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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Experimental study of turbulence, sedimentation, and coignimbrite mass partitioning in dilute pyroclastic density currents
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Experimental study of turbulence, sedimentation, and coignimbrite mass partitioning in dilute pyroclastic density currents

机译：稀热碎屑密度流中湍流，沉降和共燃物质分配的实验研究

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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.
机译：由湍流悬浮在空气中的温暖滑石粉组成的实验室密度流模拟了稀释的火山碎屑密度流（PDC）的许多方面，并证明了大电流行为，沉降和湍流结构之间的联系。理查森，弗洛德，斯托克斯和沉降数的密度和热值与天然PDC的密度和热值与动能密度之比相匹配。实验电流比自然PDC具有更低的体积雷诺数，但实验是完全湍流的。因此，实验与某些自然电流的稀释部分动态相似。通常，水流流过实验池的底部，使颗粒沉降，并以湍流的方式夹带，加热和热膨胀空气，直到所有颗粒沉积物或水流浮起并升起，形成共燃烟羽。当形成羽流时，电流经常会发生局部逆流。当前的跳动距离和升空位置随着密度理查森数和热能密度的增加而减小。随着这些参数的增加，总沉降量将减少，使得> 50％的初始电流质量通常会分流进入羽流中，这与近期火山喷发的一些观测结果是一致的。沉降曲线最好用夹带沉积模型来描述，而不是用非夹带箱模型得到的指数拟合来描述。时间序列分析表明，在实验中沉淀不是恒定速率的过程，而是一系列沉积-侵蚀对联发生，这些对联在池底传播并跟踪当前的运动和行为。在浮升过程中，上升的羽状流下方的沉积常常变得井井有条。颗粒浓度时间序列的自相关分析用于表征电流的湍流结构，并表明电流迅速分配为缓慢移动的上部和更快，更集中的下部。空气夹带在上部区域内发生。下部区域内的湍流结构跟踪沉积-侵蚀波，表明涡旋控制沉积。重要的是，涡流和沉降波都跟踪浮力升空后发生的流动方向逆转。此外，这些结果表明，PDC沉积物中的单个叠片可能会记录单个涡流的通过，因此单个PDC的持续时间可以估算为叠片数量与当前湍流时间尺度的乘积。

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