Experimental Study of Coherent Structures in Breaking Waves

提出了对冲浪带破碎波中的相干结构进行研究。主要目的是测量波浪破碎在平面沙滩上产生的间歇性连贯湍流的流动特性。将研究包括孤立波、规则波和不规则波在内的各种破波条件。实验室实验将在南达科他州立大学（SDSU）的一个二维波浪水槽中进行。水槽有有机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。在这个项目的第一部分，将由安装在一端的可编程波浪发生器产生一个孤立波，并在水槽的预定位置引起破裂。将测量整个冲浪区的波浪高度，以确定感兴趣的区域，以便进行详细的速度测量。采用立体粒子图像测速（PIV）系统，在对应于外、中、内冲浪带的三个分离位置，测量破碎波流场中水平纵向面、垂直纵向面和垂直横向面的瞬时三维速度场。采用条件平均法将实测流体速度场分解为集合平均和相干及非相干湍流分量。这些结果将用于理解大涡流的演变及其运动学和动力学流动特性。将研究一个溢出式断路器和两个浸入式断路器。孤立波的实验研究将使我们能够在没有波浪感应流和残余湍流影响的情况下，了解破碎波下大涡流演变背后的基本物理原理。本项目第二部分将研究规则波和不规则波作用下的相干结构。在涌浪带内进行规则浪、以溢浪为主的不规则浪、以倾浪为主的不规则浪和以倾浪为主的不规则浪的瞬时流速场PIV测量。实验研究将集中在近床层区域，以确定波致流、残余湍流和不规则波破对从流场核心冲击床层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，例如海滩侵蚀，沉积物运输，海岸淹没，海浪撞击和对海岸结构的影响。在冲浪带，波浪破碎过程一直是并且仍然是我们预测物理现象能力较差的区域。对破碎波中的大涡流的进一步了解与泥沙悬浮有关，并将反馈给改进的冲浪带水动力学和泥沙输运预测模型。拟议的研究还将提供高质量的数据集，以测试和改进冲浪带波浪破碎过程的详细模型，包括目前的模型和正在长期开发以供将来使用的模型。

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.