Experiments on conduit flow and eruption behavior of basaltic volcanic eruptions
Experiments on conduit flow and eruption behavior of basaltic volcanic eruptions
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玄武质火山喷发管道流及喷发行为实验
DOI：
10.1029/2000jb900096
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发表时间：
2000
影响因子：
--
通讯作者：
A. Freundt
中科院分区：
文献类型：
--
作者：
R. Seyfried;A. Freundt
采用模拟实验和理论分析相结合的方法研究了玄武质火山岩管道中的多相流动。取决于气体供应，大气泡（气栓）可能会上升通过玄武岩浆在制度的不同流体动力学行为：上升的单段塞，供应段塞从气源在上升过程中，和周期性的段塞流。环形流状态开始于最高的气体供应速率。第一组实验表明，由于静水减压的气体段塞的增长并不影响它们的上升速度，并且段塞中的过压仍然可以忽略不计。在第二组实验中评估了描述段塞上升速度作为液体和管道特性的函数的理论公式的适用性。第三组实验与连续气体供应到一个圆柱形导管的玄武岩条件下，莫顿，Eotvos，雷诺数，弗劳德数。气体流速和液体粘度在整个流动状态范围内变化，以观察流动动力学并测量气体和液体喷发速率。在表面由段塞爆破产生的泡沫和由尾流湍流引起的部分段塞破裂可以改变岩浆的气泡含量和尺寸分布。在从段塞流到环形流的过渡过程中，当气段塞之间的液体桥消失时，管道入口处的压力从流体静力学值下降到环形流的动态流阻，这可能会触发储存的岩浆中的进一步脱气，以维持环形流态，直到气体供应耗尽，喷发突然结束。当岩浆上升受到火山管道内壁摩擦的阻碍，环形气流将管道中的岩浆全部侵蚀掉时，岩浆排出也会终止。供给的段塞被发现达到更高的上升速度比未供给的段塞和崩溃的湍流环形流在表面爆裂。第四组实验使用了一个管道，该管道被内置障碍物部分堵塞，为气穴提供陷阱。一旦气穴被填充，上升的气栓就会变形，但在绕过障碍物时保持完整，而不会合并或发生显着的速度变化。气泡与被困气穴聚结的破裂导致压力信号至少比通过液体引起的气穴振荡强3个数量级。我们的实验表明，斯特隆波和夏威夷火山喷发的精细分类，根据时间依赖性的行为成零星脉动的熔岩喷泉（驱动随机上升的单段塞），周期性脉动的熔岩喷泉（导致段塞流），准稳定的熔岩喷泉（振荡的频率环流湍流）。
Multiphase flow in basaltic volcanic conduits is investigated using analog experiments and theoretical approaches. Depending on gas supply, large gas bubbles (gas slugs) may rise through basaltic magma in regimes of distinct fluid-dynamical behavior: ascent of single slugs, supplied slugs fed from the gas source during ascent, and periodic slug flow. An annular flow regime commences at the highest gas supply rates. A first set of experiments demonstrates that the growth of gas slugs due to hydrostatic decompression does not affect their ascent velocity and that excess pressure in the slugs remain negligible. The applicability of theoretical formulae describing slug ascent velocity as a function of liquid and conduit properties is evaluated in a second set of experiments. A third set of experiments with continuous gas supply into a cylindrical conduit are scaled to basaltic conditions over Morton, Eotvos, Reynolds, and Froude numbers. Gas flow rate and liquid viscosity are varied over the whole range of flow regimes to observe flow dynamics and to measure gas and liquid eruption rates. Foam generation by slug bursting at the surface and partial slug disruption by wake turbulence can modify the bubble content and size distribution of the magma. At the transition from slug to annular flow, when the liquid bridges between the gas slugs disappear, pressure at the conduit entrance drops by ∼60% from the hydrostatic value to the dynamic-flow resistance of the annular flow, which may trigger further degassing in a stored magma to maintain the annular flow regime until the gas supply is exhausted and the eruption ends abruptly. Magma discharge may also terminate when magma ascent is hindered by wall friction in long volcanic conduits and the annular gas flow erodes all magma from the conduit. Supplied slugs are found to reach much higher rise velocities than unsupplied slugs and to collapse to turbulent annular flow upon bursting at the surface. A fourth set of experiments uses a conduit partially blocked by built-in obstacles providing traps for gas pockets. Once gas pockets are filled, rising gas slugs deform but remain intact as they move around obstacles without coalescence or significant velocity changes. Bursting of bubbles coalescing with trapped gas pockets causes pressure signals at least 3 orders of magnitude more powerful than gas pocket oscillation induced by passing liquid. Our experiments suggest a refined classification of Strombolian and Hawaiian eruptions according to time-dependant behavior into sporadically pulsating lava fountains (driven by stochastic rise of single slugs), periodically pulsating lava fountains (resulting from slug flow), and quasi-steady lava fountains (oscillating at the frequency of annular-flow turbulence).