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，这是一种以飓风般的速度沿着地面流动的热灰和气体的猛烈流动。很少有人能幸存下来，许多建筑物无法承受火山碎屑流的袭击，使其成为爆炸性火山爆发产生的最致命的特征。稀释的火山碎屑流特别危险，因为它们不太受地形的限制，使它们更加不可预测。这些流动通常会在拥抱地面的气流变得比周围的大气密度小并逆转浮力时终止，此时它会上升形成充满灰尘的羽流。 了解这种浮力逆转的机制和速率，以及这些流动的最终流出距离，对于准确的灾害预测和解释过去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距离火山爆发100公里的航空和火山灰的危害是必要的。 随着世界人口的持续增长，与火山爆发的致命遭遇将迅速增加，因此科学家需要准确预测其行为，包括冲击力，跑出距离和浮力逆转，以减轻其危害。除了这些社会影响外，研究生和本科生以及博士后也得到了该奖项的支持。互动工具和视频也正在围绕这项工作，将分发给K-12教育工作者以及通过国家自然历史博物馆。由于火山碎屑流的破坏性抑制直接测量，本研究将结合联合收割机物理模拟实验与多相数值模拟，以建立如何火山碎屑流特性，动态压力和浮力演变过程中的旅行。 如果有足够的浮石和火山灰沉积，或者如果有足够的空气被夹带和加热，则地面拥抱流可以逆转它们的浮力，从而扩大流动。 目前，这种逆转通常被假定为地面上流动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，导致浮力反转、夹带和沉积的过程是异质的，并且可以改变浓度梯度，如先前的模拟实验、多相数值模型和现场证据所示。 事实上，以前的方法，把这样的电流均匀过度简化夹带，导致不准确的预测流动动力学。 由于湍流的行为和跳动强烈地取决于浮力逆转的发展，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（史密森学会）的实验将研究浮力如何演变，由于分层流夹带。使用深水盆地在形态动力学实验室（UT奥斯汀）的实验将调查如何夹带演变成分层电流修改颗粒沉降。每组实验将提供独立的和关键的信息，关于不稳定性如何发展和将环境流体纳入分层流，以及这些过程如何控制浮力反转和升空的速率和位置。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起飞条件。一套模拟也将被用来探索的混合过程和质量平衡的方式不可能与参数化的夹带模型在启动升空。这项研究的目的是调查不均匀的沉积和夹带在湍流，分层，颗粒负载电流使用互补的模拟实验和多相数值模型，以建立如何火山碎屑流特性，动态压力和浮力演变过程中transport.This奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.