Formation and Collapse of Cavity Network in a Granular Material due to Viscous Flow and Vibration

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The presence of cavity regions in an otherwise uniform granular material makes great influence on the local flow field around that cavity. We have already shown that the volume flow rate and the velocity at the center of the latter attains as large as two (three) times and three (six) times for a cylindrical (spherical) cavity, respectively, in comparison to the case without a cavity. At the same time, the local stress at the boundary of the cavity is enhanced, and the collapse of the latter is induced. We have also shown qualitatively that these effects are increased in the presence of many cavities with certain configurations. In order to examine these findings quantitatively, we performed new experiments and numerical simulations.1.Experiments on the collapse of cavities due to viscous flow : We observed the rate of change and the critical velocity of the collapse of two interacting cavities. By changing the center-to-center distance and the orientation of the two cavities, we eluci … More dated the fundamental process of the collapse of cavities, as well as the configurations in which the collapse occurs below the critical velocity of collapse for a single cavity case. These results can be applied to the prediction of landslides at the time of heavy rainfall and a new technique to predict anomalously fast convection of contaminants in underground water flow through cavity networks.2.Mathematical models of cavity network formation : We performed numerical simulation of the interaction of many cavities exposed to viscous flow, on the basis of cellular-automaton type model. Our model can explain the movement, elongation and merging of cavities, depending on the stiffness of the medium and the applied velocity, which eventually forms large-scale network of cavity regions or fluidization.3.Experiments on the deformation of granular surface : We observed regularly spaced ripples and Barchan dunes depending on the shear rate of viscous flow over the surface of the granular material. These results are applicable to the prediction of spread of desert area and the control of seabed level in the bay area.4.Experiments on the collision of vortex ring on the granular surface : Typical ring-like crater with a wavy edge or a lobe was generated, that can explain the Rampart crater observed only on Mars. This observation suggests the presence of fluid flow at the earlier time of Mars' environment as well as the mechanism of this crater formation.5.Experiments on the pattern formation of vertically vibrating granular layer : Thin layer of granular materials were vertically vibrated, which revealed ripples and undulations. We elucidated the mechanism of the formation of these patterns, and at the same time obtained the critical layer number that showed collective motion like those observed in continuum media. The behavior was partially explained by the buckling and bending motion of thin elastic layer.All our studies are expected to bridge the gap between microscopic collision process and macroscopic fluid motion of the granular material, and our works are presented at four major international conferences and domestic meetings. Less