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。      

数据驱动声场信息处理与控制

海洋环境复杂性、多变性及其模型参数获取限制需要直接从实验数据中估计参数、评估 模型。在水声物理框架下,利用信道物理规律、特性,基于数据估计/提取模型参数或模型(模型函数),实现声场信息处理和控制具有学术价值和应用意义。本文综述波导不变量、格林函数提取和单边声场聚焦三种数据驱动声场信息处理与控制方法近年国内外相关研究进展。     

浅海低频垂直相控阵多模声场聚焦研究*

为了提高低频垂直相控阵声场聚焦的环境适应性,增强环境失配情况下的声场聚焦效果,提出一种多模声场聚焦方法。该方法利用环境失配对低阶模态影响相对较小的特点,只利用前3阶模态进行发射权系数的计算。仿真结果表明,多模声场具有较大的聚焦跨度,在环境失配情况下仍能在期望的聚焦位置实现良好的聚焦效果。进一步地,根据巴斯微扰理论计算分析了多模声场的混响特性。由于多模声场聚焦将能量集中到了掠射角较小的低阶模态中,因此比最优权系数聚焦具有更低的混响平均强度。多模聚焦对聚焦深度进行扫描可以实现目标的深度分辨,这在水下目标探测方面具有较高的应用前景。     

基于干扰协方差矩阵重构的恒定束宽鲁棒自适应波束形成

针对宽带波束形成中的恒定束宽波束响应优化设计问题与鲁棒性问题展开研究。首先,提出一种基于相位补偿的恒定束宽全局优化设计方法,通过对阵列流形向量进行相位补偿来设计恒定束宽波束,与现有的一些方法相比,该方法不仅能获得全局最优解,而且物理实现简单。同时,还提出一种基于协方差矩阵重构的鲁棒自适应宽带波束形成算法。该算法采用Capon估计器估计样本数据的空间一频率谱密度函数,然后对期望信号波达方向之外的角度区间进行积分来重构干扰加噪声协方差矩阵,最后利用重构的协方差矩阵设计自适应波束形成器权系数。该波束形成器设计问题被表述成凸优化问题求解。仿真结果表明,在整个输入信噪比范围内,该算法几乎都能获得接近理想值的输出信干噪比。     

基于干扰协方差矩阵重构的恒定束宽鲁棒自适应波束形成

针对宽带波束形成中的恒定束宽波束响应优化设计问题与鲁棒性问题展开研究。首先,提出一种基于相位补偿的恒定束宽全局优化设计方法,通过对阵列流形向量进行相位补偿来设计恒定束宽波束,与现有的一些方法相比,该方法不仅能获得全局最优解,而且物理实现简单。同时,还提出一种基于协方差矩阵重构的鲁棒自适应宽带波束形成算法。该算法采用Capon估计器估计样本数据的空间一频率谱密度函数,然后对期望信号波达方向之外的角度区间进行积分来重构干扰加噪声协方差矩阵,最后利用重构的协方差矩阵设计自适应波束形成器权系数。该波束形成器设计问题被表述成凸优化问题求解。仿真结果表明,在整个输入信噪比范围内,该算法几乎都能获得接近理想值的输出信干噪比。      

逆波束形成的旁瓣抑制

为了降低普通逆波束形成旁瓣,提出了一种新的旁瓣抵消技术。该技术利用指向性函数中的旁瓣部分去估计声场的响应,然后从基阵估计的声场减去这部分指向性函数旁瓣估计的声场,从而得到最终的声场估计。由于较多的旁瓣响应被估计的声场扣除,最终估计的声场中的旁瓣大大降低。利用该技术可有效地抑制干扰,同时又可保持普通逆波束形成的方位高分辨力。湖试结果显示了该技术的有效性和可行性。     

浅海波导中目标散射的边界元方法

提出了一种计算三维散射体在声速剖面随深度变化、距离无关浅海波导中散射声场的数值方法波导边界元方法。当散射体不十分靠近波导界面因而边界多次散射可以忽略时,在边界元计算中可以用自由场格林函数近似波导格林函数。应用镜像法和球波函数加法定理推导了理想波导中球体散射声场的解析解,用来验证波导边界元方法的计算精度,证明该数值方法是准确的。对浅海波导中水下潜器散射声场数值模拟的结果表明,浅海波导海面、海底界面反射、声速剖面等对目标散射声场的幅值和方向性都有很大的影响。      

水平变化波导中的全局矩阵耦合简正波方法

提出了一种新的水平变化波导中声场的耦合简正波求解方法,该方法能够处理二维点源和线源问题,提供声场的双向解。该方法利用全局矩阵(DGM)一次性求解耦合模式的系数,消除了传播矩阵递推求解中存在的误差累积问题;此外,改善了现有模型中对距离函数的归一化方法,从而避免了泄露模式指数增长导致的数值溢出问题。本文还给出了绝对软海底理想波导中耦合矩阵的闭合表达式,并分析了单个阶梯下简正波耦合现象。此外,本文还计算了理想楔形波导中的声传播问题(ASA标准问题),并与解析解及COUPLE07计算结果进行了比较,结果表明该方法是一种稳定、精确的水平变化波导中的声场计算方法。      

采用分布式球形阵列的球谐波域声场分离方法

为从测得的混合声场信号中提取出需要的目标声场,提出一种基于分布式球形传声器阵列的声场分离方法。该方法依据声场的球谐波分解,利用阵列各传声器采集到的声场声压信号,获得目标声场与干扰声场的球谐波展开系数,进而估计目标声场。该方法利用声场以不同中心展开的球谐波系数之间的变换关系,直接建立传声器测量声压信号与整体坐标系下声场展开系数的方程,与传统的分布式球形阵列声场分离方法,即先求解局部坐标系下声场展开系数,再变换为整体展开系数的方法,进行比较。分别通过数值仿真和实验说明了提出方法的有效性。结果表明:该方法能够从混合声场中较准确地估计出目标声场,并且在干扰声场能量增大时,保持了较小的声场估计误差,相比于传统方法误差增加更少。     

声学中用于时域有限差分法的一种高效吸收边界

本文提出一种用于声场时域有限差分法（FDTD）的高效吸收边界条件。通过伪逆矩阵对截断边界附近网格线之间的转移矩阵作最小二乘估计，所得到的吸收边界条件性能明显优于多种常用方法。新方法的数值计算效率高，算法稳定。对点源辐射，尖劈衍射和圆柱体散射问题的FDTD数值计算结果显示了这种方法的优异性能。     

An optimal algorithm for single-mode close-loop excitation in shallow water

An optimal algorithm for single-mode close-loop excitation in shallow water is presented.By analyzing the covariance of estimation value of Green’s function matrix,an optimal source array weights matrix is presented to estimate Green’s function matrix.The weights matrix is a unitary matrix,and absolute values of the matrix elements are equal. Algorithm based on the weights matrix makes single-mode excitation converge at maximum speed and be steady.Advantages of the algorithm are confirmed by numerical simulations. Finally,results of shallow water experiment are presented,and the energy ratio of single mode is higher than 97 percent.     

Dispersion of the signal of a parametric array in shallow water

Results of an experimental study of shallow-water propagation are presented for a broadband acoustic signal produced by a parametric array. The narrow-beam character of the signal provides single-mode excitation of the oceanic waveguide. The possibility of compression of the broadband signal in the course of its propagation in shallow water is discussed for the case of a specially adjusted frequency modulation. The use of waveguide dispersion leads to an increase in the efficiency of parametric radiation in shallow water.     

Extension Theory for Magnon of a Ferromagnetic Superlattice

We prove that the extension theory for lattice Green's functions can be used to study Heisenberg ferromagnetic systems. As an example, the magnon spectrum of a ferromagnetic superlat tice is calculated by the extension theory for lattice Green's functions cooperated with the transfer matrix technique. The spectrum of magnetic excitation near the surface of a ferromagnetic superlattice is also presented.     

Electromagnetic volume scattering in the half-space: a moment-method analysis without Sommerfeld integrals or their approximations

Traditionally, in moment-method analyses of electromagnetic scattering, the elements of the impedance matrix are calculated as convolutions of the basis elements with the appropriate dyadic Green's function. However, for scattering in the half-space, the vertical and azimuthal copolar terms of the Green's function require evaluation of Sommerfeld integrals which are computationally burdensome. In this paper, it is shown that, in populating the impedance matrix for the half-space problem, evaluation of Sommerfeld integrals is, in fact, not necessary. For monochromatic excitation, the plane-wave expansion of the scattered field constitutes a Fourier transform, in the horizontal plane, of a vector spectral function. This vector function results from the convolution, in the vertical dimension, of the respective angular spectra of the Green's function and the equivalent current. On application of the moment method, through the Weyl identity, the impedance-matrix elements corresponding to the singular terms of the Green's function are convolutions in the horizontal plane of spherical potentials, and Fourier transforms of scalar spectral functions. These scalar functions are derived from the basis elements and, with a judicious choice of basis, they are well behaved and of compact support, and consequently their Fourier transforms can be computed as FFTs.     

On the sound field of a shallow spherical shell in an infinite baffle

A method is presented for calculating the far field sound radiation from a shallow spherical shell in an acoustic medium. The shell has a concentrated ring mass boundary condition at its perimeter representing a loudspeaker voice coil and is excited by a concentrated ring force exerted by the end of the voice coil. A Green's function is developed for a shallow spherical shell, which is based upon Reissner's solution to the shell wave equation [Q. Appl. Math. 13, 279-290 (1955)]. The shell is then coupled to the surrounding acoustic medium using an eigenfunction expansion, with unknown coefficients, for its deflection. The resulting surface pressure distribution is solved using the King integral together with the free space Green's function in cylindrical coordinates. In order to eliminate the need for numerical integration, the radiation (coupling) integrals are solved analytically to yield fast converging expansions. Hence, a set of simultaneous equations is obtained which is solved for the coefficients of the eigenfunction expansion. These coefficients are finally used in formulas for the far field sound radiation.     

Effect of energy losses on the amplitude-frequency characteristic of the electrodynamic system of optical modulators

The results of calculations of main parameters of a slot-type electrodynamic system taking into account losses in a single-mode nonresonance regime of excitation and in the regime of responance excitation in the vicinity of the cutoff frequency are presented. Analytic expressions obtained for the transmission coefficient, resonance frequency, and transmission band can be used in designing wide-band optical modulators. Experimental verification of laboratory prototypes of electrodynamic systems and their application in available modulators confirms the possibility of employment of such analysis for choosing optimal designs of ultrahigh-frequency modulators of optical radiation.     

Necessary conditions for nonlinear excitation of a high-order mode in a single-mode optical fiber

The necessary conditions for nonlinear excitation of higher-order modes in a single-mode step-index optical fiber are analyzed. The cutoff conditions for such an optical waveguide are investigated taking into account Kerr nonlinearity. The minimal power of optical pulses required for fulfilling the necessary conditions for nonlinear excitation of higher-order modes in a single-mode step-index fiber is calculated as a function of the normalized frequency. The allowed ranges of variation of the normalized frequency and optical-radiation power are estimated. It is demonstrated that the conditions necessary for nonlinear excitation of a higher-order mode in a step-index single-mode optical fiber can be created for optical pulses shorter than 500 fs.     

Excitation spectrum and structure factor of a two-component Bose-Einstein condensate in different hyperfine states

The elementary excitation spectrum of a two-component Bose-Einstein condensate in different hyperfine states is obtained by Green＇s function method. It is found to have two branches. In the long wave-length limit, the two branches of the excitation spectrum are reduced to one phonon excitation and one single-particle excitation. The single-particle one has an energy gap. When the energy gap exists, we study the Landau critical velocity and the depletion of the condensate. With the obtained Green＇s functions, we calculate the structure factor of a two-component condensate. It is found that the static structure factor comprises only the branch of the phonon excitation and the single-particle excitation makes no contribution to the structure factor.     

Calculation of electromechanical coupling coefficient of Lamb waves in multilayered plates

Two methods have been always used to calculate the electromechanical coupling coefficient of a Lamb wave in a multilayered plate: one is an approximate method using the acoustic velocity difference under different electric boundary conditions and the other is the Green's function method. The Green's function method is more accurate but more complicated, because an 8N-order matrix is used for calculating the electromechanical coupling coefficient of the Lamb wave in an N-layered plate, which induces great computation loads and some calculation deviations. In this paper, a transfer matrix method is used for calculating the electromechanical coupling coefficient of Lamb waves in a multilayered plate, in which only an 8-order matrix is needed regardless of the number of layers of the plate. The results show that the transfer matrix method can obtain the same accuracy as those by the Green's function method, but the computation load and deviation are greatly decreased by avoiding the use of a high order matrix used in the Green's function method.     

The influence of dynamical correlations in semiconductor plasmas on optical spectra

On the basis of a nonequilibrium Green's function technique we determine a Bethe-Salpeter equation describing the linear response of a semiconductor under plasma excitation. Dynamical correlations entering the effective screenede-h interaction are treated on the basis of the full RPA polarization function. Numerical results for optical gain and absorption spectra are presented for various temperatures and densities. The validity of the commonly used plasmon pole approximation with regard to the lineshape of optical spectra is critically investigated.