Imaging deviation through non-uniform flow fields around high-speed flying vehicles

The non-uniform flow field around high-speed flying vehicle “bends” the ray and imposes a deviation at the end of the propagation path. This imaging deviation is a kind of aero-optic effect. In this paper, we catalog the factors that influence the deviation into two classes: the vehicle-related factors and the flowfield-related factors. Flow density computation and density–refractive index conversion are discussed. A backward ray-tracing scheme is proposed. The deviations, the propagation path distances in the non-uniform flow field, and the density distributions along propagation paths for two different flying cases are computed. Three flowfield-related factors should be considered in order to reduce the deviation: the propagation path distance in the non-uniform flow field, which should be as short as possible; the angle of incidence at the freestream boundary, which should be as small as possible; and the density distribution along propagation path in the non-uniform flow field, which should be as flat as possible.     

Instability of parametric dynamic systems with non-uniform damping

A unified way of looking at parametric and combination resonances in systems with periodic coefficients and different amounts of damping in the various modes of vibration is presented. The stability boundaries of the coupled Mathieu equations are determined by the harmonic balance method, Fourier series with periods 2T and T being assumed. The basic characteristics of the solution are discussed and the method is applied to multiple-degree-of-freedom dynamic systems. The destabilizing effect on the combination resonances is obtained by the present method.     

Fully three-dimensional analysis of high-speed train-track-soil-structure dynamic interaction

In this paper, a general and fully three dimensional multi-body-finite element-boundary element model, formulated in the time domain to predict vibrations due to train passage at the vehicle, the track and the free field, is presented. The vehicle is modelled as a multi-body system and, therefore, the quasi-static and the dynamic excitation mechanisms due to train passage can be considered. The track is modelled using finite elements. The soil is considered as a homogeneous half-space by the boundary element method. This methodology could be used to take into account local soil discontinuities, underground constructions such as underpasses, and coupling with nearby structures that break the uniformity of the geometry along the track line. The nonlinear behaviour of the structures could be also considered. In the present paper, in order to test the model, vibrations induced by high-speed train passage are evaluated for a ballasted track. The quasi-static and dynamic load components are studied and the influence of the suspended mass on the vertical loads is analyzed. The numerical model is validated by comparison with experimental records from two HST lines. Finally, the dynamic behaviour of a transition zone between a ballast track and a slab track is analyzed and the obtained results from the proposed model are compared with those obtained from a model with invariant geometry with respect to the track direction.     

Simultaneous diffraction of two finite waves at non-uniform dynamic volume reflection gratings

Based on the two-dimensional coupled-wave theory, the simultaneous diffraction of two finite waves with non-constant amplitude distribution at a finite non-uniform dynamic volume reflection grating is described. A coupled differential equation is derived and solved numerically with the aid of a two-dimensional Runge-Kutta method. The results for the chosen example of a planar volume reflection grating both for pure phase and absorption gratings show the dependence of the fidelity and diffraction efficiency on the normalized parameter grating strength, on the phase shift with respect to the interference pattern of the read-out waves and on the absorption modulation. In contrast to the transmission grating, a high fidelity of the enhanced beam can be achieved for reflection gratings. The numerical results are compared with the solution of the one-dimensional theory.     

Dynamic response of underpasses for high-speed train lines

Underpasses are common in modern railway lines. Wildlife corridors and drainage conduits often fall into this category of partially buried structures. Their dynamic behavior has received far less attention than that of other structures such as bridges, but their large number makes their study an interesting challenge from the viewpoint of safety and cost savings. Here, we present a complete study of a culvert, including on-site measurements and numerical modeling.The studied structure belongs to the high-speed railway line linking Segovia and Valladolid in Spain. The line was opened to traffic in 2004. On-site measurements were performed for the structure by recording the dynamic response at selected points of the structure during the passage of high-speed trains at speeds ranging between 200 and 300 km/h.The measurements provide not only reference values suitable for model fitting, but also a good insight into the main features of the dynamic behavior of this structure. Finite element techniques were used to model the dynamic behavior of the structure and its key features. Special attention is paid to vertical accelerations, the values of which should be limited to avoid track instability according to Eurocode. This study furthers our understanding of the dynamic response of railway underpasses to train loads.     

Chatter resistance of non-uniform turning bars with attached dynamic absorbers—Analytical approach

Forced harmonic vibration of a non-uniform elastic beam with attached dynamic vibration absorbers (DVA) is studied. Analytical approximation of the solution is obtained by the functional perturbation method (FPM). The problem has application to cutting tools operations where the resistance of the tool holder against regenerative chatter can be enhanced by optimizing the real part of the frequency response function (FRF). A test case of a beam with step-like heterogeneity and single DVA at the tip shows that the FPM solution is very accurate for up to ∼40 percent deviation in both stiffness and mass density. Using the analytical results and Sims approach, optimal DVA tuning is found for each set of beam heterogeneity parameters by solving a set of nonlinear algebraic equations numerically. It is found that the optimum can be further improved by searching for the best step location. The system optimization is then expanded to a general heterogeneous beam with a DVA at its tip. The mass and stiffness distribution is optimized by applying the Lagrange variation method on the FPM solution yielding Fredholm integral equations. The optimized morphology is found to be approximately linear and far from the “intuitive” step-like one (Rivin and Kang, 1992) and yields better chatter-resistance.     

多光谱瞬态测温计用于动力发射系统火焰测试

 无论在民用钢铁冶炼、焊接技术或者军用近代动力学发射系统中,对于目标火焰的辐射温度测量一直有着重要意义,其对钢铁冶炼成分的判定、焊接工艺的提高和动力系统轨道烧灼的研究都有着重要的影响。该情况下火焰不仅温度极高,而且在某些场合其产生是一个瞬态过程。因此,传统的接触式测温方法不再适用。基于经典的普朗克黑体辐射定律在测量时受到光谱发射效率的影响也难以准确得到最后结果。以经典的普朗克黑体辐射定律作为理论基础,结合多波长光谱辐射方法,研制了新型的多光谱辐射瞬态高温测温计。该高温计最快响应时间可达到2 ns。通过采用高分辨率衍射光栅和光纤连接的方式,保证多光谱提取的准确性。同时将经典的普朗克黑体辐射定律结合多波长提出新型辐射温度算法,不仅解决对该目标辐射温度的精准计算,更可以同时求得目标在该温度下的实时光谱发射效率。通过对高速发射目标和可调节亮度的溴钨灯测量的实验表明,该方法满足测量动力发射目标表面辐射温度分布的同时,也保证了较高的精度,满足了对于发射瞬间物体表面瞬态温度测试的要求。     

一类相对转动非线性动力系统的混沌运动

研究一类具有同宿轨道、异宿轨道的相对转动非线性动力系统的混沌运动. 建立具有非线性刚度、非线性阻尼和外扰激励作用的一类两质量相对转动非线性动力系统的动力学方程. 利用Melnikov方法讨论了系统的全局分岔和系统进入混沌状态的可能途径,给出了系统发生混沌的必要条件,并利用最大Lyapunov指数图,分岔图,Poincare截面图和相轨迹图进一步分析了系统的混沌行为. 关键词： 相对转动 非线性动力系统 混沌 Melnikov方法     

How non-uniform tolerance parameter strategy changes the response of scale-free networks to failures

In this paper, we introduce a non-uniform tolerance parameter (TP) strategy (the tolerance parameter is characterized by the proportion between the unused capacity and the capacity of a vertex) and study how the non-uniform TP strategy influences the response of scale-free (SF) networks to cascading failures. Different from constant TP in previous work of Motter and Lai (ML), the TP in the proposed strategy scales as a power-law function of vertex degree with an exponent b. The simulations show that under low construction costs D, when b>0 the tolerance of SF networks can be greatly improved, especially at moderate values of b; When b<0 the tolerance gets worse, compared with the case of constant TP in the ML model. While for high D the tolerance declines slightly with the b, namely b<0 is helpful to the tolerance, and b>0 is harmful. Because for smaller b the cascade of the network is mainly induced by failures of most high-degree vertices; while for larger b, the cascade attributes to damage of most low-degree vertices. Furthermore, we find that the non-uniform TP strategy can cause changes of the structure and the load-degree correlation in the network after the cascade. These results might give insights for the design of both network capacity to improve network robustness under limitation of small cost, and for the design of strategies to defend cascading failures of networks.     

Tone-mapping and dynamic range compression using dynamic cone response

Over the last several decades, the tone-mapping and color correction methods have been incorporated into the display device such as LCD/LED display device or the photographic print. However, conventional methods are based on Gray-World Assumption using spatial smoothing filter to estimate the local illumination component. For this reason, most of these methods have several problems like visual perception problem, scale the contrast, gray-out, color distortion (dominate color, halo-artifact) and so on. Accordingly, this paper presents a color correction method using the dynamic cone response function based on human visual perception. The proposed method consists of tone-mapping and dynamic cone response. The tone-mapped image is obtained by using chromatic and achromatic colors. Thereafter, the resultant image is processed through the dynamic cone response function, and it is to estimate the dynamic response of human eye as well as to deal with mismatch between a real scene and the rendered image. The experimental results show that the proposed method yields better performance of color correction over the conventional methods.     

Circular motion of submillimeter-sized acoustic bubbles attached to a boundary by high-speed image analysis

The circular motion of submillimeter-sized bubbles attached to a boundary in an 18.5 kHz ultrasonic field are investigated experimentally by high-speed photography and image analysis. It is found that the vibration of gas bubbles with diameters of 0.2–0.4 mm is between spherical radial vibration and regular surface fluctuation. Different from the circular motion of suspended bubbles in water, the circular motion of gas bubbles attached to a boundary presents some new characteristics. These bubbles attached to a boundary (wandering bubbles) will rotate around a fixed bubble array (holding bubbles). Both the wondering bubbles and holding bubbles are “degas” bubbles. The primary Bjerknes force acting on wandering bubbles in the acoustic wave field and the secondary Bjerknes force between the wandering bubbles and the holding bubbles strongly affects the circular motion. The circling and residence behavior of gas bubbles is described and analyzed in detail, which is helpful to understand and improve industrial applications such as ultrasonic cleaning, sonochemical treatment, aeration and cavitation reduction.     

Onset of motion and dynamic reordering of a vortex lattice

Time resolved transport measurements on a driven vortex lattice in an undoped 2H-NbSe2 crystal show that the response to a current pulse is governed by healing of defects as the lattice evolves from a stationary to a moving steady state and that the response time reflects the degree of order in the initial vortex state. We find that stationary field cooled vortex lattices become more ordered with decreasing temperature and identify a temperature below which a qualitative change in the response signals the disappearance of topological defects.     

On classification and special features of motion of dynamic systems

A number of debatable problems of modern nonlinear physics are discussed. A classification of deterministic systems with chaotic behavior by the degree of openness and type of motion is suggested. Examples of dynamic systems illustrating the consistency of this classification are presented, and special features of functioning of these systems and problems of quantitative estimation of the degree of randomness are considered. A strict definition of quasi-deterministic chaos is given. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 49–58, November, 2006.     

On mechanical response of a non-uniform piezoelectric transducer under the influence of a body-force

The mechanical response of a non-uniform piezoelectric transducer when there is a body force acting on it has been evaluated using the methods of transform calculus.The author is grateful to Dr. D. K. Sinha of Jadavpur University for his kind help in the preparation of this note.     

A mean-field equation of motion for the dynamic Ising model

A mean-field type of approximation is used to derive two differential equations, one approximately representing the average behavior of the Ising model with Glauber (spin-flip) stochastic dynamics, and the other doing the same for Kawasaki (spin-exchange) dynamics. The proposed new equations are compared with the Cahn-Allen and Cahn-Hilliard equations representing the same systems and with information about the exact behavior of the microscopic models.     

On a dynamic effect in the collective motion of dislocations

The interaction between fast moving dislocations and vibrational modes, localized on point defects is investigated. By means of quantum mechanical calculations the existence of a force between the dislocations due to their interaction with the modes is established in the case of the coordinated motion of two or more dislocations. This force might play an important role in the case of edge dislocations, moving faster than the velocity of Rayleigh waves. As known, such dislocations of the same mechanical sign attract one another (in contrast to the static case), leading to fracture. The aforementioned force due to point defects might prevent dislocations from joining and reduce the probability of fracture formation.