VOLcanic conduit processes and their effect on PROjectile eXit dYnamics (VOLPROXY)

Volcanic projectiles are centimeter- to meter-sized clasts – both solid-to-molten rock fragments or lithic eroded from conduits – ejected during explosive volcanic eruptions that follow ballistic trajectories. Despite being ranked as less dangerous than large-scale processes such as pyroclastic density currents (hot avalanches of gas and pyroclasts), volcanic projectiles still represent a constant threat to life and properties in the vicinity of volcanic vents, and frequently cause fatal accidents on volcanoes. Mapping of their size, shape, and location in volcanic deposits can be combined to model possible trajectories of projectiles from the vent to their final position, and to estimate crucial source parameters of the driving eruption, such as ejection velocity and pressure differential at the vent. Moreover, size and spatial distributions of volcanic projectiles from past eruptions, coupled with ballistic modelling of their trajectory, are crucial to forecast their possible impact in future eruptions. The reliability of such models strongly depends on i) the appropriate physical functions and input parameters and ii) observational validations. In this study, we aimed to unravel intra-conduit processes that strongly control the dynamic of volcanic projectiles by combining numerical modelling and novel experimentally-determined source parameter. In particular, the multiphase ASHEE model (Cerminara 2016; Cerminara et al. 2016) suited for testing post-fragmentation conduit dynamics based on a robust shock tube experimental dataset. By exploding mixtures of pumice and dense lithic particles within a specially designed transparent autoclave, and by using a raft of pressure sensors, ultra-high-speed cameras and pre-sieved natural particles, we observed and quantified: i) kinematic data of the particles and of the gas front along the shock tube and outside, ii) pressure decay at 1GHz resolution. By feeding the ASHEE model with these datasets, and using initial and boundary conditions similar to that of the experiment, we defined domains composed by a pressurized shock tube and the outside chamber at ambient conditions, and tested particles particle motion according to a Lagrangian approach, as well as gas flow with a Eulerian approach (a 3D finite-volume numerical solver, compressible). The comparison between data and model yields estimate of the particle kinematic inside the tube, the pressure evolution at the top and the bottom of the tube, and the eruption source parameters at the tube exit.

火山抛射物（Volcanic projectiles）是指尺寸介于厘米至米级的岩屑——包括固态至熔融态的岩石碎屑，或是从火山通道（conduits）侵蚀下来的岩屑——在爆炸性火山喷发过程中被抛射而出，并沿弹道轨迹运动的物质。尽管其危险等级低于火山碎屑密度流（pyroclastic density currents，即气体与火山碎屑的热雪崩流）这类大规模喷发过程，但火山抛射物仍对火山喷口（vent）周边的生命与财产构成持续威胁，且常在火山活动中造成致命事故。通过对火山沉积中抛射物的尺寸、形态与分布位置进行测绘，可以结合模型还原抛射物从喷口到最终落点的轨迹，并估算驱动喷发的关键源参数，例如喷口处的抛射速度与压力差。此外，过往喷发产生的火山抛射物的尺寸与空间分布特征，结合其轨迹的弹道模型，对预测未来喷发中抛射物的潜在影响至关重要。此类模型的可靠性主要取决于两点：一是选用恰当的物理函数与输入参数，二是开展观测验证。本研究通过结合数值模拟与新的实验测定源参数，旨在阐明强烈调控火山抛射物动力学过程的通道内部机制。其中，多相ASHEE模型（Cerminara 2016；Cerminara等，2016）适用于基于可靠的激波管实验数据集，检验碎裂后的通道动力学过程。本研究在特制的透明高压釜（autoclave）内引爆浮岩与致密岩屑颗粒的混合物，并利用多组压力传感器、超高速摄像机以及经过预筛分的天然颗粒，对以下内容进行观测与量化：① 沿激波管内外的颗粒与气体前锋的运动学数据；② 1GHz分辨率下的压力衰减过程。我们将上述数据集输入ASHEE模型，并采用与实验一致的初始与边界条件，划定了加压激波管与环境条件下的外部腔室两个计算域，分别采用拉格朗日（Lagrangian）方法模拟颗粒运动，以及欧拉（Eulerian）方法（可压缩三维有限体积数值求解器）模拟气体流动。通过数据与模型的对比，可以估算激波管内部的颗粒运动学特征、激波管上下两端的压力演化过程，以及激波管出口处的喷发源参数。