(116bd) Granular Flow Instabilities in a Two-Dimensional Taylor-Couette Cell

Granular flows are common in many industrial processes, but they are not well understood. The behavior of granular materials can range from rapid, fluid like flows to slow, solid like flows depending on the system's concentration and applied shear. An added complexity in granular flows is the formation of flow instabilities such as clustering and jamming due to changes at the microscopic level. These flow instabilities can have an adverse effect on industrial processes. A parametric study was performed using particle dynamic simulations to examine the formation of granular flow instabilities in a pseudo two-dimensional Taylor-Couette cell. The Taylor-Couette cell consisted of a stationary outer cylinder and an inner cylinder that rotates at 190 rad/s with an inner to outer cylinder gap of 11 particle diameters. The height of the cell was 5 particle diameters with periodic boundaries imposed along the axial direction. Initially we characterized the granular behavior in the cell for particle properties and system conditions leading to a stable, homogeneous flow. Particle and system properties were then modified to identify factors affecting system stability and macroscopic flow. Increasing rolling friction and decreasing coefficient of restitution caused density waves instabilities to form in the cell at high solids fraction. In contrast, increasing sliding friction led to formation of similar instabilities at lower solids fraction. These results suggest that an increase in energy dissipation leads to instability formation, and that the form of energy dissipation leading to these instabilities is dependent on the particle volume fraction.