Vessels with semi-closed tanks(i.e.,well docks)are widely applied in the military operation and maritime engineer-ing.The water is bound by the semi-closed floating tank and forced by both the incident waves and ship...Vessels with semi-closed tanks(i.e.,well docks)are widely applied in the military operation and maritime engineer-ing.The water is bound by the semi-closed floating tank and forced by both the incident waves and ship’s motions.The free surface oscillations inside the flooded well dock is thus distinctive and very complicated.So far,the natural modes of semi-closed floating tanks have not yet been studied.This paper investigates the characteristics of natural modes of a floating semi-closed tank by combining a mode-resolving model based on mild-slope equations and a hydrodynamic model based on computational fluid dynamics.Results show that the first three natural periods(i.e.,74,23.6,and 14 s)of the tank fall into the band of swell and infragravity waves and they could be triggered under certain circumstance.Multi-period free surface oscillations are observed inside the tank,including the longest natural period(i.e.,74 s),though the incident waves are monochromatic.A possible generation mechanism for the long-period mode is explained on the basis of liquid sloshing and harbor oscillations.Moreover,a long-period component with a period close to the natural mode of well dock is observed in the ship motions,which is generated by the interaction between the waves and ship.展开更多
Two problems of scattering of surface water waves involving a semi-infinite elastic plate and a pair of semi-infinite elastic plates,separated by a gap of finite width,floating horizontally on water of finite depth,ar...Two problems of scattering of surface water waves involving a semi-infinite elastic plate and a pair of semi-infinite elastic plates,separated by a gap of finite width,floating horizontally on water of finite depth,are investigated in the present work for a two-dimensional time-harmonic case.Within the frame of linear water wave theory,the solutions of the two boundary value problems under consideration have been represented in the forms of eigenfunction expansions.Approximate values of the reflection and transmission coefficients are obtained by solving an over-determined system of linear algebraic equations in each problem.In both the problems,the method of least squares as well as the singular value decomposition have been employed and tables of numerical values of the reflection and transmission coefficients are presented for specific choices of the parameters for modelling the elastic plates.Our main aim is to check the energy balance relation in each problem which plays a very important role in the present approach of solutions of mixed boundary value problems involving Laplace equations.The main advantage of the present approach of solutions is that the results for the values of reflection and transmission coefficients obtained by using both the methods are found to satisfy the energy-balance relations associated with the respective scattering problems under consideration.The absolute values of the reflection and transmission coefficients are presented graphically against different values of the wave numbers.展开更多
针对水面目标检测中的噪声干扰和小目标的漏检问题,提出一种改进YOLOv8的水面小目标检测算法YOLOv8-WSSOD(YOLOv8-water surface small object detection).首先,为降低水面复杂环境在主干网络下采样过程中产生的噪声干扰,提出基于BiFor...针对水面目标检测中的噪声干扰和小目标的漏检问题,提出一种改进YOLOv8的水面小目标检测算法YOLOv8-WSSOD(YOLOv8-water surface small object detection).首先,为降低水面复杂环境在主干网络下采样过程中产生的噪声干扰,提出基于BiFormer双层路由注意力机制构建的C2fBF(C2f-BiFormer)模块,在特征提取过程中保留细粒度的上下文特征信息;其次,针对水面小目标的漏检问题,新增一个更小的检测头,提升网络对小目标的感知力,并在Neck端引入GSConv和Slim-neck,减轻模型复杂度并保持精度;最后,使用MPDIoU损失函数解决CIoU损失函数的局限性,以提高模型检测准确率.实验结果表明,相较于原始YOLOv8算法,该算法在水面小目标上平均准确率mAP@0.5提升了4.6%,mAP@0.5:0.95提升了2.2%,并且改进后的算法检测速度达到86f/s,能有效实现对水面小目标快速、准确的检测.展开更多
为解决水上漂浮垃圾种类繁多、移动端算力有限、现有识别模型较复杂等原因导致的水上清洁船识别垃圾精度低、速度慢等问题,提出一种基于YOLOv5s(you only look once version5 small)网络模型的水上漂浮垃圾识别算法。该算法采用K-means...为解决水上漂浮垃圾种类繁多、移动端算力有限、现有识别模型较复杂等原因导致的水上清洁船识别垃圾精度低、速度慢等问题,提出一种基于YOLOv5s(you only look once version5 small)网络模型的水上漂浮垃圾识别算法。该算法采用K-means聚类算法调整边界框比例,提高检测精度;以渐进式学习方式EfficientNetv2模型替代YOLOv5s的主干部分,融合高效通道注意力(efficient channel attention,ECA)机制,减少模型复杂度,提高检测速度,同时增强模型的特征提取能力;引入平衡因子φ和归一化高斯Wasserstein距离(normalized Gaussian Wasserstein distance,NWD)度量对YOLOv5s的损失函数优化,降低模型对水上远距离漂浮垃圾的检测敏感性。自制数据集的测试实验结果显示,改进算法的mAP比YOLOv5s算法提高2.2%,模型的参数量下降20.34%,检测速度提高30.84%,表明改进算法具有优越性。展开更多
针对多数高空间分辨率的影像波段数量较少,导致改进归一化水体指数(modified normalized difference water index,MNDWI)、新型水体指数(new water index,NWI)和自动水体指数(automated water extraction index,AWEI)等水体指数模型不...针对多数高空间分辨率的影像波段数量较少,导致改进归一化水体指数(modified normalized difference water index,MNDWI)、新型水体指数(new water index,NWI)和自动水体指数(automated water extraction index,AWEI)等水体指数模型不能有效应用的问题,通过分析各地物的光谱信息,构建出新阴影水体指数(new shaded water index,NSWI),旨在提高水体提取精度。在分析现有水体提取方法的基础上提出了新的水体提取方法,采用具有代表性的归一化水体指数(normalized difference water index,NDWI)、支持向量机(support vector machine,SVM)、最大似然(maximum likelihood,ML)法和本文方法,利用不同时期和不同区域的高分辨率影像GF-2以及稳健性分析时选取的中分辨率影像Landsat TM和Landsat OLI进行实验。结果表明:GF-2数据中,本文方法在渭河和玫瑰湖的总体精度分别为97.2%和95.3%,Kappa系数分别为0.966和0.936;在稳健性分析时,采用Landsat TM和Landsat OLI数据,本文方法在东湖和龙羊峡水库的总体精度分别为96.6%和93.4%,均高于其他方法。本文方法充分利用了影像的光谱信息、几何信息和纹理信息,使得提取的地表水体既有较完整的水体边界又保持了局部细节,同时有效抑制了同谱异物、同物异谱现象,减少了噪声斑块的产生,使得提取结果更准确。展开更多
基金supported by the National Natural Science Foundation of China(Grant No.51979029)。
文摘Vessels with semi-closed tanks(i.e.,well docks)are widely applied in the military operation and maritime engineer-ing.The water is bound by the semi-closed floating tank and forced by both the incident waves and ship’s motions.The free surface oscillations inside the flooded well dock is thus distinctive and very complicated.So far,the natural modes of semi-closed floating tanks have not yet been studied.This paper investigates the characteristics of natural modes of a floating semi-closed tank by combining a mode-resolving model based on mild-slope equations and a hydrodynamic model based on computational fluid dynamics.Results show that the first three natural periods(i.e.,74,23.6,and 14 s)of the tank fall into the band of swell and infragravity waves and they could be triggered under certain circumstance.Multi-period free surface oscillations are observed inside the tank,including the longest natural period(i.e.,74 s),though the incident waves are monochromatic.A possible generation mechanism for the long-period mode is explained on the basis of liquid sloshing and harbor oscillations.Moreover,a long-period component with a period close to the natural mode of well dock is observed in the ship motions,which is generated by the interaction between the waves and ship.
基金NASI (National Academy of Sciences, India) for providing financial support
文摘Two problems of scattering of surface water waves involving a semi-infinite elastic plate and a pair of semi-infinite elastic plates,separated by a gap of finite width,floating horizontally on water of finite depth,are investigated in the present work for a two-dimensional time-harmonic case.Within the frame of linear water wave theory,the solutions of the two boundary value problems under consideration have been represented in the forms of eigenfunction expansions.Approximate values of the reflection and transmission coefficients are obtained by solving an over-determined system of linear algebraic equations in each problem.In both the problems,the method of least squares as well as the singular value decomposition have been employed and tables of numerical values of the reflection and transmission coefficients are presented for specific choices of the parameters for modelling the elastic plates.Our main aim is to check the energy balance relation in each problem which plays a very important role in the present approach of solutions of mixed boundary value problems involving Laplace equations.The main advantage of the present approach of solutions is that the results for the values of reflection and transmission coefficients obtained by using both the methods are found to satisfy the energy-balance relations associated with the respective scattering problems under consideration.The absolute values of the reflection and transmission coefficients are presented graphically against different values of the wave numbers.
文摘针对水面目标检测中的噪声干扰和小目标的漏检问题,提出一种改进YOLOv8的水面小目标检测算法YOLOv8-WSSOD(YOLOv8-water surface small object detection).首先,为降低水面复杂环境在主干网络下采样过程中产生的噪声干扰,提出基于BiFormer双层路由注意力机制构建的C2fBF(C2f-BiFormer)模块,在特征提取过程中保留细粒度的上下文特征信息;其次,针对水面小目标的漏检问题,新增一个更小的检测头,提升网络对小目标的感知力,并在Neck端引入GSConv和Slim-neck,减轻模型复杂度并保持精度;最后,使用MPDIoU损失函数解决CIoU损失函数的局限性,以提高模型检测准确率.实验结果表明,相较于原始YOLOv8算法,该算法在水面小目标上平均准确率mAP@0.5提升了4.6%,mAP@0.5:0.95提升了2.2%,并且改进后的算法检测速度达到86f/s,能有效实现对水面小目标快速、准确的检测.
文摘针对多数高空间分辨率的影像波段数量较少,导致改进归一化水体指数(modified normalized difference water index,MNDWI)、新型水体指数(new water index,NWI)和自动水体指数(automated water extraction index,AWEI)等水体指数模型不能有效应用的问题,通过分析各地物的光谱信息,构建出新阴影水体指数(new shaded water index,NSWI),旨在提高水体提取精度。在分析现有水体提取方法的基础上提出了新的水体提取方法,采用具有代表性的归一化水体指数(normalized difference water index,NDWI)、支持向量机(support vector machine,SVM)、最大似然(maximum likelihood,ML)法和本文方法,利用不同时期和不同区域的高分辨率影像GF-2以及稳健性分析时选取的中分辨率影像Landsat TM和Landsat OLI进行实验。结果表明:GF-2数据中,本文方法在渭河和玫瑰湖的总体精度分别为97.2%和95.3%,Kappa系数分别为0.966和0.936;在稳健性分析时,采用Landsat TM和Landsat OLI数据,本文方法在东湖和龙羊峡水库的总体精度分别为96.6%和93.4%,均高于其他方法。本文方法充分利用了影像的光谱信息、几何信息和纹理信息,使得提取的地表水体既有较完整的水体边界又保持了局部细节,同时有效抑制了同谱异物、同物异谱现象,减少了噪声斑块的产生,使得提取结果更准确。
