Gravity current propagation through density stratified media with applications to transport in the built environment and pollution dispersion in nature

通过密度分层介质的重力流传播及其在建筑环境中的传输和自然界中的污染扩散中的应用

• 批准号: RGPIN-2014-04828
• 负责人: Flynn, Morris
• 金额: $ 1.97万
• 依托单位: University of Alberta
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2016
• 资助国家: 加拿大
• 起止时间: 2016-01-01 至 2017-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=611269
• 关键词: Gravity; current; propagation; through; density

Gravity currents are primarily horizontal flows that are driven by differences of fluid density and appear over a broad range of time- and length-scales in the natural and built environment. Examples include an airborne chemical agent that flows through a network of subway tunnels, smokestack effluent that propagates along an atmospheric inversion or the counter-flowing drafts of warm and cold air that result when a door to a solarium is suddenly opened. Whereas the flow of gravity currents through a uniform ambient is well-described by experimentally-validated models, far less is known concerning gravity current flow through a stratified ambient, for example one consisting of a dense lower and light upper layer. This lack of understanding represents a serious shortcoming: the atmosphere, most bodies of water and even voluminous building zones typically exhibit some appreciable vertical stratification and this has the effect of altering the dynamics of the flow. For instance, a gravity current propagating through a stratified medium often excites interfacial waves and these can, in turn, extract momentum from the advancing gravity current. Thus a strong feedback may arise so that the gravity current generates waves, which modulate the advance of the current, which changes the pattern of wave excitation and so on. Disentangling the above details is a long-term objective that would constitute a substantial scientific achievement with equally significant practical ramifications in terms of determining patterns of pollution dispersion, optimizing heat exchange in buildings, etc. The goals of the present research are more targeted. They consist of exploring, using theoretical, experimental and numerical techniques, the above dynamics in two specific scenarios. First, I will study axisymmetric gravity currents flowing through a two-layer ambient. This mimics the flow of a river plume into the sea or a draft of warm or cold air into an open-plan, naturally-ventilated building. As compared to the rectilinear analogue problem, here the gravity current height must continually decrease suggesting that the front cannot for long propagate at constant speed. However, preliminary laboratory experiments exhibit surprisingly rich and sometimes counterintuitive behavior depending on the particulars of the initial and source conditions. It remains to complete a more comprehensive sweep of the parameter space then to validate experimental results using appropriate theoretical and numerical models. Next, I will return to rectilinear coordinates and examine the role of bottom topography in modifying the advance of a bottom- or top-propagating gravity current, again through a density-stratified medium. Earlier research with a uniform ambient shows that obstacles exhibit a retarding influence. Conversely, in the absence of topography there are well-documented scenarios in which gravity current fluid can travel long distances at constant speed. Determining which of the above effects predominates under which particular conditions is an important task that will provide invaluable information in assessing dispersion patterns related, for example, to an accidental or malicious release of dense gas over uneven terrain. Model predictions and measured data from the above investigations may be adapted into algorithms that lack sufficient resolution to fully describe flow details at all spatial scales. Instead, parameterizations of the interactions between the advancing gravity current and ambient interfacial waves are required. To this effect, the prime objective of the current proposal is to collect, interpret and synthesize data in a way that is meaningful to other researchers, industry practitioners, regulators and policy makers.

重力电流主要是由流体密度差异驱动的水平流，并且在自然和建筑环境中出现在广泛的时间和长度尺度上。例子包括流经地铁隧道网络的机载化学剂，烟囱废水沿着大气反转传播或当突然打开太阳台的门时导致的温暖和冷空气的反流草稿。尽管实验验证的模型很好地描述了重力电流通过均匀环境的流动，但对于通过分层环境流动的重力电流流动，众所周知，例如，一个由密度下层和光上层组成的重力电流。缺乏理解代表了一个严重的缺点：大气，大多数水域，甚至大量的建筑区域通常都表现出明显的垂直分层，这具有改变流动动力学的作用。例如，通过分层培养基传播的重力电流通常会激发界面波，而这些电流可以从前进的重力电流中提取动量。因此，可能会出现强反馈，以使重力电流产生波，从而调节电流的前进，从而改变了波动激发的模式。 解开上述细节是一个长期目标，在确定污染分散的模式，优化建筑物中的热量交换等方面，具有同样重要的实践后果，构成了一项实质性的科学成就。本研究的目标是更具针对性的。它们包括使用理论，实验和数值技术探索上述动力学。 首先，我将研究流经两层环境的轴对称重力电流。这模仿了河羽流入大海或温暖或冷空气的草稿进入开放式，天然通风的建筑。与直线类似物问题相比，这里的重力电流高度必须不断降低，这表明前部不能以恒定的速度长时间传播。但是，根据初始和源条件的细节，初步实验室实验表现出令人惊讶的丰富，有时是违反直觉的行为。然后，使用适当的理论和数值模型验证了对参数空间的更全面的扫描，以验证实验结果。 接下来，我将返回到直线坐标，并检查底部地形在修改底部或顶部传播重力电流的前进中的作用，再次通过密度分层的培养基。对统一环境的较早研究表明，障碍物表现出较弱的影响。相反，在没有地形的情况下，有充分记录的场景，重力电流可以以恒定的速度长距离行驶。确定上述效应的主要效果在哪些特定条件是一项重要的任务下，将在评估相关的分散模式时提供宝贵的信息，例如，与意外或恶意释放密集的气体在不均匀的地形上的意外释放。 从上述研究中进行的模型预测和测量数据可以适应缺乏足够分辨率的算法，无法在所有空间尺度上充分描述流量细节。取而代之的是，需要进行前进的重力电流和环境界面波之间的相互作用的参数化。为此，当前建议的主要目标是以对其他研究人员，行业从业人员，监管者和政策制定者有意义的方式收集，解释和合成数据。