A numerical study of turbulent mixing in eruption clouds using a three-dimensional fluid dynamics model
A numerical study of turbulent mixing in eruption clouds using a three-dimensional fluid dynamics model
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DOI：
10.1029/2004jb003460
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发表时间：
2005-08-02
影响因子：
3.9
通讯作者：
Hachisu, I
中科院分区：
文献类型：
--
作者：
Suzuki, YJ;Koyaguchi, T;Hachisu, I
爆发性火山喷发中喷发云的动力学是由湍流混合将环境空气夹带到喷发云中所控制的。我们开发了一个新的数值伪气体模型的喷发云采用三维坐标，三阶精度的计划，和一个细网格尺寸，以调查由于湍流混合的卷吸的行为。夹带的定量特征通过将流边缘处的流入速度与平均垂直速度（即，夹带系数）。我们的模型已经成功地再现了在实验室实验中观察到的夹带的定量特征以及喷发云的动力学的基本特征，例如喷发柱和/或火山碎屑流的产生。喷发云卷吸系数的值估计从柱高度和柱崩溃的临界条件，通过比较我们的模型与以前的一维模型的结果。建议喷发云的卷吸系数的值是近似恒定的，虽然从柱倒塌的临界条件（k类似于0.07）的值是略小于基于柱高度（k类似于0.1）。这种差异反映了喷发云中气流结构的垂直变化。在上部区域的喷发云表现出蜿蜒的不稳定性，这导致有效的混合，而云附近的通风口保持一个同心的结构与内部致密的核心包围外剪切区。该模型不仅与已有的一维稳定喷发云模型相一致，而且也适用于实际喷发云的非稳态和瞬态特征。
The dynamics of eruption clouds in explosive volcanic eruptions are governed by entrainment of ambient air into eruption clouds by turbulent mixing. We develop a new numerical pseudo gas model of an eruption cloud by employing three-dimensional coordinates, a third-order accuracy scheme, and a fine grid size in order to investigate the behavior of entrainment due to turbulent mixing. The quantitative features of entrainment are measured by a proportionality constant relating the inflow velocity at the edge of the flow to the average vertical velocity (i.e., the entrainment coefficient). Our model has successfully reproduced the quantitative features of entrainment observed in the laboratory experiments as well as fundamental features of the dynamics of eruption clouds, such as the generation of eruption columns and/or pyroclastic flows. The value of the entrainment coefficient for eruption clouds is estimated from the column height and critical condition for column collapse by comparing results of our model with those of previous one-dimensional models. It is suggested that the value of the entrainment coefficient for an eruption cloud is approximately constant, although the value estimated from the critical condition for column collapse (k similar to 0.07) is slightly smaller than that based on the column height (k similar to 0.1). This difference reflects the vertical change of flow structure in the eruption cloud. The eruption cloud in the upper region exhibits a meandering instability, which leads to efficient mixing, whereas the cloud near the vent maintains a concentric structure with an inner dense core surrounded by an outer shear region. Our model is consistent with previous one-dimensional models for steady eruption clouds supported by the laboratory experiments, and it is also applicable to unsteady and transient features of actual eruption clouds.