Numerical solution of the time-dependent conservation equations for mass, momentum, specific internal energy, and granular temperature in flows involving gas-particle mixtures is used to explore the initiation of pyroclastic currents from collapsing mixtures such as during fountaining eruptions. One objective is to determine when a depth-averaged granular flow model or a box model for dilute currents is most applicable for hazards modeling of pyroclastic currents produced by column collapse; a second objective is to gain insight into the formation of proximal breccia facies of ignimbrites. Collapsing gas-particle mixtures impacting a flat surface are modeled with mixtures of coarse particles that are poorly coupled with the gas phase and well-coupled fine particles. Resulting lateral flows are sensitive to the impact speed, overall particle concentration, and proportions of fine and coarse particles. For total particle concentrations of around 1 vol.%, an impacting mixture consisting of at least ~ 50% coarse particles, relative to fines, will tend to form a concentrated lateral underflow, which can be approximated by a depth-averaged granular flow model for hazard assessment purposes starting from the impact zone. Low total particle concentrations (e.g., total concentrations of ~ 0.1 vol.%) tend to produce dilute lateral flows that could be simplified to box model approaches for dilute pyroclastic currents. Larger total particle concentrations in impacting mixtures (~ 10 vol.%) produce granular underflows if they have any coarse particles, but these can be complicated by Mach number effects. For intermediate concentrations of the impacting mixture, a rough threshold for development of a concentrated underflow versus a dilute-only current is based upon the flux per unit area of coarse particles to the impact and their Stokes numbers. In general, some knowledge of the eruption column (fountain) conditions is required in order to make an informed decision as to which hazards modeling approach is most applicable for a given scenario. Modeling indicates that proximal breccias are related to influxes of coarse wall-rock material into an eruptive mixture, which increase both the total particle concentration and the proportion of coarse, dense clasts in the mixture that subsequently collapses and impacts the ground. The breccias record concentration of dominantly coarse clasts immediately upon impact and formation of concentrated flows that propagate laterally while expelled fines and gas flow rapidly overhead as dilute currents. Lateral and vertical heterogeneity in proximal deposits likely record rapid time and space variations in avalanches of material into eruptive vents, and the occurrence of breccia hummocks might record the temporary positions of impact zones.

中文翻译：

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从坍塌的混合物及其近端沉积物的起源引发稀和浓的火山碎屑流

在涉及气体-颗粒混合物的流动中，质量、动量、比内能和颗粒温度的时间相关守恒方程的数值解用于探索从坍塌混合物（例如喷泉喷发期间）引发的火山碎屑流。一个目标是确定深度平均颗粒流模型或稀流箱模型何时最适用于柱倒塌产生的火山碎屑流的危害建模；第二个目标是深入了解冰凝灰岩近端角砾岩相的形成。撞击平坦表面的坍缩气体-颗粒混合物被建模为与气相耦合不良的粗颗粒和耦合良好的细颗粒的混合物。产生的横向流动对冲击速度敏感，总体颗粒浓度，以及细颗粒和粗颗粒的比例。对于大约 1 vol.% 的总颗粒浓度，相对于细颗粒，由至少 ~ 50% 粗颗粒组成的撞击混合物将倾向于形成集中的横向底流，这可以通过深度平均颗粒流模型近似为从影响区开始的危害评估目的。低总粒子浓度（例如，总浓度约为 0.1 vol.%）往往会产生稀释的侧向流，可以将其简化为用于稀释火山碎屑流的盒模型方法。如果撞击混合物中有任何粗颗粒，较大的总颗粒浓度 (~ 10 vol.%) 会产生颗粒底流，但这些可能会因马赫数效应而变得复杂。对于中间浓度的影响混合物，发展集中底溢与仅稀释电流的粗略阈值是基于粗颗粒每单位面积撞击的通量及其斯托克斯数。一般来说，需要对喷发柱（喷泉）条件有一定的了解，以便就哪种危害建模方法最适用于给定场景做出明智的决定。建模表明，近端角砾岩与粗粒围岩材料流入喷发混合物有关，这会增加总颗粒浓度和混合物中粗大、致密碎屑的比例，这些碎屑随后坍塌并撞击地面。角砾岩记录了在撞击和形成集中流后立即集中的主要粗碎屑，该集中流横向传播，同时排出细粒和气体作为稀流在头顶快速流动。近端沉积物的横向和纵向非均质性可能记录了物质雪崩进入喷发口的快速时间和空间变化，角砾岩丘陵的出现可能记录了撞击区的临时位置。