(210d) Taylor Vortex Flow in Dry Granular Media

Classical instabilities such as Rayleigh–Bernard rolls, or Taylor vortices are quite familiar in conventional fluids when subjected to external conditions. But the question is, do these instabilities exist in the granular flows? If they exist, are the underlying causes the same as those in conventional fluids? The shearing of fluids in the cylindrical Couette device is a classic viscometric flow as the stresses are readily related to the rheology of the fluid. These flows are characterized by the non–dimensional Taylor number and have proven critical to evaluating and establishing fluid rheological measurement protocols over years. As we celebrate the centennial of GI Taylor's groundbreaking paper on Taylor vortex flow from 1923, we demonstrate that these instabilities can exist in the dense dry granular flows using Discrete Element Method (DEM) simulations. In the figure below, panel (a) shows r and z–velocity vectors on a theta–averaged section plane and panel (b) shows radially averaged theta–velocity contours on the stretched outer cylindrical surface. We see that there are close similarities and differences between granular and conventional fluid vortices, qualitatively. Quantitatively, we characterize aspect ratio, vorticity, driving torque, and granular viscosity for various system heights filled with glass beads. The role of the external force in the system is found to be crucial. We are motivated by the fact that the stress and kinematics relationship in granular materials is complex. Investigating these granular viscometric flows will aid in building better rheological models.