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: 2018
• 资助国家: 加拿大
• 起止时间: 2018-01-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=650879
• 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.

重力流主要是由流体密度差异驱动的水平流，并在自然和建筑环境中出现在广泛的时间和长度范围内。例如，流经地铁隧道网络的空气中的化学制剂、沿大气逆流传播的烟囱排出物或日光浴室的门突然打开时产生的冷暖空气逆流。虽然通过实验验证的模型很好地描述了重力流通过均匀环境的流动，但对于通过分层环境的重力流的了解却少之又少，例如由致密的下层和轻的上层组成的重力流。这种缺乏理解代表了一个严重的缺点：大气、大多数水体甚至大量的建筑区域通常表现出一些明显的垂直分层，这会改变流动的动力学。例如，通过分层介质传播的重力流通常会激发界面波，而这些界面波反过来又可以从前进的重力流中提取动量。因此，可能会出现强烈的反馈，从而使重力流产生波浪，从而调节电流的前进，从而改变波浪激发的模式等等。**解开上述细节是一个长期目标，它将构成一个实质性的目标。在确定污染扩散模式、优化建筑物热交换等方面具有同等重要实际意义的科学成就。目前的研究目标更有针对性。它们包括使用理论、实验和数值技术在两个特定场景中探索上述动态。 **首先，我将研究流过两层环境的轴对称重力流。这模仿了流入大海的河流羽流或一股暖空气或冷空气流入开放式、自然通风的建筑物。与直线模拟问题相比，这里重力流高度必须不断减小，这表明锋面不能长时间以恒定速度传播。然而，初步的实验室实验表现出令人惊讶的丰富且有时违反直觉的行为，具体取决于初始条件和源条件的细节。仍然需要完成对参数空间的更全面的扫描，然后使用适当的理论和数值模型验证实验结果。**接下来，我将返回直线坐标并检查底部地形在修改底部或底部推进方面的作用。顶部传播的重力流，再次通过密度分层介质。早期对均匀环境的研究表明，障碍物表现出阻碍作用。相反，在没有地形的情况下，有充分记录的场景，重力流流体可以以恒定的速度长距离传播。确定上述哪种效应在特定条件下占主导地位是一项重要任务，它将为评估与不平坦地形上意外或恶意释放致密气体相关的扩散模式提供宝贵的信息。**模型预测和测量数据来自上述研究可能适用于缺乏足够分辨率来充分描述所有空间尺度的流动细节的算法。相反，需要对前进的重力流和环境界面波之间的相互作用进行参数化。为此，当前提案的主要目标是以对其他研究人员、行业从业者、监管机构和政策制定者有意义的方式收集、解释和综合数据。