Spatial processing of signals received by platform mounted sonar

Acoustic signals received by platform mounted sonar arrays can be spatially processed to enhance the detection of targets in the presence of both ambient and platform generated (self) noise. Ambient noise in the ocean, such as that due to distant shipping or biological choruses, are known to be spatially correlated. The platform generated noise will be of near-field origin and may not be received by all elements in the array. In this paper we investigate the performance of the minimum variance distortionless response (MVDR) beamformer and the recently introduced Fourier integral method (FIM) and compare their performances with the conventional beamformer. Real passive sonar data, obtained from a platform mounted sparse linear array of hydrophones, is used to study the performance of the beamformers in a typical sonar environment. It is shown that in the absence of self noise, when the array is accurately calibrated the MVDR beamformer will perform very well, but when sensor gain or phase errors are present the performance of the MVDR beamformer is degraded. Further, the MVDR beamformer is unable to reject the self noise which is not "seen" by the entire array. FIM however seems to perform well and a modified version of FIM, which we call weighted FIM (WFIM), is shown to perform better and is at worst comparable to a well calibrated MVDR beamformer     

一种新的波浪变形三维数值模式──0-1混合型边界元

为提高边界元法模拟三维波场波浪变形的数值计算精度,借鉴常数元和线性元剖分方式、波势函数及波势函数法向导数对单元节点设定的各自适应性,提出了一种新的单元剖分模式──0-1混合型边界元,以控制和减缓由于计算误差累计而造成的波浪数值计算上的“横向振动”,借此结合边界元法的分区模式可实现较大范围的波场线性波浪变形计算,并为时域内的波浪非线性变形计算提供时间步长的数值保证.     

Nonlinear effects in the process of propagation of internal waves with regard for the turbulent viscosity and diffusion

By the method of asymptotic multiscale expansions in the Boussinesq approximation, we study nonlinear effects observed in the process of propagation of internal waves with regard for the turbulent viscosity and diffusion. We determine the decrement of attenuation of waves and the boundary-layer solutions at the bottom and on the free surface. The wave-induced mean current is found in the second order of smallness in the wave steepness. The coefficients of the nonlinear Schr?dinger equation are obtained for the envelope of the wave packet. It is shown that a weakly nonlinear plane wave is stable under longitudinal modulation in the long-wave limit. If the wavelength is smaller than a certain critical value, then the wave is unstable under modulation.     

An Equivalent Transform Method for Evaluating the Effect of Water-Column Mismatch on Geoacoustic Inversion

An "equivalent transform method" for quantitatively evaluating the effect of water-column mismatch on geoacoustic inversions is presented. This method is based on the concept of error transferral from one medium to another and is derived from linear perturbative inverse theory. To illustrate the method, geoacoustic inversions using adiabatic mode data, including wave numbers, group velocities, and travel times, are considered. In the test cases, both linear and nonlinear internal waves are considered as the causes of the water-column mismatch, and the inversion errors due to the water-column mismatch in each case are discussed. In the case of linear internal waves, range-averaged inversion errors are largely eliminated at the full cycle distance of the internal wave; however, in the case of nonlinear internal waves, the range-averaged inversion errors are accumulated but scaled down with increasing range. Furthermore, the inversion errors produced by water-column mismatch will be large if the modes used for inversion are sensitive to the mismatch; for instance, using higher order modes might lead to increased error even though it would normally improve inversion results. Using lower frequency signal, which penetrates deeper into the bottom, extends the effective depth of the inversion solutions, but this also allows the water-column mismatch to transfer error into the deeper bottom     

局部非线性直接预测方法在检测海洋混沌和噪音中的应用

利用局部非线性直接预测方法及实测值与预测值的相关系数,提出了混沌时间序列及含有音（白噪音）混沌时间序列的检测混沌和噪音特征的一种方法。该方法对Ｌｏｇｉｓｔｉｃ映射资料及赤道日ＳＳＴ资料进行了混沌及噪音检测。结果表明,该检测方法优于一般谱方法对混沌资料的检测,在资料含噪信比约为３２％时,实测值与预测值的相关系数达０．５,在此可信相关系数范围内可检测资料的混沌特征。同时指出了随着资料噪信比的增加,实测     

Zebiak-Cane模式中条件非线性最优扰动对ENSO春季预报障碍的影响

使用Zebiak-Cane模式和条件非线性最优扰动(CNOP)方法,研究初始误差和参数误差共同作用对ENSO春季预报障碍现象的影响。选取模式中的8个El Ni?o事件,包括4次强事件和4次弱事件,每个El Ni?o事件又分别从8个不同的起始时间做1 a的预报,这样一共64个预报实验。对每个实验分别计算CNOP误差(初始误差和参数误差同时存在时的最优误差),通过分析误差增长,发现CNOP误差引起的1 a后的预报误差随着初始预报时间的不同有较大差异,并且不同强度的El Ni?o事件也会影响CNOP误差的发展,增长位相中强事件的预报误差要比弱事件的预报误差大一些;而衰减位相中恰恰相反,弱事件的预报误差要比强事件的预报误差要大一些;同时也发现高频El Ni?o事件对误差增长率的影响较大。本结论有助于提高Zebiak-Cane模式预报ENSO的技巧。     

Deep-water macrofaunal diversity in the Faroe-Shetland region (NE Atlantic): a margin subject to an unusual thermal regime

The Faroe-Shetland Channel, situated in the NE Atlantic, encompasses a number of different sediment types (habitats) as well as being subject to an unusual thermal regime. Our main objective was to assess variations in macrofaunal generic diversity and composition along two transects to gauge the relative influence of regional hydrography and local habitat heterogeneity. We found that generic richness and diversity on a West of Shetland (WoS) transect correlated most strongly with temperature range, whilst along a North of Shetland (NoS) transect, richness and diversity correlated negatively with sedimentary variables, notably total organic carbon. Macrofaunal composition at WoS is also strongly influenced by water temperature with specific genera ( e.g. Galathowenia positively associated with the temperature eigenvectors), whereas at NoS it is a combination of temperature, silt + clay fraction and total organic carbon that has an impact on composition ( e.g. Proclea and Pseudosphyrapus , showing a negative relationship with the temperature eigenvectors). Although the temperature regime exerts a strong control on regional ecology, local habitat heterogeneity remains a significant factor.     

Model Simplification and Parameter Modification for Offshore Platforms Using Experiment Data

- In this paper, a computation method has been developed so as to compare the finite element method (FEM) with the test results directly. The structure is divided into the "master" and "slave" degrees of freedom. The simplified model can be obtained with modal reduction. Then the design sensitivity analysis of the eigenvalues and eigenvectors has been carried out using the modal frequency and modal shape of the test. A two-story frame structure and a jacket model structure have been calculated. Meanwhile, the modified coefficient, the FEM computational and experimental values have been given. It has been shown that the FEM model modified using the test modal value is efficient.     

Stability Analysis on Speed Control System of Autonomous Underwater Vehicle

The stability of the motion control system is one of the decisive factors of the control quality for Autonomous Underwater Vehicle (AUV).The divergence of control,which the unstable system may be brought about,is fatal to the operation of AUV.The stability analysis of the PD and S-surface speed controllers based on the Lyapunov' s direct method is proposed in this paper.After decoupling the six degree-of-freedom (DOF) motions of the AUV,the axial dynamic behavior is discussed and the condition is deduced,in which the parameters selection within stability domain can guarantee the system asymptotically stable.The experimental results in a tank and on the sea have successfully verified the algorithm reliability,which can be served as a good reference for analyzing other AUV nonlinear control systems.     

A three-dimensional ocean general circulation model for mesoscale eddies——Ⅱ Diagnostic analysis

A three-dimensional density field associated with mesoscaie unstable waves generated by the 3-D, primitive-equation model (Wang and Ikeda, 1996) is provided to the quasi-geostrophic pressure tendency and ω-equations, and to the (ageostrophic) Q-vector equation. Diagnostic analyses, analogous to the approaches in meteorology: ω-equation and Q-vector method, are for the first time developed to examine the mesoscaie dynamical processes and mechanisms of the unstable waves propagating in the mid-latitude ocean. The weaknesses and strengths of these two diagnostic approaches are evaluated and compared to the model results. The Q-vector method is then recommended to diagnose the vertical motion associated with the mesoscaie dynamics from a hydrographic CTD (conductivity-temperature-depth) array, while the quasi-geostrophic equations produce some small-scale features (errors) in the diagnosed fields.     

利用小波包分析和混沌特征提取进行船舶辐射噪声分类

基于船舶辐射噪声信号具有非线性、非平稳的特征，提出采用提取船舶辐射噪声信号的非线性混沌特征量和多尺度小波能量特征，并将两者综合作为特征参数输入神经网络分类器进行船舶分类识别。实验结果表明，该方法能较好地区分不同类型的船舶。     

Re-study on recurrence period of Stokes wave train with high order spectral method

Owing to the Benjamin-Feir instability,the Stokes wave train experiences a modulation-demodulation process,and presents a recurrence characteristics.Stiassnie and Shemer researched the unstable evolution process and provided a theoretical formulation for the recurrence period in 1985 on the basis of the nonlinear cubic Schr dinger equation(NLS).However,NLS has limitations on the narrow band and the weak nonlinearity.The recurrence period is re-investigated in this paper by using a highly efficient High Order Spectral(HOS) method,which can be applied for the direct phaseresolved simulation of the nonlinear wave train evolution.It is found that the Stiassnie and Shemer’s formula should be modified in the cases with most unstable initial conditions,which is important for such topics as the generation mechanisms of freak waves.A new recurrence period formula is presented and some new evolution characteristics of the Stokes wave train are also discussed in details.     

Nonlinear analysis of traffic jams in an anisotropic continuum model

This paper presents our study of the nonlinear stability of a new anisotropic continuum traffic flow model in which the dimensionless parameter or anisotropic factor controls the non-isotropic character and diffusive influence. In order to establish traffic flow stability criterion or to know the critical parameters that lead, on one hand, to a stable response to perturbations or disturbances or, on the other hand, to an unstable response and therefore to a possible congestion, a nonlinear stability criterion is derived by using a wavefront expansion technique. The stability criterion is illustrated by numerical results using the finite difference method for two different values of anisotropic parameter. It is also been observed that the newly derived stability results are consistent with previously reported results obtained using approximate linearisation methods. Moreover, the stability criterion derived in this paper can provide more refined information from the perspective of the capability to reproduce nonlinear traffic flow behaviors observed in real traffic than previously established methodologies.     

基于CT非线性模型的水下目标跟踪算法比较

针对水下目标跟踪非线性跟踪精度问题,假设目标机动模型为恒转速运动模型,贝叶斯框架下,因扩展卡尔曼滤波跟踪方法进行模型在估计点的泰勒展开,忽略一阶以上高阶项,存在模型误差,比较了扩展卡尔曼滤波、无迹卡尔曼滤波、容积卡尔曼滤波在高斯噪声干扰下滤波误差均方根,以及3种方法运行时间。仿真证明,非线性系统下状态维度为5,容积卡尔曼滤波跟踪的精度高于无迹卡尔曼滤波,无迹卡尔曼滤波高于扩展卡尔曼滤波。该研究为海上目标非线性测量系统提供仿真实例,为进一步滤波算法改进提供基础。     

Internal wave spectrum in shallow water: measurement and comparisonwith the Garrett-Munk model

The covariance matrix of sound-speed variations is determined from yo-yo CTD data collected during the SWARM 95 experiment at a fixed station. The data covered approximately 2 h and were collected during a period when nonlinear solitary internal waves were absent or negligible. The method of empirical orthogonal functions (EOF) is applied to the sound-speed covariance matrix assuming that the internal wave modes are uncorrelated. The first five eigenvectors are found to agree well with the theoretically modeled eigenfunctions based on the measured buoyancy frequency and the internal wave eigenmode equation. The mode amplitudes for the first five modes are estimated from the corresponding eigenvalues. They agree with the Garrett-Munk model if j*=1 is used instead of j*=3. A second method is used to deduce the mode amplitudes and mode frequency spectra by projecting the sound-speed variation (as a function of time) onto the theoretical mode depth functions. The mode amplitudes estimated with this method are in agreement with the EOF results. A modified Garrett-Munk model is proposed to fit the frequency spectrum of linear internal waves in shallow water     

Critical Top Tension for Static Equilibrium Configuration of A Steel Catenary Riser

This paper aims to present the critical top tension for static equilibrium configurations of a steel catenary riser（SCR） by using the finite element method. The critical top tension is the minimum top tension that can maintain the equilibrium of the SCR. If the top tension is smaller than the critical value, the equilibrium of the SCR does not exist. If the top tension is larger than the critical value, there are two possible equilibrium configurations. These two configurations exhibit the nonlinear large displacement. The configuration with the smaller displacement is stable, while the one with larger displacement is unstable. The numerical results show that the increases in the riser＇s vertical distances, horizontal offsets, riser＇s weights, internal flow velocities, and current velocities increase the critical top tensions of the SCR. In addition, the parametric studies are also performed in order to investigate the limit states for the analysis and design of the SCR.     

带乘性噪声的一类非线性系统的滤波算法

针对带乘性噪声的一类非线性系统,给出了1种带单重渐消因子的强跟踪状态滤波算法。该算法将非线性系统线性化后,采用了线性最小方差估计方法来进行状态估计,通过运用正交原理和引入渐消因子,使得滤波效果具有强跟踪的优良性能。该算法扩展了卡尔曼滤波在带乘性噪声非线性系统状态估计中的应用范围。仿真结果表明了该算法的有效性。     

Stabilization of a linearly unstable wave participating in a three-wave resonant interaction

An analytical study of the influence of three-wave resonant interactions on the evolution of unstable wave disturbances is presented in the Kelvin-Helmholtz model. These results may be of interest in analyzing the dynamics of disturbances at the ocean-atmosphere interface and in two-layer flows which arise in the ocean and are characterized by large gradients of flow velocity at the boundary of layers. In the case under consideration, the instability arises when eigenfrequencies coincide in the framework of a single mode and the instability is algebraic. The amplitudes of the two other interacting stable waves are assumed to be small compared to the amplitude of the third, unstable, mode. The system of amplitude equations for this case is investigated using the WKB method. As a result, we obtain the formulas coupling the solutions for the time before and after a transition through a singular point, where the amplitude of the linearly unstable wave has a local minimum. These formulas give the rule of transformation of the parameter that characterizes a phase shift between fast and slow modes and determines the behavior of the system. It is shown that, in a transition through a singular point, this parameter changes randomly. As long as the parameter is positive, the amplitude of the linearly unstable wave remains limited and oscillates stochastically. In a transition of the parameter through zero, we exit the stabilization region and have an infinite growth of amplitude. The transition into the instability region is random. However, if the time interval where the amplitude remains limited is large enough, the scenario of the behavior of the system we have obtained can be treated as the partial stabilization of instability. The results make it possible for us to investigate the stochasticity caused by the nonlinear interaction of gravity-capillary waves in a two-layer model of a shear flow. These results are also of interest in analyzing secondary flows in laboratory facilities modeling the ocean and atmospheric processes.