粒径对土持水性能的影响

 进行不同粒径土样的持水特性试验,对吸湿与脱湿过程中的土–水特征曲线与滞后现象进行分析,探讨粒径对土持水性能的影响。利用D. G. Fredlund和A. Xing的土–水特征曲线模型对试验数据进行拟合,得到模型拟合参数与土–水特征曲线参数的定量关系。结果表明：参数a与进气值呈线性增加趋势,参数m随着残余基质吸力的增大而减小,参数n也随着减湿率的增大而减小。随着有效粒径的增大,进气值、进水值、残余基质吸力在半对数坐标上均随之呈现线性减小,减湿率呈二次函数增长。滞后效应随着有效粒径的增大而减小,同样随着粒径级配累积曲线斜率的增大而减小。     

Effect of particle size and particle size distribution on the post-liquefaction strength of granular soils

In practice, post-liquefaction strength is largely determined from case histories, but the case history database is limited and there are gaps in the information available in each case. As a result, laboratory tests have continued to play a key role in determining how various factors influence post-liquefaction strength. This paper presents an investigation into how factors such as particle size, particle size distribution, fines content, and compressibility influence post-liquefaction strength in simple shear tests. Eleven particle size distributions of a natural soil and a tailings material, ranging from silt to fine gravel, were studied. The results were compared with case histories.Materials of different geologic origins have meaningfully different post-liquefaction strengths. Particle size and particle size distributions were both found to significantly affect post-liquefaction response as well. Post-liquefaction strengths obtained from simple shear tests agreed well with those back calculated from case histories. Laboratory testing can be utilized to determine how the post-liquefaction strength of a given material may compare to the case history database, and can be used to guide design decisions. This is an important practical finding, given the sparse case history database.     

颗粒级配对水泥浆体强度和自收缩的影响

通过实验室球磨机制备出比表面积分别为280、370和670 m2/kg三种不同细度的水泥,与不同掺量矿粉配制成不同颗粒级配的复合水泥,并进行了复合水泥干粉压实体孔隙率、复合水泥浆体的抗压强度、孔隙率、自收缩和BSEM测试。结果表明:随着水泥细度的增加,压实体的孔隙率逐渐增大。细水泥对复合水泥浆体早期孔隙的细化效果显著,提高了大掺量矿渣复合水泥浆体早期强度。矿粉的掺入减小了复合水泥体系的自收缩,矿粉掺量越大,水泥浆体自收缩越小。     

Verification of the accuracy of CFD simulations in small-scale tunnel and atrium fire configurations

In preparation for the use of computational fluid dynamics (CFD) simulation results as ‘numerical experiments’ in fire research, the agreement with experimental data for two different small-scale set-ups is discussed. The first configuration concerns the position of smoke-free height in case of fire with vertical ventilation in an atrium. The second set-up deals with the critical velocity for smoke backlayering in case of fire in a horizontally ventilated tunnel. An N-percent rule is introduced for the determination of the presence of smoke in the simulation results, based on the local temperature rise. The CFD package FDS is used for the numerical simulations. The paper does not scrutinize the detailed accuracy of the results, as this is hardly possible with any state-of-the-art experimental data at hand. Rather, the global accuracy is discussed with current numerical implementation and models in FDS, considering continuous evolution over different version releases with time. The agreement between the experiments and numerical simulations is very promising. Even when quantitative agreement with experimental data is not perfect, the trends are very well reproduced in the simulations. While much additional work is required, both in CFD as in ‘real’ experiments, the results are encouraging for the potential of state-of-the-art CFD to be used as numerical experiments.     

The role of slurry TBM parameters on ground deformation: Field results and computational modelling

This paper examines the role of slurry TBM operational parameters, namely face, annulus and grout pressures, on ground deformation. The analysis includes both field data and computational modelling. Field data from the very successful Queens bored metro tunnels (6.9 m diameter) project in New York City is employed to illustrate phenomenologically how ground deformation was controlled via slurry TBM parameters. Computational modelling is then employed to support the experimental findings and to further investigate the relative influence of TBM parameters through parametric analysis. The results quantify the importance of the various pressures on controlling ground deformation.     

水泥颗粒分布对混凝土耐磨性能的影响

分别采用水泥胶砂耐磨性试验方法与混凝土及其制品耐磨性试验方法(滚珠轴承式)研究了不同颗粒分布的水泥胶砂及其混凝土的耐磨性能,并通过压汞法检测了不同颗粒分布的水泥配制的混凝土砂浆的孑L隙率.试验结果表明:颗粒分布较宽的水泥,其砂浆与混凝土的耐磨性能均优于颗粒分布较集中的水泥.较宽的颗粒分布使水泥砂浆或混凝土具有良好的密实性及孔结构,有利于耐磨性的提高.     

3D CFD simulations of wind flow and wind-driven rain shelter in sports stadia: Influence of stadium geometry

Sports stadia are increasingly used to host a wide variety of activities that attract large attendances, ranging from sports matches to concerts, festivities and conferences. One of the crucial aspects of spectator comfort in open stadia is protection from wind and rain. However, in many stadia this part of spectator comfort is insufficiently taken care of. The main reason is that stadia and stadium roofs are often designed with only vertical rainfall in mind, neglecting the influence of wind that can sweep the rain onto the stands. This wind-driven rain (WDR) can reach a large area of the stand underneath the roof, resulting in discomfort for the spectators in this area. For stadium design, it is important to understand the interaction between wind and rain in different types of stadium geometry and its effect on wetting of the stands.     

胶凝材料颗粒级配对水泥凝胶体结构及强度的影响

根据颗粒紧密堆积理论，探讨胶凝材料的颗粒级配对水泥凝胶体微观结构及强度的影响。研究表明，在水泥中掺入适当细度的矿物掺合料，使胶凝材料的颗粒级配接近紧密堆积状态时，砂浆的强度会有所提高，水泥凝胶体的微观结构也会得到改善。     

Effect of atmospheric temperature and droplet size variation on ice accretion of wind turbine blades

A numerical study of ice accretion and the resultant flow field characteristics of a 5 MW pitch controlled wind turbine blade profile (NACA 64618) have been carried out to understand the effects that atmospheric temperature and droplet size variations have on the rate and shape of ice growth. Resultant aerodynamic characteristics of the blade profile were analysed at different angles of attack ranging from −10° to +20°. Results show an increase in the ice growth with the increase of droplet size; whereas change in atmospheric temperature significantly affects the shape of accreted ice. Streamlined ice shapes were observed for low temperatures, whereas horn shape ice accretion was found at higher temperatures. Results show that for the iced blade profiles, changes in the aerodynamic characteristics are least prominent for the case of rime ice as compared to glaze ice.     

基于CFD的开放空间油池火燃烧速率和热辐射研究

采用混合组分燃烧模型和有限体积辐射模型,通过液体表面蒸发模型对液态燃料和火羽流进行耦合,建立开放空间油池火模型.利用CFD方法分别对不同直径的庚烷油池火进行模拟,研究其燃烧速率、热释放速率随直径的变化以及火焰中轴上的温度和单位体积热释放速率(HRRPUV)分布,并得出油池表面的热辐射反馈以及油池外部水平和垂直方向的热辐射强度分布规律.部分模拟结果与实验进行对比,验证该模型的适用性和有效性.     

原油罐区池火CFD模拟及应急对策研究

采用三维 CFD 火灾模拟软件 KFX 对某原油罐区不同尺度池火场景进行模拟。通过计算不同泄漏孔径下的泄漏速率,确定不同尺度的池火面积。在进行池火模拟时考虑不同的风向、风速以及罐区几何结构特点,得到不同热辐射通量的影响范围以及被火焰包裹的设施所受到的热辐射通量。结合模拟结果,分析了按照国内相关设计标准设计的罐区在不同池火场景中的安全状态和事故升级可能性。为罐区安全设计拓宽了思路,同时解决了应急准备分析中面临的实际问题。     

Suspended particle size distribution and the performance of deep bed filters

The paper examines the importance of suspended particle size distribution in deep bed filtration. Experiments were conducted with suspensions of different size distributions. The changes in total concentration, concentrations of particles of different sizes and the headloss were observed. The suspension with the largest mean particle size gave the greatest removal. It was also observed that the size distribution changed with depth and time, and this was shown to be crucial in understanding filter behaviour. It was concluded that in modelling filter performance it is not sufficient to use the mean characteristics of the influent suspension. Some evidence was found for the interaction between particles of different sizes, and the particle size distribution also affects the distribution of deposit throughout the bed.     

颗粒级配对水泥基材料性能的影响综述

本文综述了水泥基复合材料颗粒级配问题的研究起源、研究现状,从最紧密堆积和水化速率两个方面探讨了颗粒级配对复合水泥体系性能的影响。目前在亚微观范围内,使胶凝材料颗粒形成良好的级配,对胶凝体的结构及强度有何影响的研究还很欠缺;且水泥基材料颗粒级配的研究集中在超细矿物掺合料对水泥的填充机理和作用,而水泥熟料细粉填充矿物掺合料的研究报道较少,本文论证了超细水泥填充矿物掺合料的必要性和可行性。     

A comparative study of the grading coefficient, a new particle size distribution parameter

 The importance of the particle size distribution (grading) of a gravel material for unsealed roads has long been recognized. Various parameters have been devised to allow interpretation of grading curves in terms of a single parameter for ease of application, control and comparison. It is shown that there are significant correlations between many of the better known parameters, although it is also clear from the scatter of the data that the parameters probably indicate slightly different properties. The investigation has shown that a new parameter, the grading coefficient, is a good indicator of the grading and can be used interchangeably with many other parameters. In general terms the grading coefficient is approximately half the percentage gravel in a material and grading coefficients of 16 and 34 are roughly equivalent to grading moduli of 1.5 and 2.6, respectively. Received: 18 June 1998 · Accepted: 20 July 1998     

Study of cooling system with water mist sprayers: Fundamental examination of particle size distribution and cooling effects

A cooling system that sprays micro water droplets could prove useful in mitigating temperature increases in urban areas by using the heat of water evaporation, a process that consumes only small amounts of water and energy. If water mist is sprayed in a semi-outdoor area, for example, under a canopy, it could potentially improve conditions on hot days. However, there is little reference data concerning the design or control of such systems. In order to propose a method for designing and predicting the performance of a water mist system, we discuss differences in cooling effects in the context of particle size distribution of water mist. The results of numerical fluid analysis showed there is no significant difference in temperature reduction for different particle sizes. However, the water particles remained in a lower position with larger particles.     

Generation of irregular particle packing with prescribed statistical distribution,spatial arrangement,and volume fraction

A method for packing irregular particles with a prescribed volume fraction is proposed. Furthermore, the generated granular material adheres to the prescribed statistical distribution and satisfies the desired complex spatial arrangement. First, the irregular geometries of the realistic particles were obtained from the original particle images. Second, the Minkowski sum was used to check the overlap between irregular particles and place an irregular particle in contact with other particles. Third, the optimised advance front method (OAFM) generated irregular particle packing with the prescribed statistical distribution and volume fraction based on the Minkowski sum. Moreover, the signed distance function was introduced to pack the particles in accordance with the desired spatial arrangement. Finally, seven biaxial tests were performed using the UDEC software, which demonstrated the accuracy and potential usefulness of the proposed method. It can model granular material efficiently and reflect the mesostructural characteristics of complex granular materials. This method has a wide range of applications where discrete modelling of granular media is necessary.     

Influence of water content and dry density on pore size distribution and swelling pressure of two Indian bentonites

Bulletin of Engineering Geology and the Environment - The swelling pressure of compacted bentonites plays an important role in the design of deep geological repositories. The current study...     

Effects of particle shape and size distribution on the shear strength behavior of composite soils

The effects of particle shape and size distribution on the constitutive behavior of composite soils with a wide range of particle size were investigated. Two comparable sets of specimens were prepared: (1) mixtures of fines (clay and silt) and an ideal coarse fraction (glass sand and beads), and (2) mixtures of fines and natural coarse fraction (river sand and crushed granite gravels). Direct shear box testing was undertaken on 34 samples and the structure of the shear surfaces, change in volume and water content and the particle shape coefficient of the sheared specimens were examined. The results indicate that the contraction/dilation a specimen exhibits is restrained within the shear zone while the outer zones remain unchanged during shearing. An increased coarse fraction leads to an increase in constant volume shear strength. In addition, increasing elongation or decreasing convexity of the coarse fraction increases the constant volume friction angle. The overall roughness of the shear surface at constant volume state is negatively related to particle smoothness (convexity) and positively related to the area of the shear surface occupied by particles with particular shapes. Two equations are proposed for the estimation of constant volume friction angle based on the proportion and shape coefficient of the coarse fraction. It is hoped this will assist in considering the shear strength of mixed soils when the size of the coarse fraction makes laboratory testing difficult.     

Impact of lamp shadowing and reflection on the fluence rate distribution in a multiple low-pressure UV lamp array

Use of mathematical modeling for determination of ultraviolet (UV) fluence in disinfection reactors requires accurate knowledge of the fluence rate distribution in a multiple lamp array. A method for measuring the fluence rate among a multiple lamp array was demonstrated using spherical actinometry. A matrix of four low-pressure UV lamps in air were investigated to evaluate the potential for shadowing and reflection to impact the fluence rate within and surrounding the lamp array. Two fluence rate distribution models were tested to determine the ability to predict the fluence rate distribution measured by the actinometers. Shadowing proved to attenuate UV light. Reflection from the lamp surface added 3-9% to the fluence rate, depending upon position in the reactor. These effects, as well as the fluence rate at various points in the lamp matrix were effectively modeled using RAD-LSI and UVCalc3D fluence rate distribution models. At fluence rates above 8mWcm(-2), the actinometry measured fluence rate was lower than the modeled rate, presumably from saturation of the actinometer solution at high fluence rates (close to the lamp). With multiple lamp reactors, the impact of shadowing can significantly affect fluence rate distribution and thus the level of microbial inactivation. If shadowing is not included in fluence rate distribution models, the fluence rate will be over predicted in the shadow zone of a neighboring lamp, falsely skewing model inactivation predictions.     

On the extrapolation of CFD results for smoke and heat control in reduced-scale set-ups to full scale: Atrium configuration

It is a common practice to use reduced-scale experiments to develop formulae for the design of smoke and heat exhaust ventilation systems. Implicitly, up-scaling of results is assumed justified. A similar approach can be adopted with numerical simulations, i.e. a reduced-scale setup can be up-scaled to compare results to full-scale observations. However, both in numerical simulations and in experiments, scaling must be done in a proper way. The classical method for up-scaling results obtained in fire related experiments is based on preservation of the Froude number only. The present paper, focusing on the up-scaling of results by means of a series of CFD simulations of fire-induced flows in an atrium configuration, confirms this to be justified as long as the flows in both the reduced-scale and full-scale configurations are sufficiently turbulent. If this is not the case, it is illustrated that other dimensionless numbers must also be preserved in scaling, in addition to the Froude number.