Gravity torques for surface waves

The effects of the gravity torques acting on the angular momentum of surface gravity waves are calculated theoretically. For short crested waves the gravity torque is caused by the force of gravity on the orbiting fluid particles acting down the slopes of the crests and troughs and in the direction parallel to the crests and troughs. The gravity torque tries to rotate the angular momentum vectors, and thus the waves themselves, counterclockwise in the horizontal plane, as viewed from above, in both hemispheres. The amount of rotation per unit time is computed to be significant assuming reasonable values for the along-crest and trough slopes for waves in a storm area. The gravity torque has a frequency which is double the frequency of the waves. For long crested waves the gravity torque acts in the vertical plane of the orbit and tries to decelerate the particles when they rise and accelerate them when they fall. By disrupting the horizontal cyclostrophic balance of forces on the fluid particles (centrifugal force versus pressure force) the gravity torque accounts qualitatively for the three characteristics of breaking waves: that they break at the surface, that they break at the crest, and that the crest breaks in the direction of wave propagation.     

Acoustic measurements of the velocity field beneath shoaling and breaking waves

An acoustic current meter attached to a servo-hydraulic surface-following device was used to obtain near-surface velocity measurements beneath breaking and near breaking surface gravity waves shoaling on a 1:40 beach. The data are compared to velocities predicted by two adaptations of linear theory: superposition and stretching. For unbroken and near breaking waves, the predictions are in close agreement with the measurements. For breaking and broken waves, near surface crest velocity measurements are influenced by air entrainment; trough velocities are relatively well predicted. The problems associated with the acoustic measurement of near-surface velocities are highlighted.     

Internal flow structure of short wind waves

The internal flow structure of wind waves in a wind-wave tunnel was investigated on the bases of the measured vorticity distributions, streamline patterns, internal pressure fields, and stress distributions at the water surface for some waves in the field. In part I the experimental method and the internal vorticity structure relative to the individual wave crests are described. The measured vorticity distributions of distinct waves (waves with waveheight comparable with or larger than that of significant wavesH _1/3) in the field indicate that the surface vorticity layer is extraordinarily thickened near the crest, and the vorticity near the water surface shows a particularly large value below the crest. The flow near the crest of distinct waves is found to be in excess of the phase speed in a very thin surface layer, and the tangential stress distribution has a dominant peak near the crest. It is argued that the occurrence of the region of high vorticity in distinct waves is associated with the local generation of vorticity near the crest by tangential stress which attains a peak, under the presence of excess flow.     

An experimental investigation of the velocity field under cnoidal waves over the asymmetric rippled bed

1 IntroductionIn coastal areas a ubiquitous phenomenon is theformation of ripples in the seabed. It is now widelyaccepted that the flow and sediment transport overseabed are vital in relation to erosion, surface wavedissipation and pollution dispersion et…     

畸形波传播速度实验和数值模拟研究

畸形波的传播速度是其最重要的特征参数之一。研究畸形波的传播速度有助于深入和全面了解畸形波的生成机理及其演化过程,另外还可以用于畸形波的预报。针对现有关于畸形波传播速度计算方法(高阶Stokes波理论近似估算,Hilbert变换和两固定点的距离除以畸形波的波峰经过两点所用时间)的不准确性和局限性,使用32个测点描述畸形波波峰沿波浪水槽的运动轨迹,再用回归分析法估算波峰运动轨迹与时间的相关关系,从而计算出畸形波的传播速度。基于288组物理模拟畸形波和364组数值模拟畸形波传播速度的计算结果,使用回归分析方法得出了畸形波传播速度的半经验半理论计算公式,同时还分析了畸形波传播速度的强非线性特征。     

On the form of crests in limiting gravity waves

Properties of surface singularities and the form of wave crests of limiting gravity waves in steady-state flows of an ideal liquid are considered by analyzing the kinematic boundary condition. It is shown that, for rotational waves, the angle at the crest can have any value from 0° to 180°, while it has the only value 90° in the case of irrotational waves. Two inferences are made from Bernoulli’s integral and the properties of singularities: (i) the Stokes wave is a rotational wave and (ii) no angular points can appear on the profiles of capillary-gravity and capillary waves.     

驱动非线性浅水波的行波特征研究

采用带有外界强迫效应的浅水动力学模式研究非线性波动、获得了依赖于外界输入形式的驱动水波的行波解。研究结果表明,驱动水波仍具有非线性波动的一般性质,而当外界强迫波速与水波固有速度一致时,水波出现共振效应,并且外界强迫孤立子将导致驱动水波孤立子产生。     

Wave crest and trough distributions in a broad-banded directional wave field

It is well established that the modulational instability enhances the probability of occurrence for extreme events in long crested wave fields. Recent studies, however, have shown that the coexistence of directional wave components can reduce the effects related to the modulational instability. Here, numerical simulations of the Euler equations are used to investigate whether the modulational instability may produce significant deviations from second-order statistical properties of surface gravity waves when short crestness (i.e., directionality) is accounted for. The case of a broad-banded directional wave field (i.e. wind sea) is investigated. The analysis is concentrated on the wave crest and trough distribution. For completeness a comparison with a unidirectional wave field is presented also. Results will show that the distributions based on second-order theory provide a good estimate for the simulated crest and trough height also at low probability levels.     

Internal flow structure of short wind waves

For wind waves generated in a wind-wave tunnel, the surface pressure and also the pressure distribution along the internal streamlines were calculated from the measured internal velocity field. In distinct waves, with wave height comparable with or larger than the mean, the surface pressure is found to vary drastically in a narrow region around the crest, showing a dominant minimum near the crest. On the other hand, the pressure distribution along the streamline shows systematic variations that are nearly in phase with the streamline profile. It is shown that the occurrence of the pressure in phase with the streamline profile is linked with the internal vorticity distribution, especially with the presence of a high vorticity region below the crest described in Part I of this study. As a result of the occurrence of such pressure variations, the dispersion relation is modified by about 10% from that for linear irrotational waves. It is argued from the present measurements that the dispersion relation and also the energy transfer from wind into wind waves are strongly affected by the internal vortical structure so that the assumption of irrotational gravity waves cannot be applied to the wind waves being studied.     

Generation mechanisms for capillary-gravity wind wave spectrum

We investigate the role of different physical mechanisms in the generation of the capillary-gravity wind wave spectrum. This spectrum is calculated by integrating a nonstationary kinetic equation until the solution becomes stready. The mechanisms of spectrum generation under consideration include three-wave interactions, viscous dissipation, energy influx from wind, nonlinear dissipation, and the generation of a parasitic capillary ripple. The three-wave interactions are taken into account as an integral of collisions without additional simplifications. It is shown that the three-wave interactions lead to solution instability if the kinetic equation takes into account only linear sources. To stabilize the solution, the kinetic equation should incorporate a nonlinear dissipation term, which in the range of short gravity waves corresponds to energy losses during wave breaking and microscale wave breaking. In the range of capillary waves, the account of nonlinear dissipation is also needed to ensure a realistic level of the spectrum for large wind velocities. For the steady-state spectrum, the role of three-wave interactions remains essential merely in the range of the minimum of phase velocity, where a trough on the curvature spectrum is formed. At the remaining intervals of the spectrum, the main contribution into the spectral energy balance is provided by the mechanisms of wave injection, nonlinear dissipation, and the generation of parasitic capillaries.     

基于微结构图像的风生微尺度波频率谱初探

利用海面微结构光学测量装置实验获取到的微尺度波图像来给出风生微尺度波的波数谱,在此基础上利用频散关系建立微尺度波波数谱和频率谱之间的关系,最终得到微尺度波频率谱.对微尺度波的频率谱随频率变化的响应进行了研究,发现频率谱与频率的a次方成正比关系,a的值随风速的增加而增加,同一风速下a几乎为定值.     

Effects of three-wave interactions in the gravity-capillary range of wind waves

The formation of the spectrum of short wind waves from the gravity-capillary and capillary ranges under the effect of three-wave interactions is considered. In order to determine the spectrum, the kinetic equation for wave packets is integrated to the point where the solution is established. Three-wave interactions are described by a collision integral without introducing any additional assumptions simplifying the problem. This calculation procedure reproduces the Zakharov-Filonenko theoretical spectra, which correspond to the cases of energy equipartition and the inertial range. It is shown that the main role of three-wave interactions lies in the energy transfer from the range of short gravity waves to waves with shorter wavelengths. This transfer is accomplished both locally in the Fourier space and as a result of interactions between short and long waves. Its characteristic features are the formation of a dip on the curvature spectrum in the region of a minimum phase velocity of waves and the formation of a secondary peak in the capillary range. The dip is filled and disappears as the wind speed increases. Taking into account the interaction between short and long waves increases the spectrum in the capillary range several times, and the balance between energy input from long waves and viscous dissipation is established in the capillary range. The energy sink caused by three-wave interactions, viscous dissipation, and wind forcing cannot give the stability of the spectrum of short gravity waves.     

Simulation model of SAR remote sensing of turbulent wake of semi-elliptical submerged body

1 IntroductionThe study of SAR remote sensing of surface wake of a semi-elliptical submerged body is of great importance actually in the ocean. The dynamics and the SAR remote sensing mechanism of this wake are very complicated. Numerous researches (Bergmann et al., 1958; Miles, 1968a,b; Huppert and Miles, 1969; Bon-neton et al., 1993) indicate that in the ocean the imagery and dynamics of this wake are quite different when a semi-elliptical submerged body moves horizontally at high speed …     

楔形物体在静水与波浪中自由入水砰击试验研究

通过自由落体的入水方式,分别在静水和规则波中开展了两种不同横剖面的曲面楔形体入水砰击问题试验研究。使用高速摄像系统记录楔形体入水过程流场演变和运动特性,采用加速度传感器和压力传感器进行数据的动态采集。试验结果表明,在静水中入水时,外凸剖面楔形体入水砰击后模型两侧的射流飞溅比反曲剖面更剧烈,而在楔形体前端的水面以下部分形成的气腔更小;在规则波中入水时,对于相同模型,在波峰和上跨零点相位下模型入水砰击后两侧的射流飞溅比在波谷相位更剧烈。相同工况时,反曲剖面模型所受砰击的加速度峰值和压力峰值更小;在相同的入水速度下,对于相同模型,波浪载荷和砰击载荷的共同作用会使模型所受砰击压力显著增大。     

Peak Dynamic Pressure on Semi- and Quarter-Circular Breakwaters Under Wave Troughs

A series of physical tests are conducted to examine the characteristics of the wave loading exerted on circular-front breakwaters by regular waves. It is found that the wave trough instead of wave crest plays a major role in the failure of submerged circular caissons due to seaward sliding. The difference in the behavior of seaward and shoreward horizontal wave forces is explained based on the variations of dynamic pressure with wave parameters. A wave load model is proposed based on a modified first-order solution for the dynamic pressure on submerged circular-front caissons under a wave trough. This wave loading model is very useful for engineering design. Further studies are needed to include model uncertainties in the reliability assessment of the breakwater.     

Physical model of the spectrum of capillary-gravity waves

A model for the spectrum of capillary waves has been constructed. These waves are generated at the crests of short gravity waves and decay due to viscosity. Capillary wave generation leads to short gravity wave dissipation. Using empirical data on the short gravity wave dissipation spectrum, a relation for the capillary wave spectrum is derived from the equation of energy balance of capillary waves. The capillary wave spectrum is matched with the known Donelan-Pierson spectrum for short gravity waves. The obtained relation for the spectrum of wind-generated ripple is compared with the data of laboratory experiments. Translated by Vladimir A. Puchkin.     

基于TerraSAR-X卫星数据的内孤立波参数和海表流速信息提取的探索研究

本文利用TerraSAR-X(TSX)卫星于2010年4月22日在南海东沙岛附近海域获取的数据进行海洋内孤立波动力要素和海表流速信息的提取研究。基于TSX数据的后向散射强度信息,利用经验模态分解法得到内孤立波半波宽度,再利用两层模型法和参数化法计算得到内孤立波振幅和相速度。反演结果显示,利用参数化方法得到的振幅(约21～39 m)和两层模型法得到的相速度(约1.07 m/s)与历史实测资料较为一致。进而利用TSX的顺轨干涉数据获取研究海域内的多普勒速度,再分别采用M4S模型法和直接分离法处理,进而提取海表流速。结果显示,两种方法得到的海表流速的全场平均值较为一致,均为1.10 m/s左右。M4S模型法对流速最大值的改变量较大而直接分离法对流速最小值的改变量较大。M4S模型对内孤立波波峰线区域海表流速的修正大于无内孤立波的海域。最后,基于KdV方程计算得到内孤立波引起的表面流的流速约为0.28 m/s,对反演出的海表流速贡献占比23%。     

海底沙波特征线的最优方向剖面自动识别方法

海底沙波是发育在近海陆架上的一种常见海底地貌类型,海底沙波特征与运动规律的研究具有重要的科学意义与工程应用价值,沙波脊线与谷线是表征海底沙波的最基本特征,也是精确描述沙波运动的基本参量。本文提出了一种基于复合数字水深模型的沙波特征线自动识别方法——最优方向剖面法,基于水深曲面归算得到最优剖面方向,再依据最优剖面方向求导并判定极值,自动提取沙波形态特征点,最终形成沙波脊线和谷线。以台湾浅滩复合型沙波为例进行对比实验研究,结果表明,该方法能基于不同分辨率的数字水深模型自动准确地提取海底沙波脊线与谷线,勿需设置阈值,地形自动化识别程度得到进一步提升,具有重要的实际应用价值。     

Peak Dynamic Pressure on Semi-and Quarter-Circular Breakwaters Under Wave Troughs

A series of physical tests are conducted to examine the characteristics of the wave loading exerted on circular-front breakwaters by regular waves.It is found that the wave trough instead of wave crest plays a major role in the failure of submerged circular caissons due to seaward sliding.The difference in the behavior of seaward and shoreward horizontal wave forces is explained based on the variations of dynamic pressure with wave parameters.A wave load model is proposed based on a modified first-order solution for the dynamic pressure on submerged circular-front caissons under a wave trough.This wave loading model is very useful for engineering design.Further studies are needed to include model uncertainties in the reliability assessment of the breakwater.