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首页> 外文期刊>Journal of Volcanology and Geothermal Research2012V243-244NOCT,15 >The impact of particle shape on fall velocity: Implications for volcanic ash dispersion modelling
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The impact of particle shape on fall velocity: Implications for volcanic ash dispersion modelling

机译：颗粒形状对下落速度的影响：对火山灰弥散模型的启示

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Modelling atmospheric volcanic ash dispersion is a critical tool in mitigating the impact of large explosive eruptions; it is also useful for understanding and reconstructing past events. Most atmospheric dispersion models include a sedimentation velocity term that is sensitive to the physical properties of the particle, but many do not use particle shape as an input parameter; instead particles are assumed to be spherical. There are many empirical and semi-empirical shape-dependent drag laws. We measure the velocity of scaled analogue particles over the range of flow conditions anticipated for volcanic ash dispersion to test published formulae against an independent dataset. We use a semi-empirical formula we determined to be accurate for non-spheres to investigate the sensitivity of the modelled transport of an ash cloud to particle shape, using the atmospheric dispersion model NAME and a shape parameter we measure for non-spherical ash particles from Katla volcano, Iceland. We find that model particle trajectories are sensitive to particle shape for particles 1-3 mu m diameter; the sedimentation velocity of smaller particles is low compared to atmospheric vertical velocities. Sensitivity to shape increases with size such that 100 pm particles can travel 44% further from the source when they are highly non spherical (sphericity = 0.5). Despite the sensitivity of the fall velocity of large particles to their shape, however, forecasts of distal ash concentration using particle size distributions of 0.1-100 mu m and 0.1-250 mu m show relatively good agreement between a spherical and non-spherical case for the first 36 h after an eruption. The vertical structure of an ash cloud is more sensitive to particle shape than the horizontal extent. Model particle trajectories are also sensitive to partide size, and we find a discrepancy between different particle size parameters for non spherical ash: particle long axis L, used in cryptotephra studies, was on average twice the equivalent-volume sphere diameter d(v), used in dispersion modelling, for tephra samples from Katla volcano, Iceland. This discrepancy in size measurements could explain the observed travel distance of large distal cryptotephra shards. (C) 2018 Published by Elsevier B.V.
机译：模拟大气火山灰的扩散是减轻大型爆炸爆发影响的关键工具。它对于理解和重建过去的事件也很有用。大多数大气弥散模型都包含一个对颗粒的物理特性敏感的沉降速度项，但是许多模型并不使用颗粒形状作为输入参数。相反，假定粒子为球形。有许多经验和半经验形状相关的阻力定律。我们在火山灰弥散预期的流动条件范围内测量比例化的模拟颗粒的速度，以针对独立的数据集测试已发布的公式。我们使用确定为非球形的半经验公式，使用大气弥散模型NAME和我们测量的非球形灰粒形状参数来研究灰云对颗粒形状的运输模型的敏感性来自冰岛卡特拉火山。我们发现，对于直径大于1-3微米的粒子，模型粒子的轨迹对粒子形状很敏感。与大气垂直速度相比，较小颗粒的沉降速度较低。形状的敏感性会随着尺寸的增加而增加，这样当高度为非球形（球形度= 0.5）时，100 pm的颗粒可以从源头传播44％。尽管大颗粒的下落速度对其形状敏感，但是，使用0.1-100μm和0.1-250μm的粒径分布来预测远端灰分浓度，表明球形和非球形情况下相对较好的一致性。喷发后的头36小时。灰云的垂直结构比水平范围对颗粒形状更敏感。模型粒子的轨迹也对粒子的大小敏感，我们发现非球形灰的不同粒度参数之间存在差异：用于密码学研究的粒子长轴L平均为等效体积球体直径d（v）的两倍，用于色散建模，用于来自冰岛卡特拉火山的特非拉样品。尺寸测量结果的这种差异可以解释观察到的大远端隐睑碎片的行进距离。 （C）2018由Elsevier B.V.发布

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