Dilatancy-driven secondary flows in sheared granular systems

Granular materials are an important part of many industrial processes, yet a suitable continuum model that adequately describes their flow is lacking. Krishnaraj and Nott [1] showed that shearing granular materials in a Cylindrical Couette device gives rise to a secondary flow in the form of a system-spanning toroidal vortex. Unlike the centrifugally-driven Taylor-Couette vortices, this vortex is driven by the dilatancy of the material. The vortex leaves an experimentally measured large rheological signature [2]. In this work, we show that the dilatancy-driven vortices are a generic feature in flows where the gravity and shear directions are not co-linear. Using simulations and experiments, we show that such vortices arise in a split-bottom Couette device [3]. By varying the fill height, we can change the sense of rotation and number of vorticies. We show how the form of the primary flow can explain the nature of the secondary flow. Dilation at constant pressure is not included in any available continuum model – our results show the necessity of incorporating this feature.

References:
1. Krishnaraj, K. P. & Nott, P. R., Nat Commun, 2016.
2. Gutam, K. J., Mehandia, V. & Nott, P. R. Physics of Fluids, 2013
3. A. Dijksman, J. & Hecke, M. van. Soft Matter, 2010