Rheometry of granular materials in cylindrical Couette cells: Anomalous stress caused by gravity and shear

Abstract

The cylindrical Couettedevice is commonly employed to study the rheology of fluids, but seldom used for dense granular materials.Plasticity theories used for granular flows predict a stress field that is independent of the shear rate, but otherwise similar to that in fluids. In this paper we report detailed measurements of the stress as a function of depth, and show that the stress profile differs fundamentally from that of fluids, from the predictions of plasticity theories, and from intuitive expectation. In the static state, a part of the weight of the material is transferred to the walls by a downward vertical shear stress, bringing about the well-known Janssen saturation of the stress in vertical columns. When the material is sheared, the vertical shear stress changes sign, and the magnitudes of all components of the stress rise rapidly with depth. These qualitative features are preserved over a range of the Couette gap and shear rate, for smooth and rough walls and two model granular materials. To explain the anomalous rheological response, we consider some hypotheses that seem plausible a priori, but show that none survive after careful analysis of the experimental observations. We argue that the anomalous stress is due to an anisotropic fabric caused by the combined actions of gravity, shear,and frictional walls, for which we present indirect evidence from our experiments. A general theoretical framework for anisotropicplasticity is then presented. The detailed mechanics of how an anisotropic fabric is brought about by the above-mentioned factors is not clear,and promises to be a challenging problem for future investigations.

Analysis of powder rheometry of FT4: Effect of air flow

“…Bulk powder flow under dynamic conditions and the associated rheology has been analysed by Campbell (2004;, Delannay et al (2007), Jop et al (2006), Koval et al (2009) and Pouliquen et al (2006). The recent works of Gutam et al (2013) and Kumar et al (2013) show that the flow pattern even in a simple Couette geometry is more complex than previously perceived due to the secondary recirculating flow.…”

The lift on a disc immersed in a rotating granular bed

“…In flows such as simple shear, a density gradient is set up within the shear layers . The effect of dilatancy need not, however, be confined to the shear layers—we have recently shown in our group that dilatancy causes a large‐scale secondary flow in cylindrical Couette cells, thereby explaining a stress anomaly observed earlier . It is, therefore, of interest to study the distribution of in this problem.…”

Rheometry of dense granular materials: The crucial effects of gravity and confining walls

“…However, when the material is sheared by rotating the inner cylinder, the stress field changes dramatically. In this presentation, we shall present the findings of recent studies [4,5] of our group, wherein the stress profile for model granular materials (such as glass beads and mustard seeds) were measured at the outer cylinder using a multi-axis force sensor. The profiles are radically different from the the predictions of all plasticity theories, previous experiments, and intuitive expectation.…”

“…Our previous papers [4,5] have reported the results only for the "funneling" protocol. It is well known from prior literature that the static stress profiles for these different filling protocols will differ significantly.…”

“…The observed stress profiles are also contrary to the predictions of plasticity theories, which predict σ rz = 0. We had earlier proposed [4,5] the hypothesis that an anisotropic shear-induced fabric is responsible for the The variation of the vertical shear stress σ rz with depth z for two filling protocols: raining (green squares), innerouter (red circles). Data are for glass beads of 0.8 mm diameter with a Couette gap of 4 cm.…”