Effects of debris entrainment and recycling on explosive volcanic eruption jets and columns
Effects of debris entrainment and recycling on explosive volcanic eruption jets and columns
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碎片夹带和回收对爆炸性火山喷发射流和柱的影响
DOI：
10.1007/s00445-023-01675-8
复制
发表时间：
2023
影响因子：
3.5
通讯作者：
Valentine, Greg A.
中科院分区：
文献类型：
--
作者：
Valentine, Greg A.
多相流体动力学模型用于探索颗粒碎片夹带进入持续火山喷流的影响，例如产生亚普林尼式到普林尼式喷发柱的火山喷流。这些碎片可能来自火山口壁的雪崩或先前喷发物质的回收等过程。结果表明，碎片并没有立即均匀地混合到射流中，而是形成一个致密的鞘，围绕射流边缘向上拖动。虽然相对于喷发排放率而言非常少量的碎片随着高度的增加逐渐混合到喷射流中，但致密的鞘层可以抑制空气夹带进入喷射流的下部，这可以解释柱不稳定的迹象，例如在岩屑含量增加的沉降物沉积物中分层增加。随着碎片体积的增加，致密的鞘层可能会从一定高度范围内塌陷，为火山碎屑流提供能量。夹带碎片鞘的存在与一些假设相矛盾，例如一维模型中常用的密度和速度的顶帽剖面。从产生沉降物的浮力柱到塌陷行为的转变可能与碎片夹带有关，而初级喷发参数（例如喷口尺寸、出口速度或气体含量）没有任何变化。沸腾行为也可能是由碎片夹带引起的，包括回收先前喷发的物质，例如可能发生在出口受限的火山口中。当夹带的碎片相对细粒度时，它可以与喷发的混合物很好地耦合，由于碎片流入射流底部的挤压效应，可能会发生复杂的、高度瞬态的超压射流过程。越来越粗的碎片导致喷射在夹带材料的护套内准直。结果表明，考虑碎片夹带的影响对于许多自然喷发序列的理论评估以及潜在的亚普林尼式到普林尼式活动的危险场景评估可能很重要。
A multiphase fluid dynamic model is used to explore the effects of entrainment of granular debris into sustained volcanic jets such as those which produce sub-Plinian to Plinian eruption columns. The debris may be sourced from processes such as avalanches from crater walls or from recycling of previously erupted material. The results indicate that debris is not immediately, homogeneously mixed into a jet but instead forms a dense sheath that is dragged upward around the jet margin. While very small volumes of debris relative to the eruptive discharge rate mix progressively into the jet with increasing altitude, the dense sheath can inhibit entrainment of air into the lower portions of the jet, which may explain signs of column instability such as increased stratification in fallout deposits where lithic content increases. As debris volume increases, the dense sheath can collapse from a range of elevations to feed pyroclastic currents. The presence of the sheath of entrained debris contradicts some assumptions such as the top-hat profile for density and velocity that is commonly used in 1-D models. Transitions from fallout-producing buoyant column to collapsing behavior can be related to debris entrainment without any changes in primary eruption parameters such as vent size, exit velocity, or gas content. Boiling-over behavior can also be caused by debris entrainment, including recycling of previously erupted material such as might occur in a crater with restricted outlet. When entrained debris is relatively fine-grained such that it can couple well with the erupting mixture, complex, highly transient overpressured jet processes can occur due to the pinching effect of debris flowing into the base of the jet. Increasingly coarse debris causes collimation of the jet within the sheath of entrained material. The results suggest that accounting for the effects of debris entrainment is likely important for theoretical assessment of many natural eruption sequences and for assessment of hazard scenarios for potential sub-Plinian to Plinian activity.
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