Onset of Granular Flows by Local and Global Forcing
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This thesis focuses on the onset of granular flows and memory effects in granular materials under local and global forcing conditions. Global flows are induced in a shear cell of Taylor-Couette type by moving a boundary wall. We find that how a granular shear flow starts depends strongly on the prior shear direction. We observe that when the shear direction is reversed, the material goes through a transient period during which the material compacts, the shear force is small, and the shear band is wide. Three dimensional confocal imaging of particle rearrangements during shear reversal shows that bulk and surface flows are comparable. Local flows are induced by forcing a rod into a fluid immersed granular bed with various preparation methods. Particle rearrangements are observed in 3D by confocal microscopy and by moving a laser sheet through the sample. Image analysis indicates that rearrangements spread farthest not directly under the penetrometer but in a ring around the penetrometer. In addition, the direction of preformed stress chains in the material influences the particle rearrangements. Material compressed from one side exhibits anisotropic particle rearrangements under penetrometer testing.