Total horizontal and vertical forces of irregular waves on partially perforated caisson breakwaters

In this study, the total horizontal and vertical forces as well as the phase differences of irregular waves on a partially perforated caisson breakwater are investigated. The partially perforated caisson is located on a rubble fill foundation and filled with rock. Based on linear potential theory, a simple semi-analytical solution of the present problem is developed by means of the matched eigenfunction method and the finite element method. Experimental tests are also conducted to validate the theoretical model and examine the wave forces acting on the perforated caisson. The effects of some of the main factors on the total wave forces and the phase difference are examined. Theoretical and experimental studies show that when the total horizontal force reaches its peak, the simultaneous total vertical (upward) force is rather small or even becomes downwards. This is due to the existence of an obvious phase difference between the time histories of the total horizontal and vertical forces, which is an important advantage of perforated caissons over traditional structures.     

The irregular broken wave forces on vertical wall

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Viscous drift forces in regular and irregular waves

A method to compute wave- and current-induced viscous drift forces and moments on floating platforms in regular and random waves is presented. The relative velocity drag term of Morison's equation is used in conjunction with frequency domain first-order motion transfer functions to compute the drift forces and moments. Mean viscous drift forces and moments in regular waves in all six degrees-of-freedom of a tension leg platform are computed. The relative importance of the free-surface force integration, steady current, wave-current interaction and platform motions on the computed drift forces and moments are discussed. The results from this method, in the frequency domain, are used to compute the drift forces and responses in irregular waves using existing methods developed for potential drift computations. Comparisons with results from time-domain computations are also presented and good agreement between the frequency-domain and time-domain results is found. Some comparisons with experimental data are also made. The frequency-domain method is found to be an efficient and useful tool for the analysis of semi-submersible and tension leg platforms during the preliminary design stage in which extensive parametric studies need to be undertaken.     

直立堤前海床的局部冲刷

对不规则波作用下直立堤前海床的局部冲刷进行了研究。研究了由于波浪作用导致床面的底沙运动,从而造成提前海床的局部冲刷,即所谓的“相对粒沙”的情况。报导了;(1)堤前海床面为水平、堤下无基床情况下的堤前局部冲刷,以及堤下有明基床时基床尺度对堤前局部冲刷的影响;(2)堤前海床面为斜坡面时堤前的局部冲刷,以及当堤下有明基床时基床尺度对堤前局部冲刷的影响;(3)从波浪作用下床面边界层的理论分析出发,对于在不规则波作用下,离堤面愈远处的冲刷坑深度愈小的事实给出了解释。     

Linear and nonlinear irregular waves and forces in a numerical wave tank

A time-domain higher-order boundary element scheme was utilized to simulate the linear and nonlinear irregular waves and diffractions due to a structure. Upon the second-order irregular waves with four Airy wave components being fed through the inflow boundary, the fully nonlinear boundary problem was solved in a time-marching scheme. The open boundary was modeled by combining an absorbing beach and the stretching technique. The proposed numerical scheme was verified by simulating the linear regular and irregular waves. The scheme was further applied to compute the linear and nonlinear irregular wave diffraction forces acting on a vertical truncated circular cylinder. The nonlinear results were also verified by checking the accuracy of the nonlinear simulation.     

斜向不规则波对直墙作用的实验研究

通过长峰不规则波和方向波谱对直墙作用的实验研究 ,分别给出了每延米斜向不规则波波浪力与正向力之比 ,方向波谱波浪力与斜向长峰不规则波波浪力之间的关系 ,通过与规则波实验结果的比较给出了斜向规则波与不规则波波浪力之间的相对关系 ;并对斜向不规则波的反射系数与正向波时的变化作出了分析     

斜向和多向不规则波对直立堤平均越浪量研究

通过三维波浪模型试验研究了斜向和多向不规则波对直立堤的越浪量。分别按平均越浪量和单波最大越浪量进行研究,探讨了平均越浪量随相对堤高、波浪方向、波浪方向分布宽度、波陡和相对水深等影响因素的变化规律,导得了斜向和多向不规则波作用于直立堤上的平均越浪量的计算公式。     

High-frequency variability of vertical distribution of suspended sediment concentration under irregular waves

Based on the data of a field experiment in a storm environment, we examined the specific features of the variability of the vertical profiles of the concentration of suspended sand sediments at a discreteness shorter than the wave period. Physical mechanisms that determine the processes of suspension and shape the types of vertical distributions of sediments are considered. A mathematical model for the prediction of vertical distributions of the suspended sediments governed by irregular wave forcing is developed. The model is verified from data of field and laboratory experiments.     

Wave forces acting on a submerged horizontal circular cylinder in oblique waves and on a vertical cylinder in deep waves

This paper presents a method of estimating wave forces acting on a submerged horizontal circular cylinder fixed in oblique waves.The experiments show that drag and inertia coefficients in beam sea are available for calculating the wave forces in oblique waves.Wave forces exerted on a vertical circular cylinder in deep waves are also investigated.The experimental results show that wave forces acting on the vertical cylinder coincide approximately with hydrodynamic forces acting on a submerged circular cylinder in an oscillating fluid.     

破碎波冲击直立桩柱的大比尺试验研究

波浪破碎是海洋中最常见的现象之一,其能够对海洋中的结构物产生巨大的波浪力作用。本文在大比尺波浪水槽通过聚焦波的方法生成了极端波浪和不同破碎阶段的破碎波浪,并对其冲击桩柱过程中的点压力进行了测量,进而采用连续小波变换的方法,对桩柱上点压力的分布及大小进行了细致分析。结果表明,多次重复试验下,相比非破碎极端波浪,破碎极端波浪产生的点压力离散性更强;波浪破碎程度越大,测点位置越靠近波峰,则点压力离散程度越大;破碎波的最大点压力出现在1.2倍的最大波面附近,且其大小可达3倍的最大静水压力;基于点压力小波谱,不同破碎阶段破碎波产生冲击作用不同,对于波浪作用桩柱前波浪已经发生破碎的情况,其冲击区域更大,点压力分布更复杂;而对于桩面破碎的情况,其造成的波浪总力更大。     

Active heave compensation on drill-ships in irregular waves

This paper investigates a possible method for heave compensation on board deep-water drill-ships subjected to irregular-wave excitation. The proposed system exploits favorable interaction of coupled oscillators to achieve the desired results. This study examines an actively controlled compensator which performs well over a large wave-frequency bandwidth. Performance under certain operating conditions is investigated using a dynamic model. Simple mathematical arguments and frequency-domain computations in an irregular wave spectrum show the proposed heave compensation system to be effective within the bounds of linearity.     

Numerical calculation of forces induced by short-crested waves on a vertical cylinder of arbitrary cross-section

Most off-shore oil platforms are supported by vertical cylinders extending to the ocean floor. An important problem in off-shore engineering is the calculation of the wave loading exerted on these vertical cylinders. Analytical solutions have been found for the case of plane incident waves incident on a circular cylinder by MacCamy and Fuchs [(1954), Wave forces on piles: a diffraction theory. U.S. Army Corps of Engineering, Beach Erosion Board, Technical Memorandum No. 69] and also for short-crested waves incident on a circular cylinder by Zhu [(1993), Diffraction of short-crested waves around a circular cylinder. Ocean Engng 20, 389–407]. However, for a cylinder of arbitrary cross-section, no analytic solutions currently exist. Au and Brebbia [(1983), Diffraction of water waves for vertical cylinders using boundary elements. Appl. Math. Modelling 7, 106–114] proposed an efficient numerical approach to calculate the wave loads induced by plane waves on vertical cylinders by using the boundary element method. However, wind-generated waves are better modelled by short-crested waves. Whether or not these short-crested waves can induce larger wave forces on a structure is of great concern to ocean engineers. In this paper wave loads, induced by short-crested incident waves, on a vertical cylinder of arbitrary cross-section are discussed. For a cylinder of certain cross-section, the wave loads induced by short-crested waves can be larger than those induced by plane waves with the same total wave number.     

不规则波作用下垂直挡板式透空堤透浪系数试验研究

垂直挡板式透空堤作为一种新型的透空式防波堤结构,通过将挡浪板垂直设置于波能最集中的水体表层来消减波浪,透浪系数是其最关键的指标。通过物理模型试验,分析不规则波作用下入射波高、波周期、挡板相对入水深度、相对堤宽、相对挡板超高、相对面板超高等因素对垂直挡板式透空堤透浪系数的影响规律,并在Wiegel公式的基础上拟合了垂直挡板式透空堤透浪系数的计算公式。可作为今后类似透空式防波堤结构透浪系数的近似估算,具有一定的参考价值。     

Characteristics of breaking irregular wave forces on a monopile

The substructures of offshore wind turbines are subjected to extreme breaking irregular wave forces. The present study is focused on investigating breaking irregular wave forces on a monopile using a computational fluid dynamics (CFD) based numerical model. The breaking irregular wave forces on a monopile mounted on a slope are investigated with a numerical wave tank. The experimental and numerical irregular free surface elevations are compared in the frequency-domain for the different locations in the vicinity of the cylinder. A numerical analysis is performed for different wave steepness cases to understand the influence of wave steepness on the breaking irregular wave loads. The wave height transformation and energy level evolution during the wave shoaling and wave breaking processes is investigated. The higher-frequency components generated during the wave breaking process are observed to play a significant role in initiating the secondary force peaks. The free surface elevation skewness and spectral bandwidth during the wave transformation process are analysed and an investigation is performed to establish a correlation of these parameters with the breaking irregular wave forces. The role of the horizontal wave-induced water particle velocity at the free surface and free surface pressure in determining the breaking wave loads is highlighted. The higher-frequency components in the velocity and pressure spectrum are observed to be significant in influencing the secondary peaks in the breaking wave force spectrum.     

Reproduction of higher harmonics in irregular waves

The basic features of higher harmonic waves were originally presented by Stokes in 1847 who showed that a 2nd-order regular wave would attain a higher, sharper crest and a flatter trough than the basic linear one. In this paper, these characteristics are investigated for an irregular wave train. This was done to make it possible to reproduce in a laboratory flume a wave train which has the correct profile and rms according to 2nd-order theory. The 2nd-order wave heights and the overall horizontal asymmetries of the wave train were calculated for a 1st-order PM and a Jonswap spectrum. The theoretical predictions show that the traditional wave generation in a laboratory flume which is based on linear theory produces free 2nd-order waves in addition to the bound 2nd-order waves described by Stokes. These free, parasitic waves incorrectly distort the shape of the waves and produce a varying rms as a function of distance from the wave paddle. In the 2nd-order theory presented in this paper, these free (parasitic) waves are calculated for the entire spectrum. Knowing these, they can be eliminated by compensating for them by using a 2nd-order control signal on the wave paddle. This is verified by a series of model tests which show that realistic wave profiles can be generated only by means of 2nd-order control signals. The theory and corresponding paddle control are presented for any type of translating, rotating or combined wave generator.