Birth and growth of a granular jet
Birth and growth of a granular jet
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DOI：
10.1103/physreve.78.011305
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发表时间：
2008-07-01
影响因子：
2.4
通讯作者：
Jaeger, Heinrich M.
中科院分区：
文献类型：
--
作者：
Royer, John R.;Corwin, Eric I.;Jaeger, Heinrich M.
颗粒床中细颗粒与周围间隙气体之间的相互作用可以导致在简单的颗粒流单流体模型中无法捕获的定性新现象。这一点可以通过固体球体撞击松散细沙床形成的颗粒射流来证明。与流体中的冲击形成的射流不同，这种射流实际上由两个独立的组成部分组成，一个是由冲击物体留下的空腔坍塌形成的初始薄射流，该射流堆叠在第二个较厚的射流之上，该射流强烈依赖于环境气体压力。这种复杂的结构是环境气体、床层颗粒和撞击球之间相互作用的结果。在这里，我们提出了系统的实验结果，联合收割机测量的射流在表面上改变释放高度，球体直径，容器的大小，和床材料的表面下的X射线照相连接床的变化响应的变化结构的射流。我们发现，被困在一个细粒床的低渗透率的间隙气体在射流的形成中起着两个不同的作用。首先，颗粒之间捕获和压缩的气体阻止了压实，导致床像不可压缩的流体一样流动，并允许撞击物体深深地沉入床中。第二，射流是由撞击物体留下的空腔的重力驱动的塌陷引发的。如果空腔足够大，被塌陷空腔捕获和压缩的气体可以通过直接向上推动床层物质并产生厚射流来放大射流。作为这两个因素的结果，当环境气体压力降低时，存在从床的几乎不可压缩的流体状响应到高度可压缩的耗散响应的交叉。床在减压下的压实减小了撞击物体的最终深度，导致更小的空腔和厚射流的消亡。
The interaction between fine grains and the surrounding interstitial gas in a granular bed can lead to qualitatively new phenomena not captured in a simple, single-fluid model of granular flows. This is demonstrated by the granular jet formed by the impact of a solid sphere into a bed of loose, fine sand. Unlike jets formed by impact in fluids, this jet is actually composed of two separate components, an initial thin jet formed by the collapse of the cavity left by the impacting object stacked on top of a second, thicker jet which depends strongly on the ambient gas pressure. This complex structure is the result of an interplay between ambient gas, bed particles, and impacting sphere. Here we present the results of systematic experiments that combine measurements of the jet above the surface varying the release height, sphere diameter, container size, and bed material with x-ray radiography below the surface to connect the changing response of the bed to the changing structure of the jet. We find that the interstitial gas trapped by the low permeability of a fine-grained bed plays two distinct roles in the formation of the jet. First, gas trapped and compressed between grains prevents compaction, causing the bed to flow like an incompressible fluid and allowing the impacting object to sink deep into the bed. Second, the jet is initiated by the gravity driven collapse of the cavity left by the impacting object. If the cavity is large enough, gas trapped and compressed by the collapsing cavity can amplify the jet by directly pushing bed material upwards and creating the thick jet. As a consequence of these two factors, when the ambient gas pressure is decreased, there is a crossover from a nearly incompressible, fluidlike response of the bed to a highly compressible, dissipative response. Compaction of the bed at reduced pressure reduces the final depth of the impacting object, resulting in a smaller cavity and in the demise of the thick jet.