| Research Abstract |
1. Pattern formation of granular materials under external oscillation, and
2. deformation and movement of the boundary of granular materials due to fluid flow are investigated. In the former, we made both experimental and numerical approaches to the convection or mixing of the constituent grains under vertical vibration of the container filled with granules. We found "stretching and folding" of the layer, which is one of the chaos-producing mechanisms. We also found ripples on the surface of the layer, which grow into steep standing waves as the layer depth is reduced to less than about ten layers. Closer observation of the waves by a high-speed video camera revealed that they oscillated with a frequency one half or one fourth of the externally applied oscillation. We further obtained the dependence of the wave-length on the frequency and amplitude of the external forcing, granule size and layer depth, etc. In the horizontally thin granular layer, we found squares, stripes, triangle/hexagonal, and quasi-crystal planar patterns as the magnitude of oscillation was increased. In the latter case, we analyzed the effect of the void space in an otherwise uniform flow in an unbounded granular material, which was compared with experiment. When cylindrical circular cavity are present, the velocity at the center of the void becomes three times, while the volume flow into that region amounts to twice larger (on the other hand in the spherical void case, they become, respectively, six times and three times larger) than those without the hole. The increment of the flow near the void surface leads to the collapse of the boundary. In the multi-void case, they interact each other to form a linked voids, or waterway network. The last observation suggests a fundamental process of landslides or avalanche due to heavy rain fall on the ground.
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