Experimental study on modulational instability and evolution of crescent waves

A series of experiments on the instability of steep water wave trains in water with finite water depths and infinite water depths in a wide wave basin were performed. It was found that under the coupled development of modulational instability and class-I instability, the initial two-dimensional steep wave trains evolved into three-dimensional crescent waves, followed by the occurrence of disordered water surfaces, and that the wave energy transferred to sidebands in the amplitude spectrum of the water surface elevation. The results also show that water depth has a significant effect on the growth of modulational instability and the evolution of crescent waves. The larger the water depth, the more quickly the modulational instability suppresses class-I instability.     

AN INTEGRAL EQUATION DESCRIBING RIDING WAVES IN SHALLOW WATER OF FINITE DEPTH

1 .　INTRODUCTIONSmall scalewavessuperposedonlarge scalewavesarecalled“ridingwaves” .Small scalewavesonoceansurfacehasaroused greatinterestwiththegreatimprovementofprecisionofmeasuringinstrumentsusedinthefieldofremotesensing .Thisstudyaimsatunderstandingthemotionofthelarge scalewavesandtheenergyexchangebetweenairandseathroughsomeinformationofsmall scalewaves .Henyeyetal .[2 ] derivedthemotione quationtothefirstorderofridingwavesbyusingacanonicalHamiltonianformulationandassumingsmallwave…     

船舶在长峰和短峰不规则波中运动的三维时域数值模拟

船舶在真实海况中的航行性能越来越受到重视。此文在规则波中航行船舶运动的时域预报方法的基础上,发展了长峰和短峰不规则波的数值模拟技术,建立了航行船舶在长峰和短峰不规则波中运动的三维时域数值模拟方法,并应用该方法对一条10万吨油轮进行了数值模拟研究,计算了该油轮在长峰不规则波中的运动响应。试验和计算结果表明此文方法有一定的精度。最后还对该油轮进行了短峰不规则波中运动的数值模拟。长峰和短峰不规则波中的计算结果比较表明短峰不规则波对该油轮运动可产生较大影响。     

Ship bow waves

The bow wave generated by a ship hull that advances at constant speed in calm water is considered. The bow wave only depends on the shape of the ship bow (not on the hull geometry aft of the bow wave). This basic property makes it possible to determine the bow waves generated by a canonical family of ship bows defined in terms of relatively few parameters. Fast ships with fine bows generate overturning bow waves that consist of detached thin sheets of water, which are mostly steady until they hit the main free surface and undergo turbulent breaking up and diffusion. However, slow ships with blunt bows create highly unsteady and turbulent breaking bow waves. These two alternative flow regimes are due to a nonlinear constraint related to the Bernoulli relation at the free surface. Recent results about the overturning and breaking bow wave regimes, and the boundary that divides these two basic flow regimes, are reviewed. Questions and conjectures about the energy of breaking ship bow waves, and free-surface effects on flow circulation, are also noted.     

NUMERICAL MODELING OF WAVE EVOLUTION AND RUNUP IN SHALLOW WATER

Based on the Navier-Stokes (N-S) equations for viscous, incompressible fluid and the VOF method, 2-D and 3-D Numerical Wave Tanks (NWT) for nonlinear shallow water waves are built. The dynamic mesh technique is applied, which can save computational resources dramatically for the simulation of solitary wave propagating at a constant depth. Higher order approximation for cnoidal wave is employed to generate high quality waves. Shoaling and breaking of solitary waves over different slopes are simulated and analyzed systematically. Wave runup on structures is also investigated. The results agree very well with experimental data or analytical solutions.     

非线性波浪绕射问题的数值计算

本文给出非线性波浪绕射问题的计算模型,并用时间步进和边界元法求解。该模型能适用于严重非线性情形,且能满足初始条件和辐射条件。作为对该模型的检验,我们对孤立波绕过穿刺水面的直立圆柱的算例进行了计算,并同Isaacson的解析解、李宝元的差分解进行了比较,结果吻合很好。此外我们还用数值方法同时模拟造波机造波和求该波与直立圆柱的相互作用。结果表明,数值波浪水池能够以尽可能短的暂态过程,模拟波浪与物体的相互作用,提供良好的初始条件。数值波浪水池还能用于任意水深、任意水底形状下的相互作用问题。本文的计算模型和算法具有机时省,计算稳定的优点。     

珊瑚岛礁护岸对礁坪上极端波浪传播特性的影响

极端波浪要素是珊瑚岛礁护岸工程设计的重要参数。利用大比尺波浪水槽,采用1: 15的模型比尺开展试验,从波浪形态、增水、波速、波谱等方面研究了珊瑚岛礁护岸对礁坪上极端波浪传播特性的影响规律。护岸工程产生了明显的阻水效应:对于波浪形态,反射波的影响使得波浪在礁缘破碎更为剧烈; 显著增加了礁坪增水,最大增水值是自然地形下的4倍,影响在波周期较短时更为显著; 减小了波速约20%~35%,其影响随波周期的减小而增强; 对于波谱,护岸的存在使得波浪的非线性相互作用更加复杂,降低了波浪在礁缘处的主频能量,但同时增大了礁坪上的主频及高频能量; 护岸距礁缘距离对堤前水位具有重要影响,随该距离的增加,堤前时均水位先增加后降低。     

无反射造波数值波浪水槽

本文应用ＶＯＦ方法建立了二维数值波浪水槽模型。基于线性造波机理论在水槽左端设置了可吸收式造波机数值边界条件。即造波板的运动除了产生行进波外，同时还产生一个抵消反射波的局部波动。水槽右端除了通常的可滑移与无滑移直墙边界条件外，依据线性辐射条件建立了开边界条件。通过水槽右端为直墙时的全反射情形，比较了可吸收造波机与普通造波机的速度变化与波面变化。文中在Ｈ０／ｄ＝０．１￣０．３，ｄ／Ｌ＝０．１２４￣０．     

极平缓岸坡上的底部波能损耗

本文分别分析与计算了波浪在粉沙质海床、沙纹海床和淤泥质海床上传播时波高的衰减。研究表明，波浪在粉沙质海床上传播的渗透损耗可以忽略不计；波浪在沙纹海床上传播的底摩阻损耗比平滑粉沙底床上传播的损耗大，前者的摩擦系数约比后者值大一个数量级。波浪在淤泥质海床上传播的能量损耗最大，其损耗机遇也很复杂。为了便于比较，将波浪在淤泥质海床上的衰减换算成等价摩擦系数。并运用宾汉模型和波浪作用通量守恒原理计算了波浪在极平缓淤泥质海滩上传播时的衰减及沿程等价摩阻系数。与连云港波浪观测资料对比，符合较好。此外。理论计算发现沿程等价摩阻系数与水深关系较大，由深水至浅水逐渐增大，且水深越浅，增速越快。     

浅水波与圆弧型贯底式防波堤的相互作用

该文基于椭圆余弦波绕射理论,应用特征函数展开法,推导了浅水波对圆弧型贯底式多孔介质防波堤绕射的波势解,并据此解析计算了作用于防波堤的水平波浪载荷与浅水波波面分布。计算结果表明:浅水波入射角与非线性影响参数、海况条件以及防波堤几何条件等因素的相对变化对波浪作用均存在一定的影响,结构透空可以明显减弱防波堤所受波浪作用。与微幅波理论对比可知:在一定浅水条件下采用椭圆余弦波一阶分量模型所计算的最大无量纲波浪力、力矩和波面分布值均明显高于微幅波理论的对应估值,反映了水波非线性因素的影响效应。     

DEFORMATION AND BREAKINC OF NONLINEAR WAVE IN WATER OF GRADUALLY VARYING DEPTH

Based upon the perturbation method,this paper used the method of conformalmapping to solve the deformation and breaking of nonlinear waves in water of gradually varyingdepth.The wave characteristics,such as wave deformation,velocity field,breaker type andbreaker height are discussed by the third order approximation.Results obtained are comparedwith experimental data.     

STUDY OF NONLINEAR LONG WAVE APPOXIMATION IN UNIFORM CHANNELS VIA HAMILTONIAN STRUCTURE

ＳＴＵＤＹＯＦＮＯＮＬＩＮＥＡＲＬＯＮＧＷＡＶＥＡＰＰＯＸＩＭＡＴＩＯＮＩＮＵＮＩＦＯＲＭＣＨＡＮＮＥＬＳＶＩＡＨＡＭＩＬＴＯＮＩＡＮＳＴＲＵＣＴＵＲＥ￥ＸｕＸｉｎ－ｓｈｅｎｇ；ＺｈｏｎｇＷａｎ－ｘｉｅ；ＬｕＹｕ－ｌｉｎ（ＩｎｓｔｉｔｕｔｅｏｆＥｎｇ...     

二层流体浅水波演化模型

本文在Boussinesq的浅水波假定下，采用各层内任意点ziα速度水平分量代替传统的垂向平均速度，推导出了各层速度的三个分量及压力由各层内点ziα速度水平分量表示的沿垂向分布，将三维问题简化为二维问题，建立了二层流体浅水波演化模型，即二层流体的改进Boussinesq方程组。在该方程组中，随ziα距交界面距离不同，该二层流体浅水波演化模型具有不同的色散特性和非线性。作者证明文中参数αi皆取-0.393时，该方程组具有最佳的色散特性。该方程组不仅适于模拟波浪沿水平方向从深水域向浅水域传播时的折射、绕射和反射问题，而且也适用于研究在浅水或中等水深的水域中波浪传播问题。     

Boussinesq方程的永形波的级数解

对于用层平均速度势(或速度)与波高给出的非线性等水深Boussinesq方程,与KdV方程一样,存在两类永形波—孤立波和周期波。不同的是,对于Boussinesq方程,孤立波和周期波一样,是以反函数的积分形式给出的,这在使用上是不方便的。本文对这两类波,分别给出了相应的级数形式的正函数解。对于孤立波,级数的收敛速度是十分快的,且文中还给出了一个具有高精度的十分简单的近似解,对于周期波,只要有关参数选得合适,解的收敛速度也是相当快的。这就有效地克服了反函数形式解在使用上的不方便。     

DISSIPATION OF WAVE ENERGY ON VERY MILD SLOPE

1.　INTRODUCTIONOffshoreareasarewherepeoplefrequentlyengagevariouskindsofhumanactivities,forexamples,portbuilding.seawallconstruction,installationofoildrillingplatformandseashorepowerstation,etc.Whenstructuresplacedinshallowwateraredesigned,thedesignwaveheightalwaystakesthecriticalwaveheight-breakingwaveheight.ThecriticalwaveheightadoptedinChineseCodeisdeterminedbyGoda′smethod,whichiswidelyusedinmanycountries.Thismethodisbasedonthestudyofregularwavesonbottomslopei≥1/50.Themaximumwavehe…     

水面波动问题频域三阶解的一个实例(之二):三阶波形和水动力的求解

针对定深水域中一块直立推板简谐运动引起水面二维波动的问题,在论文[1]已给出速度势三阶解表达式的基础上,本文进一步求取波形和推板所受水动力的三阶解.通过与二阶解及完全非线性解的比较,分析了三阶解在非线性解中的地位和作用.     

THE STATISTIC CHARACTERISTICS OF IRREGULAR BREAKING WAVE FORCES ON VERTICAL WALLS

ＴＨＥＳＴＡＴＩＳＴＩＣＣＨＡＲＡＣＴＥＲＩＳＴＩＣＳＯＦＩＲＲＥＧＵＬＡＲＢＲＥＡＫＩＮＧＷＡＶＥＦＯＲＣＥＳＯＮＶＥＲＴＩＣＡＬＷＡＬＬＳ￥ＬｉＹｕ－ｃｈｅｎｇ；ＬｉｕＤａ－ｚｈｏｎｇ（Ｄｅｐｔ．ｏｆＣｉｖｉｌＥｎｇｉｎｅｅｒｉｎｇ，Ｄａｌｉａｎ...     

Ship waves in rivers: Environmental criteria and analysis methods for measurements

A literature review shows that the most important physical quantities determining environmental impact of ship waves in a waterway are the fluid velocities, maximum and minimum water levels, and size of drawdown events. Fluid velocity can vary strongly in the vertical so that the usual measurements at a single point are not enough unless made near where the effects are most important, often the bed. Customary use of wave height as a measure of impact has been misleading, because the all-important fluid velocity is of a scale given by wave height divided by wave period. A good and simple estimate of the surface velocity as a disturbance scale is shown to be given by the time derivative of the free surface height. The most important role of linear wave theory is to explain and understand the physics and measurement procedures, such as done here in several places. Its use for obtaining numerical results is criticised. Instead, three integral measures of impact are proposed, all of which use surface elevation measurements and which require no essential mathematical approximations or wave-by-wave analysis. The methods are applied to a study of ship waves on the Danube River. A number of results are presented, including examination of the effects of measurement frequency. After a ship passage, due to repeated shoreline reflections of the wake waves, the river is brought into a long-lasting state of short-crested disturbances, with finite fluid velocities. The environmental consequences of this might be important. After the primary and secondary ship waves it could be called the tertiary wave system.     

各种扰动源产生孤立子波的研究

本文就四种不同形式的扰动在浅水中生成孤立子波的问题进行了研究。这里采用的理论基础是Boussinesq型非线性色散长波模型。非线性偏微分方程组按初-边值问题用有限差分法数值求解,这里包括对开边界条件的近似处理。 数值计算实例给出了四种不同扰动源生成孤立子波的条件和规律。由对计算结果的分析比较,可以发现当外界与流体间有一定方式的质量、动量或能量(动能、位能)交换时,只要满足一定的条件均能生成孤立子波。     

Scour around submarine pipes due to high-amplitude transient waves

Estimation of scour dimensions below submarine pipelines is a vital step in designing offshore infrastructure. Extreme events like tsunami waves produce strong erosive forces below the underwater pipes, apt to create scour holes, jeopardizing the safety of the structure. Despite the importance of this issue, previous studies have mainly focused on steady flow cases, and the scour pattern below submarine pipes induced by high-amplitude transient waves has rarely been investigated. This paper reports the results of 40 experimental runs on transient wave-induced scour below a model pipe in a laboratory flume under a variety of initial conditions. The variables included the bed particle size and gradation,initial water depth, wave height, and slope of the bed layer. Waves were generated by a sudden release of water from a sluice gate, installed in the middle of the flume. A pressure transducer data acquisition system was used to record the wave heights at different time steps. The results indicate that, with a shallower initial depth of flow, the scour depth is relatively large. It was also found that there exists a direct correlation between the induced wave height and the size of the scour hole. It was observed that, in clear water conditions, the size of the scour hole in coarse sediments is smaller, while in live-bed conditions, larger scour holes are created in coarser sediments. It was also observed that at high wave amplitudes, the live-bed conditions are dominant, and consequently the bed elevation is altered.