浅海涌浪对表面声道声传播的影响

受海面强风和海-气相互作用影响,表面声道普遍存在于冬季海洋环境中,是一种天然有利于声传播的波导.但是海面波浪使得海表形成粗糙界面,会严重破坏这种优良性能.本文利用南海北部海区的一次冬季声传播实验数据,研究表面声道声传播特性.研究表明,海底底质对表面声道内声传播的影响较弱,当海面风较小时,涌浪造成的影响为主要原因.实验数据显示,考虑涌浪后的粗糙海面给70km远处带来了10dB的传播损失增长.因此在考察南海北部海区冬季声场特性时,不仅要考虑海面风浪的影响,更需要考虑周围海域传来的涌浪的影响.研究涌浪存在时的声传播特性对提升声纳设备在海况较差时的使用性能具有重要意义.     

Comparison of Surface Duct Energy Leakage with Bottom-Bounce Energy of Close Range Propagation

For close range(almost 8-15km) propagation,bottom-bounce energy(BBE) usually suffers from a great transmission loss due to a large grazing angle interacting with the ocean bottom,and the surface duct energy leakage(SDEL) might be able to make a significant contribution to shadow zone insonification.This study aims at making a comparison between SDEL and BBE in a shadow zone with a source depth of 50 m for close range propagation.Analysis of experimental data shows that for lower frequencies the SDEL can be comparable with the BBE up to a range of 12 km.Numerical simulations suggest that the transmission loss(TL) of 90 dB is a proper value to quantify the role of the SDEL in shadow zone insonification.When TL of the SDEL is about 90 dB,it is believed to be comparable with the BBE.Studies on the effect of mix layer depth(MLD) on the SDEL indicate that larger MLD and lower frequencies can help to create favorable conditions for SDEL and that there exists- an optimal MLD for SDEL at certain frequency.Statistics of MLD distribution in the South China Sea(SCS) show that winter is favorable for SDEL and the situation reverses in summer.Eddies are known to occur frequently in the SCS and the study on the effect of eddies on the SDEL suggests that the presence of an eddy will influence the SDEL significantly by modifying the MLD.     

基于联合波叠加法的浅海信道下圆柱壳声辐射研究

针对浅海信道下弹性结构声辐射预报尚无高效可靠的研究方法,提出了一种浅海信道下弹性结构声辐射快速预报的联合波叠加法.该方法结合了浅海信道传输函数、多物理场耦合数值计算法和波叠加法理论,运用该方法可对浅海信道下弹性结构辐射声场进行快速预报.经数值法和解析解法验证后,从信道下辐射源、环境影响和辐射声场测量的角度研究分析了浅海信道下弹性圆柱壳的声辐射特性,阐释了进行浅海信道下结构声辐射研究的必要性.研究结果表明,仅在低频浅海信道下弹性结构可近似等效为点源,信道上下边界对声场产生显著的耦合影响,高频段的空间声场指向性分布尤为明显,垂直线列阵进行信道下结构辐射声功率测量时,测量结果受到信道环境边界和潜深的影响较大.     

表面声道对深海风成噪声垂直空间特性的影响规律

水下风成噪声的垂直空间特性包括噪声垂直方向性和垂直相关性,研究海洋环境对其影响规律对提升声呐性能、增加海洋环境参数反演的准确性具有重要意义.本文利用Pekeris割线下的简正波理论描述噪声的传播过程,研究了深海环境下存在表面声道时,表面声道以下噪声垂直空间特性的变化规律及其原因.研究表明,在临界深度以上,表面声道的存在导致噪声垂直方向性在水平凹槽边缘靠近海底方向上的峰值升高,噪声垂直相关性随垂直距离增加先后周期地向正相干和负相干方向偏移;在临界深度以下,表面声道的存在导致水平方向上的噪声能量增强,噪声垂直相关性整体向正相干方向偏移.当表面声道的参数变化时,表面声道的厚度变化对噪声垂直空间特性影响较大,而表面声道内的声速梯度变化对噪声垂直空间特性几乎没有影响.结合各类简正波的变化分析表明,存在表面声道时,噪声源激发的折射简正波阶数增加,强度增强,是表面声道引起噪声垂直空间特性变化的主要原因.     

Non-linear effect of wall linings on sound attenuation in ducts

The equivalent surface source method is extended to the analysis of high intensity sound propagation in a duct whose wall is partially treated with a sound absorbing material. The propagation of sound in the gas is assumed to be linear, but the acoustic resistance of the sound absorbing material is assumed to be a function of the normal acoustic velocity. The problem is reduced to a non-linear integro-differential equation for the fluid particle displacement at the lined wall surface, which can be solved by a successive approximation method. Numerical examples show that the non-linear effect decreases or increases the peak sound attenuation rate of the lowest mode depending upon the linear component of the resistance. The dependence of the attenuation spectrum on modal phase difference of multi-mode incident waves is heavily affected by the non-linear effect. In the case of incident waves of multi-circumferential modes, different circumferential modes are generated by the non-linear effect.     

浅海周期起伏海底环境下的声传播

海底粗糙对水下声传播及水声探测等应用具有重要影响.利用黄海夏季典型海洋环境,分析了同时存在海底周期起伏和强温跃层条件下的声传播特性,结果表明:由于海底周期起伏的存在,对于低频(<1 kHz)、近程(10 km)的声信号,传播损失可增大5—30 dB.总结了声传播损失及脉冲到达结构随声源深度、海底起伏周期及起伏高度等因素变化的规律.当海底起伏周期不变时,起伏高度越大引起的异常声传播的影响随之变大;当起伏高度不变时,随着起伏周期变大,其对声传播的影响逐渐变小.用射线理论分析了其影响机理,由于海底周期起伏改变了声波与海底的入射和反射角度,使得原本小掠射角入射到海底的声线变为大掠射角,导致海底的反射损失增大;另一方面,声线反射角度的改变会使得原本可以到达接收点的声能量,由于与海底作用次数增加或变为反向传播而大幅度衰减.在浅海负跃层环境下,声源位于跃层上比位于跃层下对声传播影响更大.周期起伏海底对脉冲声传播的影响表现在引起不同角度的声线(或简正波号数)之间的能量发生转化,一些大角度声线能量衰减加大,多途结构变少.多途结构到达时间及相对幅度的变化进而影响声场的频谱,会使得基于匹配场定位的方法性能受到影响.所以,声呐在实际浅海环境中应用时,应对起伏海底的影响予以重视.此外,研究结果对海底地形测绘空间精度的提高也具有重要参考意义.     

一种基于模态域波束形成的水平阵被动目标深度估计

水面水下目标分辨与识别一直是被动声呐探测领域的难题.利用一种水平阵模态域波束形成算法获得己知方位目标声源的各阶模态强度,将其与不同深度的各阶参考模态强度进行匹配,最终实现了对声源的深度估计.仿真结果表明,该算法可以在信噪比为-10 dB的情况下,用300Hz带宽的信号样本,实现对声源深度的有效估计.系统分析了不同参数和不同波导条件对该方法目标深度估计性能的影响.其中,阵元数越多,模态样本数越多,计算频段越宽,方位估计精度越高,有效阵长越长,深度估计的性能越好.阵元间距和波导深度的变化不会影响该方法的深度估计性能,并且该方法的深度估计性能在声速剖面、海底参数等波导条件存在扰动时具有鲁棒性.     

考虑噪声源深度分布的海洋环境噪声模型及地声参数反演

考虑到海洋环境噪声源深度分布不集中,建立了噪声源随深度分布的海洋环境噪声模型,分析了源深度对噪声场垂向特征的影响并从简正波角度予以解释,发现海底声阻抗和声源深度都显著影响由海洋环境噪声获得的等效海底反射损失大掠射角部分,进而将该模型用于地声参数反演.两段实测噪声数据200—525 Hz频段的反演结果表明:基于海洋环境噪声的地声参数反演最优值与声传播的反演结果相近;源平均深度最优值随频率增加有变小的趋势,说明随频率增加环境噪声主要贡献源逐渐由航船转为风浪;当海况大于3级时,400 Hz以上频段噪声源深度平均值很小,与Monahan气泡理论的描述一致.     

一种基于单水听器的浅海水下声源被动测距方法

针对浅海波导中宽带脉冲声源的被动测距问题,本文在群延迟理论的基础上,与warping变换处理相结合,提出了一种适用于浅海波导中宽带声源的单水听器被动测距方法.利用warping变换可以实现对脉冲声源接收信号各阶简正波的分离提取,对分离后的简正波进行时频分析处理可以得到各阶简正波到达时刻和频率之间的关系,即各阶简正波的频散曲线,从而得到任意两阶简正波到达接收水听器的时延差.海底相移参数P是描述海底地声参数的一个重要参量,包含了海底地声参数信息,在海底环境参数未知而P已知的情况下,利用P和简正波水平波数之间的关系可以求得任意两阶简正波的?S_(g,mn)(群慢差).根据群延迟理论,利用得到的任意两阶简正波的时延和?S_(g,mn)可实现利用单水听器对水下声源进行被动测距.本文提出的测距方法测量简单、计算方便,具有较强的实用意义.数值仿真和海上实验数据处理结果的测距误差都在10%以内,证明了该方法的有效性.     

三维浅海下弹性结构声辐射预报的有限元-抛物方程法

利用多物理场耦合有限元法对结构和流体适应性强、抛物方程声场计算高效准确的特点,提出了三维浅海波导下弹性结构声振特性研究的有限元-抛物方程法.该方法采用多物理场耦合有限元理论建立浅海下结构近场声辐射模型,计算局域波导下结构声振信息,并提取深度方向上复声压值作为抛物方程初始值;然后采用隐式差分法求解抛物方程以步进计算结构辐射声场.重点介绍了该方法对浅海下结构声辐射计算的准确性、高效性以及快速收敛性后,对Pekeris波导中有限长弹性圆柱壳的声振特性进行了分析.研究得出,当圆柱壳靠近海面(海底)时,其耦合频率比自由场下的要高(低),当潜深达到一定范围时,与自由场耦合频率基本趋于一致;在低频远场,结构辐射场与同强度点源声场具有一定的等效性,且等效距离随着频率增加而增加;由于辐射声场受结构振动模态、几何尺寸和简正波模式影响,结构辐射场传播的衰减规律按近场声影响区、球面波衰减区、介于球面波和柱面波衰减区、柱面波衰减区四个扩展区依次进行.     

Stability and a fast calculation method of travel speed of pulse peak in convergence zone

A long-range sound propagation experiment was conducted in the West Pacific Ocean in summer 2013.The signals received by a towed array indicate that the travel speed of pulse peak(TSPP)in the convergence zones is stable.Therefore,an equivalent sound speed can be used at all ranges in the convergence zones.A fast calculation method based on the beam-displace-ment ray-mode(BDRM)theory and convergence zone theory is proposed to calculate this equivalent sound speed.The computation speed of this proposed method is over 1000 times faster than that of the conventional calculation method based on the normal mode theory,with the computation error less than 0.4%compared with the experimental result.Also,the effect of frequency and sound speed profile on the TSPP is studied with the conventional and fast calculation methods,showing that the TSPP is almost independent of the frequency and sound speed profile in the ocean surface layer.     

The use of phased rays in very shallow water

Numerical predictions are studied of propagation in isovelocity water over a lossy bottom with the depth and frequency such that only one, two, or three normal modes exist. Comparison of the result from a new “Phased Ray” model with the normal mode result shows that it is the phase change upon reflection that is the major cause of the disagreement between the Phased Ray and Mode models. The possibility of controlling the phase of the plane wave reflection coefficient by tuning an overlying layer of silt, or whatever, is also demonstrated. The case is then put forward, when dealing with problems where the bottom has irregular contours or where the surface is rough, i.e., where there are no normal mode solutions, for modelling the propagation with rays corrected to agree in general properties with the normal mode solutions of nearly the same problems.     

The near-ground structure of the nocturnal sound field

The near-ground behavior of the low-frequency (100 Hz to 500 Hz) sound field in the nocturnal sound duct is studied theoretically and experimentally. In the first few meters of the atmosphere, narrow-band sound fields are found to have a characteristic vertical structure. The sound field is the superposition of a "surface mode," whose magnitude decreases monotonically with altitude, with a sum of "higher modes," each of whose magnitudes has a pronounced minimum a few meters from the ground at approximately the same height. The surface mode attenuates to negligible levels after a few hundred meters from the source. Consequently, more than a few hundred meters from a narrow-band source, there is a "quiet height" at which the sound level is reduced by 10 to 15 dB relative to its value on the ground. The narrow-band quiet height is shown to be a robust feature of nocturnal sound propagation.     

风成噪声背景下垂直阵阵列信噪比随声源深度的变化规律

风成噪声是海洋中最广泛存在的环境噪声、是被动声纳处理性能的重要影响因素.它具有一个典型的物理性质是很大程度上只对应中高阶模态.阵列信噪比计入了阵列采样辐射声强、背景噪声功率和阵处理增益,是决定声纳阵列处理性能的关键物理量.本文研究了受风成噪声模态结构影响,典型夏季浅海环境中垂直阵阵列信噪比随声源深度的变化关系.在简正波模深函数采样完整的假设条件下,理论证明了阵列信噪比随声源深度的变化可近似为低阶模态幅度强度(模深函数模值的平方)随深度变化的线性叠加,且模态阶数越低,贡献越大;并且,在强风成噪声背景、显著负梯度环境下该变化规律可由1阶模态幅度强度随深度的变化近似独立表征.以上结果表明,在同一声源距离条件下,声源置于水体下半部分时的阵列信噪比比置于海面附近更大,并且在位于1阶模态峰值点所在深度附近时达到最大.典型负声速梯度浅海环境中的仿真实验结果对理论分析进行了验证,并表明在一定条件下阵列信噪比随声源深度的变化与声源距离近似无关.     

Prediction of outdoor sound transmission loss with an artificial neural network

An artificial neural network is developed for rapid prediction of sound transmission loss (TL) during propagation outdoors. The network predicts TL for a nonturbulent atmosphere from inputs involving the source/receiver propagation geometry (height range: 0-5 m, horizontal separation distance: 100-900 m), source frequency (range: 20-200 Hz), ground properties, and atmospheric refractive profile characteristics. A parabolic equation (PE) code generates the training and test data sets for the network. To ensure that a minimal set of input parameters is used in the network training, a nondimensional version of the PE and accompanying boundary, initial, and atmospheric conditions is developed. A total of 10 independent, nondimensional input parameters are found to be necessary for the training. Approximately 27,000 random cases involving these 10 parameters are generated used to train networks with varying numbers of neurons. The root mean square (RMS) error between random test cases solved by the PE and corresponding neural network predictions was 2.42 dB when a sufficient number of neurons (about 44) are included in the hidden layer. Also, only 18% of the cases resulted in RMS errors that were greater than 2 dB.     

利用简正模态相位关系的浅海声源深度分辨方法

针对浅海波导声源深度分辨问题,本文提出了一种利用简正波的相位关系进行声源深度分辨的方法。该方法通过对垂直阵接收到的信号进行简正波分解,得到简正波系数,在已知声源距离的情况下通过对简正波系数项的处理得到各阶简正波的相位关系,根据相位关系估计声源的深度区间。该方法在阵元数较少时依然可以分辨声源深度,而且对宽带信号、窄带信号、连续信号和脉冲信号均适用。理论分析和数值仿真的结果说明:只有近水面处声源激发的简正波满足相位一致的特性,利用此特性可分辨水面水下目标。仿真和海试结果证明:在海水声速随深度不变或缓变的水文环境下,该方法可以准确的分辨声源深度区间,并且在深度采样不充分的情况下仍然适用。      

线偏振高斯光束通过复合型衍射光栅的传输特性

提出一种集合浮雕和折射率周期分布调制的复合型衍射光栅,并利用角谱表示和严格的模式理论研究了线偏振高斯光束通过这种复合型衍射光栅的传输特性.数值计算结果表明,在光栅的同一透射深度处复合型衍射光栅光强的起伏频率要比浮雕型光栅光强的起伏频率小.最后使用复合型衍射光栅模型,研究了亚波长体积相位全息光栅的表面起伏和入射光束的束腰宽度对衍射效率的影响. 关键词： 复合型衍射光栅 亚波长体积相位全息光栅 高斯光束 严格模式理论     

Modal Doppler theory of an arbitrarily accelerating continuous-wave source applied to mode extraction in the oceanic waveguide

A Doppler-based method for using a moving narrow-band source to extract the modes of acoustic propagation in a range-independent shallow ocean waveguide over a partial-water-column spanning vertical line array (VLA) is introduced. Because the modal components propagate at distinct frequencies in the case of uniform radial source motion, the modal depth functions may be isolated and extracted from a frequency decomposition of the field. Because Doppler broadening due to radial source accelerations degrades the effectiveness of the extraction method, the method incorporates a technique to compensate for Doppler broadening. As the basis for the compensation technique, a theory is introduced for describing the VLA field from an accelerating cw source. By connecting the range of the source at the time a signal feature is emitted (the retarded time) to the range of the source at the time the signal feature arrives at the receiver (the contemporary time), the theory incorporates the Doppler effects associated with the finite group velocities of the modal components. The mode extraction method and compensation technique are applied to simulation and ocean data.     

Nonlinear skin effect in semimetals

The influence of nonlinearity on propagation of radio waves into semimetals in the geometry where the constant magnetic field H is directed along the trigonal axis of the crystal has been investigated. Strong magnetic Landau damping occurs in the linear mode in this geometry. It is shown that the electron capture by the magnetic field of a radio frequency wave of large amplitude decreases this absorption. As a result, the depth of the skin layer in the semimetal becomes the function of the amplitude of the exciting radio frequency field and can increase many times.     

Guided waves in a stratified half-space

The dispersion and excitation mechanisms and the energy distribution of guided waves in a stratified half-space are studied. All possible guided waves excited by a symmetric point source in two or three-layer medium models and their relation to the medium parameters are analyzed in detail. The excitation and propagation characteristics, as well as the energy distribution along the depth direction, of all modes of the surface waves and trapped waves are numerically investigated and analyzed thoroughly not only in the case when the shear wave velocity increases from up to down layers but also when a low-velocity layer is contained in halfspace, especially when the shear wave velocity decreases from up to down layers. It is found that there exist many guided wave modes in the case where the shear wave velocity of each layer increases from up to down layers. However, there is less than one guided wave mode in the case where the shear wave velocity of each layer decreases from up to down layers. The trapped waves exist and propagate along the low-velocity structure in the stratified half-space. It is also found that the characteristic of a mode is related to the source frequency. It is possible that a surface wave at one value of frequency is like a trapped wave at another value of frequency. Finally, the relation of the characteristics of all guided waves (surface waves and trapped waves) to the parameters of media is studied.