Experimental Study of Coherent Structures in Breaking Waves

建议研究海浪区破碎波中的相干结构。主要目的是测量由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。本课程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立大学(SDSU)的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。在本工程的第一部分中，将由安装在一端的可编程波浪发生器产生孤立波，并使其在水槽的预定位置处破裂。将测量冲浪区的波浪高程，以确定感兴趣的区域，以便进行详细的速度测量。利用立体粒子图像测速系统(PIV)，在外、中、内三个不同位置，测量了破碎波流场中水平纵面、垂直纵面和垂直横面的三维瞬时速度场。通过条件平均，将测量的速度场分解为集合平均湍流分量、相干湍流分量和非相干湍流分量。这些结果将被用来理解大涡的演化及其运动学和动力学流动特性。将研究一个溢流式断路器和两个下沉式断路器。通过对孤立波的实验研究，我们可以在不受波浪诱导流和残余湍流影响的情况下，了解破碎波下大涡演化的基本物理机制。在本项目的第二部分，将研究规则波和不规则波下的相干结构。瞬时流体速度场的PIV测量将在规则波、以溢流为主的不规则波和以俯冲为主的不规则波的波浪区内进行。试验研究将集中在近床区，以确定波浪诱导流、残余湍流和不规则波破碎对从流场核心撞击床面的相干结构物发生频率、时间和特性的影响。本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、泥沙输送、海岸淹没、波浪撞击和对海岸建筑物的冲击。在冲浪区，波浪破碎过程一直是我们预测物理现象能力较差的领域。更好地理解破碎波中的大涡与泥沙悬浮有关，并将反馈到改进的海浪带水动力和泥沙输送预测模型。拟议的研究还将提供高质量的数据集，以测试和改进冲浪区波浪破碎过程的详细模型，包括目前的模型和正在进行长期开发的模型，以供未来使用。

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.