粗糙度与风浪特征量关系的研究

在实验室风浪槽中观测风浪和风速,研究粗糙度与波面特征量的关系,发现风浪谱宽度增加,粗糙度增大。在窄谱时,粗糙度随谱宽的增加变化不明显,当波陡降低,粗糙度降低;在宽谱时,当谱宽度增加,即使波陡降低,粗糙度仍可增大。这一结果表明,波陡不足以完全决定粗糙度。当风浪波龄增加,粗糙度呈下降趋势,但由于谱宽度对粗糙度的影响,当波龄增加,部分波浪可有较大的粗糙度。由于这一因素,在粗糙度与波龄关系的观测结果中,数据点的散落不完全由观测误差造成。     

风浪波分量相速度的测量及其结果的解释

线性海浪谱理论认为:风浪是由多数频率和振幅不等、位相紊乱的简单波动线性叠加的结果。按照此种理论,风浪波分量的相速度应遵从线性波动弥散关系c=g/n(n为圆频率,g为重力加速度)。这种理论究竟在多大程度上符合实际风浪的情况?答案有赖于对风浪波量的相速度作准确的测量。法国马赛实验室Ramamonjiarisoa(1974)用交叉谱方法测一得组数据。此组数据表明,风浪的波分量,只有其频率接近谱峰频率的那一部分遵从线性弥散关系,具有较高频率的波分量是非弥散的,都以几乎同样的相速度传播。     

涌浪对风浪能量影响的实验研究

在实验室风浪水槽中进行纯风浪和混合浪波面位移观测,研究波长较长的规则波对风浪能量的影响.本文用混合浪和纯风浪中的风浪显著波的零阶谱矩之比代表混合浪中的风浪与纯风浪能量之比,并以此表征涌浪对风浪能量的影响.研究了该能量比随涌浪波陡S、风区x、波龄倒数u/C、涌浪频率与纯风浪谱峰频率之比fs/fwp的变化规律.结果表明,涌浪对风浪能量的抑制作用随涌浪波陡的增加、波龄倒数的增大及涌浪频率与纯风浪谱峰频率之比的增大而增强.发现该能量比依赖于无因次量R=(1+80(πS)2)1.9(fs/fwp)0.9(u/C)0.27,并拟合得到2者的经验关系.此外,本文实验还发现,在某些情况下,涌浪的存在使风浪能量增加.     

风浪谱的形式的再探讨

在现有风浪理论中,如Sverdrup与Munk的有效波理论,的三维风浪理论,Pierson与Neumann的波谱理论和文圣常的普遍风浪谱理论,对于尚处在同时随风区和风时成长的风浪的处理存在着一个共同的缺陷,即都将此阶段的风浪处理为或相对于风区或相对于风时成长。为弥补这一不足,本文作者曾于1962年沿用文氏提出的波谱-能量方法导出一同时受制于风区和风时成长的谱。可是,这种谱具有较大的经验性,且在确定谱中组成波的涡动粘滞系数时,用了一个不全是符合风浪实际的经     

四种典型方向谱模式在渤海海域的应用

本文根据仪器阵列测量和“956”测波浮标获得的风浪个例方向谱资料,从总能量的观点出发,比较和检验了Mitsuyasu、Donelan、Hasselmann由观测得到的风浪方向谱以及文圣常等由解析方法得到的理论风浪方向谱.从而得到Mitsuyasu方向谱与渤海海域充分成长状态的实测风浪方向谱吻合程度最好.Donelan和理论风浪方向谱与风浪所有成长状态的实测结果比较一致,尤其与风浪的初始成长和成长状态的实测结果一致程度为最好.     

海浪方向谱阵列资料实例分析

根据傅氏级数展开法和贝叶斯方法,对现场获得的阵列资料进行了方向谱实例分析。结果表明,实测风浪方向谱能量的相对分布与相应的波场类型密切相关。涌浪和由稳定风场引起的风浪方向谱各组成波能量相对于方向呈单峰对称分布。大风引起的风浪方向谱能量的相对分布,虽也近似为单峰,但对称性差。不同波场类型的风浪方向谱各组成波的谱密度极值方向不同,表明不同频率的组成波有不同的主方向。     

大洋中涌浪对风浪能量的影响

由于涌浪与风浪在特征物理参量及成长、衰亡上的显著不同,区分风涌浪以及研究涌浪对风浪的影响尤为重要。本文使用2013年及2015年大洋中的WaMoSⅡ测波雷达观测数据,研究了涌浪对风浪能量的影响。由于测波雷达仅使用了9s的有效周期作为谱分离判据,其所得风浪有效波高显著高于PM谱充分成长关系给出的波高。因此本文结合2D法与1D法,加入风速、风浪夹角、波龄等要素给出新的判据,重新对风、涌浪进行了分离。通过对比不同的波龄判据,发现当波龄取1.5时,所得结果与PM谱吻合良好。以Toba-3/2定律为基础,研究了不同类型涌浪对风浪能量的影响。发现三种类型涌浪存在时,风浪能量及有效波高整体上都有所增加,其中尤其以反向涌浪存在时增加最多。     

风浪频谱中的特征量

在三参量风浪频谱的基础上对谱参量进行深入地研究,给出了谱参量与风场要素、波场要素的关系,提供了依据风场要素、波场要素及实测波浪资料计算谱参量的方法。从而可以依据上述因素直接计算出三参量风浪频谱。此外还根据谱宽度的变化,描述了风浪频谱的成长方式,解释了传统的波陡、波龄关系中经验常数的不同选取所代表的物理背景。     

海浪方向谱阵列资料实例分析

根据傅氏级数展开法和贝叶斯方法,对现场获得的阵列资料进行了方向谱实例分析,结果表明,实测风浪方向谱能量的相对分布与相应的波场密切相关,涌浪和由稳定风场引起的风浪方向谱各组成波能量相对于方向呈单峰对称分布,大风引起的风浪方向谱能量的相对分布,中也近似为单峰,但对称性差,不同波场类型的风浪方向谱各组成波的谱密度极值方向不同,表明不同频率的组成波有不同的主方向。     

风浪波分量相速度的测量及其结果的解释

线性海浪谱理论认为：风浪是由多数频率和振幅不等、位相紊乱的简单波动线性叠加的结果。按照此种理论，风浪波分量的相速度应遵从线性波动弥散关系c=g／n(n为圆频率，g为重力加速度)。这种理论究竟在多大程度上符合实际风浪的情况?     

论风浪的局域结构──Ⅰ.风浪的局域结构与局域小波能谱

基于小波变换,引入了能刻画风浪局域结构的局域小波能谱。论述了风浪的整体结构与局域结构。指出了在不同时间尺度上,风浪具有不同的局域化特征。提出了风场演化过程中整体的共振在线性相互作用是否存在的质疑。     

Local balance in the air-sea boundary processes

A new growth equation for wind waves of simple spectrum is presented upon three basic concepts. The period and the wave height of significant waves in dimensionless forms, which are considered to correspond to the peak frequency and the energy level, respectively, are used as representative quantities of wind waves. One of the three basic concepts is the concept of local balance, and the other two concern the acquisition of wave energy and the dissipation of wave energy, respectively. It is shown from some actual data that the equation, together with two universal constants concerning the acquisition and the dissipation of wave energy (B=6.2×10^?2 andK=2.16×10^?5, respectively), is applied universally to wide ranges of wind waves from those in a wind-wave tunnel to fully developed sea in the open ocean. “The three-second power law for wind waves of simple spectrum”, and a few relations as the lemmas, are derived, such that the mean surface transport due to the orbital motion of wind waves is always proportional to the friction velocity in wind, and that the steepness is inversely proportional to the root of the wave age. It is also derived that the portion of wind stress which directly enters the wind waves decreases exponentially with increasing wave age and is 7.5 % of the total wind stress for very young waves. Also, equations are presented as to the increase of momentum of drift current, and as to the supply of turbulent energy by wind waves into the upper ocean.     

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.     

Comparison of experimental irregular water wave elevation and kinematic data with new hybrid wave model predictions

Surface water wave elevations and kinematics from four unidirectional irregular wave trains, with a Pierson and Moskowitz or JONSWAP random wave spectrum, were measured in the laboratory using resistance wave probes and a laser Doppler anemometer. The wave elevation data, velocity time series, extreme (largest) wave horizontal velocity profiles and extreme wave acceleration fields are compared with the predictions of a new wave kinematics model, named the hybrid wave model. Irregular waves are commonly viewed as the summation of many linear wave components of different frequencies, but more accurate predictions of downstream surface elevations (wave evolution) and wave kinematics are attained by considering the non-linear interactions among wave components. The hybrid wave model incorporates these non-linear wave component interactions, and its wave evolution predictions and kinematics estimates are compared with laboratory measurements in this study. Linear random wave theory, Wheeler stretching and linear extrapolation wave kinematic prediction techniques are also compared. Comparisons between measurements and hybrid wave model estimates demonstrate its improved capability to predict velocity and acceleration fields and wave evolution in two-dimensional irregular waves.     

A derivation of the scaling law, the power law and the scaling relation for fetch-limited wind waves using the renormalization group theory

A state of wind waves at a fetch is assumed to be transformed into another state of wind waves at a different fetch by the renormalization group transformation. The scaling laws for the covariance of water surface displacement and for the one-dimensional and two-dimensional spectrum and the power law for the growth relation are derived from the fact that the renormalization group transformation constitutes a semigroup. The scaling relation or the relation among the exponents of the power law is also derived, using the two assumptions that the renormalization group transformation is applicable to fetch-limited wind waves and that the saturated range exists, which implies that the directional distribution function of energy in the wave number region much larger than the peak wave number does not depend on wave number.     

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.     

A coupled discrete wave model MRI-II

An ocean wind-wave prediction model MRI-II is developed on the basis of the energy balance equation which contains five energy transfer processes, namely, the input by the wind, the nonlinear transfer among the components of windsea by resonant wave-wave interactions, wave breaking, frictional dissipation and the effect of opposing winds. The nonlinear energy transfer is expressed implicitly together with the wind effect by Toba's one-parameter representation of windsea, but neither swell-swell nor swell-windsea resonant interactions are considered. Hypothetical assumptions are introduced to describe wave breaking effects. The numerical constant required in the assumptions of wave breaking is determined through trial test runs for a hindcast performed on the North-western Pacific Ocean. The significant wave height, one-dimensional wave spectrum and two-dimensional wave spectrum hindcasted by this new model are in more reasonable agreement with observations than those obtained with our old model MRI.     

A novel technique in analyzing non-linear wave-wave interaction

During wave growth non-linear wave–wave interactions cause transfer of some wave energy from lower to higher wave periods as the spectrum grows. Wavelet bicoherence, which is a new technique in the analysis of wind–wave and wave–wave interactions, is used to analyze non-linear wave–wave interactions. A selected record of wind wave that contains the maximum wave height observed during 6 h of wave generation is divided into five segments and wavelet bicoherence is computed for the whole record, and for all divided segments. The study shows that the non-linear wave–wave interaction occurs at different bicoherence levels and these levels are different from one segment to another due to the non-stationarity feature of the examined data set.     

Power capture performance of an oscillating-body WEC with nonlinear snap through PTO systems in irregular waves

Compared with solar and wind energy, wave energy is a kind of renewable resource which is enormous and still under development. In order to utilize the wave energy, various types of wave energy converters (WECs) have been proposed and studied. And oscillating-body WEC is widely used for offshore deployment. For this type of WEC, the oscillating motion of the floater is converted into electricity by the power take off (PTO) system, which is usually mathematically simplified as a linear spring and a damper. The linear PTO system is characteristic of frequency-dependent response and the energy absorption is less powerful for off resonance conditions. Thus a nonlinear snap through PTO system consisting of two symmetrically oblique springs and a linear damper is applied. A nonlinear parameter γ is defined as the ratio of half of the horizontal distance between the two oblique springs to the original length of both springs. JONSWAP spectrum is utilized to generate the time series of irregular waves. Time domain method is used to establish the motion equation of the oscillating-body WEC in irregular waves. And state space model is applied to replace the convolution term in the time domain motion equation. Based on the established motion equation, the motion response of both the linear and nonlinear WEC is numerically calculated using 4th Runge–Kutta method, after which the captured power can be obtained. Then the influences of wave parameters such as peak frequency, significant wave height, damping coefficient of the PTO system and the nonlinear parameter γ on the power capture performance of the nonlinear WEC is discussed in detail. Results show that compared with linear PTO system, the nonlinear snap through PTO system can increase the power captured by the oscillating body WEC in irregular waves.