ELEMENT MODELLING TO AVOID SPURIOUS OSCILLATION

In this article, the finite element solution of quasi-three-dimensional (quasi-3-D) groundwater flow was mathematically analyzed. The research shows that the spurious oscillation solution to the Finite Element Model (FEM) is the results choosing the small time step ?t or the large element size L and using the non-diagonal storage matrix. The mechanism for this phenomenon is explained by the negative weighting factor of implicit part in the discretized equations. To avoid spurious oscillation solution, the criteria on the selection of ?t and L for quasi-3-D groundwater flow simulations were identified. An application example of quasi-3-D groundwater flow simulation was presented to verify the criteria. The results indicate that temporal discretization scale has significant impact on the spurious oscillations in the finite-element solutions, and the spurious oscillations can be avoided in solving practical quasi-3-D groundwater flow problems if the criteria are satisfied.     

灰岩人工砂石粉对混凝土微观结构及力学性能影响

采用X射线衍射、扫描电镜和混凝土实验方法,研究了灰岩人工砂中石粉含量对混凝土水化产物种类、含量及混凝土力学性能的影响。结果表明:灰岩人工砂中的碳酸钙与水泥水化产物发生水化反应,形成水化碳铝酸钙,其含量随石粉含量及龄期的增长而增加;水化碳铝酸钙在集料-浆体界面的形成影响了界面上定向氢氧化钙晶体的形成与生长,从微观上改善了混凝土的结构,强化了薄弱的界面区,宏观上提高了混凝土的力学性能。     

仿岩溶碳酸氢钙改良膨胀土试验研究

膨胀土具有吸水膨胀、失水收缩等不良工程性质,常采用石灰、水泥等土壤固化材料对其进行改良,但这些传统的土壤固化材料存在着污染环境、拌合不均匀等问题。以陕西省汉中市强膨胀土为研究对象,对仿岩溶碳酸氢钙改良膨胀土进行了自由膨胀率、无荷膨胀率、收缩、直接剪切、酸碱度和碳酸钙含量测定、粒度分析和扫描电镜试验等物理、力学、化学和微观结构试验。试验结果表明,仿岩溶碳酸氢钙可较好地降低膨胀土的胀缩性,并提高土体的抗剪强度,且仿岩溶碳酸氢钙与膨胀土的最优配比为6∶1;改良土中吸附的交换性Ca²⁺含量降低,碳酸钙含量增多,细粒含量降低,粗粒含量增多,内部孔隙减少;仿岩溶碳酸氢钙改良膨胀土的机理在于溶液中含有H⁺、Ca²⁺、HCO₃^-、CO₃^2-等离子,同时溶解有饱和的二氧化碳气体,通过离子交换作用,溶液中的H⁺置换了土颗粒吸附的Ca²⁺,减小了双电层厚度;通过岩溶作用,新形成的碳酸钙具有沉积胶结和填充作用,颗粒间联结强度和团粒化作用得到增强,从而降低了土体胀缩性。由此可见,仿岩溶碳酸氢钙对膨胀土具有良好的改良作用。     

微生物加固黏土的影响因素与机理分析

为了在岩土和环境工程中更好推广应用微生物诱导碳酸钙沉淀加固技术,本文开展了微生物加固黏土的试验研究,分析了微生物加固黏土试验的主要影响因素,并通过宏观试验结合微观分析的方法研究其加固机理.试验结果表明:适宜的培养基、接种比例和培养环境(温度、pH值)有利于微生物的生长繁殖;在一定脲酶活性条件下,养护温度、养护时间、胶结...     

渗透溶蚀作用下混凝土坝质量损失分布规律

针对渗透溶蚀作用下混凝土坝固相钙的分解问题,基于水泥基材料渗透溶蚀分析模型,提出了混凝土坝溶蚀质量的计算方法,并以石漫滩水库重建工程碾压混凝土重力坝为例,研究了大坝不同部位的溶出质量分布规律.计算结果表明:仿真结果与监测数据吻合良好,验证了该方法的正确性;石漫滩水库重建工程碾压混凝土重力坝胶凝物质溶出部位集中在面板底部...     

THE RELATIONS BETWEEN MODEL PARAMETERS AND CERTAIN PHENOMENA IN TRAFFIC FLOW

Based on the dimensionless dynamic model of traffic flow, the model parameters were compared with numerically simulating solutions, and the effects of the former on the latter was investigated. Some relations between the parameters were obtained. Investigation several idealized results from dimensionless dynamic model of traffic flow were concluded.     

NUMERICAL SIMULATION AND EXPERIMENTAL INVESTIGATION OF THE GUIDE-VANE FLOW AND THE INFLUENCE FROM THE LEAKAGE FLOW

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EXPERIMENTAL AND NUMERICAL INVESTIGATION OF FREE SURFACE VORTEX

An experimental model was set up to investigate the formation and evolution of the free surface vortex. A Particle Image Velocimetry （PIV） was used to measure the free surface vortex flow field at different development stages. Flow visualization was used to locate the vortex position and find its structure. Empirical formulas about the critical submergence and the whole field structure were obtained. It is found that the tangential velocity distribution is similar to that of the Rankine vortex and the radial velocity changes little in the vortex functional scope. Vortex starts from the free surface and gradually intensifies to air entrainment vortex. The vortex core moves during the formation and evolution of the free surface vortex. Based on the experimental model, the vortex position and structure were predicted by numerical simulation combined with a vortex model and compared with that of the experiments, which shows satisfactory agreement.     

Generation of long waves in ice-covered lakes by moving disturbances of atmospheric pressure

The effect of ice cover on the oscillations of water in a two-dimensional constant-depth basin within the linear theory of long waves is considered. The ice cover is treated as thin elastic plate in the presence of compressive force. The ice is fastened hardly to a shore. The eigenfrequencies and eigenfunctions of free oscillations (seishes) are obtained. The forced oscillations of fluid and ice cover under the action of a moving disturbance of atmospheric pressure are investigated. The input data are taken to have values typical of Lake Baikal. The time dependence of total mechanical energy of the fluid is calculated for different velocities of pressure motion. The variations in ice-bending stresses are determined and the causes of shore-ice breaking are explained.