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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Numerical analysis of the effect of topography on deposition from dilute pyroclastic density currents
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Numerical analysis of the effect of topography on deposition from dilute pyroclastic density currents

机译：地形对稀热碎屑密度流沉积影响的数值分析

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摘要

Pyroclastic density currents (PDCs) vary between two end members, concentrated and dilute. When a PDC interacts with an uneven topography, the flow field variables (velocity, pressure, bulk density, particle concentration) may drastically change near the flow-substrate boundary. These changes may significantly affect the sediment flux and the resulting deposits can record the effects in their facies architecture. Here we show, by means of numerical simulations, how a dilute pyroclastic density current interacts with four different types of simple topography, namely: flat, one hill, one valley and two hills. Our numerical scheme treats the very fine particles as being in full thermo-mechanical equilibrium with the volcanic gas, i.e. a dusty gas. A dusty gas-air mixture is defined as a mixture of dusty gas and atmospheric air. The trajectories of the coarser particles or discrete phase (three grain-size classes of 1mm, 5mm and 10mm and density of 1500kg/m~3) are tracked as Lagrangian particles that interact with the dusty gas-air mixture through two-way momentum and energy coupling. Numerical results are used to analyze the local effects of topography on the deposition of the Lagrangian particles, by monitoring with time and space the local changes at the boundary between the current and the substrate. The results show that the sediment flux in the flow boundary zone increases near the stoss sides of hills and in the valleys, relative to the flat reference case, whereas it decreases along the lee flanks and on top of the hills. We use the sediment flux in the flow boundary zone and the grain-size distribution of the Lagrangian particles as proxies of the deposit features, and by these parameters we qualitatively compare simulations with deposits of known eruptions.
机译：火山碎屑密度电流（PDC）在两个末端构件（集中的和稀的）之间变化。当PDC与不平坦的地形相互作用时，流场变量（速度，压力，堆积密度，颗粒浓度）可能会在流质边界附近急剧变化。这些变化可能会显着影响沉积物通量，产生的沉积物可以记录其相构造的影响。在这里，我们通过数值模拟显示了稀释的火山碎屑密度流如何与四种不同类型的简单地形相互作用，即：平坦，一个丘陵，一个山谷和两个丘陵。我们的数值方案将非常细的颗粒视为与火山气体（即多尘气体）处于完全热机械平衡的状态。含尘气体-空气混合物定义为含尘气体和大气的混合物。跟踪较粗颗粒或离散相（1mm，5mm和10mm三种粒度等级，密度为1500kg / m〜3）的轨迹为拉格朗日颗粒，它们通过双向动量和能量耦合。通过随时间和空间监视电流和衬底之间的边界处的局部变化，使用数值结果来分析地形对拉格朗日粒子沉积的局部影响。结果表明，相对于平坦的参考案例，流边界区中的泥沙通量在丘陵的草丛侧和山谷附近增加，而在后腰和丘陵的顶部则减少。我们使用流边界带中的泥沙通量和拉格朗日粒子的粒度分布作为沉积特征的代表，并通过这些参数定性地将模拟与已知喷发的沉积物进行比较。

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