Dispersal and air entrainment in unconfined dilute pyroclastic density currents
Dispersal and air entrainment in unconfined dilute pyroclastic density currents
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DOI：
10.1007/s00445-014-0852-4
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发表时间：
2014-09-01
影响因子：
3.5
通讯作者：
Andrews, Benjamin J.
中科院分区：
文献类型：
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作者：
Andrews, Benjamin J.
无限制规模的实验室实验表明，三维结构控制着稀薄火山碎屑密度流(PDC)在发射过程中和发射后的行为。实验包括加热和环境温度20微米的滑石粉，将其湍流悬浮在空气中，在一个8.5x6x2.6米的无障碍室内形成密度电流。比较了Richardson、热Richardson、Froude、Stokes和沉降数以及浮力热能和动能密度，结果表明实验电流与稀释PDC之间有很好的一致性。实验雷诺数低于PDC的实验雷诺数，但实验是完全湍流的，因此，两个系统的大尺度动力学相似。在三个正交平面上的高频同步观测表明，电流的行为与以前的二维(即受限)电流有很大的不同。具体地说，尽管环境温度流表现出径向扩散模式，但浮力反转和热流的升空集中在上升羽流下方的狭窄轴线上扩散。纵横比定义为电流长度除以特征宽度，在加热电流中通常为2.5-3.5，在环境温度电流中通常为1.5-2.5，反映了这两种类型电流之间的扩散差异。两股气流卷吸空气的机制有很大不同：卷吸主要发生在环境温度流的头部后面和上缘，而加热的气流则通过其侧缘卷吸空气。横向卷吸比垂直卷吸要有效得多，前者为0.5，后者类似于0.1，后者的定义是横流速度与流向速度之比。这些实验表明，共沸石羽流的生成应该沿着狭窄的传输轴聚集PDC，导致细长的而不是放射状的沉积。
Unconfined scaled laboratory experiments show that 3D structures control the behavior of dilute pyroclastic density currents (PDCs) during and after liftoff. Experiments comprise heated and ambient temperature 20 mu m talc powder turbulently suspended in air to form density currents within an unobstructed 8.5x6x2.6-m chamber. Comparisons of Richardson, thermal Richardson, Froude, Stokes, and settling numbers and buoyant thermal to kinetic energy densities show good agreement between experimental currents and dilute PDCs. The experimental Reynolds numbers are lower than those of PDCs, but the experiments are fully turbulent; thus, the large-scale dynamics are similar between the two systems. High-frequency, simultaneous observation in three orthogonal planes shows that the currents behave very differently than previous 2D (i.e., confined) currents. Specifically, whereas ambient temperature currents show radial dispersal patterns, buoyancy reversal, and liftoff of heated currents focuses dispersal along narrow axes beneath the rising plumes. The aspect ratios, defined as the current length divided by a characteristic width, are typically 2.5-3.5 in heated currents and 1.5-2.5 in ambient temperature currents, reflecting differences in dispersal between the two types of currents. Mechanisms of air entrainment differ greatly between the two currents: entrainment occurs primarily behind the heads and through the upper margins of ambient temperature currents, but heated currents entrain air through their lateral margins. That lateral entrainment is much more efficient than the vertical entrainment, >0.5 compared to similar to 0.1, where entrainment is defined as the ratio of cross-stream to streamwise velocity. These experiments suggest that generation of coignimbrite plumes should focus PDCs along narrow transport axes, resulting in elongate rather than radial deposits.