Theoretical analysis and experimental results of interference striation pattern of underwater target radiated noise in shallow water waveguide

Waveguide invariant in shallow water is an attractive topic in recent three decades. The interference phenomena of direct wave of radiated noise of underwater target and reflection wave from sea surface and sea bottom can be considered as a typical case of shallow water waveguide.The interference striation pattern of direct wave and its reflection is the effective and comprehensive figure for better understanding the essence of shallow water waveguide invariant.The theoretical analysis of interference phenomena generated by direct wave of radiated noise of underwater target and its reflection wave from sea surface and sea bottom is presented in this paper.It is shown that the interference wave resulted by sea surface reflection will produce striation pattern centered at high frequency band.But the interference of nulling frequency resulted by sea bottom reflection may be at low frequency or high frequency, it strongly depends on the acoustic behavior of sea bottom.The relationship between main parameters of interference striation pattern and target,receiver,and environment is derived.It is shown that the interference striation have the shape of hyperbolic curve.The equation set of the hyperbolic curve and its asymptotic line is presented.The at sea experiment carried out in South China sea shows some interesting results.A part of data processing results are illustrated in this paper.The results expressed in this paper show that the interference striation pattern can be used,in some conditions,as a potential means for target recognition.     

采用双方波信号和B-样条小波解调弱反射光纤布拉格光栅

提出了一种采用双方波信号和B-样条小波解调弱反射光纤布拉格光栅(WFBG)的方法,并进行了实验验证.单个方波周期设置为相邻WFBG间光纤中激光往返传输的时间,对单个方波进行猝发操作形成双方波,则前WFBG反射的后方波与后WFBG反射的前方波重叠干涉.采用B-样条小波变换降低干涉信号的噪声,利用Hilbert变换对干涉信号进行π/2相移,对原干涉信号和相移后干涉信号比值进行反正切运算,得到干涉信号的相位信息.将间隔为50m的5-WFBG阵列置于木地板上,分别接收不同振幅和频率的正弦声波,采用上述方法解调的干涉相位信号能较好地反映声波信息.该解调方法解调光路简单,数据处理简单.     

利用矢量海洋环境噪声提取声场格林函数

考虑到矢量水听器在垂直方向上具有8字形指向性,能够有效抑制远方非平稳噪声源的干扰,提出了一种矢量环境噪声相关函数(NCF)提取声场时域格林函数(TDGF)的方法。基于简正波理论建立了声压和垂直振速垂直相关性模型。在此基础上,给出了声压和垂直振速相关函数提取声场纵向格林函数的过程.数值仿真对比和实验数据分析表明,相对于声压提取方法,垂直振速提取方法能够有效消除直达波前出现的亮纹与亮区干扰。此外,对于同等时间长度噪声序列,声压提取方法只提取到直达波路径,而垂直振速提取方法还提取到了我们更为关心的海底反射路径。利用直达波与海底反射波到达时延差估计的海深与实测海深吻合较好。      

深海近距离声场频率-距离干涉结构反演海底声学参数

地声反演一般存在多解性的问题,不易有效分离沉积层声速与厚度,本文提出了一种当声源与接收水听器分别位于海面与海底附近时匹配声场干涉周期反演深海沉积层声速与厚度的方法。该声场周期结构由直达波与海底沉积层反射回波干涉形成,其干涉周期受沉积层声速和厚度影响,通过匹配实测与理论计算的声场干涉周期可有效反演沉积层声速和厚度。开展了深海实验验证,利用水听器接收到的近距离船舶噪声的声场干涉条纹,反演得到的实验海域海底沉积层声速与该海域海底表层采样平均值差小于20 m/s。数值仿真与实验结果表明该方法可以有效反演沉积层的声速与厚度。      

Active microwave radiometry and its applications

Summary In microwave bands, attenuation and reflectivity of objects are usually determined by locating the objects between two antennas, connected to a generator and a detector. If a narrowband signal is used, the radiators and an object should be adequately separated as otherwise so-called near field exists around the radiators; due to interfering wave fronts, space is filled by interference maxima and minima that prevent a reasonable measurement from being done.The described method employs wideband noise instead of a narrowband or CW (continuous wave) signal. Therefore a radiometer is used as detector and a noise generator as source.By creating a random field in the space between radiators, interesting phenomena have been observed which can find also a practical use even though no ready-to-use theory apparently exists to describe such phenomena.     

舰船辐射噪声贝叶斯方法反演浅海底层结构及地声参数EI北大核心CSCD

针对以舰船辐射噪声为参考声源的浅海海底分层结构及地声参数反演问题,研究了一种基于贝叶斯理论的浅海多层海底地声参数反演方法。反演中以舰船辐射噪声的线谱成分为研究对象,进而采用非线性贝叶斯反演方法反演浅海底层结构、层中声速、声速衰减和密度,并对反演结果的不确定性进行分析。反演结果的最大后验概率估计值和边缘概率分布分别通过拨正模拟退火算法和Metropolis-Hastings采样法在各参数先验区间内计算获得,并根据贝叶斯信息准则确定最佳海底分层结构。海上实验表明:根据该方法反演获得海底分层结构及地声参数,计算得到的声压场与实测舰船辐射噪声传播损失误差不超过10%,反演结果能够准确表征实验海区海底特征。反演结果不确定性分析表明:海底纵波声速、横波声速以及密度的不确定性更小,对声压场变化更加敏感,反演结果更有效、准确。     

拖曳线列阵声呐平台噪声的空域矩阵滤波抑制技术

针对拖曳线列阵声呐平台噪声构成近场强干扰影响声呐弱目标探测的问题,利用近场平台噪声的多途传播特性,将匹配场定位技术和平面波目标方位估计技术结合,使用平台噪声到达接收阵的拷贝向量以及平面波方向向量共同设计平台噪声零响应约束空域矩阵滤波器,实现了平台噪声抑制.推导得出滤波器设计最优化问题的最优解,利用广义奇异值分解简化最优解表达式,并给出滤波器对平面波方向向量整体响应误差以及对平台噪声拷贝向量响应。利用平台噪声拷贝向量与远场平面波方向向量相关性,解释了平台噪声构成强干扰的原因,以及滤波后存在探测盲区的原因。由仿真可知,空域矩阵滤波处理可获得更小的探测盲区,同时获得盲区外更高的探测能力.      

无人机副遥控系统连续波电磁辐照机理

上行副遥控冗余设计是无人机战场生存能力的重要保证,为检验连续波辐照环境下某型无人机副遥控数据链系统的可靠性,基于连续波辐射发射系统建立了无人机连续波辐照效应试验平台,以典型"失锁"效应作为受干扰判别依据,开展了副遥控数据链系统辐照效应研究,利用电磁仿真和注入法分析了"失锁"效应机理和主要干扰耦合路径。试验结果表明:在某些敏感频点,副遥控链路会出现通信中断现象,即表现为失锁状态,最小失锁阈值为0.098V/m,且出现失锁现象的电场阈值随辐射场极化方向改变。经过电磁仿真试验可知,天线是无人机副遥控系统的主要干扰耦合路径,导致副遥控数据链产生失锁的频点均与工作频段相关,电磁能量通过前门耦合传导进入机载收发组合内部,当信噪比达到一定程度时,解调输出误码率过大,通信随即中断,产生失锁。     

Curvature-dependent excitation propagation in cultured cardiac tissue

The geometry of excitation wave front may play an important role on the propagation block and spiral wave formation. The wave front which is bent over the critical value due to interaction with the obstacles may partially cease to propagate and appearing wave breaks evolve into rotating waves or reentry. This scenario may explain how reentry spontaneously originates in a heart. We studied highly curved excitation wave fronts in the cardiac tissue culture and found that in the conditions of normal, non-inhibited excitability the curvature effects do not play essential role in the propagation. Neither narrow isthmuses nor sharp corners of the obstacles, being classical objects for production of extremely curved wave front, affect non-inhibited wave propagation. The curvature-related phenomena of the propagation block and wave detachment from the obstacle boundary were observed only after partial suppression of the sodium channels with Lidocaine. Computer simulations confirmed the experimental observations. The explanation of the observed phenomena refers to the fact that the heart tissue is made of finite size cells so that curvature radii smaller than the cardiomyocyte size loses sense, and in non-inhibited tissue the single cell is capable to transmit excitation to its neighbors.     

Second order front tracking for the Euler equations

A second order front tracking method is developed for solving the hyperbolic system of Euler equations of inviscid fluid dynamics numerically. Meshless front tracking methods are usually limited to first order accuracy, since they are based on a piecewise constant approximation of the solution. Here second order convergence is achieved by deriving a piecewise linear reconstruction of the piecewise constant front tracking solution. The linearization is performed by decomposing the front tracking solution into its wave components and by linearizing the wave solutions separately. In order to construct a physically correct linearization, the physical phenomena of the front are taken into account in terms of the front types of the previously developed improved front interaction model. This front interaction model is also extended to include front numbers used in the wave decomposition. It is illustrated numerically for Sod’s Riemann problem, the two interacting blast waves problem, and a two-dimensional supersonic airfoil flow validation study that the proposed front tracking method achieves second order convergence also in the presence of strong discontinuities and their interactions.     

Interference properties of a light beam having a helical wave surface

The spatial wave front characteristics of a laser were examined by two beam interference after amplitude division. Complex interference patterns were observed and are interpreted as due to a helical form of the phase front. This unusual form is implicit in previous analyses of cavity eigenfunctions, but the properties of such a helical co-phasal wavefront have not apparently been previously studied.     

利用迁移规律分析迈克尔逊干涉仪

本文以洛埃德镜实验作为引入,消除学生受薄膜干涉的思维定势,通过类比引导学生理解在点光源下迈克尔逊干涉仪实现干涉的方法是分波阵面法,并进一步分析等倾干涉和等厚干涉。     

Focused antenna array in a strongly inhomogeneous medium

A focused acoustic antenna array is considered in a strongly inhomogeneous stationary medium. An opportunity is indicated to determine the coordinates of a number of objects by active location. It is assumed that, in insonifying the objects by a wave with an arbitrary wave front, they scatter spherical waves and are sufficiently separated in distance and angle to be resolved by the same array in a homogeneous medium. The procedure of determining the coordinates of the objects involves a wave front inversion for distinguishing between the signals from different objects. The coordinates are determined by estimating the parameters for each individual object. The parameter estimation procedure is shown to provide a high efficiency of extracting the argument of a complex signal. The results of the numerical modeling and solution of the problem are presented.     

Problems of interpretation of holographic interferograms near shock wave fronts

A method of avoiding ambiguity in the interpretation of interferograms near a shock wave front is proposed. The method is based on combining the double-exposure schlieren method and holographic interferometry. Relations for calculating, on the basis of data obtained by analyzing double-exposure schlieren photographs, both the density at the shock wave front and the gradient of the density directly behind the front, which is necessary for calculating the shifts of the interference fringes near the shock wave front, are presented. Zh. Tekh. Fiz. 68, 88–91 (September 1998)     

根据非线性贝叶斯理论的界面波频散曲线反演

通过时频分析法从海底环境噪声数据中提取界面波频散曲线,进而采用非线性贝叶斯反演方法估算海底沉积物厚度、剪切波速度、压缩波速度和密度等参数及其不确定性。参数的最大后验概率(MAP)估计值和边缘概率分布分别通过自适应单纯形模拟退火法和Metropolis-Hastings采样法在各参数先验区间内搜索获得,采用贝叶斯信息准则(BIC)从不同参数化模型中选择最优模型。界面波频散曲线反演结果表明:满足实测数据的最优海底模型结构为3层均匀分布剪切波速度剖面结构,海底深度的反演精度在800m以内,比起压缩波速度和密度,剪切波速度的不确定性更小,对界面波频散曲线更敏感。      

曝光系统离焦对平面全息光栅衍射波前的影响

波前像差是衍射光栅的重要技术指标，它直接影响光栅的分辨率。由光致刻蚀剂记录两束相干光干涉条纹是制作全息光栅的关键步骤。为了提高全息光栅曝光系统调整精度、减小离焦、降低光栅的衍射波前像差，从离焦对反射球面准直镜的准直光平行度的影响程度出发，分析了准直光平行度对全息光栅衍射波前像差的影响。理论分析和数值模拟结果表明，准直镜调整误差直接决定全息光栅衍射波前像差大小。以3种不同刻线密度光栅为例，得出了准直镜调整误差的允许变化范围。     

散射光数字全息检测过程的统计光学讨论

从统计光学及标量衍射的基本理论出发,研究散射光数字全息重构波面与原散射波的关系,提出非平滑物体双曝光全息过程中散射光传播及干涉的简化模型.基于简化模型及计算机数值计算,对一个双曝光全息干涉系统转动形变的检测过程进行了理论模拟,并通过与该系统实验测量的比较,证实了简化模型的可行性.     

The effect of ocean wave on the vertical spatial correlation of ocean ambient noise

The effect of the correlation function of noise sources derived from the ocean wave spectrum on the vertical spatial correlation of ocean ambient noise is investigated. The spatial correlation models of ocean ambient noise usually assume that the surface noise sources are uncorrelated. This assumption can be used to explain some physical phenomena, but it is not consistent with the real situation. Considering the relation between the ocean wave motion and the ambient noise generated by wind, the spectrum of ocean wave is introduced to calculate the vertical correlation of ocean ambient noise as the correlation function of noise sources by using the Kuperman-Ingenito(K/I) noise model. The comparison of the simulations and the experimental data shows that the simulations of vertical correlation of ambient noise have some differences with the experimental data by assuming the noise sources are uncorrelated and the simulations of vertical correlation of ambient noise have a good agreement with the experimental data by using the correlation function of noise sources derived from the ocean wave spectrum under the situation of high wind speed.     

Quantum interference of spontaneous decays: noise reduction in V lasers without inversion

For a four-level double-V laser without inversion we show that the interference of spontaneous decays leads to intensity noise squeezing. The optimum amount of noise reduction is 50%, which is the maximum squeezing in a resonant three-level inversionless Λ-laser with coherent driving and a resonant three-level Raman laser. By transforming into a combination state basis, the Mandel parameter and the output fluctuation spectrum are calculated and the mechanism is interpreted.