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1.
How to evaluate time-domain Green function and its gradients efficiently is the key problem to analyze ship hydrodynamics in time domain.Based on the Bessel function,an Ordinary Differential Equation(ODE)was derived for time-domain Green function and its gradients in this paper.A new efficient calculation method based on solving ODE is proposed.It has been demonstrated by the numerical calculation that this method can improve the precision of the time-domain Green function.Numerical research indicates that it is efficient to solve the hydrodynamic problems. 相似文献
2.
Fast and accurate computation of the free-surface Green function is of key importance for the numerical solution of linear and second-order wave-structure interaction problems in three dimensions. Integral and series expressions for the Green function are derived for which the limiting values for zero and infinite frequency are consistent with the zero and infinite frequency Green function defined in terms of infinite series of Rankine image sources. The integral expressions presented here have the advantage that they are slowly varying with the non-dimensional wave frequency, making them more efficient to approximate compared with previous expressions. 相似文献
3.
Extension of the Frequency-Domain pFFT Method for Wave Structure Interaction in Finite Depth 总被引：1，自引：0，他引：1
To analyze wave interaction with a large scale body in the frequency domain, a precorrected Fast Fourier Transform (pFFT) method has been proposed for infinite depth problems with the deep water Green function, as it can form a matrix with Toeplitz and Hankel properties. In this paper, a method is proposed to decompose the finite depth Green function into two terms, which can form matrices with the Toeplitz and a Hankel properties respectively. Then, a pFFT method for finite depth problems is developed. Based on the pFFT method, a numerical code pFFT-HOBEM is developed with the discretization of high order elements. The model is validated, and examinations on the computing efficiency and memory requirement of the new method have also been carried out. It shows that the new method has the same advantages as that for infinite depth. 相似文献
4.
A panel-free method (PFM), based on the desingularized Green’s formulae proposed by Landweber and Macagno, has been developed to solve the radiation problem of a floating body in the time domain. The velocity potential due to a non-impulsive velocity is obtained by solving the boundary integral equation in terms of source strength distribution. The singularity in the Rankine source term of the time-dependent Green function is removed. The geometry of a body surface is mathematically represented by NURBS surfaces. The integral equation can be globally discretized over the body surface by Gaussian quadratures. No assumption is needed for certain degree of approximation of distributed source strength on the body surface. The accuracy of PFM was demonstrated by its application to a classical problem of uniform flow past a sphere. The response function of a hemisphere at zero speed was then computed by PFM. The computed response function, added-mass and damping coefficients are compared with other published results. 相似文献
5.
Realistic numerical simulations of nonlinear internal waves (NLIWs) have been hampered by the need to use computationally expensive nonhydrostatic models. In this paper, we show that the solution to the elliptic problem arising from the incompressibility condition can be successfully approximated by a few terms (three at most) of an expansion in powers of the ratio (horizontal grid spacing)/(total depth). For an n dimensional problem, each term in the expansion is the sum of a function that satisfies a one-dimensional second-order ODE in the vertical direction plus, depending on the surface boundary condition, the solution to an n-1 dimension elliptic problem, an evident saving over having to solve the original n-dimensional elliptic problem. This approximation provides the physically correct amount of dispersion necessary to counteract the nonlinear steepening tendency of NLIWs. Experiments with different types of NLIWs validate the approach. Unlike other methods, no ad hoc artificial dispersion needs to be introduced. 相似文献
6.
异常事件对EMD方法的影响及其解决方法研究 总被引：4，自引：1，他引：4
赵进平 《中国海洋大学学报(自然科学版)》2001,31(6):805-814
作者指出异常事件在数据中形成局部的高频信号 ,运用经验模态分解 (EMD)方法分析这种存在异常事件干扰的数据 ,就会产生本征模函数 (IMF)的频率混叠现象 ,而造成物理过程的重叠 ,使得难以用时间过程曲线表现特定的物理过程。这一问题是 EMD方法中尚未妥善解决的问题。为解决这一问题 ,作者利用干扰信号极值及其两边的极大与极小值位置与原始数据有明显对应关系的特征 ,将相关 IMF中的异常信息直接滤除 ,再用 Spline插值方法弥补滤除时段的数据 ,得到重新拟合的该 IMF数据。采用这种方法可以提取出异常信号 ,提取的精度与异常信号的时段长度有关。而且 ,拟合结果消除了异常干扰 ,可以将该 IMF与其余 IMF一起叠加成没有异常干扰的数据。将滤除了异常干扰的数据再次进行 EMD分解 ,可以得到新的 IMF系列 ,而它与不加校正的分解结果有相当大的差别 ,可靠地反映了真实物理过程。结果表明 ,只有在有效滤除异常干扰的情况下才能获得可靠的 IMF系列 ,并准确地描述各种尺度的现象 ;消除了异常干扰的 IMF可以任意单独或组合使用 ,表现各种时间尺度的变化与过程 ;所讨论的方法只适合异常时段较小的情形。对于异常时段接近或大于正常变化周期的干扰还需要探讨其他方法 相似文献
7.
A numerical solution is developed to investigate the generation and propagation of small-amplitude water waves in a semi-infinite rectangular wave basin. The three-dimensional wave field is produced by the prescribed “snake-like” motion of an array of segmented wave generators located along the wall at one end of the tank. The solution technique is based on the boundary element approach and uses an appropriate three-dimensional Green function which explicitly satisfies the tank-wall boundary conditions. The Green function and its derivatives which appear in the integral equation formulation can be shown to be slowly convergent when the source and field points are in close proximity. Therefore, when computing the velocity potentials on the wave generators, the source points are chosen outside the fluid domain, thereby ensuring the rapid convergence of these functions and rendering the integral equations non-singular. Numerical results are shown which illustrate the influence of the various wavemaker and basin parameters on the generated wave field. Finally, the complete wave field produced by the diffraction of oblique waves by a vertical circular cylinder in a basin is presented. 相似文献
8.
In this paper we study the breaking of long waves propagating along an open channel with linear friction on the bottom. The equations governing the wave propagation consist of a pair of first-order nonlinear hyperbolic partial differential equations (PDEs). We first transformed the PDEs into a pair of ordinary differential equations (ODEs) along the characteristic directions by means of a pair of Riemann invariants. By analyzing the ODEs, we found that the breaking of waves can be identified by the singularity of the derivative of the Riemann invariants. Thus, we derived an analytical solution for the derivative of the Riemann invariants. Then, a breaking criterion and an analytical formula for the estimation of breaking time were developed and validated through numerical experiments. It is also shown in the paper that the present model includes the previous model neglecting bottom friction as a special case. 相似文献
9.
基于MODIS数据的2016年黄海绿潮灾害动态监测研究 总被引：3，自引：1，他引：2
本研究利用高分辨率的GF-1卫星影像对MODIS数据绿潮监测的精度进行验证, 并在此基础上利用MODIS数据对2016年黄海绿潮过程进行连续动态监测, 结果表明: 相较于GF-1卫星影像,MODIS数据对绿潮的监测误差高于50%; 2016年黄海绿潮移动路径总体呈先向北, 然后沿山东半岛海岸线向东北方向移动, 并最终停滞于青岛、威海附近海域; 此次绿潮持续时间为80天左右, 并呈现出与往年类似的“出现→发展→暴发→治理→消亡”的规律; 其中“出现”的时间为5月12日, “发展”阶段时间为5月中下旬, 此时绿潮主体分布于苏北浑水区, 适宜前置打捞治理, 当5月底6月初绿潮进入清水区之后才开始进入“暴发”阶段, 本年度绿潮灾害“暴发”规模较大, 对山东沿海水产养殖业及旅游业影响严重。本研究成果对于绿潮预警和防控具有科学和实际意义。 相似文献
10.
黄海绿潮分布年际变化特征分析 总被引：2，自引：1，他引：1
2008-2015年,连续8年在黄海海域暴发大规模绿潮,但因暴发时间、规模及漂移路径的不同,对沿海地区造成的环境影响和经济损失大不相同。本文利用EOS/MODIS卫星的多通道资料,采用NDVI算法获取绿潮信息,给出了2007年以来5-8月所有无云或少云晴朗天气下黄海海面绿潮的分布情况。黄海绿潮覆盖面积变化每年呈现单峰值分布,MODIS卫星在5月中旬至6月初首次发现绿潮,随后30~40 d内达到覆盖面积峰值,7月快速消退,8月上旬完全消失。2007年绿潮出现首年覆盖面积极低,发现绿潮时面积均未超过50 km2;2008年和2009年绿潮覆盖面积峰值分别为3 110 km2和4 075 km2,自此绿潮灾害成为新的海洋环境事件;2010-2012年绿潮暴发规模异常低值,各年覆盖面积峰值均未超过1 800 km2,但从2013年开始绿潮规模逐渐攀升,到2015年绿潮覆盖面积峰值达5 629 km2,持续可达98 d,覆盖面积和持续时间为历年之最。绿潮漂移路径可归纳分为3类:2008年和2011年绿潮主体先北向漂移越过34°30'N后,西北向垂直于岸线漂移,主要对连云港、日照和青岛造成较大影响;2009年和2012年绿潮主体先北向漂移越过35°N后,东北向平行于岸线漂移,故只有少许绿潮上岸;2010年、2013-2015年绿潮主体北向漂移至近岸后东北向沿岸漂移,对日照、青岛和荣成沿海造成大面积影响。所有年份绿潮影响范围均限于南黄海内,东侧边界最远未越过124.2°E。 相似文献
11.
Xiaobo Chen 《Applied Ocean Research》1994,16(6):337-345
A numerical model based on using a tank Green function, has been developed to compute the side wall effects on first- and second-order loadings upon bodies of arbitrary geometry in wave tanks. This tank Green function (TGF) is composed of a finite series of open-sea Green functions and an asymptotic part represented by two single integrals whose kernels decrease exponentially with the integral variable. This consistent expression of the TGF permits one to highlight the side wall effects and to give some criteria for the choice of tank width and the measurement duration to limit the reflection of diffraction and radiation waves.
The efficiency of the developed model is shown in the application to hemispheres and a box-shaped barge placed in the center of the wave tanks. The numerical results explain well the irregularities in the experimental measurements and show that the side walls have important effects on the first-order quantities. These effects are much more pronounced on the second-order drift loads. 相似文献
12.
A numerical model for the simulation of typhoon surge has been developed for the coastal areas of the northern Taiwan. Results from the model outputs are then used as a typhoon surge data bank for 7 main estuary areas in this region. The data bank consists of the historical typhoon events from 1980 to 2004. Both characteristic and frequency analyses of the typhoon surge in the coastal region have been studied. Using these data, a relation between the surge height and pressure distribution is obtained. It is shown that, either the numerical model, or the statistical equations presented in this paper, can be used to predict possible surge heights in the estuary areas with sufficient accuracy. 相似文献
13.
E.R. Jefferys 《Applied Ocean Research》1984,6(1):31-39
Classical frequency and time domain models of a single degree of freedom wave power device are presented. In the time domain, a convolution integral is conventionally used to represent the fluid dynamic radiation force, characterised by added mass and damping in the frequency domain. This integral is replaced by an approximate ordinary differential equation (ODE) model which is faster and more convenient in simulations. A time domain model of the fluid dynamics of an oscillating water column (OWC) device is derived to illustrate the technique. Digital simulations of the OWC are used to compare the accuracy of the classical and ODE models. The simulation of the ODE model runs about six times as fast as the classical model based on convolution, yet characterises the fluid dynamics accurately. 相似文献
14.
The long baseline (LBL) system is widely used to locate and track autonomous underwater vehicles (AUV) through acoustic communication.Three important issues are presented here in LBL system application with AUV.Those issues which regard the normal acoustic communication between LBL system and AUV are the depth of towed array,the length of beacon cable,and the effective area of the AUV.The first issue is the key of the LBL system,which ensures the normal communication between towed array and beacons.The second issue which impacts the normal communication from the AUV to beacons in available range should be considered after the first one has been settled.Then the last issue determines the safe work area of the AUV.The ordinary differential equations (ODE) algorithm of ray is deduced from Snell′s law.The ODE algorithm is applied to obtain sound rays from sound source to receiver.These problems are solved by the judgment that whether rays pinging from a sound source arrives at a receiver.The sea trial shows that these methods have much validity and practicality. 相似文献
15.
1 .IntroductionShipsrestrainedbycablesandfendersinfrontofdocksundergolargeamplitudenon harmonicmotionsinwaves.Forthiskindofnon harmonicproblem ,atime domainmethodmustbeapplied .LinandYue ( 1 990 )usedanintegralequationwiththetime domainGreenfunctionforinfinitewaterdepthtocomputetheshipmotionindeepwater.Butforthepresentproblem ,theintegralequationwiththetime domainGreenfunctionforfinitewaterdepthmustbeapplied .TheGreenfunctionisafieldwithacertainboundaryandinitialconditionsproducedbyasourceat… 相似文献
16.
On the evaluation of time-domain Green function 总被引：1，自引：0，他引：1
An analytical method has been developed to evaluate the wave part of the time-domain Green function and its derivatives. Based on Taylor series expansion, the Green function is obtained by solving a fourth-order ordinary differential equation. The method accelerates the convergence of the summation of an infinite series in the numerical computation. The accuracy of this method was demonstrated by its comparison with other method and its application to solve the radiation problem of a floating hemisphere using a panel-free method. The computed hydrodynamic coefficients agree well with the analytical solutions. 相似文献
17.
An Empirical Method to Derive Ocean Waves From Second-Order Bragg Scattering: Prospects and Limitations 总被引：2，自引：0，他引：2
Klaus-Werner Gurgel Heinz-Hermann Essen Thomas Schlick 《Oceanic Engineering, IEEE Journal of》2006,31(4):804-811
High-frequency (HF) radar wave processing is often based on the inversion of the Barrick-Weber equations, introduced in 1977. This theory reaches its limitations if the length of the Bragg-scattering wave raises to the order of the significant waveheight, because some assumptions are no longer met. In this case, the only solution is moving to lower radar frequencies, which is not possible or desirable in all cases. This paper describes work on an empirical solution which intends to overcome this limitation. However, during high sea state, the first-order Bragg peaks sometimes could not be clearly identified which avoids the access to the second-order sidebands. These cases cause problems to the algorithm which have not been solved yet and currently limit the maximum significant waveheight to about the same values as reported for the integral inversion method. The regression parameters of the empirical solution calibrated from the European Radar Ocean Sensing (EuroROSE) data set are constant values for the complete experiment and when applied to the HF radar data they reconstruct the measurements by a colocated wave buoy quite well. When including a radar-frequency-dependent scaling factor to the regression parameters, the new algorithm can also be used at different radar frequencies. The second-order frequency bands used for the empirical solution are sometimes disturbed by radio interference and ship echoes. Investigations are presented to identify and solve these situations 相似文献
18.
Low frequency variability in the tropical Atlantic is complex and hard to witness due to the weakness of this signal compared to the dominant seasonal one. TOPEX/Poseidon and Jason provide a new tool to enlighten these topics by offering more than 10 years of continuous altimetric series. In the tropical regions, due to the vanishing of the Coriolis parameter, uncertainties of a few centimeters in sea level can result in large errors on geostrophic velocity which will propagate rapidly over the entire basin. Accuracy is then a crucial problem for these areas. The ARAMIS program (Altimétrie sur un Rail Atlantique et Mesures In Situ) has been developed by the French Institut de Recherche pour le Développement (IRD) and Centre National d'Etudes Spatiales (CNES) organizations in order to get a long-term survey of temperature, salinity and pCO2 structures in the tropical Atlantic along a merchant ship line. The first two ARAMIS cruises, in July 2002 and March 2003, were dedicated to Jason validation. The dynamical contrast between ARAMIS1 and ARAMIS2 is first analyzed here in agreement with seasonal variations of surface fluxes and wind forcing. Comparisons with TOPEX/Poseidon and JASON data are then presented in terms of sea level analysis. New geopotential models such as the Earth Gravitational Model 1996 (EGM96) that have become available with a resolution of undulations on the order of 50 km, are checked to get the absolute signal. Finally, the tropical Atlantic surface circulation characteristics are used to point out the agreements/discrepancies between all in situ/satellite products, as geostrophic current will emphasize the sea level results. 相似文献
19.
Hard limiting or sign detection schemes for low-pass known and random signals in additive noise are popular as very simple signal detectors maintaining constant values for their false-alarm probabilities under the rather weak assumption that the sampled noise observations have zero median values. For nonparametric detection of narrow-band signals the natural extension of the zero medians assumption is the zero marginal medians assumption on the in-phase and quadrature noise components. Nonparametric detectors operating under this assumption are developed here for narrow-baud signals; these can be taken to be the logical counterparts of the low-pass sign correlator and polarity coincidence correlator detectors of low-pass theory. The concept of conditional testing, which has been applied previously to obtain efficient multilevel versions of low-pass sign detection schemes, is shown to enter quite naturally in the definition of narrow-band counterparts of the sign correlator and the polarity coincidence correlator detectors. It is also shown that under the diagonal symmetry extension of the low-pass univariate symmetry condition on the noise probability density function, multilevel extensions of these conditional test narrow-hand detectors may also be defined as counterparts of the multilevel conditional test schemes for low-pass signals. 相似文献
20.
Jin-Yuan Liu Ping-Chang Hsueh Chen-Fen Huang 《Oceanic Engineering, IEEE Journal of》2002,27(4):853-861
The problem of coherent reflection of an acoustic plane wave from a rough seabed with a randomly inhomogeneous sediment layer overlying a uniform elastic basement is considered in this analysis. The randomness of the sound field is attributable to the roughness of the seabed and the sound-speed perturbation in the sediment layer, resulting in a joint rough surface and volume scattering problem. An approach based upon perturbation theory, combined with a derived Green's function for a slab bounded above and below by a fluid and an elastic half-space, respectively, is employed to obtain an analytic solution for the coherent field in the sediment layer. Furthermore, a boundary perturbation theory developed by Kuperman and Schmidt (1989) is applied to treat the problem of rough surface scattering. A linear system is then established to facilitate the computation of the coherent reflection field. The coherent reflection coefficients for various surface roughness, sediment randomness, frequency, sediment thickness, and basement elasticity have been generated numerically and analyzed. It was found that the higher/larger size of surface and/or medium randomness, frequency, thickness, and shear-wave speed, the lower the coherent reflection. Physical interpretations of the various results are provided. 相似文献