How does stably-stratified shear-driven turbulence mix our oceans and estuaries?

稳定分层的剪切驱动湍流如何混合我们的海洋和河口？

基本信息

批准号：
NE/W008971/1
负责人：
Adrien Lefauve
金额：
$ 89.48万
依托单位：
University of Cambridge
依托单位国家：
英国
项目类别：
Fellowship
财政年份：
2023
资助国家：
英国
起止时间：
2023 至无数据
项目状态：
未结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FW008971%2F1
关键词：
How does stably stratified shear

项目摘要

This research is ultimately motivated by reducing the harmful consequences of climate change on society, in the UK and worldwide.The root of the problem is global warming, caused by the greenhouse effect of carbon dioxide from fossil fuels. As our atmosphere warms, so do our oceans, which directly affects biodiversity and causes sea levels to rise. As our oceans warm, the balance of forces that keep them in constant motion changes too, disrupting their worldwide circulation. This disruption is worrying, both in the short and long term, because the present circulation patterns perform at least two functions vital to our hospitable climate. First, vertical currents store excess heat and carbon deep into the ocean (slowing global warming). Second, North-South currents redistribute tropical heat to more temperate regions (reducing extreme weather and climate). Therefore, a weakening of these currents could accelerate climate change, with long-lasting societal consequences.To mitigate this, scientists try to predict how the world's climate will evolve by using advanced mathematical and computer models of the ocean circulation. However, these models and their predictions need to be improved to be of greater benefit to society and decision-makers. A serious cause of uncertainty in these models lies in the mixing between water currents that have different salinity or temperature (and thus density). Currents of different densities organise into vertically-stacked (or "stably-stratified") layers which flow past one another at different speeds (creating a "shear" flow). These flows are always turbulent, which means that a vast number of tiny chaotic "eddies" mix the salinity and temperature of much larger layers in complex and unpredictable ways.This fundamental but extremely challenging process of turbulent mixing in stably-stratified shear flows needs to be better understood. To do this, I will employ the following scientific approach in three steps.First, I will use an accurate, reduced-scale model of such flows in the laboratory. This has two great benefits: it gives full control over the flow geometry, the density difference, flow speed, etc, allowing me to test and understand the influence of each parameter separately; and it allows me to use high-tech measurements to quantify the chaotic eddies and their mixing better than ever before.Second, I will interpret these new laboratory measurements with mathematical models of turbulent mixing to generalise (or "extrapolate") my findings to real-scale flows found in the ocean. This crucial step relies on the power of "dimensional analysis" in fluid dynamics, which is also routinely used by engineers to develop new aircraft or ship designs from smaller-scale laboratory prototypes.Third, I will verify the validity of my real-scale predictions by comparing them to actual measurements taken from ships (which are usually sparse, expensive, and less accurate). This step is similar to engineers performing a full-scale test before production, except that we have no control over the ocean. Although challenging, this "validation" step will help ensure that my whole approach succeeds in providing climate scientists with more accurate models for ocean mixing.In addition to the long-term effects of global warming, I will also apply the above three steps to a shorter-term consequence: saltwater intrusions in estuaries. Sea level rise, more frequent droughts, extreme storm surges, and stronger tides will all increase the gradual encroachment of seawater in densely-populated deltas (including the important Thames Basin in the UK). The upstream intrusion of a dense saltwater layer beneath the fresh river water, and their vertical mixing reduce the availability of surface freshwater, with devastating consequences for coastal communities already felt around the world. I will develop more accurate models of mixing in saltwater intrusions to help mitigate this urgent problem.

这项研究的最终目的是减少气候变化对英国和全世界社会的有害影响。问题的根源是全球变暖，这是由化石燃料产生的二氧化碳的温室效应引起的。随着大气变暖，海洋也变暖，这直接影响生物多样性并导致海平面上升。随着海洋变暖，维持海洋持续运动的力量平衡也会发生变化，从而扰乱海洋的全球循环。无论从短期还是长期来看，这种破坏都令人担忧，因为目前的环流模式至少发挥着两个对我们宜人的气候至关重要的功能。首先，垂直流将多余的热量和碳储存到海洋深处（减缓全球变暖）。其次，南北洋流将热带热量重新分配到更温带的地区（减少极端天气和气候）。因此，这些洋流的减弱可能会加速气候变化，从而产生长期的社会后果。为了缓解这一问题，科学家们试图利用先进的海洋环流数学和计算机模型来预测世界气候将如何演变。然而，这些模型及其预测需要改进，才能为社会和决策者带来更大的利益。这些模型中不确定性的一个严重原因在于具有不同盐度或温度（以及密度）的水流之间的混合。不同密度的水流组织成垂直堆叠（或“稳定分层”）的层，这些层以不同的速度流过彼此（产生“剪切”流）。这些流动始终是湍流，这意味着大量微小的混沌“涡流”以复杂且不可预测的方式混合了更大层的盐度和温度。稳定分层剪切流中的湍流混合这一基本但极具挑战性的过程需要得到更好的理解。为此，我将分三个步骤采用以下科学方法。首先，我将在实验室中使用此类流动的精确、缩小规模的模型。这有两个很大的好处：它可以完全控制流动几何形状、密度差、流速等，使我能够分别测试和了解每个参数的影响；它使我能够使用高科技测量来比以往更好地量化混沌涡流及其混合。其次，我将用湍流混合的数学模型来解释这些新的实验室测量结果，以将我的发现推广（或“外推”）为真实的结果- 海洋中发现的规模流动。这一关键步骤依赖于流体动力学中“尺寸分析”的力量，工程师也经常使用它来通过较小规模的实验室原型开发新的飞机或船舶设计。第三，我将验证我的实际规模预测的有效性通过将它们与从船上获得的实际测量值（通常稀疏、昂贵且不太准确）进行比较。这一步类似于工程师在生产前进行全面测试，只不过我们无法控制海洋。虽然具有挑战性，但这个“验证”步骤将有助于确保我的整个方法成功地为气候科学家提供更准确的海洋混合模型。除了全球变暖的长期影响之外，我还将把上述三个步骤应用到短期后果：海水入侵河口。海平面上升、更频繁的干旱、极端风暴潮和更强的潮汐都会加剧海水对人口稠密的三角洲（包括英国重要的泰晤士盆地）的逐渐侵蚀。上游淡水河水下方稠密咸水层的入侵及其垂直混合减少了地表淡水的可用性，对世界各地的沿海社区造成了毁灭性后果。我将开发更准确的盐水入侵混合模型，以帮助缓解这一紧迫问题。