Collaborative Research: Experimental and Numerical Constraints on Density Evolution, Buoyancy Reversal, and Runout Distance in Pyroclastic Density Currents

合作研究：火山碎屑密度流中密度演化、浮力反转和跳动距离的实验和数值约束

• 批准号: 1852471
• 负责人: Benjamin Andrews
• 金额: $ 5.41万
• 依托单位: Smithsonian Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-06-01 至 2024-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1852471&HistoricalAwards=false
• 关键词: Collaborative; Research; Experimental; Numerical; Constraints

Explosive volcanic eruptions often generate pyroclastic flows, which are violent currents of hot ash and gas that travel along the ground at hurricane force speeds. Few people survive and many structures cannot withstand encounters with pyroclastic flows, making them the deadliest features generated from explosive volcanic eruptions. Dilute pyroclastic flows are particularly hazardous because they are less constrained to follow topography, making them much more unpredictable. These flows typically terminate when the ground hugging current becomes less dense than the surrounding atmosphere and reverses buoyancy, at which point it rises up to form ash-laden plumes. Understanding the mechanism and rate of this buoyancy reversal, and ultimately the runout distance of these flows, is necessary for accurate hazard prediction and for interpreting deposits of past eruptions. Likewise understanding the formation of buoyant plumes is necessary to understand the aviation and ashfall hazards 100's of kilometers from the eruptions. As the world's population continues to grow, deadly encounters with volcanic eruptions will increase rapidly, and thus scientists need to accurately predict their behavior, including impact force, runout distance, and buoyancy reversal in order to mitigate their hazards. In addition to those societal impacts, graduate and undergraduate students as well as a postdoc are supported by this award. Interactive tools and videos are also being developed around this work, to be distributed to K-12 educators as well as via the National Museum of Natural History.Because the destructiveness of pyroclastic flows inhibits direct measurements, this study will combine physical analog experiments with multiphase numerical modeling to establish how pyroclastic flow properties, dynamic pressures, and buoyancy evolve during travel. Ground hugging flows can reverse their buoyancy if enough pumice and ash is deposited and/or if enough air is entrained and heated, expanding the flow. At present, this reversal is often posited as an abrupt terminating condition for the progression of the flow on the ground. Counter, however, to assumptions used in parameterized models, the processes that result in buoyancy reversal, entrainment and deposition, are heterogeneous and can alter concentration gradients as indicated by previous analog experiments, multiphase numerical models, and field evidence. Indeed, previous approaches that treat such currents as uniform oversimplify entrainment, resulting in inaccurate predictions of flow dynamics. As the behavior and runout of turbulent currents are strongly dictated by the development of buoyancy reversal, a better understanding of the interactions and feedbacks between sedimentation and entrainment is needed to develop more realistic models. Experiments in the Experimental Volcanology Laboratory (Smithsonian Institution) will investigate how buoyancy evolves due to entrainment in stratified currents. Experiments using the Deep Water Basin in the Morphodynamics Laboratory (UT Austin) will investigate how entrainment evolves in stratified currents modified by particle settling. Each set of experiments will provide independent and crucial information about how instabilities develop and incorporate ambient fluid into stratified currents, and how these processes control the rate and location of buoyancy reversal and liftoff. Three dimensional data from the experiments will be directly compared to multiphase numerical simulations (UOregon) to validate stratification evolution and liftoff conditions under end-member scenarios. A suite of simulations will also be used to explore the mixing processes and mass balance during the initiation of liftoff in ways not possible with parameterized entrainment models. This study aims to investigate heterogeneous deposition and entrainment in turbulent, stratified, particle-laden currents using complementary analog experiments and multiphase numerical models to establish how pyroclastic flow properties, dynamic pressures, and buoyancy evolve during transport.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

爆炸性的火山喷发通常会产生火山碎屑流，它们是热灰和气体的暴力电流，以飓风速度沿着地面传播。很少有人生存，许多结构无法承受与火山碎屑流的相遇，这使其成为爆炸性火山喷发产生的最致命的特征。稀释的火山碎屑流特别危险，因为它们对遵循地形的限制较小，从而使它们变得更加不可预测。当地面拥抱电流变得比周围的气氛较少并逆转浮力时，这些流量通常会终止，此时它会升起形成灰烬的羽毛。 了解这种浮力逆转的机制和速率，以及最终这些流动的跳动距离，对于准确的危害预测和解释过去喷发的沉积是必要的。 同样，必须了解浮力羽毛的形成，以了解从喷发的航空和灰烬危害100公里。 随着世界人口的不断增长，与火山喷发的致命相遇将迅速增加，因此科学家需要准确预测其行为，包括影响力，跳动距离和浮力逆转，以减轻其危害。除了这些社会影响外，该奖项还支持研究生和本科生以及博士后。 Interactive tools and videos are also being developed around this work, to be distributed to K-12 educators as well as via the National Museum of Natural History.Because the destructiveness of pyroclastic flows inhibits direct measurements, this study will combine physical analog experiments with multiphase numerical modeling to establish how pyroclastic flow properties, dynamic pressures, and buoyancy evolve during travel. 如果足够的浮肿和灰分沉积和/或加热足够的空气并扩大了流量，则地面拥抱流可以扭转其浮力。 目前，这种逆转通常被认为是地面流动进展的突然终止条件。 但是，对参数化模型中使用的假设，导致浮力逆转，夹带和沉积的过程是异质的，可以改变浓度梯度，如先前的模拟实验，多相数字模型和现场证据所示。 实际上，以前的方法将这些电流视为均匀简化的夹带，从而导致流动动力学的预测不准确。 由于浮力逆转的发展强烈决定了湍流的行为和跳动，因此需要更好地理解沉积和夹带之间的相互作用和反馈，以开发更现实的模型。实验火山学实验室（史密森尼机构）的实验将研究浮力如何由于分层电流夹带而演变。使用形态动力学实验室（UT Austin）中使用深水盆地的实验将研究夹带在通过粒子沉降修饰的分层电流中如何演变。每组实验都将提供有关如何发展并将环境流体融入分层电流的独立和关键信息，以及这些过程如何控制浮力逆转和升降的速度和位置。实验中的三维数据将直接与多相数字模拟（Uoregon）进行比较，以验证分层的演变和升降条件在末端成员的情况下。一套模拟还将使用参数化的夹带模型以不可能的方式启动升降机期间探索混合过程和质量平衡。这项研究旨在研究使用互补的类似实验和多相数值模型来研究异质沉积和夹带湍流，分层，含粒子的电流，以确定在运输过程中如何发展热塑性流动性能，动态压力和浮力在运输过程中的进化。这种奖励反映了NSF的法定任务和审查的范围，该奖项是通过评估的范围来弥补的。