Gravity-driven granular free-surface flow around a circular cylinder
Gravity-driven granular free-surface flow around a circular cylinder
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DOI：
10.1017/jfm.2013.42
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发表时间：
2013-04-01
影响因子：
3.7
通讯作者：
Gray, J. M. N. T.
中科院分区：
文献类型：
--
作者：
Cui, X.;Gray, J. M. N. T.
雪崩和其他危险的地球物理粒状流，如泥石流、火山泥流和火山碎屑流，在沿着斜坡流下时经常撞击障碍物，使其附近的水流高度和速度发生迅速变化。了解颗粒状物质如何绕过这些障碍物，对于改进偏转和截流水坝的设计以及正确解释现场观测结果非常重要。本文采用小尺度实验和数值模拟的方法研究了颗粒状雪崩在重力驱动下绕圆柱体的超临界自由表面流动。我们的实验表明，在圆柱体前产生了一个非常尖锐的弓形激波和一个滞止点。激波距离通过激波捕获数值模拟精确再现，近似等于弗劳德数的倒数，与先前浅水流动的近似结果一致。当颗粒在圆柱体周围移动时，流动扩大，压力梯度迅速将颗粒再次加速到超临界速度。内部压力不足以立即将颗粒推入圆柱体后面的空间，相反，在背风侧形成无颗粒区域或颗粒真空。对于适度的上游弗劳德数和坡度，颗粒真空迅速闭合形成一个三角形区域，但在较陡的坡度上，实验和数值模拟都表明，夹断距离向下游移动很远。
Snow avalanches and other hazardous geophysical granular flows, such as debris flows, lahars and pyroclastic flows, often impact on obstacles as they flow down a slope, generating rapid changes in the flow height and velocity in their vicinity. It is important to understand how a granular material flows around such obstacles to improve the design of deflecting and catching dams, and to correctly interpret field observations. In this paper small-scale experiments and numerical simulations are used to investigate the supercritical gravity-driven free-surface flow of a granular avalanche around a circular cylinder. Our experiments show that a very sharp bow shock wave and a stagnation point are generated in front of the cylinder. The shock standoff distance is accurately reproduced by shock-capturing numerical simulations and is approximately equal to the reciprocal of the Froude number, consistent with previous approximate results for shallow-water flows. As the grains move around the cylinder the flow expands and the pressure gradients rapidly accelerate the particles up to supercritical speeds again. The internal pressure is not strong enough to immediately push the grains into the space behind the cylinder and instead a grain-free region, or granular vacuum, forms on the lee side. For moderate upstream Froude numbers and slope inclinations, the granular vacuum closes up rapidly to form a triangular region, but on steeper slopes both experiments and numerical simulations show that the pinch-off distance moves far downstream.