STUDY OF THE IMPINGEMENT OF IMPULSE WAVE UPON A FLAT PLATE

The impulse wave is made by the weak normal shock discharge from an open end of a shock tube. In order to understand the dynamic characteristics of the impulse wave impinging upon a flat plate, experiments are performed over the shock Mach number range from 1·01 to 1·20. A flat plate is placed downstream, normal to the axis of the shock tube, to simulate the impulse wave impingement on an object. The distance between the exit of the shock tube and flat plate is changed. The baffle plate is installed at the exit of the shock tube. The sizes of the baffle plate and impinging flat plate are varied to investigate the magnitude of the impulse wave impinging upon the flat plate. Computational analysis is applied to model the flow field subject to unsteady, axisymmetric, inviscid, compressible, equations. Computational results well predict the experimented dynamic behavior of the impinging impulse wave. The impulse wave impinging upon the flat plate has a sharp peak of very short rising time and its magnitude decreases with distance from the center of the flat plate. The magnitude of the impulse wave impinging upon the flat plate can be predicted by the empirical equations that are developed in the present study.     

Magnetic-Flux-Induced Tunable Completely Flat Band and Topological Nearly Flat Band in Square Kagome Lattice

The flat bands in a square kagome lattice containing square and triangle plaquettes, are respectively investigated, and the origin of the doubly degenerated completely flat bands and the corresponding compact localized states are elucidated. It is found that the introduction of external magnetic flux enriches the modulation parameters, making the system present many interesting results. In the case that the magnetic flux penetrates through each square plaquette, the tunable completely flat band has one more tunable parameter. And when the external magnetic flux penetrates through each triangle plaquette, excepting a completely flat band, the band dispersions of the system present the topological nearly flat band, which is very useful to realize the interesting fractional quantum Hall physics. The average density of states is also calculated to corroborate the completely FB generated by the highly localized eigenstates. Furthermore, the implementation of the square kagome lattice system based on the current photonic waveguide network technology is demonstrated. The scheme opens up a way to generate the tunable completely flat band and topological nearly flat band in square kagome lattice under multi-parameter variable conditions.     

Novel method for high accuracy figure measurement of optical flat

Phase Measuring Deflectometry (PMD) is a non-contact, high dynamic-range and full-field metrology which becomes a serious competitor to interferometry. However, the accuracy of deflectometry metrology is strongly influenced by the level of the calibrations, including test geometry, imaging pin-hole camera and digital display. In this paper, we propose a novel method that can measure optical flat surface figure to a high accuracy. We first calibrate the camera using a checker pattern shown on a LCD display at six different orientations, and the last orientation is aligned at the same position as the test optical flat. By using this method, lens distortions and the mapping relationship between the CCD pixels and the subaperture coordinates on the test optical flat can be determined at the same time. To further reduce the influence of the calibration errors on measurements, a reference optical flat with a high quality surface is measured, and then the system errors in our PMD setup can be eliminated by subtracting the figure of the reference flat from the figure of the test flat. Although any expensive coordinates measuring machine, such as laser tracker and coordinates measuring machine are not applied in our measurement, our experimental results of optical flat figure from low to high order aberrations still show a good agreement with that from the Fizeau interferometer.     

Ion guiding through a flat insulating channel

Guiding of beams through flat insulating capillaries has been investigated. A flat capillary guides a beam at inclination of the capillary plane with respect to the beam direction. It is found that a beam can be guided by rotation of a flat capillary around the axis oriented perpendicularly to its plane, i.e., without changing the plane orientation. This effect has been referred to as double guiding. Theoretical models are proposed to explain guiding and double guiding of an ion beam through a flat capillary.     

Radial conformal killing vectors in spherically symmetric shear-free space-times

The existence of “radial” conformai Killing (RCK) vector fields is discussed for metrics describing spherically symmetric, shear-free, perfect fluids. It is shown that conformally flat metrics admit three RCK fields, while nonconformally flat metrics might admit, at most, one such field. An RCK vector parallel to the 4-velocity of the fluid occurs in a subclass of conformally flat metrics containing the Friedmann-Robertson-Walker space-times as particular cases. A new class of nonconformally flat, self-similarity solutions is found. The necessary conditions for the existence of an RCK field in other nonconformally flat metrics are given in full.     

Spherically symmetric space-times transparent to scalar multipole waves

Using nonscattering potentials of Chang and Janis, a large class of spherically symmetric space-times is constructed on which all multipole solutions to the minimally coupled scalar wave equation are expressible in terms of characteristic data functions in essentially as simple a fashion as for flat space-time. The space-times are transparent to multipole waves in the same sense that flat space-time is. Both conformally flat and not conformally flat space-times are obtained. Some examples are discussed which show that the variety of transparent space-times is large even within the class of Robertson-Walker spaces.     

基于一维耦合腔光子晶体的声光可调谐平顶滤波器的研究

本文利用一维耦合腔光子晶体,提出了一种声光可调谐平顶滤波器.该滤波器利用声光效应,通过改变超声波频率使一维耦合腔光子晶体透射谱的平顶滤波器的中心波长产生漂移,从而实现可调谐的滤波功能.基于传输矩阵法和声光效应理论,建立了这种平顶滤波器的理论模型;利用COMSOL软件,对平顶滤波器的矩形度、通带带宽、插入损耗、可调谐特性、加工精度进行仿真研究.研究结果表明,通过施加频率为6-11 MHz的超声波,可实现通带带宽为5-6 nm及中心波长在1514-1562 nm范围内可调谐的平顶滤波器;在可调谐范围内通带带宽内插入损耗不超过2.23 dB,最低仅为0.78 dB,矩形度最低可达1.4;加工误差在±10 nm内平顶滤波器的中心波长、矩形度、插入损耗、通带带宽出现的偏差很小.该平顶滤波器具有易于设计和集成、通带平坦、可调谐范围宽、通带带宽稳定、插入损耗低、品质因素高的特点,在光开关、可调谐光纤激光器、光纤传感等光通信领域有重要应用.     

Wavelength dependence of focusing properties of two-dimensional photonic quasicrystal flat lens

We investigated the wavelength dependence of the focusing properties of a germanium-cylinder-based two-dimensional (2D) decagonal Penrose-type photonic quasicrystal (PQC) flat lens for the first time, to the best of our knowledge. We found that near the second bandgap and in the high-frequency side (between the bandgap boundary and the first light intensity peak) of the pass band, the flat lens can exhibit a focusing effect for a point light source and that the focusing wavelengths can directly be drawn from the photonic band structure. For all the focusing wavelengths, the summation of the object distance and the image distance is less than the thickness of the flat lens when the object distance is half the thickness of the flat lens. As the wavelength increases, the image distance, the image quality, and the effective refractive index of the flat lens increase, whereas the image power of the point light source decreases. The effective refractive index of the flat lens is less than -1.     

高精度大口径平面镜瑞奇-康芒定量检测方法研究

瑞奇 康芒法是检测大口径平面时的行之有效的方法。由于被检平面处于发散光路中,这就使得平面面形与系统波像差之间的关系(即影响函数)变得十分复杂,推导起来十分困难,故长期以来该方法只能作为一种定性或半定量的检测手段。给出了数学算法,推导出了被检平面镜面形误差与检验系统波像差之间的相互关系,实现了瑞奇 康芒的定量检测。     

On the stability of flat space

It is shown that: (1) there exists, near flat space, a neighborhood of nonsingular asymptotically flat weak field solutions of the initial value equations of General Relativity; the solutions have physically appropriate generality; and (2) this neighborhood is complete and flat space is stable; every geometry representing a weak perturbation and satisfying the varied constraints is tangent to the space of solutions.     

一个新的改善电容器放电平顶的电路

本文提出了一个旨在改善电容器放电平顶的简单电路。从基本电路的分析出发,推出了补偿后的输出电压、平顶峰值电压、峰值电压出现时间以及平顶时间同电压补偿系数k和放电时间常数比λ间关系式和曲线族。结果表明,如参数选择得当,在过补偿工作状态下,可以获得满意的平顶和较长的平顶持续时间。     

基于背景光调制的复合光傅里叶变换轮廓术

提出一种基于背景光调制的用于傅里叶变换轮廓术测量范围的复合光栅,该光栅通过调制一正弦条纹和不含任何相位信息的背景光来抑制零频,较基于相移技术的复合光栅有更大的优势:背景光只含直流分量,使得复合光栅的频谱更加简单,有利于滤出载波信息,提高测量精度;对从复合光栅中解调出来的背景光只涉及平均值校准,校准过程更为简单;解调出的背景光与物体表面的反射率成正比,具有潜在的应用价值。采用Matlab程序对该复合光栅进行了数值模拟,并对该光栅实用性进行了实验研究,结果证实了该光栅用于抑制零频、扩大傅里叶变换轮廓术测量范围的有效性,且提高了测量精度。     

平响应X射线二极管辐射能流还原率提高方法

惯性约束聚变实验中使用平响应X射线二极管(XRD)阵列从不同方向测量黑腔辐射能流的时间演化过程和角分布。平响应XRD的谱响应曲线存在局部不平整,给辐射能流的还原引入了误差。为减小由于能谱形状和平响应局部不平带来的还原误差,提出了加权求解平响应XRD的平均响应值的方法。对该方法提高辐射能流的还原率的必要条件进行了研究。改进后的方法应用于实验数据的处理,结果表明:修正后的等效积分温度随时间的演化过程与多道软X光能谱仪吻合较好。     

Uniqueness and nonuniqueness of static black holes in higher dimensions

We prove a uniqueness theorem for asymptotically flat static charged dilaton black-hole solutions in higher-dimensional space-times. We also construct infinitely many nonasymptotically flat regular static black holes on the same space-time manifold with the same spherical topology. An application to the uniqueness of a class of flat p-branes is also given.     

The nearly flat universe

We study here what it means for the Universe to be nearly flat, as opposed to exactly flat. We give three definitions of nearly flat, based on density, geometry and dynamics; all three definitions are equivalent and depend on a single constant flatness parameter ɛ that quantifies the notion of nearly flat. Observations can only place an upper limit on ɛ, and always allow the possibility that the Universe is infinite with k = −1 or finite with k = 1. We use current observational data to obtain a numerical upper limit on the flatness parameter and discuss its implications, in particular the “naturalness” of the nearly flat Universe.     

Curvature perturbation from supersymmetric flat directions

We show that a contribution to the total curvature perturbation may be due to the presence of flat directions in supersymmetric models. It is generated at the first oscillation of the flat direction condensate when the latter relaxes to the minimum of its potential after the end of inflation. We also point out that, if the contribution to the total curvature perturbation from supersymmetric flat direction is the dominant one, then a significant level of non-Gaussianity in the cosmological perturbation is also naturally expected.     

300mm平面标准镜装卡结构的关键参数

为了实现大口径平面标准镜的高精结构装卡,对其在重力作用下的面形变化进行了研究。首先,对结构胶的有限元建模进行了理论分析,建立了大口径平面标准镜胶结装卡结构有限元模型。然后,分析了不同胶点数量及分布、不同胶接面积以及不同镜框支撑方式等关键结构参数对参考面面形的影响。最后,设计了大口径平面标准镜胶结及支撑的结构。结果表明,采用胶点直径为5 mm,12×3胶点分布形式胶结时,参考面面形的PV值为24.06 nm,RMS值为6.78 nm,满足了大口径平面标准镜面形精度的要求。     

Wave function collapses and 1/n energy spectrum induced by a Coulomb potential in a one-dimensional flat band system

We investigate the bound state problem in a one-dimensional flat band system with a Coulomb potential. It is found that, in the presence of a Coulomb potential of type I (with three equal diagonal elements), similarly to that in the two-dimensional case, the flat band could not survive. At the same time, the flat band states are transformed into localized states with a logarithmic singularity near the center position. In addition, the wave function near the origin would collapse for an arbitrarily weak Coulomb potential. Due to the wave function collapses, the eigen-energies for a shifted Coulomb potential depend sensitively on the cut-off parameter. For a Coulomb potential of type II, there exist infinite bound states that are generated from the flat band. Furthermore, when the bound state energy is very near the flat band, the energy is inversely proportional to the natural number, e.g.,$E_n\propto$ 1/n, n=1,2,3,... It is expected that the 1/n energy spectrum could be observed experimentally in the near future.     

利用倾斜镜对太阳像作平场处理

提出一种利用高速倾斜反射镜对太阳像进行平场处理的新方法。介绍了此方法的原理和实际工作过程，并给出了实验结果。结果表明，应用高速倾斜反射镜可以非常有效地对太阳像进行平场处理，提高太阳像成像质量。     

高精度,大口径平面波像差标准—移相式数字平面干涉仪

介绍研制成的一台移相式数字平面干涉仪，其孔径为φ２４５ｍｍ，用液面作基准标定并消除仪器的系统误差，准确度优于λ／５０（峰谷值，λ＝０．６３２８μｍ），利用计算机辅助干涉术，被测件口径可扩展到φ５００ｍｍ，该仪器将用于建立高精度，大孔径平面波像差标准。文中阐述了光学干涉仪，移相器及精度校核等问题。