Characteristics of Bolus formation from Breaking Internal Waves on Shelf Slopes

[133380 . koseff]从海洋温跃层下面向上坡、向岸上输送较冷、营养丰富的海水，被认为对珊瑚礁等沿海生态系统至关重要。坡面上浅滩内波的破裂所产生的内孔或球体为密集水的跨陆架输送提供了一种机制。事实上，观察证实，当内波传播到浅水时，波浪会变陡，这可能导致破裂，有时还会产生密集流体的湍流，这些湍流继续向岸边移动，与周围的地表水混合。这一机制被认为是营养物、沉积物和幼虫在深水和陆架环境之间转移的重要机制。此外，丸驱动的运输有可能将近海污水排放口的污染物深入到近岸环境中，通过捕鱼和在海滩直接接触对人类健康构成威胁。然而，关于内孔的传播和运输，包括什么条件导致内孔的形成，仍然存在一些问题。因此，该项目将利用实验室实验研究内部钻孔形成和质量传输过程。迄今为止，关于内部钻孔形成、传播和相关的质量输运的知识相当有限。因此，本研究的总体目标是：(1)定量确定控制浅滩内波形成内孔的物理条件，并为此建立模型和参数化；(2)将初始流动参数与内孔的质量输运和流体携带联系起来。在斯坦福环境流体力学实验室的内波设备中进行的一系列实验室实验将研究均匀斜坡/陆架地形下两层体系内波的浅滩化。定量流动可视化技术，包括平面激光诱导荧光（PLIF）和粒子图像测速（PIV）将允许详细研究初始流动条件变化的影响，例如入射波的振幅和频率，对颗粒形成和传播的影响。额外的速度场和密度场数据将量化井眼内部传播的质量传递，以及量化周围流体的体积夹带。总的来说，该项目将有力地补充正在进行的审查沿海生态系统质量转移问题的实地研究。此外，预期的结果不仅将为浅海内波的物理机制提供见解，而且还将对浅海沿岸系统中受流体动力学和质量传递影响的生物过程产生影响。最后，像这样的研究对于解释海洋数据经常提供的“快照”，以及指导进一步的海洋测量，是非常宝贵的。这项工作的结果将对研究珊瑚礁、海带森林、海草群落的近岸运输和混合过程的研究人员以及那些关注海洋排放物和其他人为排放物对人类健康影响的研究人员有用。最后，本文进一步发展和完善的分层流动中PLIF和PIV的方法和技术将对实验流体力学界有所帮助。

1133380KoseffThe upslope, shoreward transport of colder, nutrient-rich waters from below the ocean thermocline is thought to be critical to coastal ecosystems such as coral reefs. Internal bores, or boluses, generated by the breaking of shoaling internal waves on the slope provide a mechanism for this cross-shelf transport of dense water. Indeed, observations confirm that as internal waves propagate into shallow water, the waves steepen, which can lead to breaking and, at times, the generation of turbulent boluses of dense fluid which continue to travel inshore, mixing with ambient surface water. This mechanism is thought to be a significant mechanism for the transfer of nutrients, sediment, and larvae between deep waters and the shelf environment. Furthermore, bolus-driven transport has the potential to transport contaminants from offshore sewage outfalls at depth into the near-shore environment, posing risks for human health through fishing and direct exposure at beaches. However, questions still remain regarding internal bore propagation and transport, including what conditions cause the formation of internal bores. Thus, this project will investigate the internal bore formation and mass transport processes using laboratory experiments.To date, the knowledge of internal bore formation, propagation, and associated mass transport is fairly limited. The overall goals of the research, therefore, are to (1) quantitatively determine the physical conditions which control the development of internal bores from shoaling internal waves, and develop models and parameterizations for this, and (2) relate initial flow parameters to the mass transport and fluid entrainment of internal bores. A series of laboratory experiments in the internal wave facility of the Stanford Environmental Fluid Mechanics Laboratory will study the shoaling of internal waves on uniform slope/shelf topography in a two-layered system. Quantitative flow visualization techniques, including Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) will allow a detailed study of the effect of variations in the initial flow conditions, such as incoming wave amplitude and frequency, on bolus formation and propagation. Additional velocity and density field data will quantify the mass transport of internal bore propagation, as well as quantify the bolus entrainment of ambient fluid.Overall, the project will strongly complement ongoing field studies examining mass transfer issues in coastal ecosystems. Additionally, the expected results will not only provide insight to the physical mechanisms of shoaling internal waves, but will also have implications for biological processes affected by hydrodynamics and mass transfer in shallow coastal systems. Finally, studies such as these can be invaluable to interpreting the "snapshots" that ocean data often gives, and in perhaps directing further ocean measurements. Results from this work will be of use to researchers studying near-coastal transport and mixing processes in coral reefs, kelp forests, seagrass communities, as well as those concerned with human health impacts from ocean outfalls and other antrhopogenic discharges. Finally, the methodologies and techniques for PLIF and PIV in stratified flows that are further developed and refined here will be of use to the experimental fluid mechanics community.