Experimental Study of Two-Phase Suspended Sediment Transport in Breaking Waves

本项目旨在研究破碎波中大尺度湍流的结构以及这些大涡与可蚀床的相互作用。实验室试验将在一个长25米、宽0.9米、深0.75米的倾斜式水槽中进行，该水槽配备了可编程的波浪发生器。将在水槽底板的凹槽中放置一个由泥沙颗粒组成的平坦床面。利用体积三分量测速系统(V3V)，在可侵蚀床上方的三维水流中，同时测量溢流和下沉波条件下的泥沙颗粒速度、泥沙颗粒位置和流体速度。两相流测量将被用来了解大涡的拓扑结构，大涡在床面泥沙收集过程中的作用，以及悬浮泥沙在水柱上的分布。它将涉及破浪的几个方面，这些方面仍然知之甚少，对于制定成功的数值方案以预测海浪带中的泥沙输送和海滩改造至关重要。测量三维流动中的三组分流体速度将使我们能够研究大涡的演化以及它们是如何从上层对流到底部的。结合使用高速数字电影摄像机的流动显示，这些测量将揭示大涡的产生机制以及沿波峰的非均匀波破裂对其结构的影响。体积流体速度测量还将使研究人员能够检查大涡对底部的冲击。这个过程是瞬变的和三维的，其特点是撞击区的瞬时湍流动能和剪应力很大。使用V3V系统同时测量可侵蚀床上的颗粒位置、颗粒速度和流体速度是一项创新，将使该项目的研究人员能够解决破碎波中泥沙运动的三个基本问题。第一个问题涉及悬沙输移的合适底边界条件；具体地说，如何参数化破碎波湍流对拾沙速率的影响。第二个问题涉及大涡和悬浮粒子之间的相互作用。这项拟议的研究将回答以下问题：沉积物颗粒的瞬时速度与流体颗粒的瞬时速度有多接近？悬浮泥沙输移与湍动输沙之间有直接联系吗？泥沙分布过程能否模拟为扩散过程？第三个问题涉及破碎机类型对泥沙输送的影响。调查员将回答以下问题：外海浪带悬移质输运量和方向如何与初破浪时的波浪特性有关？与板流输送等其他机制相比，这种输送方式有多重要？更广泛的影响：该项目将提供一个高质量的数据集，以测试和改进海浪带破碎波和泥沙输送的详细模型。这样的模型将促进人们对破碎的海浪如何侵蚀或沉积沉积物以及影响海岸带性质的理解。V3V在两相流测量方面的进展也将促进其在其他研究领域的发展和应用。研究人员将能够从PI的研究网站下载测量数据。该项目将通过邀请本科生和研究生参与实验研究来培训下一代研究人员。收购拟议的V3V系统将为学生提供最先进的测量仪器，以进行流体力学的实验研究。将为本科生和研究生班开发两个使用V3V的新实验，以加强学生对流体力学原理的学习。公安部还将使用这些设备在有K-12学生参加的工作坊中现场演示流体流动现象。这些活动的结果将是吸引更多的高中生(包括美国原住民学生)攻读理工科大学学位，为本科生提供研究经验，通过支持有能力的学生进行基础研究来建立具有全国竞争力的研究生课程，并保持一个现代流体力学实验室，以继续进行研究和教育。

This project is to investigate the structure of large-scale turbulence in breaking waves and the interaction of these large eddies with an erodible bed. Laboratory experiments will be carried out in a 25 m-long, 0.9 m-wide and 0.75 m-deep tilting flume equipped with a programmable wave generator. A flat bed of sediment particles will be placed in a recess in the flume floor. A volumetric three-component velocimetry (V3V) system will be used to measure sediment particle velocities, sediment particle locations, and fluid velocities simultaneously in a three-dimensional (3-D) flow volume above the erodible bed under spilling and plunging wave conditions. The two-phase flow measurements will be used to understand the topology of large eddies, and the role of large eddies in the sediment pickup process at the bed and the distribution of suspended sediment over the water column.Intellectual Merit: This project will build on prior work of the principal investigator (PI). It will address several aspects of breaking waves that are still poorly understood and are critical to the development of successful numerical schemes for predicting sediment transport in the surf zone and beach transformation. Measurement of three-component fluid velocities in a 3-D flow volume will enable thestudy the evolution of large eddies and how they are convected from the upper layer to the bottom. In conjunction with flow visualization using a high-speed digital movie camera, the measurements will shed light on the generation mechanisms of large eddies and the effect on their structure of non-uniform wave breaking along the wave crest. The volumetric fluid velocity measurements will also allow the investigator to examine the impingement of large eddies on the bottom. This process is transient and three-dimensional, characterized by large instantaneous turbulent kinetic energy and shear stresses in the impingement area.The simultaneous measurement of particle location, particle velocity, and fluid velocity over an erodible bed using a V3V system is innovative and will allow the project's investigator to address three fundamental problems in sediment transport in breaking waves. The first problem relates to the appropriate bottom boundary conditions for suspended sediment transport; specifically, how to parameterize the effect of breaking wave turbulence on the sediment pick-up rate. The second problem concerns the interaction between large eddies and suspended particles. The proposed research will answer questions such as: How closely do the instantaneous velocities of sediment particles follow those of fluid particles? Is there a direct link between suspended sediment transport and turbulence transport? Can the sediment distribution process be modeled as a diffusion process? The third problem concerns the effect of breaker types on sediment transport. The investigator will address the following questions: How do the quantities and direction of suspended load transport in the outer surf zone relate to the wave characteristics at incipient breaking? How important is this mode of transport compared to other mechanisms such as sheet-flow transport?Broader Impacts: This project will provide a high quality data set to test and improve detailed models of breaking waves and sediment transport in the surf zone. Such models will advance understanding of how breaking waves erode or deposit sediments and impact water-front property. Advances in two-phase flow measurements using V3V will also stimulate their development and application in other research fields. Researchers will be able to download the measured data from PI's research website.The project will train the next generation of researchers by engaging both undergraduate and graduate students in experimental investigation. Acquisition of the proposed V3V system will provide students with a state-of-the-art measurement apparatus to conduct experimental investigation in fluid mechanics. Two new experiments using V3V will be developed for both an undergraduate and a graduate class to enhance student learning of fluid mechanics principles. The PI will also use the equipment to conduct live demonstrations of fluid flow phenomena in workshops involving K-12 students. The outcomes of these activities will be to attract more high school students (including Native American students) to pursue a college degree in science and engineering, to provide research experiences to undergraduate students, to build a nationally competitive graduate program by supporting capable students to conduct basic research, and to maintain a modern fluid mechanics laboratory for continued research and education.