舰船尾流图像的边缘分割算法研究

舰船尾流是指舰船航行过程中由于螺旋桨空化、水与舰船侧壁摩擦等原因,在舰船尾部形成的气泡群。利用舰船尾流多波束成像技术,可以探测到尾流的声散射强度数据。为了更好地研究尾流的特性,对原始声呐数据进行图像可视化处理。由于处理后的原始尾流图像掺杂着噪声,尤其当海面反射较强时,造成部分尾流图像与海面的反射图像完全混合在一起,因此需要寻找合适的算法将尾流图像的边缘提取出来。文章研究了微分算子、数学形态学算法、分形算法等边缘分割算法在尾流图像处理中的应用,并通过比较总结了每种算法的优缺点。     

Two-photon excitation of atoms by ultrashort electromagnetic pulses in a discrete spectrum

Abstract

The paper is devoted to the theoretical investigation of two-photon excitation of atom in a discrete energy spectrum by ultrashort electromagnetic pulses of femto- and subfemtosecond ranges of durations. An analytical expression for the total probability of the process is derived. Numerical simulations are made for hydrogen and sodium atoms. It is shown that the total probability of the process is nonlinear function of pulse duration and character of this function depends strongly on the frequency detuning of pulse carrier frequency from two-photon resonance.     

Pulsed Eddy Current Testing of Carbon Steel Pipes’ Wall-thinning Through Insulation and Cladding

Pulsed eddy current testing of wall-thinning through cladding and insulation was studied from both theoretical and experimental aspects. The analytical solution was derived for a simplified four-layered structure and was used to conduct simulations to ascertain the feasibility of this method. A pulsed eddy current testing probe consisting of a circular excitation coil and an AMR-sensor-embedded differential detector was fabricated to measure the time-varying magnetic field signals on the axisymmetric excitation coil??s axis. The measurement system was able to measure magnetic field down to a few hundred micro-Gausses in an unshielded environment. Simulation and test results showed that over a certain time after turning off the excitation current the magnetic field signal??s decay behavior is almost merely relevant to the pipe??s wall thickness. Future development of a carbon steel pipe??s wall-thinning can be evaluated by using decay coefficients estimated from previously obtained test data.     

拖船辐射噪声的海试数据分析及特性仿真

针对海试数据分析了拖船辐射噪声的特性,进而对拖船噪声进行了建模仿真。首先利用自相关的AR模型产生宽带连续谱噪声序列,并迭加线谱成分,构造一维时间序列,而后根据海洋环境条件以及拖线阵与拖船空间位置关系建立声传播路径模型,构建拖线阵的接收声场。仿真结果在时频特性以及空间特性上都与海试数据非常逼近。     

Scattering-induced changes in the degree of polarization of a stochastic electromagnetic plane-wave pulse

The scattering of a stochastic electromagnetic plane-wave pulse on a deterministic spherical medium is investigated. An analytical formula for the degree of polarization (DOP) of the scattered field in the far zone is derived. Letting pulse duration T(0) → ∞, our formula can be applied to study the scattering of a stationary stochastic electromagnetic light wave. Numerical results show that the DOP of the far zone field is closely determined by the size of the spherical medium when the incident field is a stochastic electromagnetic plane-wave pulse. This is much different from the case when the incident field is a stationary stochastic electromagnetic light wave, where the DOP of the far zone field is independent of the size of the medium. One may obtain the information of the spherical medium by measuring the scattering-induced changes in the DOP of a stochastic electromagnetic plane-wave pulse.     

Maximum roll angle estimation of a ship in confused sea waves via a quasi-deterministic approach

This paper considers the maximum roll motion of a ship in confused sea waves. The ship motion is described by a nonlinear differential equation including quadratic damping and cubic restoring force. The excitation of the ship is represented by a stationary mean-zero Gaussian process of a given power spectral density function. It is shown that a reliable estimate of the maximum roll motion is found considering the ship response to an approximate deterministic representation of an appropriately large and adequately rich (frequency-wise) load. Specifically, the time variation of the load is approximated by a normalized autocovariance function; the maximum amplitude of the load is taken as a certain multiple of the standard deviation of the stochastic load process. This approximation relates to the method of quasi-deterministic representation of extreme realizations of a stationary Gaussian process; the method is interpreted as a tool for generating deterministic time histories of the load which are compatible with a certain power spectral density function. The efficacy of this perspective is shown by comparison with the results from pertinent Monte Carlo simulations.Next, the paper addresses the ship stability problem in the space of initial conditions. In this context, it shows that the proposed approximation can be adequately utilized for a ship safety assessment.     

Anatomical analysis of the light transmission through a single subwavelength metal slit by the FDTD method

Transverse-magnetic (TM) polarized light transmission through a single subwavelength metal slit is re-examined with finite-difference time-domain (FDTD) simulations. In contrast to previous studies, we derive an anatomical view of the electromagnetic field distribution in different cross-sections and emphasize the generation of a field coupling mode in the slit. Numerical modeling reveals that both peak and dip transmissions are features of the field interference in the slit. The slit width and depth are mainly responsible for establishing the amplitude and phase, respectively, of the coupled mode. Moreover, it is found that the output energy dispensation between the radiative and surface components is actually determined by the slit width. Analysis of the physical properties of the slit, including the coupled mode structure in the transverse plane, the effective refractive index and Ohmic absorption losses, provides new insights into the light transmission processes.     

Spurious principal components in the set of spectra subjected to disturbances: I. Presentation of the problem

It has been shown that a kind of imperfections which can be met in spectrometers, or small shifts of the recorded lines, can influence properties of spectra sets in such a way that the number of principal components (PCs) necessary to describe the non-noisy part of the total variance of the set increases compared to the number when the disturbances are absent. The number of additional (spurious) PCs depends on the magnitude of disturbances and on the signal-to-noise ratio (SNR). The mechanism which generates spurious PCs is discussed, and a criterion enabling an evaluation when the effect becomes important, is given. Numerical simulations of the effect enabled the inference of an empirical relationship between the number of PCs, the magnitude of imperfections, and SNR. Finally, a procedure is proposed to check if a spectrometer is free of the kind of disturbances discussed.     

Dislocation dynamics in an isotropic,paramagnetic and electrically conducting solid in a large magnetic field

The authors are concerned with the mechanical and electrical fields produced by moving dislocations in a paramagnetic solid with electric conduction in the presence of a large magnetic field. It is assumed that the magnetic field is constant throughout the body, and the materials are isotropic in both the mechanical and electromagnetic features. The fields of displacement and electric scalar potential are given in terms of Green's functions and the plastic distortion tensor. Finally, the expressions for the fields produced by an infinite straight dislocation, which is moved by a uniform subsonic velocity, are given. Numerical computations are made to estimate the electrical fields.     

舰船尾流侧向声检测方法

舰船尾流声检测是水下航行体实现自动搜索、跟踪、攻击水面舰艇目标的最有效途径之一,当前工程应用和相关研究主要集中于从尾流下方实施的顶视声尾流检测方法.为了进一步研究水下航行体新型尾流检测技术,提高尾流自导性能及抗干扰能力,研究了舰船尾流的侧向声散射检测方法.分析了舰船尾流的几何特性和声特性,进行了实验室模拟尾流的气幕声散...     

Feasibility study of an electromagnetic thruster for ship propulsion

A comprehensive computational feasibility study is presented for an outer-type electromagnetic thruster intended for ship propulsion. A thruster previously presented in the literature is modeled in a way to keep computer power needed in reasonable limits despite wide coverage of the efficiency estimates. For this purpose a two-dimensional approach is adopted. It is also shown how to generalize the results to similar vessels by means of scaling. Water salinity and magnetic flux density generated by the coils are included in the scaling model. The results are presented in a way to make comparison with the conventional ship propeller straightforward.<>     

Effect of annular secondary conductor in a linear electromagnetic stirrer

This paper presents the variation of average axial force density in the annular secondary conductor of a linear electromagnetic stirrer. Different geometries of secondaries are considered for numerical and experimental validation namely, 1. hollow annular ring, 2. annular ring with a solid cylinder and 3. solid cylinder. Experimental and numerical simulations are performed for a 2-pole in house built 15 kW linear electromagnetic stirrer (EMS). It is observed for a supply current of 200 A at 30 Hz the force densities in the hollow annular ring is 67% higher than the equivalent solid cylinder. The same values are 33% for annular ring with a solid cylinder. Force density variation with supply frequency and current are also reported. Numerical simulations using finite element model are validated with experimental results.     

Feasibility investigation regarding evaluation of external wall-thickness loss in nonmagnetic tubes using a bobbin-typed electromagnetic acoustic transducer

ABSTRACT

During fabrication and practical service, nonmagnetic tubes are prone to the External Wall-thickness Loss (EWL) which leaves the tubes vulnerable to structural failure. In this paper, a bobbin-typed electromagnetic acoustic transducer is proposed for tube inspection. The feasibility of EWL evaluation by using the proposed transducer is investigated via simulations and experiments. In order for efficient simulations of Electromagnetic Acoustic Transduction (EMAT), the hybrid modeling integrating the analytical modeling and finite element modeling is established. Closed-form expressions particularly regarding the EMAT-related field quantities of electromagnetics are formulated via the Extended Truncated Region Eigenfunction Expansion (ETREE) modeling. Simulations by using the hybrid model indicate that the proposed transducer is capable of evaluating EWL in nonmagnetic tubes. In parallel, experiments with the fabricated transducer have been carried out. The experimental results are supportive of the conclusion drawn from simulations. From simulations and experiments, it can be found that EWL evaluation with the proposed bobbin-typed electromagnetic acoustic transducer is feasible, which could benefit the real-time and in-situ nondestructive evaluation of nonmagnetic tubes.     

一种针对未知参数系统的实时混沌化方法

针对未知参数系统,本文提出一种基于频谱优化和时延反馈控制的实时混沌化方法。首先,基于系统稳态响应构造频谱性能指标,量化系统稳态行为（周期或混沌等）;然后,随着时间进程的推进,按照直接寻优算法（Hooke-Jeeves 方法）实时调整反馈控制器的时延,直至算法收敛。当性能指标达到最小值时,系统进入最佳混沌状态。数值算例验证了本方法的有效性。本方法最大优点在于无需知道系统参数,仅基于系统响应状态便可实施混沌化控制,易于实现工程化应用。     

一种被动拖曳声纳阵列信号模拟器设计

声纳基阵信号模拟器是一种实用的可仿真实际检测目标及使用环境的阵元级信号发生器。设计的被动拖曳声纳阵列信号模拟器,仿真了拖曳阵的拖船干扰时域信号和空间传播特性、目标辐射信号以及环境噪声。通过预设定拖船干扰宽带功率谱,以AR模型拟合该功率谱,利用海底、海面对声场的一次反射作用仿真了拖船干扰的空间多途信道。采用插值滤波器,解决了宽带信号阵元间延时的时延精确控制问题。     

Numerical simulations of a tanker collision with a bergy bit incorporating hydrodynamics,a validated ice model and damage to the vessel

Numerical simulations of a collision between a loaded tanker and a bergy bit have been conducted using LS-Dyna™ software. The simulations incorporated hydrodynamics, via LS-Dyna's ALE formulation, and a validated crushable foam ice model. The major portion of the vessel was treated as a rigid body and a section of the hull, located on the starboard side of the forward bow where the ice contact occurred, was modeled as typical ship grillage that could deform and sustain damage as a result of the collision. Strategies for dealing with the highly varying mesh densities needed for the simulations are discussed as well as load and pressure distribution on the grillage throughout the course of the collision. Realistic movement of the bergy bit due to the vessel's bow wave prior to contact with the ice was observed and the damage to the grillage resembled published results from actual grillage damage tests in the lab. A load measurement from the lab tests compared reasonably well with a rough estimate from the simulation. The collision eventually ruptured the hull in a ripping fashion resembling documented incidents of vessel impacts with ice masses.     

Remote Field Eddy Current Technique: Phantom Exciter Model Calculations

Abstract

High resolution results of finite element calculations for remote field eddy current “phantom exciter” simulations of slit defect interactions using single through wall transit are presented. These show that fine circumferential slits cause almost no field perturbations in the case of nonferromagnetic tubes but big perturbations in ferromagnetic tubes where high magnetic H fields occur in the slits. Defect-induced magnetic field perturbations must therefore be considered in addition to eddy current perturbations when ferromagnetic materials are inspected, particularly in the case of fine slits orthogonal to the magnetic field direction. Additional details seen are the funnelling of energy into slits in ferromagnetic pipes and precursor disturbances of fields approaching defects. It is suggested that these are due to the reflection of the electromagnetic waves dictates by boundary conditions at the near side defect boundary.     

A method of domain decomposition for calculating the steady flow past a cylinder

For flow past a cylinder it is known that the vorticity is only significant in a thin boundary-layer adjacent to the surface and within a parabolic wake far from the cylinder. To address this behaviour of the vorticity a numerical method is implemented whereby the flow field is decomposed into two regions: an inner region to deal with boundary-layer phenomena and an outer region to model wake phenomena. This method equally applies to any cylinder cross section. The equations of motion are solved in each region and are matched at the boundary. Numerical solutions have been carried out for the trial case of a circular cylinder and the agreement with existing results is good.     

一种海洋环境地磁场三分量的测量方法

提出了一种利用安装在舰船上的三分量磁传感器进行地磁场测量的方法．从理论上解决了在任意舰船姿态下，消除测量中舰船磁场干扰的问题．为测量船在海上测量地磁场的三分量奠定了理论基础．根据我国舰船普遍安装的导航设备的精度，进行了计算机仿真．从仿真结果看，其误差不超过50nT，能够满足舰栽消磁系统对三分量磁场的精度要求．     

A brief history of mathematical ship-squat prediction, focussing on the contributions of E.O. Tuck

This article traces the contributions of Prof. E.O. Tuck to the field of mathematical ship-squat prediction. The review expands on Tuck??s own review of his early work (Ann. Rev. Fluid Mech. 10:33, 1978) and describes the use of his formulae in modern squat-prediction methods. A method for calculating Tuck??s sinkage and trim coefficients using easily obtainable ship parameters is also described.