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首页> 外文期刊>Journal of Volcanology and Geothermal Research >The effect of intensity of turbulence in umbrella cloud on tephra dispersion during explosive volcanic eruptions: Experimental and numerical approaches
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The effect of intensity of turbulence in umbrella cloud on tephra dispersion during explosive volcanic eruptions: Experimental and numerical approaches

机译：爆发性火山喷发中伞云湍流强度对特发拉扩散的影响：实验和数值方法

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摘要

During an explosive volcanic eruption, tephra fall out from the umbrella region of the eruption cloud to the ground surface. We investigated the effect of the intensity of turbulence in the umbrella cloud on dispersion and sedimentation of tephra by performing a series of laboratory experiments and three dimensional (3-D) numerical simulations. In the laboratory experiments, spherical glass-bead particles are mixed in stirred water with various intensities of turbulence, and the spatial distribution and the temporal evolution of the particle concentration are measured. The experimental results show that, when the root-mean-square of velocity fluctuation in the fluid (W_(rms)) is much greater than the particle terminal velocity (v_t), the particles are homogeneously distributed in the fluid, and settle at their terminal velocities at the base of the fluid where turbulence diminishes. On the other hand, when W_(rms) is as small as or smaller than v_t, the particle concentration increases toward the base of the fluid during settling, which substantially increases the rate of particle settling. The results of the 3-D simulations of eruption cloud indicate that W_(rms) is up to 40 m/s in most of the umbrella cloud even during a large scale plinian eruption with a magma discharge rate of 10~9 kg/s. These results suggest that relatively coarse pyroclasts (more than a few mm in diameter) tend to concentrate around the base of the umbrella cloud, whereas fine pyroclasts (less than 1/8 mm in diameter) may be distributed homogeneously throughout the umbrella cloud during tephra dispersion. The effect of the gradient of particle concentration in the umbrella cloud explains the granulometric data of the Pinatubo 1991 plinian deposits.
机译：在爆炸性火山喷发期间，特非拉从喷发云的伞状区域掉落到地面。我们通过进行一系列的实验室实验和三维（3-D）数值模拟，研究了伞云中湍流强度对特发拉的扩散和沉降的影响。在实验室实验中，将球形玻璃珠颗粒在具有各种湍流强度的搅拌水中混合，并测量颗粒浓度的空间分布和时间演变。实验结果表明，当流体中速度波动的均方根（W_（rms））远大于颗粒末端速度（v_t）时，颗粒均匀地分布在流体中并沉降在流体中湍流减弱处的流体底部的最终速度。另一方面，当W_（rms）小于或等于v_t时，在沉降期间颗粒浓度朝着流体的基部增加，这实质上增加了颗粒沉降的速率。喷发云的3D模拟结果表明，即使在规模较大的普利尼喷发期间，岩浆喷出速率为10〜9 kg / s，W_（rms）在大多数伞云中也高达40 m / s。这些结果表明，相对较粗糙的火山碎屑（直径大于几毫米）倾向于集中在伞状云的底部，而细的火山碎屑（直径小于1/8毫米）可能在特非拉虫期间均匀地分布在整个伞状云中。分散。伞状云中颗粒浓度梯度的影响解释了Pinatubo 1991年普林尼安矿床的粒度数据。

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