一种浅海声简正波近场分离方法

针对浅海环境下声简正波的近场分离问题,提出了一种基于频率-波数(Frequency-Wavenumber,F-K)变换的分离方法。该方法通过对由近场水平多道接收信号所组成的水声信号矩阵进行F-K变换,将二维接收信号矩阵从时间-空间域转换至频率-波数域,由提取频率-波数域上各阶简正波各频点的波数实现对信号中各阶简正波的分离及频散特性提取。数值模拟和水池实验对本方法在实际研究中的可靠性和有效性进行了验证,表明在500 m距离内利用本方法能可靠分离Pekeris波导中200 Hz以下各阶简正波。     

浅海波导中目标散射的简正波-Kirchhoff近似混合方法

提出了计算浅海波导中复杂目标散射的数值方法:简正波-Kirchhoff近似混合方法。通过把目标散射的Kirchhoff近似方法和简正波声传播模型相结合,可对大尺寸复杂目标在浅海波导中的散射声场进行计算。以浅海波导中刚性球体散射的解析解为标准解验证了本方法,说明简正波-Kirchhoff近似混合方法是有一定计算精度的工程预报方法。数值计算Pekeris浅海波导中球体目标散射声场在深度-频率平面和距离-频率平面上的二维干涉结构及其与自由空间中的差异。进而通过FFT获得目标时域回波随深度的分布图,分析浅海波导中目标姿态、声速剖面对Benchmark潜艇目标散射的影响。     

Extracting the interference components of normal modes in shallow water waveguide using singular value decomposition method

The normal mode interference characteristic in shallow water waveguide is a valuable topic in the fields of underwater acoustic.A method for extracting the interference components of normal modes from broadband acoustic propagation data recorded by a single hydrophone without any prior information is present in this paper.First,a Hermitian matrix is formed by the power spectral density.Second,a singular value decomposition(SVD) is performed on the Hermitian matrix to obtain the orthonormal eigenvectors,which are proportional to the interference components of normal modes.The fundamental equations of the new extracting method are derived based on normal mode and waveguide invariant theory.And the validity of the present method is verified by the numerical simulation and experimental results.In addition,the extracted results of normal-mode interference components are intended to be used for passive ranging of broadband sources.     

Coherence of acoustic modes propagating through shallow water internal waves

The 1995 Shallow Water Acoustics in a Random Medium (SWARM) experiment [Apel et al., IEEE J. Ocean. Eng. 22, 445-464 (1997)] was conducted off the New Jersey coast. The experiment featured two well-populated vertical receiving arrays, which permitted the measured acoustic field to be decomposed into its normal modes. The decomposition was repeated for successive transmissions allowing the amplitude of each mode to be tracked. The modal amplitudes were observed to decorrelate with time scales on the order of 100 s [Headrick et al., J. Acoust. Soc. Am. 107(1), 201-220 (2000)]. In the present work, a theoretical model is proposed to explain the observed decorrelation. Packets of intense internal waves are modeled as coherent structures moving along the acoustic propagation path without changing shape. The packets cause mode coupling and their motion results in a changing acoustic interference pattern. The model is consistent with the rapid decorrelation observed in SWARM. The model also predicts the observed partial recorrelation of the field at longer time scales. The model is first tested in simple continuous-wave simulations using canonical representations for the internal waves. More detailed time-domain simulations are presented mimicking the situation in SWARM. Modeling results are compared to experimental data.     

浅海不确定声场的随机多项式展开法研究

为获得求解浅海不确定声场的普适模型,建立了随机多项式展开法与Helmholtz方程的非嵌入式耦合模型,其间运用概率配点法求解多项式系数。针对仅当海水深度不确定时的Pekeris波导、声速剖面和海深均不确定时的Pekeris波导以及下限深度不确定温跃层等几种情形,计算了传播损失概率密度分布。结果表明所建模型对声场计算模型普适性强,计算精度和计算效率高,可用于研究含多个不确定环境参数、声速剖面复杂的浅海环境中声传播的不确定性。     

Nonlinear effects in trapped modes of standing waves on the surface of shallow water

It was shown that traveling waves may coexist with standing waves in a planar infinitely long channel filled by ideal liquid with a free surface. The standing waves are localized near a dynamic inclusion—a massive die on an elastic base. The amplitude of the traveling waves may be turned to zero by appropriately selecting the vibration frequency of the die. The standing waves arise because the vibration eigenfrequencies have a mixed spectrum; that is, the discrete and continuous spectra superpose. Nonlinear effects were observed for the first time when standing waves form in shallow water. In particular, a relationship between the die weight necessary to excite trapped modes, die dimensions, and vibration frequency was derived. It was shown that the nonlinear effects cause double-frequency traveling waves with amplitudes of the next order of smallness. These traveling waves vanish if the die geometry is properly chosen, as for the waves of the zeroth order.     

浅海波导环境失配下的协方差矩阵拟合稳健匹配场声源功率估计

实际浅海波导中环境噪声为相干噪声,最小方差匹配场声源功率估计方法能在相干噪声背景下准确估计声源辐射功率,但该方法受环境不确定性影响较大;此外,由于最小方差匹配场声源功率估计方法使用信号幅度作为中间量估计声源功率,信号幅度估计误差会二次放大并传递到声源功率估计结果中。本文提出一种协方差矩阵拟合稳健最小方差匹配场声源功率估计方法,该方法引入信道传递函数不确定集,结合协方差矩阵拟合思想将声源功率估计问题建模为在信道传递函数不确定集约束下对函数取极值的问题,使用Lagrange乘子法求解该问题得到信道传递函数估计值和声源辐射功率估计值。环境失配影响声源辐射功率估计性能的根本原因在于信道传递函数偏差较大,协方差矩阵拟合稳健匹配场声源功率估计方法有效减小了环境失配时信道传递函数的偏差,从而显著提升环境失配稳健性。此外,该方法使用权值直接估计声源功率,无需使用信号幅度作为中间量,避免了估计误差的传递。仿真验证了协方差矩阵拟合稳健匹配场声源功率估计方法的环境失配稳健性。     

Surface reverberation at sound field focusing in shallow water

A numerical experiment is carried out to study the long-range surface reverberation in the presence of intense surface waves for the case of using vertical transmitting arrays providing sound field focusing at different depths. To focus the field, a phase conjugation of acoustic waves from a probe source positioned at the focusing point is used. It is demonstrated that surface waves considerably affect the focusing quality at a distance of several tens of kilometers from the transmitting array. This prevents the efficient suppression of long-range reverberation by increasing the focusing depth.     

Characteristic features of the sound field formation near the bottom of a shallow sea

Detailed calculated data are presented for the angular, temporal, and energy structures of the sound field at the points of localtion of a horizontal line array of receivers (at 250-m intervals) in a shallow sea. The array is deployed near the bottom of a 200-m-deep waveguide along a sound propagation track, at distances from 20 to 30 km from a source generating a signal in the kilohertz frequency range. The influence of a sand or mud bottom on the intensity of signals arriving along single rays is numerically estimated for winter and summer conditions of sound propagation in a specific region of the Barents Sea.     

浅海波导中折射类简正波的warping变换

Warping变换可以实现单水听器浅海低频声波导的简正波分离.本文讨论了海水声速随深度线性减小浅海波导中折射类简正波的warping变换.理论推导了海水折射类简正波水平波数、频域相位及瞬时相位的表达式,由此提出了相应的时域和频域warping变换算子,并由仿真数据进行了验证.本文的理论推导过程同样适用于海水声速线性增大或海水折射系数的平方随深度线性变化的浅海波导.     

Interference structure of the field of infrasonic directed sources in shallow water

The effect of the beam pattern of sources on the spatial interference structure of the infrasonic field in shallow water has been studied numerically. Calculations have been performed for a monopole and differently oriented dipoles and quadrupoles in the near-field zone, where the modes have not yet formed, as well as in the far-field zone, where the mode interference is observed. Dependence of the interference patterns at different frequencies on the emission and reception horizons, the multipole type, and the wave distance has been demonstrated. The importance of taking into account the horizontal beam pattern of sources in the far-field zone has been shown.     

Properties of the acoustic intensity vector field in a shallow water waveguide

Acoustic intensity is a vector quantity described by collocated measurements of acoustic pressure and particle velocity. In an ocean waveguide, the interaction among multipath arrivals of propagating wavefronts manifests unique behavior in the acoustic intensity. The instantaneous intensity, or energy flux, contains two components: a propagating and non-propagating energy flux. The instantaneous intensity is described by the time-dependent complex intensity, where the propagating and non-propagating energy fluxes are modulated by the active and reactive intensity envelopes, respectively. Properties of complex intensity are observed in data collected on a vertical line array during the transverse acoustic variability experiment (TAVEX) that took place in August of 2008, 17 km northeast of the Ieodo ocean research station in the East China Sea, 63 m depth. Parabolic equation (PE) simulations of the TAVEX waveguide supplement the experimental data set and provide a detailed analysis of the spatial structure of the complex intensity. A normalized intensity quantity, the pressure-intensity index, is used to describe features of the complex intensity which have a functional relationship between range and frequency, related to the waveguide invariant. The waveguide invariant is used to describe the spatial structure of intensity in the TAVEX waveguide using data taken at discrete ranges.     

Cavitation in water under tension near the freezing point

Experiments are reported on cavitation in water at an initial temperature of 0.7°C under the dynamic tension created when a compression wave interacts with a free liquid surface. It is found that the tensile strength of water increases from 20 to 50 MPa as the strain rate is varied from 1.8 × 10⁴ to 5.2 × 10⁴ s^?1. It is shown that the phase state of water obtained in experiments is in a double metastable region.     

An experimental study on matched field processing in shallow water

I.IntroductionPassiverangeanddepthlocalizationofanacousticsourceinshallowwaterisadifficult,yetinterestingproblemthathasreceivedagreatdealofattentioninthelastfewyears['5].Thesimu1taneousestimationofrangeanddepthrequirestheuseofnumericalpropagationmodels.Theclassicalapproachtothisproblemisto"match"thereceivedacousticdatawiththesoundfieldpredictedbythepropagationmodelforanumberofhypotheticalrange/depthsourcelocatiolls.Thistechniqueiscalledmatchedfieldprocessing.Itiscommonlyacceptedthatthewavepro…     

Magnetization switching modes in nanopillar spin valve under the external field

The current-induced magnetic switching is studied in Co/Cu/Co nanopillar with an in-plane magnetization traversed under the perpendicular-to-plane external field.Magnetization switching is found to take place when the current density exceeds a threshold.By analyzing precessional trajectories,evolutions of domain walls and magnetization switching times under the perpendicular magnetic field,there are two different magnetization switching modes:nucleation and domain wall motion reversal;uniform magnetization ...     

Josephson dynamics for coupled polariton modes under the atom–field interaction in the cavity

We consider a new approach to the problem of Bose–Einstein condensation (BEC) of polaritons for atom–field interaction under the strong coupling regime in the cavity. We investigate the dynamics of two macroscopically populated polariton modes corresponding to the upper and lower branch energy states coupled via Kerr-like nonlinearity of atomic medium. We found out the dispersion relations for new type of collective excitations in the system under consideration. Various temporal regimes like linear (nonlinear) Josephson transition and/or Rabi oscillations, macroscopic quantum self-trapping (MQST) dynamics for population imbalance of polariton modes are predicted. We also examine the switching properties for time-averaged population imbalance depending on initial conditions, effective nonlinear parameter of atomic medium and kinetic energy of low-branch polaritons. PACS 03.75.Lm; 71.36.+c; 42.50.Fx