利用字典学习的浅海被动声层析

针对被动海洋声层析,提出了一种利用字典学习从海洋环境噪声反演浅海声速剖面的方法。首先,通过海洋噪声互相关函数提取出两个水平阵列间的经验格林函数;其次,通过字典学习从数据生成字典矩阵来稀疏表征声速剖面;最后通过搜索稀疏的系数来实现对浅海声速剖面的反演。通过南海实验数据对本方法进行了验证,相对于传统被动海洋声层析方法实际反演结果的均方根误差降低为0.53 m·s^-1。而且搜索参数更少,同时具有较高的准确性。     

基于参考点之间水声传播的声速剖面反演

针对海洋声速剖面测量成本高、长期观测困难的难题,文章初步研究了利用水下固定参考点与水面已知位置之间的声信号传播时延来反演海水声速剖面的方法,提出了一种等声速分层模型下的声速剖面反演方法。将海水分层,对声信号传播过程进行建模,推导反演声速的非线性方程组;再利用牛顿迭代法,对非线性方程组进行求解。通过仿真和海试试验数据处理,分层数不同时,反演声速与实际声速之间的误差随着分层数的增加而变小,声速误差最小为0.80 m·s^-1左右,验证了反演方法的有效性与准确性。     

少量声速剖面时浅海声传播不确定性研究

当只有少量的浅海实测声速剖面时,不能直接利用Monte Carlo法研究声传播的不确定性。基于经验正交函数法,提出扩展向量法对少量实测声速剖面进行扩展,产生与实测声速剖面统计量基本一致的扩展声速剖面,基于MonteCarlo法,利用扩展后的声速剖面计算了声传播损失概率密度分布。结果表明,扩展向量元素服从标准正态分布,扩展后的数据捕捉到了未能测量到、但可能存在的声速剖面,该方法为浅海环境下实测声速剖面数据较少时,研究声传播不确定性提供了一种新的途径。     

扩展经验正交函数沿海声层析的声速反演

为了实现沿岸海域声速的实时连续监测,针对目前单收发换能器沿海声层析系统的剖面反演声线不足和接收信号不稳定的问题,提出了双收发换能器沿海声层析（Double-transcever Coast Acoustical Tomography,DCAT）系统。通过计算机进行仿真模拟,结果表明DCAT系统声速反演精度提高了一个数量级,验证了该系统对存在问题的改善及其有效性。在声速反演过程中,通过使用扩展经验正交函数（Extension Empirical Orthogonal Function,EEOF）表示声速剖面,实现了剖面全水深的声速反演。DCAT系统的提出和EEOF方法的应用,为海上实时连续观测系统的建立及数据处理,提供了理论基础和有效的实施方法。     

距离依赖的声速场反演与运动声源的跟踪定位

借助运动声源位置的时变性,利用具有时空变化特性的声速剖面在一定条件下可近似建模为经验正交函数系数随时间/距离演化的特点,提出一种运动声源参数与声速场参数联合估计的方法,解决距离依赖浅海海域快变声速场反演问题。该方法采用扩展卡尔曼滤波和集合卡尔曼滤波算法在重构真实声速场的同时对运动声源进行跟踪定位。仿真结果和实测数据验证了算法的有效性,并对两种非线性算法进行了对比。研究表明:集合卡尔曼滤波表现出较优越的估计性能,即使在接收阵元数目减少的局限条件下,仍然能够很好的估计出移动声源的位置和声速参数在时间/距离维度上的演化轨迹。该方法对研究动态海洋参数反演和观测系统的布设具有一定的参考意义。     

基于爆炸声传播时间的声速剖面反演

特征声线搜索以及传播时间测定的精确性是基于声传播时间的声速剖面反演的关键。在具有倾斜海底的三维海域,声线在海底的反射会导致水平偏转,给特征声线搜索和声传播时间计算带来了困难。为此,首先提出了一种三维空间特征声线搜索方法。通过对南海海洋环境反演实验数据的处理,分析了声线的水平偏转对声传播时间的影响,并用爆炸声传播时间作为代价函数,用量子粒子群算法作为优化算法进行了声速剖面反演。结果表明,海底坡度较大时,声线的水平偏转对声传播时间影响较大,考虑声线的水平偏转能有效地减小声传播时间计算的误差,进而使得声速剖面反演的精度得到显著提高。     

抚仙湖各个月份的温度剖面

对多年来抚仙湖温度剖面测量数据进行了统计分析,给出了抚仙湖各个月份温度剖面,并利用己有的声速经验公式算出了相应的声速剖面。还对试验中发现的一些现象进行了简要叙述。     

利用残缺样本声速重构声速剖面

用经验正交函数(experiential orthogonal functions,EOF)表示声速剖面受限于样本声速的测量深度,应用该方法重构声速剖面只能计算到样本中最浅剖面的深度。要想进行全海深声速剖面的重构,必须对残缺的样本声速进行合理地外延。为此,首先对样本中温度和盐度进行了外延,然后根据声速经验公式计算得到了全海深的样本声速。在此基础上,通过解多元方程组的办法求解经验正交函数系数达到了声速剖面重构的目的。结果表明,提出的声速剖面外延方法是有效的。另外,只要知道声速剖面变化较剧烈深度上的3个点的声速值就能重构声速剖面,对于文中的数据来说,重构的均方根误差可达到0.872 m/s;增加经验正交函数的阶数能提高重构精度,但5阶以上,阶数的继续增加对精度的提高将不会有显著的影响。     

利用匹配场方法反演海底单参数

用海底反射损失对掠射角的斜率作为单个参数描述海底可以简化地声反演过程,快速获得海底的地声性质。基于单参数模型及其相干声场的反射相位近似关系,利用匹配场处理技术反演海底性质。通过Hamilton地声模型中的密度声速关系,同时获得海底的密度和声速值。对2001年中美东海联合实验(ASIAEX 2001)实验数据进行了反演,获得海底地声参数。最后利用反演结果进行传播损失预报及海底沉积物辨别,其效果证实了基于单参数海底模型反演方法的有效性及实用性。     

由矢量水听器阵列反演浅海地声参数

基于矢量水听器能够比传统的声压水听器提供更多的声场信息,文章提供了一种利用矢量水听器阵列（AVS）进行浅海地声参数反演的方法。首先,对声场矢量的传播规律进行了研究;其次,利用矢量匹配场（MFP）方法进行了海底声速的反演：最后利用声压和质点垂直振速的传播损失差反演了海底吸收。基于矢量水听器的海底参数反演方法主要具有下述优点：一是利用矢量匹配场反演海底声速能够有效减小参数估计误差：二是利用声场矢量传播损失差进行海底吸收反演能够排除信号源级起伏对反演的干扰。实验结果表明,基于矢量水听器阵列的海底参数反演能够很好的进行声场传播预报工作。     

基于K-means聚类分析与单经验正交函数回归法的南海声速剖面估计方法研究

基于遥感参数和Argo历史数据对水体声速剖面(Sound Speed Profile, SSP)进行重构,对单经验正交函数回归(single Empirical Orthogonal Function-regression, sEOF-r)法在南海的适用性进行了研究。由于南海动力活动的复杂性,SSP扰动相对复杂,同时海域内SSP样本稀疏,相关的SSP统计学估计方法在南海区域还难以有效应用。文章基于K-means对样本进行聚类分析,讨论南海海域正交经验函数模态的一致性。通过扩大重构实验网格解决样本稀疏的问题。利用经典的sEOF-r对南海SSP进行反演,对重构SSP的误差分析说明了该方法在南海海域应用的有效性。SSP重构的均方根误差为2.341 1 m·s^-1,较大误差主要出现在深度40～200 m,其原因是海域内混合层深度发生变化。实验证明在南海区域内利用遥感参数可以有效地估计SSP。     

Coherent sea-level fluctuations along the global continental slope

Signals in sea-level or, more properly, sub-surface pressure (SSP; sea-level corrected for the inverse barometer effect) are expected to propagate rapidly along the continental slope due to the effect of sloping topography on wave modes, resulting in strongly correlated SSP over long-distances. Observations of such correlations around the Arctic and Antarctic are briefly reviewed, and then extended using satellite altimetry to the rest of the global continental slope. It is shown that such long-distance correlations are common, especially in extra-tropical regions. Simple correlations from altimetry cannot, however, establish the wave speed, or whether waves are responsible for the correlations as opposed to large-scale coherence in the forcing. A case study around South America is used to highlight some of the complications, and is found to strengthen the case for the importance of wave modes in such long-distance SSP coherence, although more detailed in situ data are required to resolve the cause of the correlations.     

Characteristics of flow field near sea bottom in a combined wave and current motion

Abstract

A reasonable time‐invariant eddy viscosity model is proposed to describe turbulent flow over a rough sea bottom in a combined wave‐current system. The linearized governing equations are solved for the wave and current kinematics both inside and outside the wave boundary layer. The results of velocity profile, friction factor and apparent roughness in a wave‐current motion are presented. The friction factors are shown to be consistent with previous theoretical results for the limiting cases of pure wave and pure current motions. Present results are compared favorably with the available data.     

分谱处理算法在基于超声导波损伤识别中的应用

利用分谱处理（SSP）算法,通过比较基准波信号和检测波信号的瞬时相位变化度（IPVD）来评估损伤散射的基础阶对称（S0）模式的飞行时间（ToF）,最终实现基于超声导波的损伤识别。首先在理想的工作环境中（无噪声）,对无缺陷的铝板进行检测,并把所采集的波信号作为基准信号。然后在不同的工作环境中（无噪声和有噪声）检测带有切缝缺陷的铝板,并把所采集的波信号作为检测信号。实验结果表明,当检测波信号的信噪比较低时,噪声能量严重干扰了检测波信号的能量分布。然而,SSP算法所提取的检测波信号的IPVD几乎不受噪声的干扰,因此利用SSP算法能够有效地抵抗强噪声的干扰,精确地评估损伤散射的S0模式的ToF,并进一步结合三角定位算法成功地定位出铝板中的切缝缺陷。     

Thermoacoustic effect of traveling-standing wave

In order to improve thermoacoustic efficiency, the thermoacoustic devices have been developed from standing wave devices to traveling wave devices. Actually, the acoustic field in practical thermoacoustic devices is neither a pure traveling wave nor a pure standing wave. The thermoacoustic effect is the hybrid effect of traveling wave component and standing wave component. Therefore, the thermoacoustic effect of traveling-standing wave will be study in this paper. Firstly, the thermoacoustic conversion performance of the traveling-standing wave are analyzes qualitatively by combining the thermoacoustic conversion performance of the traveling wave with those of the standing wave. Then, based on the basic thermoacoustic formulas, the influence of the parameters of the acoustic field and the regenerator’s structure on the thermoacoustic conversion is analyzed, and the optimum condition for the thermoacoustic conversion is discussed. The results are consistent with the qualitative analysis. Additionally, our theoretical results also show a good agreement with the experimental data [Biwa et al. Phys Rev E 2004;69(6):066304(6)], which indicates the validity of the analysis in this paper. Furthermore, the analysis in this paper further provides a more intensive understanding of these experimental results. The conclusions obtained in this paper are significant to guide for the design of new thermoacoustic devices.     

浅海沉积层中界面波的传播以及掩埋目标的回波

根据地声界面波的相关理论,采用时域有限差分法建立浅海沉积层中界面波传播模型。以沉积层中掩埋小目标为例,计算界面波对空腔掩埋物和实心掩埋物的不同作用结果。根据运动方程和应力应变关系采用中心差分网格建立声场的有限差分模型,计算了声场瞬态情况和质点振动的时间序列结果,从不同角度比较分析了地声界面波对掩埋目标的作用。根据浅海沉积层的性质计算了界面波的传播损失和对掩埋物体的目标强度,由于界面波传播衰减主要由沉积层的能量耗散引起,其规律接近横波的传播衰减,在高频情况下界面波在沉积层中传播的距离不远。文章试图建立一种对浅海沉积层中掩埋小尺度目标的探测新方法。通过一些典型模型的分析,从理论角度证明地声界面波对目标探测的可行性。     

Dynamic strain response of lake and sea ice to moving loads

The results from two experiments to measure the strains due to a vehicle moving over ice are discussed in the context of theoretical work derived from existing solutions in the literature. The experiments took place on two very different types of ice; the lake ice of Femund in Norway, and sea ice near Scott Base in the Antarctic. In both cases, strain was measured directly by means of strainmeters developed specifically for use on ice. The existence of a critical velocity at which the strain is resonant is discussed, and using values derived from the data, a dispersion equation for free waves is solved in the super-critical domain to provide wavelength estimates. At subcritical speeds a moving static load calculation provides the equivalent theory. The experimental results for lake ice and sea ice are similar, although some differences do exist. The magnification factor of the critical strain over static strain is, for example, larger for lake ice (2.25) than for sea ice (1.45). A critical velocity of 15.2 ms^?1 was observed for lake ice, for sea ice the value was 19.6 ms^?1; both are associated with the minimum phase velocity of free wave propagation.     

Evanescent wave holographic recording in a photopolymer

We present a study of holographic recording with an evanescent reference and a homogeneous (plane) object wave. The grating step was 324 nm. The dependence of the diffraction efficiency on exposure was investigated. The maximum value obtained was 0.01% at 5 mJ cm^?2 exposure. The optimal pre-exposure, needed for grating adhesion to the glass substrate, was 1 μJ cm^?2.     

Modelling of progressive short waves using wave envelopes

We consider progressive waves such that the time independent potential satisfies the Helmholtz equation, for example, the travelling wave diffracted from a body. In order to model the wave potential using finite elements it is usual to discretize the domain such that there are about ten nodal points per wavelength. However, such a procedure is computationally expensive and impractical if the waves are short. The goal is to be able to model accurately with few elements problems such as sonar and radar. Therefore we seek a new method in which the discretization of the domain is more economical. To do so, we express the complex potential ϕ in terms of the real wave envelope A and the real phase p such that ϕ=Ae^ip, and expect that in most regions the functions A and p vary much more gradually over the domain than does the oscillatory potential ϕ. Therefore instead of modelling the potential we model the wave envelope and the phase. The usual approach then uses the well known geometrical optics approximation (see p. 109 of Reference 1) : if the wave number k is large then the potential can be expanded in decreasing powers of k. The first two terms give the eikonal equation for the phase and the transport equation for the wave envelope respectively (see p. 149 of Reference 2). However, using the geometrical optics approximation (or ray theory) gives no diffraction effects. This approach shall therefore not be considered. (We note though that Keller's theory of geometrical diffraction, an extension to geometrical optics, does allow for diffraction effects and this may be considered at a later date.) We shall consider a new method which shall be described in the present paper and apply it to two-dimensional problems, although the method is equally valid for arbitary three-dimensional problems. (The method has already been validated for the case of one-dimensional problems.) An iterative procedure is described whereby an estimate of the phase is first given and from the resulting finite element calculation for the wave envelope a better estimate for the phase is obtained. The iterated values for the phase and wave envelope converge to the expected values for the test progressive wave examples considered. Even if a very poor estimate for the phase is first given the iterated values converge to the exact values but very slowly. © 1997 John Wiley & Sons, Ltd.     

Dynamics and interference of fine-structure wave packets created by strong ultrashort pulses

This work presents a theoretical study of two coherent schemes which allow the manipulation of wave packets created in atomic systems by strong ultrashort pulses. Our three-state system is composed of a ground state and two excited states simultaneously excited by the laser pulses. The corresponding dynamics are described in the bright state-dark state formalism where spectacular effects appear. (1) The wave packet created initially by a first pulse can be completely frozen through the action of a second strong pulse. (2) For pulses with a generalized pulse area equal to (2 p +1)2π (p integer), the population is preferentially transferred from the ground state to the dark state whatever the pulse duration. This is in complete disagreement with the physical interpretation valid in the weak field regime where the wave packet created by an ultrashort pulse is localized in the bright state at the end of the pulse. This effect can be revealed with wave packet interference induced by a second identical pulse. A simple analytical model using squared pulses is used to highlight the physical insight. This model is ‘reinforced’ by numerical simulations on the (4s–4p ² P 1/2,1/3) transitions in potassium atoms excited with Gaussian pulses.