Experimental Study of Coherent Structures in Breaking Waves

提出了碎波区破碎波相干结构的研究方法。本文的主要目的是测量波浪破碎在平面海滩上产生的间歇性相干湍流的流动特性。将研究各种破碎波条件，包括孤立波、规则波和不规则波。实验室实验将在南达科他州州立大学的二维波浪水槽中进行。水槽有树脂玻璃侧壁和底部延伸其全长，它可以从水平倾斜到3%的坡度。在本项目的第一部分中，将由安装在一端的可编程波浪发生器产生孤波，并使孤波沿水槽沿着在预定位置破碎。将测量整个碎波带的波浪高度，以确定详细速度测量的感兴趣区域。在对应于外、中、内碎波区的三个分离位置，利用立体粒子图像测速（PIV）系统测量破碎波流场的水平纵向、垂直纵向和垂直横向平面上的瞬时三维速度场。通过条件平均，将测量的流体速度场分解为系综平均、相干和非相干湍流分量。结果将用于了解大涡的演变及其运动学和动力学流动特性。将研究一个溢流式破碎器和两个倾翻式破碎器。对孤立波的实验研究将使我们能够了解在破碎波下大涡旋演变背后的基本物理学，而不受波生流和残余湍流的影响。本计画的第二部份将探讨规则波与不规则波下的相干结构。瞬时流体速度场的PIV测量将在碎波区内进行，用于溢出和倾翻规则波、主要为溢出破碎波的不规则波和主要为倾翻破碎波的不规则波。实验研究将集中在近床区域，以确定波生流、残余湍流和不规则波浪破碎对流场核心撞击床面的相干结构的发生频率、时间和性质的影响。拟议研究的影响源于波浪破碎在许多海岸过程中的重要性，如海滩侵蚀、泥沙输运、海岸淹没、波浪冲击和对岸边建筑物的影响。在碎波带中，波浪破碎过程一直是并且仍然是我们预测物理现象的能力很差的领域。对破碎波中的大涡旋的进一步了解与沉积物悬浮有关，并将反馈到改进的碎波带流体动力学和沉积物输运预测模型。拟议的研究还将提供一个高质量的数据集，以测试和改进碎波带波浪破碎过程的详细模型，包括目前的模型和未来使用的长期开发模型。

It is proposed to study coherent structures in breaking waves in the surf zone. The primary objective is to measure the flow properties of the intermittent, coherent turbulence produced by wave breaking on a plane beach. A wide range of breaking wave conditions including solitary waves, and regular and irregular waves will be studied. Laboratory experiments will be carried out in a two-dimensional wave flume at South Dakota State University (SDSU). The flume has Plexiglas sidewalls and bottom extending its full length and it can be tilted from horizontal to 3% slope. In the first part of this project, a solitary wave will be generated by a programmable wave generator installed at one end and caused to break at a predetermined location along the flume. Wave elevations across the surf zone will be measured to define regions of interest for detailed velocity measurements. At three separated locations corresponding to the outer, middle and inner surf zone, instantaneous three-dimensional velocity fields in the horizontal longitudinal, vertical longitudinal and vertical transverse planes in the breaking wave flow field will be measured using a stereoscopic particle image velocimetry (PIV) system. The measured fluid velocity fields will be decomposed into ensemble average and coherent and incoherent turbulence components by conditional averaging. The results will be used to understand the evolution of the large eddies and their kinematical and dynamical flow properties. One spilling breaker and two plunging breakers will be studied. The experimental study on solitary waves would allow us to understand the essential physics behind the evolution of large eddies under breaking waves without the influence of wave-induced currents and residual turbulence. In the second part of this project, coherent structures under regular and irregular waves will be investigated. PIV measurements of instantaneous fluid velocity fields will be conducted inside the surf zone for spilling and plunging regular waves, irregular waves with predominantly spilling breakers, and irregular waves with predominantly plunging breakers. The experimental investigation will focus on the near-bed region to determine the effects of wave-induced currents, residual turbulence and irregular wave breaking on the occurrence frequency, timing, and properties of coherent structures impinging on the bed from the core of the flow field.The impact of the proposed research stems from the importance of wave breaking in many coastal processes such as beach erosion, sediment transport, coastal inundation, and wave impingement and impact on shore-sited structures. In the surf zone, the wave breaking process has been and remains an area in which our ability to predict the physical phenomena is poor. Improved understanding of the large eddies in breaking waves is relevant to sediment suspension and would feed back to improved predictive models for surf zone hydrodynamics and sediment transport. The proposed research would also provide a high quality data set to test and improve detailed models of surf zone wave breaking process, including both present day models and models that are under long-range development for use in the future.