(133h) Slow Granular Flows in a Split-Bottom Couette Device

In this work, we present DEM simulations and experiments of slow shear of granular materials in a split-bottom Couette device. This device is a cylindrical cup with a split base, wherein a central disc that is flush with the bottom rotates, while all other walls of the device are stationary [1]. Shear originates from the split between this disc and the rest of the base. This device has been widely used to validate rheological models for granular flows since it exhibits wide shear bands. We show that by changing the fill height of the split-Bottom Couette device, we change the location of shear in the material. We then show the presence of large, system spanning vortices that appear in addition to the primary azimuthal flow. These vortices are like those seen in the cylindrical Couette device [2]. We show how the number and form of these vortices are highly dependent on the where the system shears. We show that the form of the vortices can be explained by accounting for shear-induced dilation in the system, validating the arguments made for the vortices seen in the Cylindrical Couette device [2]. It was shown by [2] that these vortices have a large rheological signature, wherein the components of stress increase exponentially with depth in a sheared Couette device [3]. None of the current rheological models for slow granular flow can capture these vortices because none of them allow for shear-induced dilation. We thus make the argument that dilation needs to be incorporated in any general rheological model for slow granular flows.
References

1. Dijksman, Joshua A., and Martin van Hecke. "Granular flows in split-bottom geometries." Soft Matter 6.13 (2010): 2901-2907.
2. Krishnaraj, K. P., & Nott, P. R. (2016). A dilation-driven vortex flow in sheared granular materials explains a rheometric anomaly. Nature communications, 7, 10630.
3. Gutam, K. J., Mehandia, V., & Nott, P. R. (2013). Rheometry of granular materials in cylindrical Couette cells: anomalous stress caused by gravity and shear. Physics of Fluids, 25(7), 070602.