声跃层控制声线传播弯曲模型的构建

基于斯涅耳折射定律,将海水垂向等分成若干层,利用傅里叶步近算法,构建受声速剖面唯一控制的声线传播弯曲模型。将该模型用于模拟研究不同浅海声跃层类型对声线传播弯曲的影响,得出声线波长和轨迹长度按负跃层、无跃层、正跃层的顺序逐渐增加。并利用该模型定量研究跃层深度、跃层强度、跃层厚度三特征参量对声线传播弯曲的影响,得出负跃层强度越大、厚度越大、深度越浅,声线弯曲越大,波长越小。正跃层三特征参量对声线作用相反。

基于BELLHOP声线模型的水声信道特性研究

论文以BELLHOP声线模型为理论研究依据,研究了水声信道的相关特性。首先详细地讲述了BELLHOP声线模型的基本原理,推导了声线方程的基本公式,并以此公式作为BELLHOP计算的理论依据;接着运用BELLHOP计算了10条声线的传播路径;而后研究了声线的传播损失和时延随传播距离的变化关系;其次研究了海底反射损失与海底掠射角的变化关系;然后研究了声线的传播损失与深度的变化关系;最后研究了声线传播损失随传播距离的变化关系。

水下定位声线扰动分析与分段指数权函数设计

声速误差是水下声学定位的重要误差来源,主要包括声速测量本身的不确定性及声速时空变化误差。本文基于常梯度声线跟踪模型,导出入射角、声速梯度及深度变化产生的声线扰动数学模型。已有研究表明,对于相同的水深和声速梯度,入射角越大其扰动对声线影响越大,声线弯曲影响越明显。依据导出的入射角扰动对声线扰动的函数响应关系,构建了入射角相关的水下定位分段指数函数随机模型。试验结果表明:分段指数函数随机模型的定位结果相较于等权模型、指数模型有明显改善,与分段余弦随机模型定位结果相当,但本文随机模型会保留更多观测信息。

基于射线模型的典型海底地形下的声传播

海洋中检测、通信、定位和导航主要利用声波。声波是目前水下信息传输的主要载体。文章基于射线模型对深海环境下四种典型海底地形声传播展开研究。仿真结果表明,可利用海底地形对声波的反射和折射效应,获得声源达到空间任意位置的声线传播轨迹和传播损失,基于射线模型的声传播研究可为实际海洋环境下的水下信息传输以及探测提供技术支持。

基于益损分析的鱼雷战术效能的海洋环境影响评估

针对不同海洋环境中海水阻力系数和声线传播的变化特性,根据鱼雷航行及自导过程中的守恒原理,提出了一种评估海洋水文环境对鱼雷战术效能影响的新方法,并根据海洋水文环境对介质参数的影响,结合战术效能益损评价指标体系,建立了海洋环境对鱼雷战术效能影响的评估模型。通过仿真算例,对该评估方法进行了仿真实验,验证该方法的合理性和可靠性,进而发展运用于鱼雷作战使用的海洋环境影响评估和保障决策之中。

微扰法声速剖面反演改进算法

为了降低传播时间测量误差对微扰法声速剖面反演精度的影响,本文提出了一种基于微扰法的声速剖面反演改进算法。该算法通过调整平均声速剖面以增大声速扰动量,进而增大声速扰动所引起声线传播时间的扰动量,降低了测时误差对反演算法的影响,提高了反演算法对声速扰动的敏感性。数值算例表明,相同条件下传统微扰法反演的声速剖面的最大误差为1.478 m/s,而改进算法的最大误差降低至0.403 m/s,验证了改进算法可以有效降低传播时间测量误差对声速剖面反演结果的影响,比微扰法的反演精度更高。

浅海海底声学参数反演方法研究

海底声学参数反演问题近年来已经成为水声界的研究热点之一。论文针对不同的海底模型和海底参数提出了两种反演方法。浅海多途效应导致声场在距离-频率平面形成干涉谱,利用距离域干涉谱提取出干涉准周期作为反演匹配物理量,提出一种基于无限均匀沉积层海底模型的海底反演方法。针对倾斜海底,提出一种基于特征声线传播时间的海底反演方法。

声速跃变层对吊放声纳搜潜的影响

除自身性能外,吊放声纳的搜潜效果很大程度上会受到水文环境的影响,跃变层是夏季浅海经常出现的一种现象,且对声纳探测效果影响巨大。阐述了跃变层中的声传播特点,仿真了其声线传播的轨迹,分析了吊放声纳入水深度对其探测效果的影响。仿真分析表明同深探测可有效提升声纳探测效果。

Noise Reduction of Welding Defect Image Based on NSCT and Anisotropic Diffusion

In order to reduce noise effectively in the welding defect image and preserve the minutiae information, a noise reduction method of welding defect image based on nonsubsampled contourlet transform(NSCT) and anisotropic diffusion is proposed. Firstly, an X-ray welding defect image is decomposed by NSCT. Then total variation(TV) model and Catte_PM model are used for the obtained low-pass component and band-pass components, respectively. Finally, the denoised image is synthesized by inverse NSCT. Experimental results show that, compared with the hybrid method of wavelet threshold shrinkage with TV diffusion, the method combining NSCT with P_Laplace diffusion, and the method combining contourlet with TV model and adaptive contrast diffusion, the proposed method has a great improvement in the aspects of subjective visual effect, peak signal-to-noise ratio(PSNR) and mean-square error(MSE). Noise is suppressed more effectively and the minutiae information is preserved better in the image.

Contrast enhancement of medical ultrasound imaging with reversal phase-inversion pulse technology

A new ultrasound contrast imaging technique was proposed for eliminating the harmonic components from the emission signal transmitted by the broadband ultrasonic system.Reversal phase-inversion pulse was used for the first time to separate the contrast harmonics from the harmonics in the emission signal to improve the detection of contrast micro-bubbles.Based on the nonlinear acoustic theory of finite-amplitude effects and the associated distortion of the propagating wave,the Bessel-Fubini series model was applied to describe the nonlinear propagation effects of the reversal phase-inversion pulse,and the Church's equation for zero-thickness encapsulation model was used to produce the scattering-pulse of the bubble.For harmonic imaging,the experiment was performed using a 64-element linear array,which was simulated by Field II.The results show that the harmonic components from the emission signal can be completely cancelled,and the harmonics generated by the nonlinear propagation of the wave through the tissue,can be reduced by 15-30 dB.Compared with the short pulse,the reversal phase-inversion pulse can improve the contrast and definition of the harmonic image significantly.

Electrostatic Nonplanar Positron-Acoustic Shock Waves in Superthermal Electron-Positron-Ion Plasmas

The basic properties of the nonlinear propagation of the nonplanar （cylindrical and spherical） positronacoustic （PA） shock waves （SHWs） in an unmagnetized electron-positron-ion （e-p-i） plasma containing immobile positive ions, mobile cold positrons, and superthermal （kappa distributed） hot positrons and electrons are investigated both analytically and numerically. The modified Burgers equation （roBE） is derived by using the reductive perturbation method. The basic features of PA SHWs are significantly modified by the cold positron kinematic viscosity （U）, superthermal parameter of electrons （ke）, superthermal parameter of hot positrons （kp）, the ratio of the electron temperature to hot positron temperature （or）, the ratio of the electron number density to cold positron number density （μe）, and the ratio of the hot positron number density to cold positron number density （μph）. This study could be useful to identify the basic properties of nonlinear electrostatic disturbances in dissipative space and laboratory plasmas.

Observation and analysis of whistler-mode wave and electrostatic solitary waves within density depletion near magnetic reconnection X-line

The PWI/WFC data onboard Geotail during one burst time interval when Geotail is skimming a magnetic reconnection diffusion region in the near-Earth magnetotail is carefully analyzed.Both the whistler-mode wave and the electrostatic solitary wave are found within the region with density depletion on the boundary layer near the magnetic reconnection X-line.The whistler-mode wave is electromagnetic whistler wave propagating quasi-parallel to the ambient field with a small angle between the wave vector and the ambient magnetic field.The whistler-mode wave associated with ESWs suggests that enhanced electromagnetic whistler-mode fluctuations can also be generated after the decay of the ESWs,which is different from the 2-D PIC simulation results.