(378e) A Dilation-Driven Secondary Flow in Sheared Granular Materials, and Its Rheological Signature

The cylindrical Couette device is commonly used for viscometric measurements of fluids, but its use for granular materials is problematic: previous studies showed anomalous wall stress profiles^1,2, wherein the vertical shear stress changes sign and all components of the stress increase nearly exponentially with depth. In a recently study³, we have shown the presence of a remarkable dilation-driven secondary flow when a dense granular material is sheared slowly in a cylindrical Couette cell. The secondary flow appears in the form of a single toroidal vortex that spans the entire granular column, and is fundamentally different in origin and manifestation from the Taylor-Couette vortices seen fluids. Most importantly, the vortex explains the anomalous, nearly exponential, rise of the stress with depth.

In this presentation, we report the robustness of the vortex to variations in the parameters that determine inter-grain interactions, and to confining pressures exerted at the top of the granular column. We present the results of DEM simulations and experiments for granular columns confined at the top by a freely movable weight. We show that the strength of the vortex decreases with increasing confining pressure, but its essential features, such as the sense of rotation and symmetry, remain unchanged. We show how the character of the vortex changes from anti-centrifugal to centrifugal as the shear rate is increased, establishing that the vortex is indeed driven by dilation in the dense, slow flow regime. Finally, we propose a continuum plasticity model that accounts for dilation in viscometric flows, and provides a plausible explanation for how the vortex may arise as an instability.

¹Mehandia, V. ,Gutam, K. J. & Nott, P. R., Anomalous stress profile in a sheared granular column. Phys. Rev. Lett. 109, 128002 (2012).

²Gutam, K. J., Mehandia, V. & Nott, P. R., Rheometry of granular materials in cylindrical Couette cells: anomalous stress caused by gravity and shear. Phys. Fluids 25, 070602 (2013).

³Krishnaraj, K. P. & Nott, P. R., A dilation-driven vortex flow in sheared granular materials explains a rheometric anomaly, Nat. Commun. 7, 10630 (2016)