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.