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

火山爆发通常会产生火山碎屑流，这是一种由炽热的火山灰和气体组成的猛烈气流，以飓风的速度沿着地面移动。很少有人幸存下来，许多建筑物无法承受火山碎屑流的冲击，这使它们成为火山爆发产生的最致命的特征。稀释的火山碎屑流尤其危险，因为它们不太受地形的限制，这使得它们更加难以预测。这些气流通常会在环绕地面的气流密度低于周围大气密度时终止，并与浮力相反，此时它会上升形成富含火山灰的羽流。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，对于准确预测危险和解释过去火山喷发的沉积物是必要的。同样，了解浮力羽流的形成对于了解火山喷发100公里外的航空和火山灰危害是必要的。随着世界人口的持续增长，与火山爆发的致命接触将迅速增加，因此科学家需要准确预测火山爆发的行为，包括冲击力、跳动距离和浮力逆转，以减轻其危害。除了这些社会影响之外，该奖项还支持研究生和本科生以及博士后。围绕这项工作也正在开发互动工具和视频，分发给K-12教育工作者，并通过国家自然历史博物馆分发。由于火山碎屑流的破坏性阻碍了直接测量，本研究将结合物理模拟实验和多相数值模拟来确定火山碎屑流的特性、动压力和浮力在移动过程中是如何演变的。如果有足够多的浮石和火山灰沉积，或者有足够多的空气被吸入并加热，那么紧贴地面的气流就会逆转其浮力。目前，这种反转常被假定为地面上气流进展的突然终止条件。然而，与参数化模型中使用的假设相反，先前的模拟实验、多相数值模型和现场证据表明，导致浮力逆转、卷带和沉积的过程是不均匀的，可以改变浓度梯度。事实上，以前将这种电流视为均匀的方法过于简化了夹带，导致对流动动力学的预测不准确。由于湍流的行为和跳动强烈地取决于浮力反转的发展，因此需要更好地了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。在实验火山学实验室（史密森学会）进行的实验将研究在分层洋流夹带下浮力是如何演变的。在形态动力学实验室（UT Austin）使用深水盆地进行的实验将研究在颗粒沉降改变的分层水流中夹带是如何演变的。每组实验都将提供关于不稳定性如何发展并将环境流体纳入分层流，以及这些过程如何控制浮力逆转和上升的速率和位置的独立和关键信息。实验的三维数据将直接与多相数值模拟（UOregon）进行比较，以验证端元情景下的分层演化和升空条件。还将使用一套模拟来探索在发射开始期间的混合过程和质量平衡，这是参数化夹带模型无法实现的。本研究旨在利用互补模拟实验和多相数值模型研究湍流、分层、颗粒负载流中的非均质沉积和夹带，以确定火山碎屑流特性、动压力和浮力在运输过程中的演变。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.