(339y) Rheological Behavior of Dense Granular Matter

The main objective of this research is to study the rheological behavior of granular matter in the Quasi-Static (solid-like) and Intermediate (liquid-like) regimes of flow using different flow-geometries. We used a Couette cell where powder is sheared between two concentric cylinders and modified it by providing continuous powder feed and discharge that enabled a small axial flow. This allowed a slight decrease of bed solid fraction (measured by a capacitance probe) and shifting of the flow from the quasi-static to intermediate regime. Furthermore, by measuring the normal stresses inside the shearing zone and defining the coefficient of apparent friction as the ratio of shear to normal stresses, we suggest a visco-plastic constitutive law for dense granular materials. We studied the effects of size and its distribution, shape, stiffness and surface roughness of the particles on the rheology of bulk [1,2].

We also used in this work a classical Jenike cell and a slightly modified version of it that allows faster shearing. Wall-friction coefficients were measured by adding one or several normal force (stress) transducers on the shearing wall and applying known stresses to the free surface of the powder. Using this experimental technique we were able to compare the pressure (force) applied to the top surface of the powder with stresses measured on the opposite side of the cell as sensed by the normal stress transducer. The experimental results were compared with a DEM theoretical model. We found that stresses are transmitted through thin layers without significant attenuation if the material is in continuous shear but that large fluctuations are introduced especially for rigid materials such as glass and steel. Particles of different size, shape, roughness and stiffness were tested in layers of different thicknesses and the response of the transmitted stress was recorded and analyzed.

Several other geometries of flow were also analyzed by measuring stresses, stress fluctuations and porosity. The studied geometries were, centrifugal flow in a ?spheronizer? and ?roping? flow in a high-shear mixer.

Reference

1.M. Kheiripour Langroudi, S. Turek, A. Ouazzi, and G. Tardos; Submitted to Powder Technology, 2009

2.M. Kheiripour Langroudi, G. Tardos, J. Michaels, and Paul Mort; Accepted to be published in American Institute of Physics Journal, 2009