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？

• 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; How; does; stably; stratified; shear

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.

最终，这项研究是通过减少英国和全球气候变化对社会对社会的有害后果的影响。问题的根源是全球变暖，这是由于化石燃料的二氧化碳效应引起的。随着我们的大气变暖，我们的海洋也会直接影响生物多样性，并导致海平面上升。随着我们的海洋温暖，使它们保持不断运动的力量的平衡也破坏了他们的全球循环。在短期和长期的情况下，这种破坏令人担忧，因为当前的循环模式至少在热情好客的气候中发挥了至少两个功能。首先，垂直电流将多余的热量和碳深入海洋（减缓全球变暖）。其次，南北电流将热带热量重新分配到更温和的地区（减少了极端天气和气候）。因此，这些电流的削弱可能会加速气候变化，并带来持久的社会后果。为了减轻这一点，科学家试图通过使用海洋循环的先进数学和计算机模型来预测世界气候如何发展。但是，这些模型及其预测需要改进，以对社会和决策者更大。这些模型中不确定性的严重原因在于，盐度或温度不同（因此密度）的水流之间的混合。不同密度的电流组织成垂直堆叠的（或“稳定分层”）层，这些层以不同的速度（创建“剪切”流）相互流过。这些流总是动荡的，这意味着大量的混乱的“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。这种基本但极具挑战性的在稳定分层的剪切流中湍流混合的过程需要更好地理解。为此，我将在三个步骤中采用以下科学方法。首先，我将在实验室中使用此类流量的准确，减少的模型。这有两个很大的好处：它可以完全控制流程几何形状，密度差，流速等，使我能够分别测试和理解每个参数的影响；它使我能够使用高科技测量值来量化混乱的涡流及其混合物比以往任何时候都更好。第二，我将使用湍流混合的数学模型来解释这些新的实验室测量值，以将我的发现（或“外推”）概括为在海洋中发现的实际规模流量。这一关键步骤依赖于流体动力学中“维分析”的力量，工程师也通常使用它来开发新飞机或船舶设计的新飞机或船舶设计。三分之二，我将通过比较我的实际规模预测的有效性，通过将它们与从ships（通常是稀疏，昂贵，昂贵的，廉价的，较少的）进行比较来进行实际测量。此步骤类似于工程师在生产前进行全面测试，只是我们无法控制海洋。尽管具有挑战性，但这种“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了更准确的海洋混合模型。除了全球变暖的长期影响外，我还将将上述三个步骤应用于较短的后果：埃斯塔伊尔人的盐水入侵。海平面上升，更频繁的干旱，极端的风暴潮和更强烈的潮汐都将增加人口稠密的三角洲（包括英国重要的泰晤士河盆地）中海水的逐渐侵占。新鲜河水下方的浓盐水层的上游侵入，它们的垂直混合降低了地表淡水的可用性，对世界各地的沿海社区的造成了毁灭性的后果。我将开发更准确的盐水侵入中混合模型，以帮助缓解这一紧急问题。