Transient and Oscillatory Granular Shear Flow

The forces and particle motion during transient and oscillatory shear of granular material are investigated experimentally. In a shear cell of Taylor-Couette-type we find that how a granular shear flow starts depends strongly on the prior shear direction. If 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. Repeated reversals, or oscillations of the shear direction, lead to additional compaction, which can be described by a stretched exponential, similar to compaction induced by tapping.

Figure 1

Schematic of the shear cell.

Figure 2

Average flow velocity of particles $V(r)$ vs shear displacement of the inner cylinder in units of particle diameter $d$. Lines without markers track the inner cylinder motion. Lines with markers trace average velocities in three regions as labeled in the schematic of the top surface. When shear is restarted in the same direction (a), a steady state is reached immediately. When shear is reversed (b), the shear band is initially wider.

Figure 3

Extra displacement $l_e(r)$ as a function of distance $r$ from the shear surface. Compared to steady state shear, particles travel extra displacements due to transiently wider shear band after reversal of shear direction.

Figure 4

Confocal image of the cross section through a granular layer $15d$ inside a granular shear flow (a). Spacetime plot of the $\theta$-averaged intensity of difference images, a measure of the extent of the moving region (b).

Figure 5

Shear stress as a function of shear displacement. When the shear direction is reversed, the shear stress takes about $(5–10)d$ to reach a steady state. Same direction plot shown for comparison.

Figure 6

Column height vs shear displacement. The material compacts over strain of less than one particle diameter and then gradually expands again. Density or volume fraction is shown in square brackets.

Figure 7

Density development under repeated shear reversals as a function of the number of oscillations. Reversal of shear is repeated before full recovery takes place, resulting in further compaction.

Figure 8

Shear strain as a function of time. Depending on the amplitude of oscillation, gradual strengthening in both directions was observed.