Modelling of wave transmission through a pneumatic breakwater

A theoretical approach is derived to study interaction of linear water waves with an air bubble curtain used as a pneumatic breakwater. Modelling of wave transmission through an aerial barrier is a complex task due to a need to cover processes associated with wave-current interaction, effects of two-phase flows, wave damping, etc.. An initial boundary-value problem is solved by applying an efficient eigenfunction expansion method and a time-stepping procedure. The derived semi-analytical solution is used to study the effect of basic parameters of the model on wave dissipative properties of the pneumatic breakwater. Results show that wave damping by the breakwater is mainly affected by an air flow rate. The increased air discharge results in higher velocities of ascending bubbles and increases aerial barrier width. This leads to a substantial reduction of transmitted wave heights, especially for waves of intermediate length and short waves. In order to verify the applicability of the presented theoretical approach, laboratory experiments are conducted in a wave flume for different wave regimes and pneumatic breakwater characteristics. The analysis of a wave transmission coefficient calculated numerically and measured in the laboratory confirms that the derived model can be used for a certain range of wave conditions.     

斜坡上梯形潜堤附近流动结构的数值研究

波浪通过斜坡上潜堤时的波形和非线性特性均有显著变化,同时潜堤附近会出现涡流的生成及消散现象,有必要对其深入研究.基于1:20的斜坡上的波浪水槽实验,分析规则波在斜坡上不可渗梯形潜堤附近的动力特性.本文垂向二维波浪数学模型以RANS方程为控制方程,采用非线性涡黏性(各向异性) 紊流传输模型封闭雷诺应力项,利用VOF方法追踪自由表面以及采用源函数造波技术来模拟规则波对斜坡上潜堤的作用,数值计算结果与实验测量数据进行对比,吻合良好.利用数值计算进一步分析了流场、流线、紊动动能及紊动动能耗散率等特性,重点探讨了堤顶水深变化的影响,得到随堤顶水深增大,潜堤的消浪作用将明显减弱.     

柔性浮式防波堤消浪特性研究

针对圆柱形柔性浮式防波堤的消浪效果展开断面试验研究,分析其在规则波和不规则波作用下的消浪特性。通过大量水槽试验给出了圆柱形柔性浮式防波堤透射系数、反射系数、衰减系数与锚固方式、试验水深、相对宽度的变化关系。研究结果表明:该结构形式的浮式防波堤对短周期波浪消浪效果明显,而对长周期波浪消浪效果不佳;锚固方式和水深对消浪效果的影响较小;相对宽度是影响消浪效果的主要因素。研究成果可为由该结构连接成的"V"形浮式防波堤的锚固方式和摆放角度的设计提供参考。     

EXPERIMENTAL STUDY ON A NEW KIND OF OPEN TYPE BREAKWATERS

A new kind of open type breakwaters was proposed based on the concept of wave energy conversion. The excellent feature of the structure is that it can absorb and utilize the wave energy to dissipate waves. The effects of some factors, such as the configuration insides, the geometrical parameters and the relative width (the ratio of the width to the wave length), on the performance of the breakwater were discussed based on a series of regular wave tests in the wave flume. The hydrodynamic characteristics of the new open type breakwater were analyzed in detail, and the structure was optimized to improve the performance of the breakwater to effectively dissipate the waves with long period. The variation of the coefficients of wave transmission and reflection along with attenuation with respect to relative width was examined. The results of the physical model tests indicate that the transmission coefficient significantly decreases by about twenty percent at the range of relative width from 0. 1 to 0.2, compared with the traditional open type breakwaters.     

水平斜插板透空式防波堤消波性能数值模拟

通过改进已有的防波堤模型，建立了一种新型水平斜板透空式防波堤模型。基于VOF方法建立了波浪与水平斜板防波堤相互作用的数值模型。经数值模拟研究了该新型水平斜板防波堤结构对波浪透射系数的影响，主要考虑了斜板角度、波陡、相对板间距、相对板宽和相对出水高度等因素对防波堤消波效果的影响。模拟结果显示:①水平斜插板透空式防波堤比水平板透空式防波堤的消波效果好。在斜插板作用下，随着斜板角度增大，透射系数逐渐增大，但增幅逐渐减小；②随着波陡增大，透射系数先减后增；③随着水平板间距增加，透射系数增大；④随着防波堤相对板宽的增大，透射系数逐渐减小；⑤随着防波堤出水高度增加，透射系数先减后增。结果表明，该新型结构防波堤的消波效果显著，可以很好地消减波浪，应用前景良好。     

Effect of the PTO damping force on the wave pressures on a 2-D wave energy converter

The information of the wave loads on a wave energy device in operational waves is required for designing an efficient wave energy system with high survivability. It is also required as a reference for numerical modeling. In this paper, a novel system, which integrates an oscillating wave energy converter with a pile-restrained floating breakwater, is experimentally investigated in a 2-D wave flume. The measurements of the wave pressure on the wet-surface of the device are made as the function of the power take-off(PTO) damping force. It is shown that the wave pressure is significantly affected by the PTO system, in particular, at the edges, and the wave pressure varies under different wave conditions. From the results, conclusions can be drawn on how the PTO damping force and wave conditions affect the loads on the device, which is of engineering concern for constructing safe and reliable devices.     

新型透空组合板式防波堤结构型式及消浪特性分析

为提高透空式防波堤的消浪性能并减小结构受力,在前人研究基础上提出了一种新型透空组合板式防波堤,比较分析了该型式的防波堤(模型c)与另外两种型式的防波堤(模型a和模型b)的结构特点。通过物理模型试验,探讨了模型c在不同影响因素下的消浪特性,并对比分析了模型a与模型c的透射系数、反射系数和能量耗散系数在不同相对堤宽下的变化关系,比较了两种模型的消浪效果。分析得出,由于模型c兼具开孔水平板式防波堤和开孔挡板式防波堤的作用,模型c的波浪反射小于模型a,消浪效果优于模型a,且模型c这种结构能有效减小防波堤结构受力;当模型宽度和相对间隙比同时较大时消浪效果更好,防波堤出水后的消浪性能要优于淹没状态下的防波堤。     

气幕防波堤消波性能影响因素数值模拟研究

为研究气幕防波堤消波性能影响因素,将空气和水组成的两相流体处理为变密度单流体,通过UDF在连续方程中添加质量源项和动量方程中添加动量源项建立了气幕防波堤的数学模型,并将数值模拟结果与试验结果进行对比分析,二者具有较好的一致性,验证了气幕防波堤数学模型建立的合理性。以此为基础,对影响气幕防波堤消波性能的因素如入射波高、管道淹没深度进行了数值模拟计算,并通过分析、比较得出相关结论。这些结论对气幕防波堤的实际工程设计、施工等具有一定参考价值和指导意义。     

NUMERICAL SIMULATION OF PROPAGATION AND BREAKING PROCESSES OF A FOCUSED WAVES GROUP

A two-dimensional numerical wave flume is developed to study the focused waves group propagation and the consequent breaking processes. The numerical model is based on the Reynolds-Averaged Navier-Stokes (RANS) equations, with the standard k-ε turbulence model to simulate the turbulence effects. To track the complicated and broken free-surface, the Volume Of Fluid (VOF) method is employed. The numerical model combines the "Partial Cell Treatment (PCT)" method with the "Locally Relative Stationary (LRS)" concept to treat the moving wave paddle so that various waves can be generated directly in a fixed Cartesian grid system. The theoretical results of the linear and nonlinear waves are used to validate the numerical wave flume firstly, and then a plunging breaking wave created by a focused waves group is simulated. The numerical results are compared to the experimental data and other simulation results, with very good agreements. The turbulence intensity, the flow field and the energy dissipation in the breaking processes are analyzed based on the numerical results. It is shown that the present numerical model is efficient and accurate for studying the waves group generation, the waves packet propagation, and the wave breaking processes.     

数字波浪槽在防波堤预设计中的应用

传统的以物理模型进行的防波堤断面实验被以计算机进行的数字波浪槽代替,边界造波为Bretschneider/Mit-suyasu波浪谱,防波堤断面根据海港水文规范(简称“规范”)设计。以改进的Boussinesq波浪方程,通过引入波浪反射项以数值模拟实现了波浪在水槽中传播的全过程。通过数值水槽获得了波浪在本文防波堤断面前0.45的反射系数,将数值模拟的结果和规范中的反射系数进行了对比得出,数字波浪水槽有满意的精度以及在规范中使用了一定的安全系数,表明数字波浪槽在防波堤的预设计中有重要作用。     

规则波作用下潜堤透浪系数数值模拟

潜堤透浪系数是衡量潜堤削弱波浪能力的主要指标。基于Boussinesq型方程的MIKE21-BW模型建立波浪数值水槽,进行规则波作用下潜堤透浪系数的数值模拟。根据Dingemans物理模型试验结果对BW模型进行参数率定和验证,验证结果良好。在此基础上,进一步分析了影响潜堤透浪系数的无因次影响参数,包含相对淹没深度、波陡、相对堤顶宽度、斜坡堤坡度等,研究不同因素组合情况下潜堤透浪系数的变化规律。建立了规则波作用下潜堤透浪系数的计算式,应用MATLAB数学软件对式中的系数进行拟合,拟合式计算值与BW模型计算值相关系数为0.858,相关性较好。另外,将拟合式与杨正己公式进行比较,并分析数据的差异与原因。研究表明:MIKE21-BW模型可以进行规则波作用下潜堤透浪系数的数值模拟。     

基于SWASH模型的近岸波浪传播变形数值模拟

SWASH模型是一种新型的非静压时域波浪模拟。为了探讨SWASH模型对于解决近岸波浪传播变形问题的适用性,在对其控制方程、边界条件、数值解法等进行介绍的基础上,采用该模型分别模拟了正向规则波、斜向规则波和斜向不规则波入射条件下L形防波堤附近水域的波浪场和波生流场,并与物理试验结果进行对比。结果表明,SWASH模型较好地复演了波浪在近岸区域所发生的浅水变形、折射、破碎,以及堤前反射、堤内绕射等物理现象,波高沿断面的定量分布与试验结果吻合良好,同时较好地模拟了不同波况下防波堤附近水域的波生流场,说明该模型适用于复杂岸线和地形条件下波浪传播变形的数值模拟。     

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.     

NUMERICAL SIMULATION OF TWO-DIMENSIONAL OVERTOPPING AGAINST SEAWALLS ARMORED WITH ARTIFICIAL UNITS IN REGULAR WAVES

A new mathematical model for the overtopping against seawalls armored with artificial units in regular waves was established. The 2-D numerical wave flume, based on the Reynolds Averaged Navier-Stokes (RANS) equations and the standard k-ε turbulence model, was developed to simulate the turbulent flows with the free surface, in which the Volume Of Fluid (VOF) method was used to handle the large deformation of the free surface and the relaxation approach of combined wave generation and absorbing was implemented. In order to consider the effects of energy dissipation due to the armors on a slope seawall, a porous media model was proposed and implemented in the numerical wave flume. A series of physical model experiments were carried out in the same condition of the numerical simulation to determine the drag coefficient in the porous media model in terms of the overtopping discharge. Compared the computational value of overtopping over the seawall with the experimental data, the values of the effective drag coefficient was calibrated for the layers of blocks at different locations along the seawalls.     

游泳水槽的气泡运动规律及去除方法研究

游泳水槽作为一个先进的工具，可有效地提高竞技游泳水平。目前游泳水槽建造技术尤其是对水流的控制技术已趋于成熟，但水槽内的气泡将会影响对运动员技术动作观测和图象解析，并对运动员游泳姿态测试、分析和改进带来困难。本文通过对游泳水槽结构及水槽中水流流动状况分析得到了气泡产生的主要原因，并研究了气泡运动的规律，提出了减少气泡产生的合理方法以及去除气泡的合理设计，为现有的游泳水槽改进提供良好方案，为今后自主设计先进的游泳训练用水槽提供技术支撑。     

缓坡方程的有限分析数值模式

基于时间关联的双曲型缓坡方程,采用九点有限分析法,建立近岸波浪传播变形的数值模式,在对时间关联的双曲型缓坡方程数值离散时,空间导数采用九点有限分析法,时间导数仍采用有限差分法,再采用质量集中方法得到本文的数值模式。为了验证本模式的数值精度和有效性,分别对Berkhoff(1972)实验和突堤地形进行数值模拟,结果表明本模式的模拟值与物理模型试验值、解析解及有限差分模式的模拟值吻合良好,表明本模式可以对近岸波浪传播过程中折射、绕射、反射、浅水变形、波浪破碎和弱非线性等因素的影响进行较好的预测。     

波浪通过斜坡上潜堤的波能演化实验研究

潜堤是防止海岸侵蚀的常用护岸建筑物,准确的掌握潜堤前后波浪的传播变形规律十分必要.该文以斜坡上梯形潜堤为研究对象,基于波浪水槽实验,通过谱分析探求线性波、非线性波、非规则波通过斜坡上潜堤内部组成能量的变化规律,分析相同的入射波情况下波浪倍频能量演变与堤顶水深大小的关系,得出堤顶水深越小主频能量衰减越大的规律.同时,比较研究线性波与非线性波作用下潜堤前后波能的变化,发现非线性波高倍频能量产生更加剧烈.     

Experimental research on unstable movements of sandbars under wave actions

This study is motivated by recognition of complex sandbar evolution patterns under wave actions inside the surf zone. A series of physical model experiments were conducted in a wave flume to investigate sandbar migration under various wave conditions, including wave groups, regular waves, and random waves. It was observed that under certain wave conditions sandbars move alternately shoreward and seaward rather than continuously in the same direction. The measurements show that the unstable movement of sandbars is closely related to the amplitude modulation of waves. Smaller amplitude modulation tends to produce more intense unstable bar movements. Data analysis further shows that the sandbar migration does not seem to be a passive response of the sea bed to wave forcing, but is most likely caused by the feedback interaction between waves and bed topography.     

三孤立波的斜坡爬高特征

该文采用推板式造波机和改进的Goring造波方法在波浪水槽中生成了由三个等波高孤立波组成的波列,首先在坡度为1/10的斜坡上进行了多个孤立波连续爬高的实验研究。实验测量了等波高双孤立波和三孤立波在等水深波浪水槽中传播和在斜坡上的爬高过程,给出了沿程多点浪高仪测量的波面时间序列和高速摄像记录的水线在斜坡上爬高与回落过程影像。分析了来波相对波高和无量纲波峰间距对每个孤立波爬高放大系数的影响。在基于RANS方程和VOF方法的数值波浪水槽中,模拟了三孤立波爬高过程,给出了最大爬高与破碎特征。实验和数值模拟结果表明,孤立波在斜坡上回落时会产生强劲的回流水流,该反向流动导致后随孤立波破碎,消耗波动能量,使得后随孤立波的爬高放大系数明显小于没有回落水流影响的首个孤立波的爬高放大系数。当孤立的波峰间较大时,若来波相对波高较小,后两个的孤立波爬高基本相同;若来波相对波高较大时,第三个孤立波的爬高大于第二个孤立波的爬高。     

采用乳化空气泡做示踪粒子的PIV测速技术

本文介绍一种适用于水洞力学实验的采用乳化空气泡作为示踪粒子的ＰＩＶ测速技术。选用乳化空气泡做ＰＩＶ测速示踪粒子，取代了通常采用的固体颗粒，由两台ＹＡＧ大功率脉冲式激光光源组合系统做为ＰＩＶ底片记录的双脉冲曝光光源。乳化空气泡的最大优点是对实验水洞或水槽无污染，且产生方便、经济，又可以形成高质量的杨氏条纹图像。这一技术能够使得ＰＩＶ测速技术在水洞力学应用测量中真正达到实用。