Detection of nonlinear waves and their contribution to ocean wave spectra Part I: Theoretical consideration

This is a Part I of a paper of nonlinearities of wind waves in the deep open ocean. First, considerations are given in order to verify the theoretical expression for bound waves from observed data. We compare the contribution of bound waves and double Bragg scattering to the second-order scattering, and we show that the contribution of bound waves is larger, and that bound waves can be detected by measuring the Doppler spectra of HF (high-frequency) radio wave scattering from the sea surface. Moreover, if the theory of the HF radio wave scattering from the sea surface is verified, so is the second-order perturbation theory for bound waves. Then, the contributions of bound waves to ocean wave spectra are investigated on the basis of the nonlinear theory. The bound waves are shown to modify frequency spectra and wave directional distributions at higher frequencies, and it is shown that although the modifications of frequency spectra are smaller for a two-dimensional field case than for a one-dimensional field case, they are not negligible at higher frequencies. On the other hand, the modifications of wave directional distributions are shown to be significant at higher frequencies. These discussions become significant only when bound wave predictions are verified in the open ocean. Consequently, it is shown that nonlinearities of water waves are important in considering both radio wave scattering from the sea surface and the detailed structures of ocean wave spectra at high frequencies.     

风浪和涌浪分离方法的比较（英文）

海浪通常以风浪和涌浪混合的形式存在,如何进行分离风浪和涌浪一直是海浪理论研究和海洋工程应用中的重要问题。本文利用模型试验和实测资料,对目前提出的一种二维谱风涌浪分离方法（2D法）和三种一维谱风涌浪分离方法（PM法、WH法、JP法）进行了检验,分析发现：2D法给出的结果整体而言最为可靠,与2D法相比,PM法明显高估了风浪成分,WH法低风速时高估了风浪,高风速时跟2D法比较接近,而JP法在整体上高估了风浪成分。通过调整分割频率的比例系数,改进了PM法,改进后的PM法给出的分离结果与2D法最为一致。     

A Calculation Method for Incidence and Reflection of Irregular Waves Along Seawalls

Some researches have been made in this aspect.In the method by Walton Jr.(1992),incidentwaves are supposed to be the overlapping result of M component waves with different frequencies whichmay take different directions,the direction of incident waves should be available in advance,but in fact thedirection of incident waves is not available.In our study,incident waves are supposed to be composed ofM overlapping component waves with different frequencies,and different frequencies have different direc-tions.Based on the irregular wave reflection theory,the calculation formulas of wave direction,complexamplitude of incident waves,and complex amplitude of reflected waves in surface which are composed ofcomponent waves are derived by means of discrete Fourier transform.Then,the frequency spectra of inci-dent waves and reflected waves and the reflection coefficient of waves with corresponding frequencies areobtained.Verification of the method and the calculation results from in-situ measured data indicate thatt     

Short-wave measurements by a fixed tower-based and a drifting buoysystem

The small-scale roughness of the sea surface acts as an important link in air-sea interaction processes. Radar and sonar waves are scattered by short surface waves providing the basis for remote sensing methods of the sea surface. At high wind speeds, breaking waves occur. Bubbles penetrate into the water and drastically increase acoustical reverberation, transmission loss and ambient noise. Thus, the development of short waves and wave breaking have to be known to apply radar remote sensing to the surface and to deduce from radar backscatter which sonar conditions prevail. To measure the wind dependence of short waves an experimental device was constructed for use from stationary platforms. It is nearly all-weather capable and can easily be handled by a crane. On the other hand, frequencies of short waves measured in a fixed position are extremely frequency shifted by currents. This limits the usefulness of tower-based measurements, e.g., the short wave modulation by wind and waves or currents can only be estimated in a rough approximation. Consequently, a buoy was developed to reduce the frequency shifts. The principle of the buoy is to drift in the local surface current and to follow the amplitudes of long waves. Therefore, short waves are measured in facets of long waves and the Doppler shifts are minimized. The wind is measured at a constant height above the long wave profile and relative to the moving facets. The paper describes the conventional measuring device and points out the necessity of the drifting buoy system. Examples of wind and wave spectra are presented and short wave modulations by long waves are depicted, too. From these measurements, new insights in short wave behaviour have to be expected     

Instability of Two Interacting Quasi-Monochromatic Waves in Shallow Water

The nonlinear interactions of waves with a double-peaked power spectrum have been studied in shallow water.The starting point is the prototypical equation for nonlinear unidirectional waves in shallow water,i.e.the Korteweg de Vries equation.By means of a multiple-scale technique two defocusing coupled Nonlinear Schrdinger equations are derived.It is found analytically that plane wave solutions of such a system are unstable for small perturbations,showing that the existence of a new energy exchange mechanism which can influence the behavior of ocean waves in shallow water.     

Structure of a modulated narrow light beam in seawater: Monte Carlo simulation

Dispersion properties of photon density waves propagating in seawater from a unidirectional point source have been studied using the Monte Carlo technique. It is shown that the spatial distribution of irradiance of the photon density waves at high modulation frequencies is significantly different from that of the stationary light field. Principal dispersion effects predicted earlier in the approximation solutions of the nonstationary radiation transfer equation are confirmed. An increase in the wave decrement and a decrease in the transversal cross section of the initially narrow beam at the modulation frequency with the increase of the modulation frequency are demonstrated. Frequency dependencies of phase and group velocities of photon density waves are calculated. It is shown that seawater possesses anomalous dispersion in a wide frequency range with respect to these waves.     

Acoustic wave scattering from two solid boundaries at the oceanbottom: reflection loss

An acoustic wave scattering model is formulated and solved for three homogeneous layers consisting of a thin solid sediment layer sandwiched by semi-infinite water and solid basalt media. The model is applied to two cases to analyze both the physical parameters affecting reflection loss and the effects of interface roughness scattering. It is shown that effects of attenuation in the sediment layer, especially of S-waves, combine with conversion and scattering processes of the basalt interface to constitute the dominant mechanism of reflection loss, especially in the small grazing angle directions. The scattering process is found not only to produce the well-known acoustic energy loss from specular to nonspecular directions, but also to alter the conversion efficiency between P and S waves with a resulting loss or gain     

Laboratory wave reflection analysis using co-located gages

Most coastal and ocean engineering laboratories employ techniques that use two or three spatially separated wave gages to estimate reflection of irregular waves in two-dimensional wave flumes. This paper presents a frequency domain method for separating incident and reflected wave spectra from co-located gages (gages located on the same vertical line). The technique is based on linear wave theory, and it can be applied to time series of sea surface elevation and horizontal water velocity collected in a vertical array, or it can be used with horizontal and vertical water velocity time series collected at the same point in the water column. Application of the method is limited to those frequencies showing good coherence between time series signals. Outside the range of good coherence, gross inaccuracies occur.The utility of the co-located gage method is illustrated using water velocity data collected in a wave flume with a laser Doppler velocimeter, and the method is validated for the case of complete reflection by a vertical wall. Side-by-side comparison to the spatially-separated wave gage method of Goda and Suzuki (1976) exhibited close agreement for a variety of irregular wave trains being weakly reflected by a mild sloping beach. The co-located gage method is useful in situations where there are spatial variations in the wavelength, such as on a mildly sloping bottom, or in the region close to highly reflective structures where errors arising from spatial variations in characteristic wave parameters would corrupt estimates made using the spatially-separated wave gage method.     

双曲余弦海脊上海啸俘获波的解析与数值研究

海啸能被大洋海脊引导以俘获波的形式沿其传播上万千米,且因其特殊的运动方式,携带巨大能量影响远场地区的港口,严重威胁海岸安全。本文首先基于线性浅水方程,推导了双曲余弦平方海脊上俘获波的波面解,其为μ阶ν次的连带勒让德函数的第一类解和第二类解的组合。进一步推导出其对应的频散关系,其中对于确定的频率ω,存在无穷多个波数k_y与之对应。采用MIKE21-BW模型,模拟了产生于海脊脊顶处的海啸在理想双曲余弦平方海脊上的传播变形过程。结果表明,小部分能量以自由先驱波进行传播,海啸波的波能大部分被海脊俘获。海脊俘获波沿着海脊方向为行进波,随着海啸波传播时间的增加,波浪在沿着海脊方向的延展范围也逐渐增大,波高逐渐减小、波的个数逐渐增加。俘获波能量主要由不同频率以相同速度传播的具有孤立波特性的波浪成分和能量主要集中在特定频率范围内的波浪成分组成。     

A 10-year wave energy resource assessment and trends of Indonesia based on satellite observations

Wave energy resource assessment and trends around Indonesian's ocean has been carried out by means of analyzing satellite observations. Wave energy flux or wave power can be approximated using parameterized sea states derived from satellite data. Unfortunately, only some surface parameters can be measured from remote sensing satellites, for example for ocean surface waves: significant wave height. Others, like peak wave period and energy period are not available, but can instead be estimated using empirical models. The results have been assessed by meteorological season. The assessment shows clearly where and when the wave power resource is promising around Indonesian's ocean. The most striking result was found from June to August, in which about 30–40 kW/m(the 90 th percentile: 40–60 kW/m, the 99th percentile: 50–70 kW/m) wave power energy on average has been found around south of the Java Island. The significant trends of wave energy at the 95% level have also been studied and it is found that the trends only occurred for the extreme cases, which is the 99th percentile(i.e.,highest 1%). Wave power energy could increase up to 150 W/m per year. The significant wave heights and wave power have been compared with the results obtained from global wave model hindcast carried out by wave model WAVEWATCH III. The comparisons indicated excellent agreements.     

海洋雷达探测技术综述

通过简要回顾半个世纪以来海洋雷达技术发展历程的特点和规律,结合海洋雷达技术发展现状,介绍海洋雷达涉及的关键技术并进行应用场景分析。总结我国海洋雷达技术近30 a来的发展及其与国际水平的主要差距,提出"十三五"期间我国海洋雷达技术优先发展项目的建议,即超视距雷达广域海洋监测技术、浮动/机动平台高分辨率海洋雷达技术、新体制微波海洋雷达技术以及海洋雷达数据管理与应用技术等。     

一种实体嵌入的三维海浪可视化方法

针对实体嵌入对海面的随机干扰问题,提出一种海浪三维可视化方法。该方法采用扰动的思想改进了海浪的数学模型,并利用信息更加全面的方向谱描述海浪,进而将方向谱和改进海浪模型共同应用于海浪的三维可视化,形象展示实体嵌入时对海浪的随机性影响,并给出了三维实体的建模方法。通过多组实体嵌入的海浪可视化仿真试验的比较与分析,验证了本文提出方法的有效性。     

High-frequency radar measurements of the ocean wave-directionalspectrum

Measurements of the ocean wave directional spectrum using a dual, high-frequency (HF) radar system are presented. A model-fitting technique is used to obtain wave measurements from the radar Doppler spectra. Over 100 h of data, collected NURWEC2 (Netherlands-UK Radar Wavebuoy Experimental Comparison), have been compared with measurements using a WAVEC directional wave buoy. The amplitude and directional characteristics of long-wave components at frequencies of 0.07-0.1 Hz in general show good agreement. Reasonable estimates of the directional spectrum across the whole frequency range are obtained when the assumptions of the model-fitting technique are appropriate. Remaining problems in radar measurement and difficulties in assessing accuracy are discussed     

X-band low-grazing-angle ocean backscatter obtained during LOGAN1993

This paper reviews X-band ocean microwave backscatter data from the LOGAN (LOw Grazing ANgle) experiment conducted on the Chesapeake Light Tower by the Naval Air Warfare Center. The data were collected under varied wind, sea, and swell conditions that provide some new insights into low-grazing-angle backscatter phenomena. Transient backscatter peaks called “sea spikes” have long been associated with deep-water breaking waves; however, they have yet to be fully reconciled with backscatter and hydrodynamic theories. New analysis techniques have been applied to the LOGAN data that take advantage of the unique characteristics of sea spikes and their dynamics. High-resolution Doppler spectra are organized relative to the space-time centroids of the sea-spike clusters and conditionally averaged by RCS strength. The mean Doppler variation of the strongest sea spikes then map the breaking-wave structure just as Doppler histories measured at moderate grazing angles map the dynamics of the dominant linear surface-wave components. While breaking waves are manifest to some degree in backscatter data at all grazing angles, a non-Bragg-scatter mechanism accentuates the crest scattering at low grazing angles. The phenomena potentially can be exploited for remote ocean sensing and imaging     

Efficient computation method for two-dimensional nonlinear waves

I~IOWNUmerics wave tank is a ~ tOOl by which variouS nonlinear wave - ac interactionpwhleln can be treated in time domain. As the face stage, we f~ our efforts on the devious of an effita saution tee~ of fully nofor waves. aam numtried of water - wavesconnected PwhlemS, es~ in a wave tank, is alwayS comas Of PartS: wave generation, P~ and a~. The effi~ Of a nUm~ wave tankdepends not Only on the qwhty Of the wave abotion tecboaue, which allowS the lOng times~ and keePS finite tank leng…     

On the synthetic aperture radar imaging of ocean surface waves

A process of synthetic aperture radar imaging of ocean surface waves is considered on the basis of the two-scale model of microwave scattering by a disturbed sea surface. Analytical expressions are obtained to relate characteristics of a large-scale wave image, averaged over an ensemble of realizations of the small-scale ripple, with the wave, radar, and viewing scheme as parameters. It is shown that the wave image would be defocused as an image of a target moving in the along-track direction with a speed equal to a half of the wave phase speed projection on the line of flight. The defocusing magnitude was measured experimentally for the ocean swells images, obtained with an airborneS-band radar, and the results are found to be in satisfactory agreement with the model prediction.     

风浪和涌浪分离方法的比较

海浪通常以风浪和涌浪混合的形式存在。文中利用模型试验和实测资料,对目前提出的一种二维谱风涌浪分离方法(2D法)和3种一维谱风涌浪分离方法(PM法、WH法、JP法)进行了检验,分析发现:2D法给出的结果整体而言最为可靠,与2D法相比,PM法明显高估了风浪成分,WH法低风速时高估了风浪,高风速时跟2D法比较接近,而JP法在整体上高估了风浪成分。通过调整分割频率的比例系数,改进了PM法,改进后的PM法给出的分离结果与2D法最为一致。     

Dynamic Analysis of Turret-Moored FPSO System in Freak Wave

Freak wave is the common wave which has significant wave height and irregular wave shape, and it is easy to damage offshore structure extremely. The FPSOs (Floating Production Storage and Offloading) suffer from the environment loads, including the freak wave. The freak waves were generated based on the improved phase modulation model, and the coupling model of FPSO-SPM (Single Point Mooring) was established by considering internal-turret FPSO and its mooring system. The dynamic response characteristics of both FPSO and SPM affected by the freak wave were analyzed in the time domain. According to the results, the freak waves generated by original phase modulation model mainly affect the 2nd-order wave loads. However, the freak waves which are generated by random frequencies phase modulation model affect both 1st-order and 2nd-order wave loads on FPSO. What is more, compared with the irregular waves, the dynamic responses of mooring system are larger in the freak waves, but its amplitude lags behind the peak of the freak wave.     

Draft effect on wave action with a semi-infinite elastic plate

1Introduction Wave action on coastal and offshore structuresis investigated by many researches. In the recent pa-pers we can seen that Li et al. (2006) developed athree-dimesional(3D) numerical model for obliquewave action on vertical walls, Li et al. (20…     

Probability density functions for non-linear random waves and responses

Non-linear waves have been a research topic for a long time, but have not yet entered common ocean engineering practice. In this article the problem of calculating the probability density function (pdf) for non-linear waves is investigated. The ‘saddle point approximation’ is applied to a second-order wave model to estimate the pdf. The performance of the approximation is compared with the performances of the more commonly used methods for different types of input spectra. In addition, a non-parametric transformation method, with extrapolation of the level crossing intensity, is proposed to estimate the pdf. One simulation and three different data sets are used to investigate the performance of the model.