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The contact force model consisting of a linear spring dashpot with a frictional glider has been widely adapted to simulate granular flows. Real contact mechanics between two solid bodies is very complicated. Extensive theoretical and experimental studies exist for binary contacts. Very little work has been reported that addresses the effect of contact mechanics on the bulk behavior of granular materials. We first briefly summarize the difference of binary contacts between a linear spring--dasbpot model and the Hertzian nonlinear spring with two nonlinear dashpot models. We then compare the constitutive behaviors of a granular material using a linear and a nonlinear model. The stress- and strain-rate relation in simple shear flow and the resulting coordination number are calculated using the discrete element method. It is found that although at the grain level binary contact between two particles depends on whether a linear or a nonlinear model is used, the bulk behavior of granular materials is qualitatively similar with either model. DOI: 10.1061/(ASCE)0733-9399(2006)132:11(1252) CE Database subject headings: Models; Granular materials; Solids flow; Stress strain relations.

Dense two-phase flows Occur in many material processing Situations. In these flows, particle collision presents a mechanism for internal resistance In the literature, restitution coefficients are given for collisions between dry objects. Little is known about two colliding objects placed in a fluid. Thus, although two-phase flow models exist for dense mixtures, realistic values of the required restitution coefficient are not available. Recent theoretical and experimental analyses of colliding spheres in a viscous fluid show that collisional rebound depends greatly on the interstitial fluid. At very high viscosity, contact may not occur. This paper presents an experimental study of collisional restitution between steel and nylon, and a range of Newtonian viscous fluids. The fluids have a viscosity range from 27 [mm.sup.2]/s to 2,600 [mm.sup.2]/s. The normal impact velocity ranges from 0.015 m/s to 0.75 m/s. Preliminary results show that the behaviour of the impact and the resulting restitution coefficient depend on the interstitial fluid. These results will be useful to the mathematical modeling of dense two-phase flows., A experiment is conducted to determine the restitution coefficient of colliding objects with viscous fluid at the contact surface. Results show that restitution coefficient values are comparable to values from dry collisions, for oil viscosity up to 10 sq mm/s. For greater oil viscosity however, the restitution coefficient can decrease. Softer materials are also more dependent on viscosity changes. For modeling two-phase flows, the restitution coefficient for both the solid and liquid must be taken into account.

A simple shear flow of granular materials can have a range of behavior from a rate-independent plastic material to a rate-dependent viscous material. From physical experiments and computer simulations, it is known that this constitutive relation is a consequence of the shear rate, the solid concentration, and the micro-mechanical properties of the particles. Simple shear tests of granular materials show that in the rate-independent case, a shearing granular assembly forms crystallized regions and shearing occurs locally in a narrow band. In the rate-dependent case, these crystallized zones 'melt' and the whole granular assembly participates in the shear motion. This study utilizes computer simulation to address yet another effect on the constitutive relation: the sample size. A 2D uniform disk assembly is simulated using periodic boundary conditions. Through investigating details of the kinematics the source of the transition is examined from a rate-independent to a rate-dependent fluid as the sample size increases. Results of this study have implications on the design of equipment handling granular materials. That is, rheological properties of a granular flow can change due to a change of the relative size of the equipment and the grains.

The contact force model consisting of a linear spring dashpot with a frictional glider has been widely adapted to simulate granular flows. Real contact mechanics between two solid bodies is very complicated. Extensive theoretical and experimental studies exist for binary contacts. Very little work has been reported that addresses the effect of contact mechanics on the bulk behavior of granular materials. We first briefly summarize the difference of binary contacts between a linear spring–dashpot model and the Hertzian nonlinear spring with two nonlinear dashpot models. We then compare the constitutive behaviors of a granular material using a linear and a nonlinear model. The stress- and strain-rate relation in simple shear flow and the resulting coordination number are calculated using the discrete element method. It is found that although at the grain level binary contact between two particles depends on whether a linear or a nonlinear model is used, the bulk behavior of granular materials is qualitatively similar with either model. [ABSTRACT FROM AUTHOR]

The stability of a shear layer inside a granular material in a gravity field is studied experimentally and numerically. A shear cell is built of transparent acrylic to visualize the motion of the granular material. This shear cell consists of two concentric cylinders containing layers of uniform spheres in the annular space between the cylinders. The shearing motion of the spheres is produced by rotating the bottom boundary of the cell. Friction of the cylinder walls resists the shear motion, thus creating a single shear layer adjacent to the bottom boundary, while the rest of the layers above move with constant speed as a solid body. As the rotation speed of the bottom boundary increases, two layers adjacent to the bottom boundary begin to shear. This shearing zone quickly thickens and dilates as the rotational speed increases. The transition of this shear motion from a single layer to many layers of shearing is studied by video recording. The initiation of this transition is observed to depend on the material properties and the number of layers overlaying the shear layer. A one-dimensional numerical model is constructed to bring insight into this transitional phenomenon. [ABSTRACT FROM AUTHOR]