(77c) Computational and Experimental Studies of Stress-Controlled Shear Flows of Fibrous Particles

The Discrete Element Method (DEM) has been utilized to simulate stress-controlled shear flows of fibrous particles, in which a shear flow of the material is driven by the relative movement between two parallel plates and a constant normal stress is applied to the material by one of the plates. A typical stress-controlled shear flow is granular material flow in a Schulze shear cell test. The DEM simulations of stress-controlled shear flows of fibrous particles are calibrated with the Schulze shear cell tests of cut fishing wires. The shear flow simulations and experiments are performed with various normal stresses in order to explore the dependence of the yield shear stress on the pressure applied to compress the material. Using the DEM simulations, parametric studies are conducted to examine the effects of particle-particle friction, fiber bending stiffness, and particle aspect ratio on the yield shear stress. The influence of moisture is also investigated by performing DEM simulations and Schulze shear cell tests using wet fibrous particles (i.e. the wetted cut fishing wires). It is found that in dense flows, the effect of cohesion, which is caused by the moisture, is minimal compared to the effect of particle-particle friction.

The stress-controlled shear flows are then compared with concentration-fixed shear flows, in which the solid volume fraction remains constant during the flow process. The average particle velocity profiles are very different between these two types of shear flows: a linear velocity profile is observed in the concentration-fixed flows while the velocity profile is non-linear in the stress-controlled flows, for which much a larger velocity gradient is observed close to the boundary plate that can move perpendicularly to the direction of material flow. With the same normal stress, the average shear stress and solid volume fraction (at steady state) obtained in the stress-controlled shear flow are the same as those in the concentration-fixed shear flow.