应力波和动光弹等差条纹的分析与判读

介绍了动光弹的基本原理，并从理论上回答了动光弹应用中的一些问题，如惯性力可否忽略等．并在总结回顾动光弹等差条纹级次判读方法的基础上，提出了把应力波理论与等差条纹分析判读相结合的方法，这样不仅使条纹判读的概念更清晰，而且可提高条纹分析和级次判读的准确性．     

Determining the three individual stress components from measured isochromatic fringes

This paper combines measured isochromatic information, complex stress functions and numerical concepts into a new and effective hybrid method of stress analysis. The technique simultaneously smooths the measured isochromatic data, provides accurate boundary information, and separates the isochromatic information into normal and shear stresses at nonboundary locations. No additional experimental data such as the isoclinics are needed. The technique is illustrated experimentally by application to a tensile plate containing a hole.Paper was presented at the 1990 SEM Spring Conference held in Albuquerque, NM on June 3–6.     

Experimental visibility measurement technique

Experimental Techniques -     

A critical review of methods for determining stress-intensity factors from isochromatic fringes

Two-parameter methods of fracture analysis for determining the stress-intensity factor from photoelastic isochromatic-fringe data were critically reviewed. The methods of Irwin, Bradley and Kobayashi, and Smith were developed in detail and differences in the three approaches were noted. Theoretical fringe loops were generated for a crack of length 2a in a semi-infinite plate with biaxial loading. These fringe loops were used to compare the three analysis methods and to determine the accuracy of each method. All three methods give a close estimate of the stress-intensity factor, with the Bradley-Kobayashi differencing procedure providing the most precise estimate ofK. However, if measurement errors become excessive (larger than 2 percent) the differencing procedure magnifies these errors and the original method proposed by Irwin is the recommended approach. The two-parameter methods can be employed to determineK to within ±5 percent, provided the angle of tilt of the isochromatic-fringe loop is 73 ≤θ _m < 139 deg. Ifθ _m is outside this range, the two-parameter methods should not be employed.     

Gas Permeability of Fractured Sandstone/Coal Samples under Variable Confining Pressure

Rock fractures transmit underground gas effectively once they have sufficient widths and interconnection. However, the fracture geometries needed for gas transport are strongly influenced by surrounding pressure conditions. In order to inspect and quantify the influence of surrounding pressure, we design and manufacture a set of gas flow apparatus that can be connected to the MTS815 material testing system, which provides loads and exhibits external pressures in the experiment. With the apparatus and MTS815, we test the fractured samples of sandstone and coal and obtain their relationship between permeability and external pressure. In particular, our permeability calculation based on collection of gas flux and pressure difference has involved the influence of non-Darcian flow. In addition, our study also includes a numerical simulation in the RFPA software platform to display the internal field changes of cracks. The results show that fracture permeability strongly depends on confining pressure, and a critical pressure probably occurs, about 1.5–2 MPa in our experiments, to split each of the permeability curves into two stages, a slow climb and an exponential rush. As a complement, the numerical simulation also demonstrates one more stage for the permeability curve, the post-rush steady fluctuation.     

Non-Newtonian Flow in a Variable Aperture Fracture

The transmissivity of a variable aperture fracture for flow of a non-Newtonian, purely viscous power-law fluid with behavior index n is studied. The natural logarithm of the fracture aperture is considered to be a two-dimensional, spatially homogeneous and correlated Gaussian random field. We derive an equivalent fracture aperture for three flow geometries: (1) flow perpendicular to aperture variation; (2) flow parallel to aperture variation; (3) flow in an isotropic aperture field. Under ergodicity, results are obtained for cases 1 and 2 by discretizing the fracture into elements of equal aperture and assuming that the resistances due to each aperture element are, respectively, in parallel and in series; for case 3, the equivalent aperture is derived as the geometric mean of cases 1 and 2. When n=1 all our expressions for the equivalent aperture reduce to those derived in the past for Newtonian flow and lognormal aperture distribution. As log-aperture variance increases, the equivalent aperture is found to increase for case 1, to decrease for case 2, and to be a function of flow behavior index n for case 3.     

Variable viscosity in peristalsis of Sisko fluid

This paper aims to examine variable viscosity effects on peristalsis of Sisko fluids in a curved channel with compliant characteristics. Viscous dissipation in a heat transfer is studied. The resulting problems are solved using perturbation and numerical schemes to show qualitatively similar responses for velocity and temperature. A streamline phenomenon is also considered.     

Some Ambiguities in the Complex Variable Method in Elasticity

The application ad litteram of the complex variable method for solving plane elastic problems according to the classical procedure of analytic continuation may present some ambiguities. These can be resolved after a careful application of the Schwarz Reflection Principle in constructing the stress functions.     

Complex Dynamics in Pendulum-Type Equations with Variable Length

We prove the existence of complex dynamics for a generalized pendulum type equation with variable length. The solutions we find switch from an oscillatory behavior around the stable vertical position to a rotational type behavior crossing the unstable position with positive or negative velocity following any prescribed two-sided sequence of symbols. Moreover, to any periodic sequence of symbols corresponds a periodic solution of the equation. The proof is based on a topological approach and the results are robust with respect to small perturbations. In particular a small friction term can be added to the equation.     

Effect of chemical composition of PM2.5 on visibility in Guangzhou,China, 2007 spring

The object of this study was to investigate the correlation of visibility with chemical composition of PM2.5 in Guangzhou. In April 2007, 28 PM2.5 samples were collected daily at the monitoring station of the South China Institute of Environmental Sciences （SCIES）, in urban Guangzhou. Water-soluble ionic species （CI^-, NO3^-, SO4^2-, NH4^＋, K^＋, Na^＋, Ca^2＋, and Mg^2＋） and carbonaceous contents （OC and EC） of the PM2.5 samples were determined to characterize their impact on visibility impairment. The results showed that sulfate was the dominant species that affected both light scattering and visibility. The average percentage contributions of the visibility-degrading species to light scattering coefficient were 40% for sulfate, 16% for nitrate, 22% for organics, and 22% for elemental carbon. Because of its foremost effect on visibility, sulfate reduction in PM2.5 would effectively improve the visibility of Guangzhou.     

PM2.5 in China: Measurements,sources, visibility and health effects,and mitigation

Concern over the health effects of fine particles in the ambient environment led the U.S. Environmental Protection Agency to develop the first standard for PM_2.5 (particulate matter less than 2.5 μm) in 1997. The Particle Technology Laboratory at the University of Minnesota has helped to establish the PM_2.5 standard by developing many instruments and samplers to perform atmospheric measurements. In this paper, we review various aspects of PM_2.5, including its measurement, source apportionment, visibility and health effects, and mitigation. We focus on PM_2.5 studies in China and where appropriate, compare them with those obtained in the U.S. Based on accurate PM_2.5 sampling, chemical analysis, and source apportionment models, the major PM_2.5 sources in China have been identified to be coal combustion, motor vehicle emissions, and industrial sources. Atmospheric visibility has been found to correlate well with PM_2.5 concentration. Sulfate, ammonium, and nitrate carried by PM_2.5, commonly found in coal burning and vehicle emissions, are the dominant contributors to regional haze in China. Short-term exposure to PM_2.5 is strongly associated with the increased risk of morbidity and mortality from cardiovascular and respiratory diseases in China. The strategy for PM_2.5 mitigation must be based on reducing the pollutants from the two primary sources of coal-fired power plants and vehicle emissions. Although conventional Particulate Emission Control Devices (PECD) such as electrostatic precipitators in Chinese coal-fired power plants are generally effective for large particles, most of them may not have high collection efficiency of PM_2.5. Baghouse filtration is gradually incorporated into the PECD to increase the PM_2.5 collection efficiency. By adopting stringent vehicle emissions standard such as Euro 5 and 6, the emissions from vehicles can be gradually reduced over the years. An integrative approach, from collaboration among academia, government, and industries, can effectively manage and mitigate the PM_2.5 pollution in China.     

Schlieren imaging of loud sounds and weak shock waves in air near the limit of visibility

A large schlieren system with exceptional sensitivity and a high-speed digital camera are used to visualize loud sounds and a variety of common phenomena that produce weak shock waves in the atmosphere. Frame rates varied from 10,000 to 30,000 frames/s with microsecond frame exposures. Sound waves become visible to this instrumentation at frequencies above 10 kHz and sound pressure levels in the 110 dB (6.3 Pa) range and above. The density gradient produced by a weak shock wave is examined and found to depend upon the profile and thickness of the shock as well as the density difference across it. Schlieren visualizations of weak shock waves from common phenomena include loud trumpet notes, various impact phenomena that compress a bubble of air, bursting a toy balloon, popping a champagne cork, snapping a wooden stick, and snapping a wet towel. The balloon burst, snapping a ruler on a table, and snapping the towel and a leather belt all produced readily visible shock-wave phenomena. In contrast, clapping the hands, snapping the stick, and the champagne cork all produced wave trains that were near the weak limit of visibility. Overall, with sensitive optics and a modern high-speed camera, many nonlinear acoustic phenomena in the air can be observed and studied.     

Effect of chemical composition of PM_(2.5) on visibility in Guangzhou,China,2007 spring

The object of this study was to investigate the correlation of visibility with chemical composition of PM2.5 in Guangzhou. In April 2007, 28 PM2.5 samples were collected daily at the monitoring station of the South China Institute of Environmental Sciences (SCIES), in urban Guangzhou. Water-soluble ionic species (Cl-, NO3-, SO42-, NH4+, K+, Na+, Ca2+, and Mg2+) and carbonaceous contents (OC and EC) of the PM2.5 samples were determined to characterize their impact on visibility impairment. The results showed that sulfate was the dominant species that affected both light scattering and visibility. The average percentage contributions of the visibility-degrading species to light scattering coefficient were 40% for sulfate, 16% for nitrate, 22% for organics, and 22% for elemental carbon. Because of its foremost effect on visibility, sulfate reduction in PM2.5 would effectively improve the visibility of Guangzhou.     

Complexity analysis of multiscale multivariate time series based on entropy plane via vector visibility graph

Nonlinear Dynamics - Vector visibility graph (VVG) is an algorithm that transforms multivariate time series into directed complex networks. However, at present, the researches of VVG mainly focus...     

Variable speed synergetic control for chaotic oscillation in power system

Chaotic oscillation is an undesirable phenomenon in power system and it can destroy the stability of power system. The objective of this paper is to propose variable speed synergetic control to eliminate chattering phenomenon in sliding-mode control and avoid undesirable phenomena when suppressing chaotic oscillation in power system. The prominent advantage of proposed control scheme is that it can adjust convergence speed according to the system response thus avoiding undesirable phenomena in the control process. Simulation results show that our control scheme avoids undesirable phenomena in the control process and speeds up convergence rate.     

Variable density and viscosity,miscible displacements in capillary tubes

The displacement of a more viscous fluid by a miscible, less viscous one of lower density in a horizontal capillary tube is studied by means of Stokes flow simulations. Both axisymmetric and three-dimensional simulations are conducted at Péclet numbers up to 10⁴, in order to resolve discrepancies between earlier simulations by [C.Y. Chen, E. Meiburg, Miscible displacements in capillary tubes. Part 2. Numerical simulations, J. Fluid Mech. 326 (1996) 57] and corresponding experiments of [P. Petitjeans, T. Maxworthy, Miscible displacements in capillary tubes. Part 1. Experiments, J. Fluid Mech. 326 (1996) 37] and [J. Kuang, T. Maxworthy, P. Petitjeans, Miscible displacements between silicone oils in capillary tubes, Eur. J. Mech. B Fluids 22 (2003) 271–277]. An initial set of simulations addresses the influence of different viscosity–concentration relations on the quasisteady finger tip velocity. The results indicate that steeper relations generally result in a higher tip velocity. However, the effect is too small to explain the above discrepancies. Further simulations show that a concentration-dependent diffusion coefficient results in a slight reduction of the tip velocity at moderate Pe, but again the effect is too small to fully account for the observed differences.Three-dimensional simulations that include gravitational forces yield a much more significant effect. Consistent with the experiments of [P. Petitjeans, T. Maxworthy, Miscible displacements in capillary tubes. Part 1. Experiments, J. Fluid Mech. 326 (1996) 37], at moderate Pe the tip slows down as the gravity parameter increases, an effect that becomes more pronounced as Pe decreases. However, the three-dimensional simulations do not produce the longitudinal splitting phenomenon observed by [P. Petitjeans, T. Maxworthy, Miscible displacements in capillary tubes. Part 1. Experiments, J. Fluid Mech. 326 (1996) 37]. In order to check for the existence of gravitational instabilities that might cause such a splitting, additional two-dimensional simulations are conducted in cross-sections of the tube. A comparison of these two-dimensional results with corresponding three-dimensional simulations demonstrates that for a wide range of parameters the evolution of the trailing finger sections is governed by a two-dimensional balance between gravitational and viscous forces. However, a gravitational instability along the lines suggested by [P. Petitjeans, T. Maxworthy, Miscible displacements in capillary tubes. Part 1. Experiments, J. Fluid Mech. 326 (1996) 37] was not observed. On the other hand, for some parameter combinations the evolution of a 'dimple' is observed on the lower side of the finger, and close to its tip. This dimple may signal the evolution of a splitting phenomenon after long times, which are beyond the reach of the current simulations. Taken together, the two- and three-dimensional simulations suggest that the splitting phenomenon observed by [P. Petitjeans, T. Maxworthy, Miscible displacements in capillary tubes. Part 1. Experiments, J. Fluid Mech. 326 (1996) 37] likely is caused by the gravity-induced modification of the flow around the tip of the finger, rather than by a gravitational instability per se.     

一种改进的变结构控制器在转台中的应用

针对变结构控制的抖振问题,本提出了一种新的解决方案,采用多切换面的改进变结构控制,在保证了理想滑动模态的抗干扰能力强、鲁棒性好的优点的同时,很好的削弱了系统抖振。仿真结果表明,这种控制规律应用在转台上取得了良好的控制效果。     

Modeling Transverse Dispersion and Variable Density Flow in Porous Media

A two-dimensional numerical model is used to study the nonlinear behavior of density gradients on transverse dispersion. Numerical simulations are conducted using d ³ f, a computer code for simulation of density-dependent flow in porous media. Considering a density-stratified horizontal flow in a heterogeneous porous media, a series of simulations is carried out to examine the effect of the density gradient on macro-scale transverse dispersivity. Changing salt concentration significantly affects fluid properties. This physical behavior of the fluid involves a non-linearity in modeling the interaction between salt and fresh water. It is concluded that the large-scale transport properties for high density flow deviate significantly from the tracer case due to the spatial variation of permeability, described by statistical parameters, at the local-scale. Indeed, the presence of vertical flow velocities induced by permeability variations is responsible for the reduction of the mixing zone width in the steady state in the case of a high density gradient. Uncertainties in the model simulations are studied in terms of discretization errors, boundary conditions, and convergence of ensemble averaging. With respect to the results, the gravity number appears to be the controlling parameter for dispersive flux. In addition, the applicability and limitations of the nonlinear model of Hassanizadeh (1990) and Hassanizadeh and Leijnse (1995) (Adv Water Resour 18(4):203–215, 1995) in heterogeneous porous media are investigated. We found that the main cause of the nonlinear behavior of dispersion, which is the interaction between density contrast and vertical velocity, needs to be explicitly accounted for in a macro-scale model.