Effects of internal waves on mixing and transport by gravity currents

由于全球水资源日益短缺，各国政府越来越多地考虑将海水淡化作为饮用水的主要来源。因此，沿海排放的浓盐水废水的命运正成为一个日益严重的问题。海水淡化设施产生的盐水羽流影响沿海海洋的面积范围仍然是一个主要的不确定因素，特别是因为存在高能内波场，这可能导致许多沿海海洋中存在或拟议的海水淡化设施的显著混合。因此，对烟羽与周围接收水混合的基本了解对于预测烟羽和周围水域的盐水和化学添加剂浓度至关重要。特别是，确定斜坡下密集重力流的混合和稀释受到迎面来的内波影响的途径和机制是一个悬而未决的问题。尽管我们对斜坡上的波浪如何破裂以及重力流如何在分层环境中下降的理解近年来取得了相当大的进展，但令人惊讶的是，在日益重要的耦合问题上所做的工作很少。本项目将利用实验室实验结合互补的数值模拟和理论建模来解决这一需求。实验和理论方法将重力电流分析扩展到环境中破坏界面波的重要设置。了解这种环境运动对重力流色散的影响对许多工业和科学领域都很重要。例如，这项工作的结果将使工业界和其他利益相关者能够更好地评估海水淡化设计替代方案的环境影响，包括可能创建基于gis的工具。暑期研究学生将通过土木与环境工程系参与。“本科生研究经验项目”和由工程学院运营的斯坦福暑期工程学院。该项目还将直接支持一名博士后研究员和一名研究生的培养和教育，他们将参与本科生的教学。拟议的实验室实验和数值模拟将确定重力流沿着斜坡下降时会发生什么，通过具有两层密度剖面和内部重力波的环境。相同实验装置的数值模拟将根据实验室观察结果进行验证，然后用于进一步研究实验室无法实现的参数体系。将建立一个基于改变电流内密度分布所需势能的理论框架，以理解从第一原理出发的色散观测。该研究将使用内波槽和数值模拟设备进行，并将研究界面波如何影响倾斜重力流的物理特性。浅滩内波有不同的破碎机制，包括陡峭斜坡上的非破碎塞；相干丸;更不连贯、更动荡的丸；还有汹涌的巨浪。重力流通过两层大气下降，在斜斜处形成侵入体；形成渗透到下层下面的溢流；或者分成侵入和下流动的部分。然而，当重力流和内波同时存在时，我们所知道的就少得多。这个项目将在两个方面带来实质性的新见解：通过识别和系统地编目内波影响重力流物理的方式，以及通过建立和验证它们产生这种影响的机制的数学模型。三个关键参数（波的弗劳德数、iribare数和电流的密度Richardson数）将被用来定义研究的参数空间。

Because of increased global water scarcity governments are increasingly considering the contribution made by desalination as a primary source of drinking water. Thus, the fate of dense brine effluent from coastal discharges is becoming an increasingly acute issue. The areal extent over which brine plumes from desalination facilities impact the coastal ocean remains a major uncertainty, particularly because there are energetic internal wave fields which can cause significant mixing in many coastal oceans where desalination facilities are present or proposed. Fundamental understanding of the mixing of the plume with the ambient receiving waters is thus critical to predicting the concentrations of brine and chemical additives in the plume and surrounding waters. In particular, determining the pathways and mechanisms in which the mixing and dilution of dense gravity-current underflows down slopes are influenced by oncoming internal waves is an outstanding question. Although our understanding of how waves on slopes break and how gravity currents descend through stratified environments have independently moved forward considerably in recent years, surprisingly little work has been done on the increasingly important coupling problem. This project will address this need using laboratory experimentation in combination with complementary numerical simulations and theoretical modeling. The experiments and theoretical approach extends gravity current analysis to the important setting of breaking interfacial waves in the environment. Understanding the influence of this kind of ambient motion on dispersion in gravity currents is important to many industrial and scientific fields. For example, the results of this work will enable industry and other stakeholders to make better assessments of the environmental impacts of desalination design alternatives, including the potential creation of GIS-based tools. Summer research students will be engaged through the Department of Civil and Environmental Engineering?s Research Experience for Undergraduates Program and the Stanford Summer Engineering Academy run by the School of Engineering. This project will also directly support the training and education of a post-doctoral researcher and one graduate student, who will both be involved in teaching undergraduates.The proposed laboratory experiments and numerical simulations will establish what happens to a gravity current that descends down a slope through an ambient that has a two-layer density profile and internal gravity waves. Numerical simulations of the same experimental setup will be validated against the laboratory observations, and then used to further investigate parameter regimes unattainable in the laboratory. A theoretical framework based on the potential energy required to change the density profile within the current will be created to understand the detrainment observations from first principles. The investigation will be performed using internal wave tank and numerical modeling facilities, and will examine the physics of how interfacial waves influence inclined gravity currents. Shoaling internal waves have different regimes of breaking, including non-breaking seiches on steep slopes; coherent boluses; less-coherent and more turbulent boluses; and turbulent surges. Gravity currents descending through two-layer ambients can either form intrusions at the pycnocline; form underflows which penetrate beneath the lower layer; or split into an intruding and an under flowing part. When gravity currents and internal waves are present at the same time, however, much less is known. This project will lead to substantial new insight in two ways: by identifying and systematically cataloging the ways that internal waves can influence the physics of gravity currents, and by building and validating mathematical models of the mechanisms by which they do so. Three key parameters (the wave Froude number, the Iribarren number, and the density Richardson number of the current) will be used to define the parameter space of the investigation.