An investigation of matched index of refraction technique and its application in optical measurements of fluid flow

Optical distortions caused by non-uniformities of the refractive index within the measurement volume is a major impediment for all laser diagnostic imaging techniques applied in experimental fluid dynamic studies. Matching the refractive indices of the working fluid and the test section walls and interfaces provides an effective solution to this problem. The experimental set-ups designed to be used along with laser imaging techniques are typically constructed of transparent solid materials. In this investigation, different types of aqueous salt solutions and various organic fluids are studied for refractive index matching with acrylic and fused quartz, which are commonly used in construction of the test sections. One aqueous CaCl₂·2H₂O solution (63 % by weight) and two organic fluids, Dibutyl Phthalate and P-Cymene, are suggested for refractive index matching with fused quartz and acrylic, respectively. Moreover, the temperature dependence of the refractive indices of these fluids is investigated, and the Thermooptic Constant is calculated for each fluid. Finally, the fluid viscosity for different shear rates is measured as a function of temperature and is applied to characterize the physical behavior of the proposed fluids.     

A blood-mimicking fluid for particle image velocimetry with silicone vascular models

For accurate particle image velocimetry measurements in hemodynamics studies, it is important to use a fluid with a refractive index (n) matching that of the vascular models (phantoms) and ideally a dynamic viscosity matching human blood. In this work, a blood-mimicking fluid (BMF) composed of water, glycerol, and sodium iodide was formulated for a range of refractive indices to match most common silicone elastomers (n = 1.40–1.43) and with corresponding dynamic viscosity within the average cited range of healthy human blood (4.4 ± 0.5 cP). Both refractive index and viscosity were attained at room temperature (22.2 ± 0.2°C), which eliminates the need for a temperature-control system. An optimally matched BMF, suitable for use in a vascular phantom (n = 1.4140 ± 0.0008, Sylgard 184), was demonstrated with composition (by weight) of 47.38% water, 36.94% glycerol (44:56 glycerol–water ratio), and 15.68% sodium iodide salt, resulting in a dynamic viscosity of 4.31 ± 0.03 cP.     

轮轨材料硬度匹配性能试验研究

利用滚动磨损试验机研究了车轮钢与U71 Mn热轧钢轨的硬度匹配性能,分析了不同硬度车轮与U71 Mn钢轨匹配时的摩擦磨损与表面损伤行为.结果表明:车轮硬度对轮轨试样滚动摩擦系数基本无影响;随车轮硬度增加,车轮磨损量呈下降趋势,钢轨磨损量表现为线性增加,轮轨总磨损量呈先降低后增加的趋势,轨轮硬度相同时轮轨系统总磨损量达到最小.车轮硬度对车轮和钢轨试样表面损伤形貌有一定影响,车轮硬度低时车轮表面损伤以麻点式剥落损伤为主,随车轮硬度增加试样表面发生大块剥落损伤,对摩副钢轨试样主要表现为表面剥落损伤机制.     

Optical measurement uncertainties due to refractive index mismatch for flow in porous media

Application of optical techniques such as PIV, PTV, and LDA for velocity field estimation in porous media requires matching of refractive indices of the liquid phase to that of the solid matrix, including the channel walls. The methods most commonly employed to match the refractive indices have been to maximize the transmitted intensity through the bed or to rely on direct refractometer measurements of the indices of the two phases. Mismatch of refractive indices leads to error in estimation of particle position, ε _PD, due to refraction at solid–liquid interfaces. Analytical ray tracing applied to a model of solid beads placed randomly along the optical path is used to estimate ε _PD. The model, after validating against experimental results, is used to generate expression for ε _PD as a function of refractive index mismatch for a range of bead diameters, bed widths, bed porosity, and optical magnification. The estimate of ε _PD, which is found to be unbiased, is connected to errors in PIV measurement using the central limit theorem. Mismatch in refractive indices can also lead to reduction in particle density, N _s, detected light flux, J, and degrade the particle image. The model, verified through experiments, is used to predict the reduction in N _s and J, where it is found that particle defocusing caused by spherical beads in refractive index mismatched porous bed is the primary contributor to reductions of N _s and J. In addition, the magnitude of ε _PD is determined for the use of fluorescent dye emission for particle detection due to wavelength-dependent index of refraction.     

Particle image velocimetry in a variable density flow: application to a dynamically evolving microburst

A fondamental difficulty in the experimental study of gravity-driven flows using particle image velocimetry (PIV) and other optical diagnostic techniques is the problem associated with variations in thé refractive index within the fluid. This paper discusses a method by which the refractive indices of two fluids are matched while maintaining density differences of up to 4%. Aqueous solutions of glycerol and potassium phosphate are used to achieve precise index matching in the presence of mixed and unmixed constituents. The effectiveness of the method is verified in a PIV study of a laboratory-scale model of an atmospheric microburst where planes of two-dimensional velocity vectors are obtained in thé evolving flow field.This work was sponsored by the National Science Foundation under grant CTS-9209948. We also thank TSI, Inc. for the use of its facility     

缺口件疲劳寿命分布预测的有效应力法

本文提出了一种由光滑件疲劳寿命试验数据预测缺口件疲劳寿命分布的有效应力法。该方法中缺口件的裂纹可能萌生表面被分解成一个个微元,整个表面可看成是这些微元组成的一个串联模型,按照串联概率失效模型,缺口件的疲劳强度失效概率就可以由各微元的疲劳强度失效概率计算得到,其中微元的疲劳强度失效概率是由光滑件的疲劳强度失效概率通过最弱环节理论计算得到的。在缺口件的疲劳强度失效概率表达式中,引入了有效应力的概念,用它查取光滑件的疲劳寿命试验数据就可以直接得到缺口件的疲劳寿命分布。该方法可以同时考虑到应力梯度和试件尺寸对缺口件疲劳寿命分布的影响。进行了材料LY12CZ的带中心孔缺口件的寿命算例分析,预测结果和试验结果吻合良好,表明该方法是有效的。     

Direct interferometric measurement of streaming birefringence

The two refractive indices in the flow of a colloidal birefringent liquid are measured separately by means of a Mach-Zehnder interferometer. For a quantitative evaluation of the resulting interferograms it is not necessary to linearize the respective equations relating the refractive index distribution to the deformation velocity in the flow. Therefore it becomes possible to perform velocity measurements in the non-Newtonian flow range. An additional measurement of the mean flow rate enables one to determine the velocity field without the need of a calibration of the observed interference fringes.     

A thin-sheet, combined tension and bending specimen

Validating stress intensity factor solutions for combined tension and bending is an arduous task because the necessary experimental data are not readily available. Toward this end, a tension and bending test specimen was designed to produce controllable levels of both tension stress and bending stress at the crack location. The specimen was made from 2024-T3 clad aluminum, which is commonly used in aircraft structures. The need for testing multiple specimens of various geometries and stress levels prompted the development of an analytical tool for specimen design. An extention of the Schijve line model, based on simple beam theory, was developed to calculate the stress distributions of tension and bending through the length of the specimen. A comparison of measured static strain levels with those predicted by the model showed the model to be accurate to within 5 percent, confirming its efficacy for specimen design. As expected, for the same remote stress (100 MPa), cracks in the tension and bending specimens grew faster than those in middle-cracked tension specimens.     

Diffusion of Gold Nanoparticles in Inverse Opals Probed by Heterodyne Dynamic Light Scattering

The diffusive behavior of nanoparticles inside porous materials is attracting a lot of interest in the context of understanding, modeling, and optimization of many technical processes. A very powerful technique for characterizing the diffusive behavior of particles in free media is dynamic light scattering (DLS). The applicability of the method in porous media is considered, however, to be rather difficult due to the presence of multiple sources of scattering. In contrast to most of the previous approaches, the DLS method was applied without ensuring matching refractive indices of solvent and porous matrix in the present study. To test the capabilities of the method, the diffusion of spherical gold nanoparticles within the interconnected, periodic nanopores of inverse opals was analyzed. Despite the complexity of this system, which involves many interfaces and different refractive indices, a clear signal related to the motion of particles inside the porous media was obtained. As expected, the diffusive process inside the porous sample slowed down compared to the particle diffusion in free media. The obtained effective diffusion coefficients were found to be wave vector-dependent. They increased linearly with increasing spatial extension of the probed particle concentration fluctuations. On average, the slowing-down factor measured in this work agrees within combined uncertainties with literature data.

     

Measurement of 3D pore-scale flow in index-matched porous media

We present the experimental analysis of fluid flow at the pore-scale of a transparent porous medium with matched refractive indices of the solid and liquid phases. The planar laser-induced fluorescence (PLIF) technique described is the first to simultaneously visualize the 3D pore-scale flow of two immiscible liquid phases in porous media. Through the application of a highly precise index matching method and the employment of up-to-date CCD imaging hardware, the system features a high spatial resolution and sampling rate. The method was used to study the dispersion of a tracer dye in single-phase flow and the displacement of oil by water in an imbibition process.     

试验确定自增强圆筒的表面裂纹K1因子和疲劳扩展规律

本文提供了一种可以进行自增强处理的圆筒试样,通过实验用柔度法确定了该试样的表面裂纹的应力强度因子,介绍了含表面裂纹试样的柔度测试技术,导出了柔度与表面裂纹尖端前缘各点的应力强度因子的关系式,该式可作为各种含表面裂纹试样柔度法测表面裂纹 K_1因子的参考,本文还测定了不同自增强程度下,自增强圆筒的表面裂纹疲劳扩展规律。     

角钢约束混凝土中长柱轴压力学性能试验及承载力计算

为了研究角钢约束混凝土中长柱在轴压荷载作用下的力学性能,以长细比、缀板间距、混凝土强度等级为变化参数,完成了8个试件的静力加载试验。通过试验观察了试件的破坏形态,获取了其极限承载力、刚度、位移延性和耗能系数等力学性能指标;分析了各变化参数对力学性能指标的影响,利用统一强度理论、极限理论和叠加理论对试件的承载力进行计算。研究结果表明:长细比越小越容易发生柱端破坏,长细比加大后易发生柱中破坏;减小缀板间距,试件的极限承载力和变形能力均得到提高。随着混凝土强度等级的提高,试件的极限承载能力和初始弹性刚度得以提高,但位移延性和耗能能力有所降低。采用统一强度理论的计算值略大于试验值,采用极限分析理论和叠加理论的计算值均小于试验值。     

Miniature load-cell instrumentation for finite-deformation biaxial testing of elastomers

Accuracy of biaxial-test equipment to examine the nonlinear mechanical behavior of thin-sheet specimens of elastomeric materials has been hampered by lack of precise determination of the force distribution along the sides of the specimen. It has been necessary to use an effective width established by approximate means to obtain the stress from the overall force applied along the edges. To avoid this experimental difficulty, individual miniature proof-ring load cells utilizing semiconductor strain gages have been designed and applied to the support hooks for the thin-sheet specimens. Typical results are shown for time-dependent stress distributions for all degrees of biaxiality. An accurate evaluation of the effective specimen width is now possible, a fact which offers improvement in both accuracy and economy for future testing.     

Influence of Specimen Geometry on Split Hopkinson Tensile Bar Tests on Sheet Materials

In recent years numerous studies on the high strain rate behaviour of sheet materials using split Hopkinson tensile bar set-ups have been reported in literature. For these experiments mostly dogbone-shaped specimens are used. However, widely divergent specimen dimensions can be found. In the present study the influence of this specimen geometry on the test results is investigated experimentally. An extensive series of Hopkinson tests on a steel sheet material using different specimen geometries is performed. An advanced optical technique is used to obtain the true distribution of the deformation along the length of the specimen. Important issues such as the contribution of the deformation of the transition zones to the total deformation and the (non-)homogeneity of the strain in the specimen are thus determined. From the experiments it is clear that the influence of the specimen geometry on the observed behaviour cannot be neglected. It is shown that inconsistencies between the assumed and real specimen behaviour account for these differences. For the TRIP steel considered in the study, accurate deformation values are only guaranteed if the length to width ratio of the central zone is larger than 1.25 and if the radius of the transition zone is sufficiently small.     

高温下轻质泡沫铝动态力学性能实验

对传统的分离式Hopkinson压杆装置加以改进,设计了一种长杆直接撞击Hopkinson杆的实验方案,检测出低波阻抗材料在高温动态加载下的应力均匀性。对轻质泡沫铝材料的实验表明,在同一撞击速度下,温度越高,试件两端的应力均匀性越差,增加温度与提高撞击速度均会导致泡沫铝材料冲击端与支撑端的应力不均匀性。根据高温下应力均匀性的实验结果,确定高温下试件均匀变形对应的冲击速度,再通过传统的分离式Hopkinson压杆实验得出泡沫铝在高温动态下的力学性能。     

M-shaped Specimen for the High-strain Rate Tensile Testing Using a Split Hopkinson Pressure Bar Apparatus

An experimental technique is proposed to determine the tensile stress–strain curve of metals at high strain rates. An M-shaped specimen is designed which transforms a compressive loading at its boundaries into tensile loading of its gage section. The specimen can be used in a conventional split Hopkinson pressure bar apparatus, thereby circumventing experimental problems associated with the gripping of tensile specimens under dynamic loading. The M-specimen geometry provides plane strain conditions within its gage section. This feature retards necking and allows for very short gage sections. This new technique is validated both experimentally and numerically for true equivalent plastic strain rates of up to 4,250/s.     

Ultrasonic detection of high-temperature fatigue-crack growth

An ultrasonic technique is described for continuously monitoring fatigue-crack growth at temperatures up to 300°C in wedge-opening-load type fracture-toughness specimens. The sound-wave energies reflected from the leading edge of the fatigue crack and from the bottom surface of the specimen are electronically compared and used to control the crack-measurement system.     

冲击载荷下瞬态温度的实时测量方法

将红外瞬态测温装置引入SHPB冲击实验,确定了不同材料试件的温度标定曲线,并实时测量了冲击下Al合金和伪弹性TiNi合金试样的表面温度。结果表明,2种试样温度变化都经历了加载过程的温度升高,主要不同在于卸载过程,Al合金卸载过程中温度保持最大加载温度不变,而TiNi合金试样卸载过程中温度降低,这反映了2种材料不同的物理变形过程和温度变化机制。直接红外测温的实验结果与根据能量守恒理论计算的温度较好吻合,说明采用的红外测温方法实时测量冲击瞬态温度是可行的。     

SHPB帽形试样尺寸效应研究

利用ANSYS/LS-DYNA软件开展不同尺寸帽形试样的SHPB数值模拟,研究帽形试样的尺寸效应。结果表明,当帽形试样剪切变形区域宽度大于0.2 mm时,根据经典公式处理得到的应力值将随t的增大而显著增大, 严重偏离理论值,当t=1.0 mm时,计算得到的应力值甚至接近理论值的2倍。采用分体模型进行的进一步计算表明,数据处理结果中应力偏差主要来源于帽形试样中环状部位的膨胀变形。提出改进的数据处理方法,并采用圆柱样与几种不同尺寸帽形试样开展了验证实验,结果与计算结果基本一致。     

Bubble behavior in a three-phase fluidized bed

Bubble size measurements by off-axis holography are reported for the first time in three-phase fluidized beds. The solid material was glass cylinders, with the liquid in one case having matching refractive index. Two different columns, 76 and 152 mm dia, were employed. An interpolative correlation was developed for the solid volume fraction of the bed as a function of liquid flux. A generalized dimensionless model was developed to predict the equilibrium bubble size in both fixed and fluidized beds.