The flow structure of jets from transient sources and implications for modeling short‐duration explosive volcanic eruptions
The flow structure of jets from transient sources and implications for modeling short‐duration explosive volcanic eruptions
复制标题
瞬态源射流的流动结构及其对模拟短时火山喷发的影响
DOI：
10.1002/2014gc005471
复制
发表时间：
2014
期刊：
影响因子：
3.7
通讯作者：
J. Phillips
中科院分区：
文献类型：
--
作者：
K. Chojnicki;A. Clarke;R. J. Adrian;J. Phillips
我们使用实验室实验来研究中性浮力喷流的上升过程，这是由不稳定的动量供应造成的，这种情况定义了离散的火山和斯特龙博利亚式喷发产生的羽流。我们同时测量了模拟射流的放电率(动量供应速率)和模拟射流内部的速度分布(动量输运和稀释的结果)。然后，我们研究了模拟喷流速度分布随时间的变化，以评估供应速率变化对动量驱动的上升动力学的影响。我们发现，随着供给速率的变化，模拟射流的速度分布发生了显著而迅速的变化，以至于在任何时刻的全场分布都与时间平均值有很大的不同。我们还发现，在单个非定常射流中，卷吸在空间和时间上都是不同的，瞬时卷吸系数值在0到0.93之间。因此，我们得出结论，供给速率变化对喷流动力学起到一级控制作用，因此在不影响模型预测大范围喷发行为的情况下，不能忽略该变化。这些结果强调了非定常和定常射流动力学之间的根本区别，并清楚地表明：(I)源动量通量的变化直接控制所产生的流动的动力学；(Ii)由变通量的源驱动的脉冲流不能合理地用准定常流动模型来近似。在能够可靠地计算火山喷发羽流的轨迹、稀释度和稳定性以进行危险管理之前，需要能够描述瞬时火山喷发羽流随时间变化的新的建模方法。
We used laboratory experiments to examine the rise process in neutrally buoyant jets that resulted from an unsteady supply of momentum, a condition that defines plumes from discrete Vulcanian and Strombolian‐style eruptions. We simultaneously measured the analog‐jet discharge rate (the supply rate of momentum) and the analog‐jet internal velocity distribution (a consequence of momentum transport and dilution). Then, we examined the changes in the analog‐jet velocity distribution over time to assess the impact of the supply‐rate variations on the momentum‐driven rise dynamics. We found that the analog‐jet velocity distribution changes significantly and quickly as the supply rate varied, such that the whole‐field distribution at any instant differed considerably from the time average. We also found that entrainment varied in space and over time with instantaneous entrainment coefficient values ranging from 0 to 0.93 in an individual unsteady jet. Consequently, we conclude that supply‐rate variations exert first‐order control over jet dynamics, and therefore cannot be neglected in models without compromising their capability to predict large‐scale eruption behavior. These findings emphasize the fundamental differences between unsteady and steady jet dynamics, and show clearly that: (i) variations in source momentum flux directly control the dynamics of the resulting flow; (ii) impulsive flows driven by sources of varying flux cannot reasonably be approximated by quasi‐steady flow models. New modeling approaches capable of describing the time‐dependent properties of transient volcanic eruption plumes are needed before their trajectory, dilution, and stability can be reliably computed for hazards management.