Experimental study of turbulence, sedimentation, and coignimbrite mass partitioning in dilute pyroclastic density currents
Experimental study of turbulence, sedimentation, and coignimbrite mass partitioning in dilute pyroclastic density currents
复制标题
DOI：
10.1016/j.jvolgeores.2012.02.011
复制
发表时间：
2012-05-01
影响因子：
2.9
通讯作者：
Manga, Michael
中科院分区：
文献类型：
--
作者：
Andrews, Benjamin J.;Manga, Michael
实验室密度电流包括温暖的滑石粉悬浮在空气中模拟稀释的火山碎屑密度电流（PDCs）的许多方面，并证明散装电流行为，沉积和湍流结构之间的联系。密度和热Richardson，弗劳德，斯托克斯，和解决数匹配的自然PDCs的热动能密度比。实验电流具有较低的体积雷诺数比自然PDC，但实验是完全湍流。因此，实验在动力学上类似于一些自然海流的稀释部分。一般来说，水流穿过实验槽的底板，沉积颗粒，并有害地夹带、加热和热膨胀空气，直到所有颗粒沉积物或水流变得有浮力并升空形成coignimbrite羽状物。当羽状流形成时，海流经常发生局部的逆流。随着密度Richardson数和热能密度的增加，电流的跳动距离和提离位置减小。随着这些参数的增加，总沉积量减少，以至于超过50%的初始电流质量通常会分离成羽流，这与最近火山爆发的一些观察结果一致。沉积剖面最好用夹带沉积模型来描述，而不是用非夹带箱模型得出的指数拟合。时间序列分析表明，在实验中，沉积不是一个恒定速率的过程，而是发生作为一系列的沉积-侵蚀对，在罐底传播跟踪当前的运动和行为。在浮力升空过程中，上升羽流下方的沉积物往往变得不那么有组织。颗粒浓度的时间序列的自相关分析是用来表征湍流结构的电流，并表明电流迅速分区成一个缓慢移动的上部和更快，更集中，下部。空气夹带发生在上部区域内。下部区域内的湍流结构跟踪沉积侵蚀波，并表明涡流控制沉积。重要的是，漩涡和沉积波跟踪浮力升空后发生的流动方向的逆转。此外，这些结果表明，PDC沉积物内的各个叠层可以记录单个涡流的通过，因此各个PDC的持续时间可以被估计为叠层的数量和电流的湍流时间尺度的乘积。由爱思唯尔公司出版
Laboratory density currents comprising warm talc powder turbulently suspended in air simulate many aspects of dilute pyroclastic density currents (PDCs) and demonstrate links between bulk current behavior, sedimentation, and turbulent structures. The densimetric and thermal Richardson, Froude, Stokes, and settling numbers match those of natural PDCs as does the ratio of thermal to kinetic energy density. The experimental currents have lower bulk Reynolds numbers than natural PDCs, but the experiments are fully turbulent. Consequently, the experiments are dynamically similar to the dilute portions of some natural currents. In general, currents traverse the floor of the experimental tank, sedimenting particles and turbulently entraining, heating, and thermally expanding air until all particle sediments or the currents become buoyant and lift off to form coignimbrite plumes. When plumes form, currents often undergo local flow reversals. Current runout distance and liftoff position decrease with increasing densimetric Richardson number and thermal energy density. As those parameters increase, total sedimentation decreases such that >50% of initial current mass commonly fractionates into the plumes, in agreement with some observations of recent volcanic eruptions. Sedimentation profiles are best described by an entraining sedimentation model rather than the exponential fit resulting from non-entraining box models. Time series analysis shows that sedimentation is not a constant rate process in the experiments, but rather occurs as series of sedimentation-erosion couplets that propagate across the tank floor tracking current motion and behavior. During buoyant liftoff, sedimentation beneath the rising plumes often becomes less organized. Auto-correlation analysis of time series of particle concentration is used to characterize the turbulent structures of the currents and indicates that currents quickly partition into a slow-moving upper portion and faster, more concentrated, lower portion. Air entrainment occurs within the upper region. Turbulent structures within the lower region track sedimentation-erosion waves and indicate that eddies control deposition. Importantly, both eddies and sedimentation waves track reversals in flow direction that occur following buoyant liftoff. Further, these results suggest that individual laminations within PDC deposits may record passage of single eddies, thus the duration of individual PDCs may be estimated as the product of the number of laminations and the current's turbulent timescale. Published by Elsevier B.V.