(493d) Parametric Analysis of the Effects of Particle Properties On the Granular Flows of Cylindrical Particles

Authors: 
James, B. D., University of Florida


In this work, the simple shear flows of cylindrical particles are simulated using the Discrete Element Method (DEM) to study the effects of particle properties (e.g., particle aspect ratio, coefficient of restitution, friction coefficient and Young’s modulus) on the granular shear flow behavior and the constitutive relations of stresses. It is found that a decrease in the coefficient of restitution can cause a decrease in the stresses while an increase in the apparent friction coefficient, which is defined as the ratio of shear stress to normal stress. This is because the loss of kinetic energy increases after interparticle collisions with smaller coefficients of restitution.

The friction coefficient has a significant impact on the granular flow. At low solid volume fractions, smaller stresses are obtained for the particles with friction due to the energy dissipation through the sliding friction. However, the system-spanning force chains are suddenly formed as a critical solid volume fraction is reached for the frictional particles, leading to a sharp increase in the stresses. This critical solid volume fraction decreases as the particle aspect ratio increases and/or the friction coefficient increases, illustrating the system-spanning force chains and sharp stress increases occur at a lower solid volume fraction for the larger aspect ratio particles with larger friction coefficient. When the force chains are formed, the relative movement of particles becomes more difficult, so that some particles move and rotate together like a solid body. As a result, the vortex-like circulation of particles is obtained for the dense flows with the frictional particles. Due to this circulating motion of particles, the apparent friction coefficient is significantly reduced as the solid volume fraction increases. In contrast, the system-spanning force chains, sharp stress increase and vortex structure are not observed for the frictionless particle systems.

The Young’s modulus of particles determines the contact stiffness when collisions occur. In the dilute granular flows for which the force chains are not formed yet, the stresses are independent of the Young’s modulus. However, as the solid volume fraction increases and the force chains are formed, the contacts become frequent and long lasting, and thus the stresses strongly depend on the contact forces, which increase as the Young’s modulus increases. In addition, the apparent friction coefficient for both dilute and dense systems is not dependent on the Young’s modulus in the range of Young’s moduli considered.