Rapid detection to long ship wake in synthetic aperture radar satellite imagery

The maritime administrative department employs synthetic aperture radar (SAR) satellite remote sensing technology to obtain evidence of illegal discharge of ships. If the ship is discharged during navigation, it forms a long dark wake on the SAR image due to the suppression of the Bragg wave by the oil fi lm. This study investigates key techniques for rapid detection of long ship wakes, thereby providing law enforcement agencies with candidate ships for possible discharge. This paper presents a rapid long ship wake detection method that uses satellite imaging parameters and the axial direction of the ship in images to determine the potential detection area of the wake. Then, the threshold of long ship wake detection is determined using statistical analysis, the area is binarized, and isolated points are removed using a morphological filter operator. The method was tested with ENVISAT Synthetic Aperture Radar and GF-3 SAR data, and results showed that the method was eff ective, and the overall accuracy of the decision reaches 71%. We present two innovations;one is a method that draws a Doppler shift curve, and uses the SAR imaging parameters to determine the detection area of the long wake to achieve rapid detection and reduce the image detection area. The other is where a classical linear fitting method is used to quickly and accurately determine whether the detected dark area is a long ship wake and realizes the twisted long ship wake detection caused by the sea surface flow field, which is otherwise diffi cult to detect by the traditional Radon and Hough transform methods. This method has good suppression performance for the dark spot false alarm formed by low speed wind region or upward flow. The method is developed for maritime ship monitoring system and will promote the operational application of maritime ship monitoring system.

Numerical Simulation on Ship Bubbly Wake

Based on a volume of fluid two-phase model imbedded in the general computational fluid dynamics code FLUENT6.3.26, the viscous flow with free surface around a model-scaled KRISO container ship （KCS） was first numerically simulated. Then with a rigid-lid-free-surface method, the underwater flow field was computed based on the mixture muitiphase model to simulate the bubbly wake around the KCS hull. The realizable k-e two-equation turbulence model and Reynolds stress model were used to analyze the effects of turbulence model on the ship bubbly wake. The air entrainment model, which is relative to the normal velocity gradient of the free surface, and the solving method were verified by the qualitatively reasonable computed results.

DETECTION OF SHIP WAKES IN SAR IMAGE USING ROTATED WINDOW RADON TRANSFORM

A novel method of rotated window Radon transform is developed for identifying the linear texture in SAR image.It is applied to automatic detection of the ship wakes of SEASAT SAR image.The location and direction of the traveling ship can be quickly and accurately detectec,In some cases, the ship velocity can also be obtained.

A Non-geometrically Similar Model for Predicting the Wake Field of Full-scale Ships

The scale effect leads to large discrepancies between the wake fields of model-scale and actual ships, and causes differences in cavitation performance and exciting forces tests in predicting the performance of actual ships. Therefore, when test data from ship models are directly applied to predict the performance of actual ships, test results must be subjected to empirical corrections. This study proposes a method for the reverse design of the hull model. Compared to a geometrically similar hull model, the wake field generated by the modified model is closer to that of an actual ship. A non-geometrically similar model of a Korean Research Institute of Ship and Ocean Engineering （KRISO）’s container ship （KCS） was designed. Numerical simulations were performed using this model, and its results were compared with full-scale calculation results. The deformation method of getting the wake field of full-scale ships by the non-geometrically similar model is applied to the KCS successfully.

A Novel Ship Wake Detection Algorithm Based on WTHT and Radon Transform

This paper proposes a novel ship wake detection algorithm based on the White Top-hat Transform(WTHT)and the Radon transform,which aims to improve the contrast between the ship wake and the background so as to improve the detection performance on Synthetic Aperture Radar(SAR)images.The proposed algorithm includes two major processes,and one is to improve the contrast and another one is to locate the ship wake.In high sea state conditions,the contrast of ship wake and background can be very low,which makes it difficult to detect.In the first step,the proposed contrast improvement algorithm is applied to improving the contrast which helps for improving the detection performance.An attribute filter based on edge detection result is adopted here.In the second step the contrast improved image is transformed into the Radon domain followed by peak extraction process to find the wake,the WTHT is used once more in this step.Finally,in the last step,the wake is overlapped on the original image.Experimental results on Tiangong-2 Interferometric Imaging Radar Altimeter(InIRA)images are presented and compared with that obtained by using the classical algorithm,and in this way,the better performance of our algorithm is demonstrated.

A NOVEL SHIP WAKE DETECTION METHOD OF SAR IMAGES BASED ON FREQUENCY DOMAIN

Moving ships produce a set of waves of "V' pattern on the ocean. These waves can often be seen by Synthetic Aperture Radar (SAR). The detection of these wakes can provide important information for surveillance of shipping, such as ship traveling direction and speed. A novel approach to the detection of ship wakes in SAR images based on frequency domain is provided in this letter. Compared with traditional Radon-based approaches, computation is reduced by 20%-40% without losing nearly any of detection performance. The testing results using real data and simulation of synthetic SAR images test the algorithm's feasibility and robustness.

基于同步累积法的舰船尾流微弱信号检测方法研究

为适应复杂的动态变化的尾流气泡场环境,提高水下探测装置对舰船尾流微弱信号的探测信噪比与检出率,本文提出了一种基于同步累积法的舰船尾流微弱信号检测方法。利用周期信号的重复性与噪声的随机性,对连续多个周期信号做累积归一化处理,降低随机噪声对探测性能的干扰,提升探测信噪比。建立了针对舰船尾流微弱信号多时间尺度检测能力评估模型,评估本方法在多参量耦合下的探测性能。通过在室内水池、室外湖泊条件下开展大量模拟舰船尾流探测实验,验证了该方法适配稀疏微小的远场尾流气泡至高湍流扰动下的大尺度近场气泡检测。本文方法可实现全时域舰船尾流的跟踪检测,有效提升水下兵器的打击能力,为舰船尾流激光探测识别工程实践提供支撑。

多船尾流磁异常特性分析与仿真系统设计

为研究多艘舰船同时航行时尾流磁异常特性,基于舰船尾流基本理论知识研究了舰船尾流的波形、速度以及感应磁场的分布,得到了多船尾流感应磁场的变化特性和分布规律。以3艘舰船编队为例,分析了舰船在运动过程中感应磁场分布及在x、y方向上的磁场分布情况,求解了3艘船同时航行时形成的尾流感应磁场分布特性,并对各种舰船组合航行时产生的感应磁场异常情况进行分析。仿真所得实验结果符合现有测量结果。利用APP Design工具对多舰船尾迹磁异常特性进行仿真系统软件的开发,该系统不仅能够准确、直观、快速地模拟设定3艘船型尾流在任意方向的磁感应强度大小和变化规律,而且具有操作简便、节省成本的优势,为舰船尾流实验提供了理论参考和更为有效的仿真平台。

基于阻力和伴流不均匀度的船型优化

为获得影响大方形系数船舶艉部线型的主要船型特征参数,以某肥大型船为研究对象,在考虑排水体积约束的情况下,利用CAESES软件半参数化变形技术实现艉部线型自动变换,构建系列船型方案,并采用SHIPFLOW软件评估这些方案的阻力性能和伴流。应用软件集成的优化算法组合(Sobol与Nelder Mead Simplex优化算法组合)构建船型优化策略,回归分析系列船型方案的优化效果,并采用Star CCM+软件验证优选方案在船舶阻力和伴流不均匀度优化方面的有效性。通过回归分析发现,艉轴上方最大反曲位置的丰满度和艉封板变形对伴流的影响更显著,优选方案相比原方案有效功率下降约2%,伴流不均匀度下降约6.4%。

基于激光主动照明的舰船尾流图像探测系统

通过探测舰船尾流与环境不同的激光后向散射特性,可反演出舰船的航向、航速、吨位等关键信息,具有极高的军事价值。本文基于激光主动照明探测模式,结合工业相机与高重频激光器构建了一套舰船尾流成像探测系统,实现了针对舰船尾流气泡的大视场、大动态范围的有效探测。利用蒙特卡洛方法构建了舰船尾流成像探测仿真平台,验证了该方法的可行性;搭建了基于激光主动照明的舰船尾流图像探测系统样机,并在室内实验室水池条件下开展了大量实验,验证了系统性能,并就实验结果分析了不同系统参数对探测性能的影响;同时开展了可见光干扰条件下探测性能的评估,验证了本系统同时存在较强的抗可见光干扰能力,为激光主动照明成像探测模式在舰船尾流探测中的实践提供了理论支撑。

四桨船舶尾流场特性数值预报研究

四桨船尾部附体的复杂性及内外桨空间布置位置的差异性会导致内外桨载荷分配不均衡。针对上述问题,本文采用CFD方法对四桨船舶的粘性尾流场进行数值模拟研究,从船体周围流场分布、湍流特性等方面详细分析内外桨盘面伴流场差异产生的原因,并与试验结果进行对比分析。研究结果表明,内桨处于船尾湍流区中,受船体边界层影响更甚,这使得内桨桨盘面轴向速度小于外桨盘面,进而导致内外桨负荷分配不均匀;外桨盘面流动为船体边界层流动及湍流尾流区的混合流动,这使得外桨伴流场更不均匀。上述研究可为四桨船舶设计及尾部布局优化提供基础和支撑。

Coupling of RANS and BEM Solvers for Simulation of Propeller Behind the Hull in Full Appended Model

To design a propeller for ship power plant,the interaction between ship hull and propeller must be taken into account.The main concern is to apply the wake effect of ship stern on the propeller performance.In this paper,a coupled BEM(Boundary Element Method)/RANS(Renolds-Averaged Navier−Stokes)solver is used to simulate propeller behind the hull in the self-propulsion test.The motivation of this work is to develop a practical tool to design marine propulsion system without suffering long computational time.An unsteady boundary element method which is also known as panel method is chosen to estimate the propeller forces.Propeller wakes are treated using a time marching wake alignment method.Also,a RANS code coupled with VoF equation is developed to consider the ship motions and wake field effects in the problem.A coupling algorithm is developed to interchange ship wake field to the potential flow solver and propeller thrust to the RANS code.Based on the difference between hull resistance and the propeller thrust,a PI controller is developed to compute the propeller RPM in every time step.Verification of the solver is carried out using the towing tank test report of a 50 m oceanography research vessel.Wake factor and trust deduction coefficient are estimated numerically.Also,the wake rollup pattern of the propeller in open water is compared with the propeller in real wake field.

基于有限元的KCS船尾流感应磁场分析

利用有限元方法,基于国际集装箱标模KCS船舶对船体感应磁场和尾流感应磁场进行了研究。结果表明:船体感应磁场的大小取决于船舶形状与测量位置;船体和尾流感应磁场的量级都为nT,在可探测范围内;尾流感应磁场在远场范围随距坐标原点的距离增加呈指数级衰减。

Data Augmentation of Ship Wakes in SAR Images Based on Improved CycleGAN

The study on ship wakes of synthetic aperture radar(SAR)images holds great importance in detecting ship targets in the ocean.In this study,we focus on the issues of low quantity and insufficient diversity in ship wakes of SAR images,and propose a method of data augmentation of ship wakes in SAR images based on the improved cycle-consistent generative adversarial network(CycleGAN).The improvement measures mainly include two aspects:First,to enhance the quality of the generated images and guarantee a stable training process of the model,the least-squares loss is employed as the adversarial loss function;Second,the decoder of the generator is augmented with the convolutional block attention module(CBAM)to address the issue of missing details in the generated ship wakes of SAR images at the microscopic level.The experiment findings indicate that the improved CycleGAN model generates clearer ship wakes of SAR images,and outperforms the traditional CycleGAN models in both subjective and objective aspects.

舰船尾流场实船测量技术浅析

舰船尾流场是影响舰载直升机安全起降的关键环境条件,其实船测量技术的发展一直备受关注。本文从分析国内外舰船尾流场实船测量研究现状入手,梳理出拉烟形态显示、方向动压探针阵列测量、超声波测风、激光测风雷达、量子测风雷达等5种典型尾流场实船测量技术,并依据主要性能指标对这5种技术进行对比分析,最后对舰船尾流场实船测量技术发展进行总结与展望。

海浪背景下舰船尾流感应磁场特性

基于麦克斯韦方程组以及海浪和尾流的速度场表达式得到无限深海水中海浪背景下舰船尾流感应电磁场的表达式,根据分界面处感应电磁场的连续性条件确定表达式中的待定系数。利用所得到的表达式分析无限深海水中尾流感应磁场、海浪感应磁场和海浪背景下舰船尾流感应磁场的特性。结果表明:舰船尾流感应磁场各分量的幅度在靠近海面的海水中均存在极大值点,且这些极大值点的数值和对应的海水深度均随船速的增大而增加;在海水中某一深度处舰船尾流感应磁场主要集中分布在一个“V”形平面区域内,且船速越大,“V”形区域内感应磁场周期性振荡的幅度越大,沿尾流运动方向振荡的空间间隔越大;海浪对舰船尾流感应磁场z分量的影响大于对x分量的影响。风向对尾流感应磁场的影响存在周期为π的周期性;当风速小于船速时,海浪对舰船尾流感应磁场的影响较弱,在“V”形区域内尾流感应磁场占主导作用;当风速继续增加时,海水中海浪感应磁场的强度显著增大,对尾流感应磁场的影响明显增强,在“V”形区域内尾流感应磁场逐渐被海浪感应磁场所覆盖;与风速变化所产生的影响差异比较明显相比,风向变化对尾流感应磁场影响的差别相对要小。

KCS船舶尾流感应电磁场仿真分析

为了更系统地研究船舶在航行过程中引起的尾流感应电磁场,文中以有限元法为基础,利用磁流体力学、麦克斯韦方程组等电磁场和流体力学理论,对KCS船舶尾流运动产生的感应电磁场进行仿真分析。结果表明:KCS船舶尾流感应磁场量级在可探测范围内有明显的分布规律;尾流感应磁场在传播方向上存在极值,其大小与测量位置相关;尾流感应磁场先随距离增大到极值,再沿传播距离方向逐渐衰减;尾流电流密度沿传播距离方向逐渐衰减。文中的研究可为海洋电磁探测及船舶跟踪和定位等研究提供参考。

Numerical Study on Flow Around Modern Ship Hulls with Rudder-Propeller Interactions

Reducing the fuel consumption of ships presents both economic and environmental gains. Although in the past decades,extensive studies were carried out on the flow around ship hull, it is still difficult to calculate the flow around the hull while considering propeller interaction. In this paper, the viscous flow around modern ship hulls is computed considering propeller action. In this analysis, the numerical investigation of flow around the ship is combined with propeller theory to simulate the hull-propeller interaction. Various longitudinal positions of the rudder are also analyzed to determine the effect of rudder position on propeller efficiency. First, a numerical study was performed around a bare hull using Shipflow computational fluid dynamics(CFD) code to determine free-surface wave elevation and resistance components.A zonal approach was applied to successively incorporate Bpotential flow solver^ in the region outside the boundary layer and wake, Bboundary layer solver^ in the thin boundary layer region near the ship hull, and BNavier-Stokes solver^in the wake region. Propeller open water characteristics were determined using an open-source MATLAB code Open Prop, which is based on the lifting line theory, for the moderately loaded propeller. The obtained open water test results were specified in the flow module of Shipflow for self-propulsion tests. The velocity field behind the ship was recalculated into an effective wake and given to the propeller code that calculates the propeller load. Once the load was known, it was transferred to the Reynolds-averaged Navier-Stokes(RANS) solver to simulate the propeller action. The interaction between the hull and propeller with different rudder positions was then predicted to improve the propulsive efficiency.

舰船尾流激光探测跟踪方法与试验

舰船尾流激光探测跟踪是水下航行器对舰船进行探测、识别、跟踪的新手段。论文基于舰船尾流分布特性、气泡目标特性,采用蒙特卡洛仿真方法,实现了多尺度、宽数密度、大厚度舰船尾流气泡群的后向散射回波信号特性仿真,得到了水下航行器载激光探测系统在搜索、跟踪阶段信号的变化趋势,以及不同目标舰船的激光后向回波信号变化强度,可有效模拟激光探测系统对舰船尾流目标特性的真实跟踪状态。对于大型船只,当激光探测系统位于尾流之下时,航行器距舰船目标越近,尾流气泡激光回波越强,脉冲宽度展宽幅度越大;当激光探测系统位于尾流之中时,航行器距舰船目标越近,尾流气泡激光回波越弱,脉冲宽度变窄幅度越大。探测系统位于尾流之下时与探测系统位于尾流之中时,信号变化相反。小型船只信号变化趋势基本与大型船只保持一致,但尾流激光探测回波强度变低。开展了湖泊环境下船舶尾流激光探测跟踪试验,当探测系统在尾流之下时,大型船只尾流激光回波信号信噪比高,小型船只尾流激光难以检测。探测系统位于尾流之中时,大小船只尾流激光探测系统都可实现有效探测。论文可为舰船尾流探测实际工程应用提供支撑。

排水型船标称伴流的数值模拟与分析

本文分析了集装箱船和散货船的船舶阻力和标称伴流的性能。采用CFD方法比较网格分布模式和湍流模型等影响因素,并将计算结果与实验值进行了比较。通过分析三种网格方案和两种湍流模型的结果,得到计算标称伴流的最优方案。以某散货船的标称伴流为例进行计算,得到“钩状”伴流。同时,对“钩状”伴流产生的原因进行了较好的分析。预测结果表明:VOF两相流模型的船体阻力计算值与实验值有很好的一致性;集装箱船标称伴流场的轴向伴流、切向伴流与径向伴流的计算值与实验值吻合较好,变化趋势保持一致,验证了本方法设置的可行性。