A Method for Calculation of Low-Frequency Slow Drift Motions Based on NURBS for Floating Bodies

Through a higher-order boundary element method based on NURBS （Non-uniform Rational B-splines）, the calculation of second-order low-frequency forces and slow drift motions is conducted for floating bodies. In the floating body＇s inner domain, an auxiliary equation is obtained by applying a Green function which satisfies the solid surface condition. Then, the auxiliary equation and the velocity potential equation are combined in the fluid domain to remove the solid angle coefficient and the singularity of the double layer potentials in the integral equation. Thus, a new velocity potential integral equation is obtained. The new equation is extended to the inner domain to reheve the irregular frequency effects; on the basis of the order analysis, the comparison is made about the contribution of all integral terms with the result in the second-order tow-frequency problem; the higher-order boundary element method based on NURBS is apphed to calculate the geometric position and velocity potentials; the slow drift motions are calculated by the spectrum analysis method. Removing the solid angle coefficient can apply NURBS technology to the hydrodynamic calculation of floating bodies with complex surfaces, and the extended boundary integral method can reduce the irregular frequency effects. Order analysis shows that free surface integral can be neglected, and the numerical results can also prove the correctness of order analysis. The results of second-order low-frequency forces and slow drift motions and the comparison with the results from references show that the application of the NURBS technology to the second-order low-frequency problem is of high efficiency and credible results.

从x出发的漂移Brownian Motion的极值分布

该文研究了从x出发的正漂移Brownian Motion的极值问题,给出了关于这种随机过程的两种极大值的定义,并主要利用Brownian Motion的一些重要性质,比如正交不变性、时空齐次性及在有限停时上的强Markov性等,获得了两种极大值的分布函数的精确表达式.

Tectonically asymmetric Earth:From net rotation to polarized westward drift of the lithosphere

The possibility of a net rotation of the lithosphere with respect to the mantle is generally overlooked since it depends on the adopted mantle reference frames, which are arbitrary. We review the geological and geophysical signatures of plate boundaries, and show that they are markedly asymmetric worldwide. Then we compare available reference frames of plate motions relative to the mantle and discuss which is at best able to fit global tectonic data. Different assumptions about the depths of hotspot sources （below or within the asthenosphere, which decouples the lithosphere from the deep mantle） predict different rates of net rotation of the lithosphere relative to the mantle. The widely used no-net-rotation （NNR） reference frame, and low （〈0.2°-0.4°/Ma） net rotation rates （deep hotspots source） predict an average net rotation in which some plates move eastward relative to the mantle （e.g., Nazca）. With fast （〉1°/Ma） net rotation （shallow hotspots source）, all plates, albeit at different velocity, move westerly along a curved trajectory, with a tectonic equator tilted about 30° relative to the geographic equator. This is consistent with the observed global tectonic asymmetries.

Control of vacuum arc source cathode spots contraction motion by changing electromagnetic field

This paper investigates the magnetic field component impact on cathode spots motion trajectory and the mechanism of periodic contraction.Electromagnetic coils and permanent magnets were installed at the different sides of cathode surface,the photographs of cathode spots motion trajectory were captured by a camera.Increasing the number of magnets and decreasing the distance between magnets and cathode both lead to enhancing cathode spots motion velocity.Radii of cathode spots trajectory decrease gradually with the increasing of electromagnetic coil＇s current,from 40 mm at 0 A to 10 mm at 2.7 A.Parallel magnetic field component intensity influence the speed of cathode spots rotate motion,and perpendicular magnetic field component drives spots drift in the radial direction.Cathode spot＇s radial drift is controlled by changing the location of the ‘zero line＇ where perpendicular magnetic component shifts direction and the radius of cathode spots trajectory almost equal to ‘zero line＇.

Image Motion Compensation of Off-Axis TMA Three-Line ArrayAerospace Mapping Cameras

To enhance the image motion compensation accuracy of off-axis three-mirror anastigmatic( TMA)three-line array aerospace mapping cameras,a new method of image motion velocity field modeling is proposed in this paper. Firstly,based on the imaging principle of mapping cameras,an analytical expression of image motion velocity of off-axis TMA three-line array aerospace mapping cameras is deduced from different coordinate systems we established and the attitude dynamics principle. Then,the case of a three-line array mapping camera is studied,in which the simulation of the focal plane image motion velocity fields of the forward-view camera,the nadir-view camera and the backward-view camera are carried out,and the optimization schemes for image motion velocity matching and drift angle matching are formulated according the simulation results. Finally,this method is verified with a dynamic imaging experimental system. The results are indicative of that when image motion compensation for nadir-view camera is conducted using the proposed image motion velocity field model,the line pair of target images at Nyquist frequency is clear and distinguishable. Under the constraint that modulation transfer function( MTF) reduces by 5%,when the horizontal frequencies of the forward-view camera and the backward-view camera are adjusted uniformly according to the proposed image motion velocity matching scheme,the time delay integration( TDI) stages reach 6 at most. When the TDI stages are more than 6,the three groups of camera will independently undergo horizontal frequency adjustment. However, when the proposed drift angle matching scheme is adopted for uniform drift angle adjustment,the number of TDI stages will not exceed 81. The experimental results have demonstrated the validity and accuracy of the proposed image motion velocity field model and matching optimization scheme,providing reliable basis for on-orbit image motion compensation of aerospace mapping cameras.

基于CFD方法的FPSO的波浪荷载和运动响应研究

针对浮式生产储油船(Floating production storage and offloading, FPSO)在深水作业时面临的恶劣海况问题,为保证其服役期间的安全性及作业效率,需要准确计算非线性波浪荷载和运动响应。本文应用基于计算流体动力学(Computational fluid dynamics, CFD)的软件STAR-CCM+建立了针对实尺度FPSO模型的数值水池,基于Richardson外推法,针对时间步长和网格尺寸,分析了二者的收敛性和数值不确定度。使用该数值水池模拟了不同波浪工况作用下FPSO的运动响应,计算其受到的二阶波浪力,并与势流结果进行对比。结果发现,随着波陡增大,平均波浪力的CFD结果逐渐大于势流结果,且呈现差异化增大。而波陡大于1/15时,纵摇的CFD结果明显大于势流结果。说明大波陡条件下,黏性效应对平均波浪力和纵摇影响明显,为恶劣海况下预报及修正FPSO的二阶波浪力和运动提供了参考。

风浪联合作用下OOstar平台低频运动响应分析

基于FAST建立OOstar平台的时域模型,研究风浪联合作用下平台的运动响应,采用统计学方法分析平台的不规则运动规律,利用傅里叶变换对平台运动进行频谱分析,重点关注波浪载荷差频分量及风载荷对平台低频慢漂运动的影响。结果表明:在不同海况下差频分量与风载荷均会使平台运动振幅增加,并使大振幅对应运动周期的离散性增强,而对小振幅对应运动周期的影响较小;差频分量对振幅及运动周期的影响随着海况的恶劣而增强,风载荷则相反。频谱分析表明,差频分量在不同海况下对平台自振频率处运动的激励影响均较大,风载荷则在四级海况下对平台自振频率处运动的激励影响较大,在八级海况下对平台自振频率处的激励影响较小。

The joint Laplace transforms for killed diffusion occupation times

The approach of Li and Zhou(2014)is adopted to find the Laplace transform of occupation time over interval(0,a)and joint occupation times over semi-infinite intervals(-∞,a)and(b,∞)for a time-homogeneous diffusion process up to an independent exponential time e_(q)for 0<a<b.The results are expressed in terms of solutions to the differential equations associated with the diffusion generator.Applying these results,we obtain explicit expressions on the Laplace transform of occupation time and joint occupation time for Brownian motion with drift.

深水缓波形立管的非线性动力分析

应用三维集中质量模型,考虑重力、浮力、流体拖曳力、拉力、惯性力、剪切力及弯曲刚度、海床作用等对缓波形立管的特性的影响,考察缓波形立管与传统悬链线立管在有效张力、弯矩、剪力等分布上的差异。着重分析了应用于2 000m水深干树半潜平台的缓波形立管在平台慢漂作用下疲劳特征点—顶端节点有效张力的变化趋势,平台运动对立管顶端有效张力的频域响应影响,以及平台的动态特性对立管疲劳特征点时域响应的影响。

消除多次透射公式飘移失稳的措施

基于对双曲型偏微分方程数值解稳定性的 ＧＫＳ （Ｇｕｓｔａｆｓｓｏｎ， Ｋｒｅｉｓｓ ａｎｄ Ｓｕｎｄｓｔｒｏｍ）准则的一种物理解释，提出了在波动数值模拟中消除多次透射公式飘移失稳的一个简单措施，并用波源问题和散射问题的数值试验检验了这一措施在稳定实现多次透射公式中的有效性．

带电粒子在地磁场中的漂移运动

利用数值计算方法模拟带电粒子在地磁场中的运动轨迹,分析了入射角度和起始位置对带电粒子在地磁场中的横向漂移运动的影响并将结果可视化。

视频码流转换编码器结构及其分析

随着数字音视频及多媒体技术的发展,迫切需要压缩巨大的视频信号数据,适用于不同领域的MPEG-1、MPEG-2、MPEG-4、H.261、H.263、H.264等国际编码标准应运而生。为了适应用户终端的多样性以及网络自身的传输特性,常需将已压缩的视频码流进行转换编码,以实现网络间资源共享。主要介绍了对已有的压缩视频流根据目标的要求如何进行相应的变换,如通过改变量化参数获得码率的变换;通过下变换获得缩小的图像,实现空间分辨率的变换,以及跳帧获得时间分辨率的变换。对不同变换器结构形式进行了分析,以获得结构复杂性和图像质量的均衡,并重点剖析了转换编码器核心技术——运动矢量的确定。

空间相机调偏流机构的设计与控制

为了减小像移对空间相机成像质量的影响,提高相机分辨率,对空间相机像移补偿方法进行了研究。分析了偏流角产生的原因及调整原理,根据本相机自身特点,设计了高精密像移补偿机构。系统采用正弦机构作为传动形式,以80C31作为偏流角控制器,以步进电机为执行元件,以绝对式编码器作为偏流角测量元件,实现了偏流角位置的闭环控制。由于偏流角调整范围在-4°～+4°之间,以-4°、-2°、0°、+2°、+4°作为假想偏流角期望值,用编码器测得了10组偏流角调整实际数据。实验结果表明:偏流角控制系统精度可达到2′,满足系统对控制系统精度<3′的要求,可以实现高精度的像移补偿。

Penning阱中H_n^+团簇离子运动的分析

根据在实验中所采用的Penning离子阱的结构及电磁场分布特点,列出阱内离子运动方程并进行求解,并对Pen-ning阱内团簇离子的二种运动进行详细分析,最后对所作的研究进行了总结.

基于速度矢量模型的圆轨道空间相机偏流角实时补偿

偏流角是影响线阵CCD相机推扫成像性能的重要因素。为实现在轨有限计算资源环境下偏流角的快速计算,根据圆轨道卫星飞行与地物随地球自转的速度矢量运动关系,建立了适用于圆轨道正视与前后视成像的偏流角模型,给出卫星飞行过程中星下点相对地物的运动速度矢量作角向振动的数学表达式。根据该模型提出基于旋转CCD方式的偏流实时补偿方案,并搭建偏流校正成像模拟平台进行了实验验证。仿真和实验表明,速度矢量模型具有更高的计算效率,计算精度和稳定性与坐标变换模型和轨道要素模型一致,能够将偏流导致的图像MTF下降抑制到5%。该方案适用于TDICCD相机与三线阵CCD立体测绘相机的偏流实时调整机构开发工作。

基于全球板块运动模型分析大西洋扩张变化

基于现代空间测量技术 SL R、VL BI和 GPS实测资料 ,解算出大西洋中脊海底扩张速率 ,其中北大西洋的东西向扩张速率平均为 35 mm/ a,赤道大西洋东西向扩张速率分别为 2 0～ 2 5 mm/ a,南大西洋东西向扩张速率为 2 2～2 8mm/ a,证实全球板块运动的存在及大西洋扩张学说 ,并基于全球几百万年地质模型 NNR- NU VEL1A,北大西洋的东西向扩张速率平均为 2 4.3m m/ a,基于最新全球板块运动模型 ITRF2 0 0 0 VEL,北大西洋的东西向扩张速率平均为 2 0 .8mm/ a,总体上大西洋实测东西扩张速度与根据地学资料推出的地球板块运动模型和最新ITRF2 0 0 0 VEL

偏流角对TDICCD相机像质的影响分析

为分析TDICCD相机成像过程中偏流角对相机像质的影响,根据卫星轨道方程写出了偏流角随纬度变化的具体形式。基于此分别研究了其在卫星穿航方向和沿航方向引起的像移量和对相机成像质量的影响,并利用极地圆轨道参数详细计算了偏流角引起的像移对像质的影响程度和在MTFdrift>0.95的情况下调整偏流角的时间间隔。结果表明,偏流角是纬度的函数,由它引起的像移在穿航方向对相机像质的影响较为严重,必须采取补偿措施加以校正。

A Nonlinear Restoring Effect Study of Mooring System and its Application

Mooring system plays an important role in station keeping of floating offshore structures. Coupled analysis on mooring-buoy interactions has been increasingly studied in recent years. At present, chains and wire ropes are widely used in offshore engineering practice. On the basis of mooring line statics, an explicit formulation of single mooring chain/wire rope stiffness coefficients and mooring stiffness matrix of the mooring system were derived in this article, taking into account the horizontal restoring force, vertical restoring force and their coupling terms. The nonlinearity of mooring stiffness was analyzed, and the influences of various parameters, such as material, displacement, pre-tension and water depth, were investigated. Finally some application cases of the mooring stiffness in hydrodynamic calculation were presented. Data shows that this kind of stiffness can reckon in linear and nonlinear forces of mooring system. Also, the stiffness can be used in hydrodynamic analysis to get the eieenfrequencv of slow drift motions.

Penning阱中离子运动及存储条件的研究

根据在实验中所采用的Penning离子阱结构及电磁场分布特点,列出阱内离子运动方程并进行求解,对其中各种运动进行详细分析,本文还分析了离子的存储条件.最后对所作的研究进行总结,得到如下结论:阱中离子的运动为简谐振动、回旋运动和漂移运动的叠加,这3种运动是稳定的.

偏流角对空间相机影响研究

研究了卫星运行中偏流角对TDICCD相机像质的影响。根据卫星运动状态,推导了偏流角的具体变化形式,以及由其在穿航方向和沿航方向引起的相机像移和由此造成的像质退化调制传递函数(MTF)。并针对高度为500 km的圆形卫星轨道,定量计算了偏流角导致的像移对相机像质的影响程度。结果表明:偏流角在赤道附近最大,随着纬度的升高,偏流角呈非线性减小。并且,由它引起的像移在穿航方向对相机像质的影响远大于沿航方向。对于给定的轨道参数,偏流角最大值约为3.7°,当积分级数为32时,由其引起的像移调制传递函数接近于零,严重影响了相机的成像质量。为此,提出了使用二维摆镜和调整焦平面方向两种实时补偿偏流角影响的方法。