长波条件下二维声子晶体中的弹性波传播及各向异性

利用多散射方法研究了长波条件下二维声子晶体中弹性波的传播特性.由长波条件下散射体幂散射特性和结构周期特性分析得到了等效波速的解析表达式.通过对幂散射波各阶分波之间由于结构周期特性而产生的耦合作用进行分析,发现幂散射波的正负二阶分量之间的耦合作用具有明显的方向性,长波条件下弹性波的传播特性是否具有各向异性取决于幂散射波的二阶分量在多散射过程中是否有明显的作用.     

混合物介质中声波尾波成因的随机过程分析

该文基于声波在混合物介质中传播时反射及散射的随机特性，把混合物介质抽象为三维各向同性的马尔科夫链，把声波在混合物介质中传播过程抽象为声波在三维马尔科夫链中以声速进行“随机游走”的随机过程。用空间内某点接收到声波的概率类比该点接收波振幅，以声波到达该点所走过的步数类比接收波时域曲线的时间。此理论模型可较好解释声波在混合物介质中传播时“峰波延后”及“尾波”等现象。     

液面位置对流-固交界面上伪瑞利波幅度影响的数值模拟

研究了流体半空间中存在气-液交界面并且其位置发生改变时,流-固交界面上伪瑞利波的传播特性和幅度响应特征。借助有限差分的方法,实现了对不同气-液交界面位置时振动方向垂直于流-固交界面的脉冲面源所激发表面波声场的数值模拟,分别讨论了气-液交界面自身的散射作用与伪瑞利波传播过程中的能量泄漏对伪瑞利波幅度的影响。数值模拟结果表明:气-液交界面自身对伪瑞利波的散射作用对伪瑞利波幅度仅有极微小的影响,而能量泄漏才是引起伪瑞利波衰减的决定性因素;随着气-液交界面位置的升高,脉冲伪瑞利波幅度呈指数规律下降,与此同时主频向低频偏移。所得研究结果可以发展为一种利用伪瑞利波幅度进行液位检测的可靠方法。      

经ORC处理银衬底上核酸碱基的SERS研究

利用天线共振子模型对银的表面增强喇曼散射(SERS)特性进行了计算和分析.结果显示，银增强因子在近红外的1.38 eV处为最大，达140以上；近紫外3.35 eV、3.90 eV、4.30 eV位置处依次减小，分别为60、48、45；而在可见光区，银增强因子的最大值为70. 在经氧化还原过程(ORC)处理的银片上做了核酸碱基腺嘌呤、尿嘧啶的SERS谱，腺嘌呤的SERS谱虽然存在荧光的影响，但SERS效应明显.尿嘧啶的SERS谱出现4个明锐的峰值，1035 cm－1是所出现的最强的模式，该模式应是尿嘧啶在核酸中存在的SERS环呼吸模式，可作为检测尿嘧啶存在的判定依据.     

太赫兹波在雾中的多重散射特性

太赫兹(THz)波作为微波和毫米波的延伸,它所提供的通信带宽和容量远大于毫米波。在随机介质中传播时,不但会发生时域和空域的形变,介质中的粒子还会对入射波发生散射,这些都会使得脉冲信号发生衰减。根据Mie理论与随机离散分布粒子的波传播与散射理论,计算了THz波信号入射下雾滴粒子的消光系数。结合雾滴粒子谱分布,得到了雾媒质的平均体系散射特性,采用蒙特卡罗法得到了平流雾对THz信号的多重散射特性,计算了THz波段信号对平流雾的透过率与反射率,分析了THz波段信号的前向、后向散射特性随散射角的分布。结果表明,低能见度大气环境中,雾对THz波产生的吸收和衰减不容忽视。相关研究结果对THz在大气传输、通信等方面的应用具有重要意义。     

椭球三维液态声子晶体完全声波禁带研究

弹性波在声子晶体中传播时，会产生一定的频率禁带，声和振动在禁带中被禁止传播。本文将椭球散射体引入三维声子晶体中，采用平面波展开法计算了该系统的声波禁带结构。对于不同椭球半径的系统，其声波禁带的位置与大小有很大不同。结果表明：当晶格常数和填充率确定时，禁带受到椭球半径的影响；当确定椭球体的某一个半径和填充率时，第一禁带的最大值出现在另2个半径相等的情况下。     

强激光与亚临界密度等离子体相互作用中的近前向散射驱动光子加速机制

通过二维粒子模拟(particle-in-cell)方法研究了强激光与亚临界密度等离子体相互作用中的近前向光子加速机制.该机制利用强激光在亚临界密度气体传输过程中的电离效应产生在纵向和横向上密度分布不均匀的电子等离子体.在纵向上,入射激光电离氦气产生一个陡峭的电子密度前沿分布.在密度前沿处,入射激光与电子等离子体波作用发生近前向散射.散射光频率较激光频率增大,在频谱中产生了第一个特征峰.在横向上,密度不均匀造成电子等离子体波具有不同的相速度并与入射激光相互作用,使入射激光发生近前向散射,在频谱中产生了第2个特征峰.由于密度分布的不均匀性较电子等离子体波的密度扰动大得多,因此基于微扰理论的散射模型和色散关系,如受激拉曼散射,无法解释频谱中两个特征峰的出现.进一步研究发现:在密度不均匀的情况下,入射激光、电子等离子体波和散射光三者之间仍满足动量和能量守恒的三波耦合关系.这能够解释两个特征峰对应的频率和强度增长过程.该研究对于强激光在亚临界密度气体传输过程中的频谱演化具有重要参考意义.     

激光在不均匀随机媒质中的传播——多重散射的m-n阶矩方程的解

本文给出了受到多重散射的激光,在波传播方向上的物理参数的涨落是不均匀的、而垂直于传播方向上的物理参数的涨落又是均匀的随机媒质中传播时,当波受到前向小角度散射时,具有不同波数不同位置的场的矩方程的解析解.同时讨论了方程的解在激光传播研究中的一些应用.     

局部固体填充的水中复杂目标声散射计算与实验

利用二维有限元方法研究水中局部填充的带球冠柱体目标声散射特性,所采用的数值方法可高效地实现精细化、宽带、复杂轴对称模型散射声场计算.根据数值结果解释壳体、填充物以及入射方位对目标散射远场的影响,确定复杂目标散射研究中所必须考虑的重要物理和几何构成.完成水中悬浮目标自由场声散射实验,收发合置条件下将目标旋转360?接收并测量不同传播路径回波到达时刻得到距离-角度伪彩色图像.以表面环绕波和"回廊波"产生理论为基础,解释内真空和局部填充模型正横入射时目标散射信号中几何回波和各种弹性波成分产生的机理.由于固体填充与弹性壳的耦合作用,频率-角度谱的正横方向两侧呈现外八字"碗"形共振曲线.通过对比,理论计算和实验得到的散射函数关键频谱峰值特性符合较好.     

光在二维无序介质中的动态传播过程

用有限时域差分 FDTD 法研究了光在二维无序介质中的动态传播过程.数值模拟了某一高斯脉冲光束入射到无序介质中光场的空间分布随时间的演化,结果表明:在光束入射的初期,能量从光源区向外传播并出现了清晰的波前传播的特点;经过一定的时间后,有部分能量从光源区向外扩散,而大部分能量逐渐聚集在介质中的一些小区域内,这与整体散射模型关于无序介质中光子局域化的研究相吻合,而且更清晰地呈现了形成光子局域化过程中的物理现象及整体散射干涉效应的重要机理作用.     

大气风场和温度对无线电声波探测系统探测高度影响的数值研究

从声波扰动介质中的电波波动方程出发, 使用时域有限差分(FDTD)方法, 结合声波传播的FDTD 模型, 构建了描述声波和电波相互作用的数值模型, 并运用该模型分析风场和温度对无线电声波探测系统的探测高度的影响. 数值模拟结果表明: 温度与风场剖面的存在改变声波和电波散射回波的传播轨迹; 温度梯度剖面主要影响声波的传播速度, 风场剖面导致作为电波散射体的声波波阵面的偏移, 降低电波散射回波的强度并改变回波路径, 使得接收数据减少, 限制无线电声波探测系统的探测高度; 在强风背景下, 若降低声波散射体高度, 电波散射回波“聚束点”的偏移会有较大的改善, 但同时意味着探测高度的降低. 为了改善风场背景下无线电声波探测系统的探测高度, 可以使用双基地雷达或者增大接收天线面积等方法来实现.     

On the possibility of reconstructing an anisotropic wind wave spectrum by the method of double-position sonar

We investigate the possibility of reconstructing the wind wave spectrum, with anisotropy taken into account, by the method of ocean acoustic sounding, using horizontally deployed antennas in the continuous mode of operation. This method uses marrow-beam horizontally spaced antennas for radiating a monochromatic sound signal and receiving signals scattered by the surface. By this method, we can localize a definite area of the scattering surface and reconstruct the wind wave spectrum owing to the direct dependence of the wind wave harmonic, which forms the scattered signal, on the position of this area relative to the radiator and the receiver. Expressions that relate the energy spectrum of the scattered signal formed at the receiver output to the wind wave spectrum are found. On the basis of these expressions, an algorithm has been developed for gaining information both about the spatial isotropic spectrum of wind waves and about the dependence of the angular spectrum on the value of the wind wave vector and its direction relative to the wind velocity.Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 1–2, pp. 139–145, January–February, 1995.     

Lump,lumpoff and predictable rogue wave solutions to the (2+1)-dimensional asymmetrical Nizhnik-Novikov-Veselov equation

This paper mainly uses Hirota bilinear form to investigate the (2+1)-dimensional asymmetrical Nizhnik-Novikov-Veselov equation. We obtain the general lump solutions and discuss its positiveness, the propagation path, amplitude and position at any time. Based on the general lump solutions, lumpoff solutions which a combination of lump solitons and stripe solitons, are also triumphantly acquired. Similarly, according to the general lump solutions, we are also consider a particular rogue wave by introducing a pair of stripe solitons, and research its predictability which include the time of the rogue wave appearance, position at time, propagation path and the maximum value of wave height. Finally, some figures are given to explain the movement mechanism of these solutions.     

声表面波检测铝板表面微缺陷深度实验研究

为了探究声表面波与不同深度微裂纹缺陷相互作用的关系,将脉冲激光作用于一系列不同缺陷的试件铝板上进行线光源激励,激发激光超声波。用超声传感器接收在铝板中传播的激光超声信号,通过数字荧光示波器采集激光超声在铝板中的传播数据。对采集到的反射波数据进行分离谱分离过程得到的铝板中激光超声的时域分布和透射波数据进行频域分析。实验发现:缺陷深度影响着反射回波两峰值特征点到达时间差,两者之间近似线性关系,也影响着透射波的截止频率且二者呈现递减关系。     

Multiple focused-waves production by parametric mixing in a layer with hysteretic quadratic nonlinearity

Nonlinear scattering of a weak probe wave by a strong pump wave in a layer with nonlinearity hysteresis is analysed. It is demonstrated that a spherical probe wave from a point source after nonlinear process of difference frequency excitation is scattered in several foci. Different foci correspond to different frequencies of the transformed wave spectrum. In the case of a plane pump wave and under the condition that nonlinear scattering proceeds without acoustic mode conversion wave front reversal is possible. To achieve wave front reversal in a layer with hysteretic quadratic nonlinearity the pump wave should have a frequency equal to the frequency of the probe wave or to any of its subharmonics.     

Stochastically excited waves in a 1D random medium - the mean field and power spectrum

Scalar wave propagation is examined when both the wave source and the propagation speed are random. Results are derived for the mean field and the power spectrum using the second-order Born approximation. The results depend on whether the source S(x, t) and the propagation speed c(x, t) are correlated or not. When they are uncorrelated, the mean field is zero. When they are correlated, the mean field is non-zero only when the source is non-stationary. The power spectrum is incoherent to leading order. There is a transfer of energy from lower to higher frequencies owing to wave scattering. The corresponding frequency upshift of the power profile in the (k, ω) domain is mainly caused by the cross power between the direct and the twice scattered field, which represents a second-order incoherent power contribution. The results are confirmed using a numerical solution of the wave equation where the scattered field is expanded to fifth order.     

同振球型矢量水听器声波接收理论研究

对同振球型矢量水听器声压和质点振速的声波接收理论进行了研究。以同振球型振速水听器测量原理为基础,推导了自由运动刚性球体和弹性球体的声波接收响应数学表达式,分析了振速水听器几何尺寸、平均密度与其频响特性曲线之间的关系;另外,根据球面接收器的声波接收理论,推导了矢量水听器声压接收响应数学表达式,通过理论分析和数值计算,研究了振速水听器表面上的声压分布规律以及声压水听器的声波接收压力系数与其接收面的大小、质点振速水听器的半径、布放的位置和半径等参数之间的关系;从理论上建立了矢量水听器声波接收理论模型和分析方法,为矢量水听器的设计和研制提供了理论依据。      

Light scattering from a fluid with a stationary temperature gradient

The spectrum of light, scattered from a fluid with a stationary temperature gradient, is calculated on the basis of fluctuating hydrodynamics. Explicit expressions are obtained for the spectrum of the scattered light which is no longer symmetric around the frequency of the incident light. In particular the difference in height and intensity of the Brillouin lines is given. Furthermore the shift in the position of the maximum of the Rayleigh line is calculated.     

Acoustic emission source location in plates using wavelet analysis and cross time frequency spectrum

In this study, the theories of wavelet transform and cross-time frequency spectrum (CTFS) are used to locate AE source with frequency-varying wave velocity in plate-type structures. A rectangular array of four sensors is installed on the plate. When an impact is generated by an artificial AE source such as Hsu–Nielsen method of pencil lead breaking (PLB) at any position of the plate, the AE signals will be detected by four sensors at different times. By wavelet packet decomposition, a packet of signals with frequency range of 0.125–0.25 MHz is selected. The CTFS is calculated by the short-time Fourier transform of the cross-correlation between considered packets captured by AE sensors. The time delay is calculated when the CTFS reaches the maximum value and the corresponding frequency is extracted per this maximum value. The resulting frequency is used to calculate the group velocity of wave velocity in combination with dispersive curve. The resulted locating error shows the high precision of proposed algorithm.     

Stochastically excited waves in a 1D random medium – the mean field and power spectrum

Scalar wave propagation is examined when both the wave source and the propagation speed are random. Results are derived for the mean field and the power spectrum using the second-order Born approximation. The results depend on whether the source S(x, t) and the propagation speed c(x, t) are correlated or not. When they are uncorrelated, the mean field is zero. When they are correlated, the mean field is non-zero only when the source is non-stationary. The power spectrum is incoherent to leading order. There is a transfer of energy from lower to higher frequencies owing to wave scattering. The corresponding frequency upshift of the power profile in the (k, ω) domain is mainly caused by the cross power between the direct and the twice scattered field, which represents a second-order incoherent power contribution. The results are confirmed using a numerical solution of the wave equation where the scattered field is expanded to fifth order.