Statistics of Wave Crest Characteristics

The statistical distribution of wave crest characteristics such as crest length, crest height, joint crest height and length are analyzed based on numerical simulation of 3-D random waves. The effects of directional functions and wave crest defining methods on crest characteristics are also studied. The results show that wave crests are no longer uniform and continuous in directional wave field; the distribution of crest length is obviously influenced by the directional function; the statistics of crest characteristics obtained by the two different methods are almost the same.

Numerical Simulation of 3-D Wave Crests

A clear definition of 3-D wave crest and a description of the procedures to detect the boundary of wave crest are presented in the paper. By using random wave theory and directional wave spectrum, a MATLAB-platformed program is designed to simulate random wave crests for various directional spectral conditions in deep water. Statistics of wave crests with different directional spreading parameters and different directional functions are obtained and discussed.

Estimation of Wave Crest Amplitudes Distribution and Freak Wave Occurrence in A Short Crested Mixed Sea

In this study we have for the first time proposed a novel transformed linear simulation method for the estimation of wave crest amplitudes distribution and freak wave occurrence in a short crested mixed sea with a bimodal 3D spectrum. For implementing the proposed transformed linear simulation method, a Hermite transformation model expressed in a monotonic cubic polynomial has been constructed so that the first four moments of the original true process match the corresponding moments of the transformed model. The proposed novel simulation method has been applied to forecast the freak wave occurrence in two short crested mixed sea states, one with a directional wave spectrum based on the measured surface elevation data at the coast of Yura, and the other one with a typical directional bimodal Torsethaugen wave spectrum. It is shown in the two cases that the proposed novel simulation method can offer more accurate forecasting results than those obtained from the traditional linear simulation method or by using Rayleigh distribution model. It is also demonstrated in this article that the proposed novel simulation method is more efficient than the nonlinear simulation method.

A Novel Method for Forecasting Freak Wave Occurrence in A Short Crested Coastal Sea

In order to forecast the distribution of crest amplitudes and the occurrence of freak waves in a short crested coastal sea,a novel transformed linear simulation method is initially proposed in this paper.A Hermite transformation model expressed as a monotonic cubic polynomial serves as the foundation for the novel simulation technique.The wave crest amplitude exceedance probabilities of two sea states-one with a directional wave spectrum based on the measured wave elevation data at the Yura coast and the other with a typical directional JONSWAP wave spectrum-have been predicted using the novel simulation method that has been proposed.The likelihood that a particular critical wave crest amplitude will be exceeded is directly correlated with the probability that freak waves will occur.It is shown that the novel simulation approach suggested can provide predictions that are more precise than those obtained from the Rayleigh crest amplitude distribution model,the Jahns and Wheeler crest amplitude distribution model,or the conventional linear simulation method.This study also demonstrated that the nonlinear simulation method is less effective than the novel simulation method in terms of efficiency.

Crest-Height Distribution of Nonlinear Random Waves

The wave crest is an important factor for the design of both fixed and floating marine structures.Wave crest height is a dominant parameter in assessing the likelihood of wave-in-deck impact and resultant severe damage.Many empirical and theoretical distribution functions for wave crest heights have been proposed,but there is a lack of agreement between them.It is of significance to develop a better new nonlinear wave crest height distribution model.The progress in the research of wave crest heights is reviewed in this paper.Based on Stokes' wave theory,an approximate nonlinear wave crest-height distribution formula with simple parameters is derived.Two sets of measured data are presented and compared with various theoretical distributions of wave crests obtained from nonlinear wave models and analysis of the comparison is given in detail.The new crest-height distribution model agrees well with observations.Also,the new theoretical distribution is more accurate than the other methods cited in this paper and has a greater range of applications.

Portable Dynamic Positioning Control System on A Barge in Short-Crested Waves Using the Neural Network Algorithm

This paper develops a nonlinear mathematical model to simulate the dynamic motion behavior of the barge equipped with the portable outboard Dynamic Positioning （DP） system in short-crested waves. The self-tuning Proportional- Derivative （PD） controller based on the neural network algorithm is applied to control the thrusters for optimal adjustment of the barge position in waves. In addition to the wave, the current, the wind and the nonlinear drift force are also considered in the calculations. The time domain simulations for the six-degree-of-freedom motions of the barge with the DP system are solved by the 4th order Runge-Kutta method which can compromise the efficiency and the accuracy of the simulations. The technique of the portable alternative DP system developed here can serve as a practical tool to assist those ships without being equipped with the DP facility while the dynamic positioning missions are needed.

The patterns of surface capillary-gravity short-crested waves with uniform current fields in coastal waters

A fully three-dimensional surface gravitycapillary short-crested wave system is studied as two progressive wave-trains of equal amplitude and frequency, which are collinear with uniform currents and doubly-periodic in the horizontal plane, are propagating at an angle to each other. The first- and second-order asymptotic analytical solutions of the short-crested wave system are obtained via a perturbation expansion in a small parameter associated with the wave steepness, therefore depicting a series of typical three-dimensional wave patterns involving currents, shallow and deep water, and surface capillary waves, and comparing them with each other.

Laboratory Research on Effective Test Area of Short-Crested Waves Generated by Two-Sided Segmented Wavemakers

The size and shape of the effective test area are crucial to consider when short-crested waves are created by segmented wavemakers. The range of the effective test area of short-crested waves simulated by two-sided segmented wavemakers is analyzed in this paper. The experimental investigation on the wave field distribution of short-crested waves generated by two-sided segmented wavemakers is conducted by using an array of wave gauges. Wave spectra and directional spreading function are analyzed and the results show that when the main direction is at a certain angle with the normal line of wave generators, the wave field of 3D short-crested waves generated by two-sided segmented wavemakers has good spatial uniformity within the model test area. The effective test area can provide good wave environments for seakeeping model tests of various ocean engineering structures in the deep ocean engineering basin.

Effect of relaxation times on circular crested waves in thermoelastic diffusive plate

The propagation of circularly crested thermoelastic diffusive waves in an infinite homogeneous transversely isotropic plate subjected to stress free, isothermal/insulated and chemical potential conditions is investigated in the framework of different thermo- elastic diffusion theories. The dispersion equations of thermoelastic diffusive Lamb type waves are derived. Some special cases of the dispersion equations are also deduced.

Short-Crested Waves Interaction with A Concentric Porous Cylinder System with Partially Porous Outer Cylinder

In this paper, based on the linear wave theory, the interaction of short-crested waves with a concentric dual cylindrical system with a partially porous outer cylinder is studied by using the scaled boundary finite element method （SBFEM）, which is a novel semi-analytical method with the advantages of combining the finite element method （FEM） with the boundary element method （BEM）. The whole solution domain is divided into one unbounded sub-domain and one bounded sub-domain by the exterior cylinder. By weakening the governing differential equation in the circumferential direction, the SBFEM equations for both domains can be solved analytically in the radial direction. Only the boundary on the circumference of the exterior porous cylinder is discretized with curved surface finite elements. Meanwhile, by introducing a variable porous-effect parameter G, non-homogeneous materials caused by the complex configuration of the exterior cylinder are modeled without additional efforts. Comparisons clearly demonstrate the excellent accuracy and computational efficiency associated with the present SBFEM. The effects of the wide range wave parameters and the structure configuration are examined. This parametric study will help determine the various hydrodynamic effects of the concentric porous cylindrical structure.

Estimation of JONSWAP Spectral Parameters by Using Measured Wave Data

- This paper presents the method of determining JONSWAP spectrum by using measured wave data. If Hs, Tz and Tc are the measured significant wave height, average zero-up crossing wave period and average period between wave crests respectively and let y = T Z / TC; this paper provides equation to solve y fromf(y,y) = 0. From the solutions of this equation and by using LSM, the expression relating y as a function of y (for 2.54<y< 15.34 and 1.6 <y < 1,79) may be written as y -5546.721 - 9586.533y + 5568.168/- 1089/+ 2/, for other intervals the related formulas are also given. When y is known, the rest of parameters in JONSWAP spectrum can be obtained. In addition, this paper also provides alternatives for determining JONSWAP spectral parameters by using Hs, Tz and (, or Hs, Tz and S(w0) or other three given data. The JONSWAP spectra given in this paper satisfy the following formulas HS= 4.0 = 2 Tc = 2

细水雾抑制石化污水系统内油气爆炸的实验与模拟

为研究石化污水系统内细水雾抑制油气爆炸的冲击规律,搭建10 m×1.5 m×1.5 m真实尺度污水系统模型,并开展细水雾抑制油气燃爆实验,结合数值模拟方法研究了细水雾覆盖区域、细水雾喷雾流量、可燃气初始浓度等因素爆炸冲击的影响。结果表明,通道前端覆盖细水雾使得腔体峰值超压明显增强,后端覆盖细水雾腔体内部爆炸超压被显著抑制;随着水雾起始位置距点火点距离D的增加,通道内爆炸超压峰值出现时间明显延后,且爆炸峰值超压逐渐减弱;腔体前端施加细水雾时,随着喷雾流量的增加爆炸超压峰值出现时间明显提前,且爆炸峰值超压逐渐增加,而后端施加细水雾时,规律相反;细水雾施加下,随着可燃气初始燃料配比ER增加,通道内爆炸超压峰值呈现先增后减、超压峰值出现时间先减小后增大。

经验小波变换的激光超声定量识别表面裂纹

针对增材制造检测领域中激光超声检测技术定量识别金属增材制件表面裂纹时,激光超声信号具有多模态混叠严重、波形复杂和信噪比低等问题,提出了一种经验小波变换分解激光超声信号,并使用表面波的缺陷回波波峰与波谷差值技术进行金属增材制件表面裂纹定量识别。分析激光超声信号的时频特性,利用经验小波变换自适应分解并提取激光超声信号中的表面波模态。对原始信号和经验小波变换提取的表面波信号在有无裂纹反射回波的时间范围内波峰与波谷差值变化进行分析,绘制原始信号和表面波信号的扫查位置——波峰与波谷差值图。经对比发现,后者能更好地确定裂纹长度的起始和结束位置,其裂纹检测结果的绝对误差不超过0.4 mm,相对误差不超过6.00%。此方法在激光超声信号的缺陷特征提取与定量识别方面有效可行,为复杂增材制造环境中对金属增材制件表面裂纹进行非接触检测提供了有力工具。

允许越浪斜坡式防波堤顶高程优化研究

基于南海应急抢险打捞保障基地工程对允许越浪斜坡式防波堤顶高程进行优化,在保证不恶化港内泊稳条件、码头结构和防波堤结构安全的前提下,最大限度降低堤顶高程以达到降低工程投资的目的。通过波浪断面物理模型试验模拟堤后次生波并与波浪数学模型推算的绕射波进行叠加,计算不同堤顶高程对应码头结构所受的波浪力,并通过有限元软件分析码头结构的安全性,同时通过波浪断面物理模型试验验证防波堤结构的稳定性。结果表明:对于码头和防波堤采用分离式布置的工程,防波堤防护要求可适当降低。堤顶高程按设计高水位+2 a一遇H13%基本不越浪控制,防波堤和码头结构安全可靠。

Rheological fluid motion in tube by metachronal waves of cilia

This paper presents a theoretical study of a non-linear rheological fluid transport in an axisymmetric tube by cilia. An attempt has been made to explain the role of cilia motion in the transport of fluid through the ductus efferent of the male reproductive tract. The Ostwald-de Waele power-law viscous fluid is considered to represent the rheological fluid. We analyze pumping by means of a sequence of cilia beats from rowto-row of cilia in a given row of cells and from one row of cells to the next(metachronal wave movement). For this purpose, we consider the conditions that the corresponding Reynolds number is small enough for inertial effects to be negligible, and the wavelengthto-diameter ratio is large enough so that the pressure can be considered uniform over the cross section. Analyses and computations of the fluid motion reveal that the time-average flow rate depends on ε, a non-dimensional measure involving the mean radius a of the tube and the cilia length. Thus, the flow rate significantly varies with the cilia length.Moreover, the flow rate has been reported to be close to the estimated value 6 × 10ml/h for human efferent ducts if ε is near 0.4. The estimated value was suggested by Lardner and Shack(Lardner, T. J. and Shack, W. J. Cilia transport. Bulletin of Mathematical Biology, 34, 325–335(1972)) for human based on the experimental observations of flow rates in efferent ducts of other animals, e.g., rat, ram, and bull. In addition, the nature of the rheological fluid, i.e., the value of the fluid index n strongly influences various flow-governed characteristics. An interesting feature of this paper is that the pumping improves the thickening behavior for small values of ε or in free pumping(?P = 0) and pumping(?P > 0) regions.

RAYLEIGH LAMB WAVES IN MICROPOLAR ISOTROPIC ELASTIC PLATE

The propagation of waves in a homogeneous isotropic micropolar elastic cylindrical plate subjected to stress free conditions is investigated. The secular equations for symmetric and skew symmetric wave mode propagation are derived. At short wave limit, the secular equations for symmetric and skew symmetric waves in a stress free circular plate reduces to Rayleigh surface wave frequency equation. Thin plate results are also obtained. The amplitudes of displacements and microrotation components are obtained and depicted graphically. Some special cases are also deduced from the present investigations. The secular equations for symmetric and skew symmetric modes are also presented graphically.

Experimental Study of Sand Bed Configurations in Front of a Vertical Wall Under Wave Action

When a 2-D progressive wave train normally or obliquely approaches a vertical wall and then is normally or obliquely reflected from it, the combination of the approaching and reflected waves may result in a standing wave or a short-crested wave in front of the wall. This paper presents the experimental observations of sand bed configurations under the action of these water waves in front of the wall. The geometry of sand ripples under these water waves in front of the vertical wall is presented as a function of flow parameters, such as the water particle semi-excursion and the mobility number.

短峰畸形波生成、演化过程的外部特征研究

在实际海况中,由于波浪组分方向不同,使海浪常表现为三维短峰波,与二维长峰波相比,有诸多异同特征。为了研究三维畸形波生成和演化过程中异常大波的外部特征以及大波间的关联,本文采用色散和方向聚焦方法在物理水池中模拟了短峰畸形波的整个生成和演化过程。研究结果表明,生成和演化过程中出现的异常大波沿着方向分布函数峰值方向演化,具有一定程度“对称”特征;根据异常大波的特征参数可将该过程分成3~4个典型的阶段,这意味着可以根据异常大波的特征参数判断其所处阶段及预测其演化趋势;三维畸形波生成和演化过程可能会跳过“波群”或者“深谷”阶段,与二维畸形波相比,时−空范围较小,这意味着在三维波浪场中,与畸形波相关的深谷和波群的发生概率较低。

圆筒形浮式生产储油装置在短峰波中水动力性能试验研究

采用不同方向分布参数的三维短峰波浪对圆筒形浮式生产储油装置(FPSO)迎浪方向的水动力性能开展模型试验研究,对圆筒形FPSO运动响应和系泊系统受力进行分析,并与长峰波浪中的试验结果进行对比.试验结果显示:在短峰波环境条件中,圆筒形FPSO的纵荡运动响应小于长峰波试验结果,横荡和横摇运动响应大于长峰波试验结果,系泊缆最大张力小于长峰波试验结果.在短峰波与长峰波环境条件中,圆筒形FPSO的水动力性能存在较为显著的差异.试验结果可为深入开展三维短峰波中海洋结构物水动力性能研究提供参考.

孤立波低顶海堤越浪的数值模拟研究

灾害性波浪是中国沿海地区最具破坏性的自然灾害之一。采用开源程序OpenFOAM中interFoam求解器,对低顶海堤(在风暴潮和海平面上升情况下所面临的不利工况)的孤立波越浪特性开展数值模拟研究。通过孤立波冲击海堤的基准算例,验证模型在模拟波浪爬升和越浪过程中大变形波面以及剧烈波浪力方面的精度。基于验证的数值模型,对孤立波在低顶海堤上的越浪特征以及防浪墙高度对越浪的影响开展参数化研究。结果表明堤顶超高减小导致更为剧烈的越浪。针对尚无低顶海堤孤立波越浪量经验公式的问题,提出新的适用于堤顶超高小或为0的孤立波越浪量经验公式。此外,研究发现增加防浪墙高度可有效减少越浪,但防浪墙所受的波浪力也增大。综合考虑防浪墙减少越浪以及自身所受波浪力,针对文中研究采用的海堤截面和波浪条件,建议无量纲防浪墙高度取为1.00。