Abstract
In this work, granular flow rheology is investigated by means of discrete numerical simulations of a torsional, cylindrical shear cell. Firstly, we focus on azimuthal velocity profiles and study the effect of (i) the confining pressure, (ii) the particle-wall friction coefficient, (iii) the rotating velocity of the bottom wall and (iv) the cell diameter. For small cell diameters, azimuthal velocity profiles are nearly auto-similar, i.e. they are almost linear with the radial coordinate. Different strain localization regimes are observed : shear can be localized at the bottom, at the top of the shear cell, or it can be even quite distributed. This behavior originates from the competition between dissipation at the sidewalls and dissipation in the bulk of the system. Then we study the effective friction at the cylindrical wall, and point out the strong link between wall friction, slip and fluctuations of forces and velocities. Even if the system is globally below the sliding threshold, force fluctuations trigger slip events, leading to a nonzero wall slip velocity and an effective wall friction coefficient different from the particle-wall one. A scaling law was found linking slip velocity, granular temperature in the main flow direction and effective friction. Our results suggest that fluctuations are an important ingredient for theories aiming to capture the interface rheology of granular materials.
Highlights
FzH and on the flow regimes as a function of the main system parameters, and on the effective wall friction, and related scalings, at the cylindrical wall.Simulation methodNumerical simulations are performed using the non smooth contact dynamics method [1], as implemented in the LMGC90 open source framework [2]
The rheological properties of granular matter submitted to torsional shear were investigated by means of discrete numerical simulations
The torsional shear flow configurations consists in a a shear cell made of a cylinder filled by grains which are sheared by a bumpy bottom and submitted to a vertical pressure which is applied at the top
Summary
FzH and on the flow regimes as a function of the main system parameters, and on the effective wall friction, and related scalings, at the cylindrical wall.Simulation methodNumerical simulations are performed using the non smooth contact dynamics method [1], as implemented in the LMGC90 open source framework [2]. H and on the flow regimes as a function of the main system parameters, and on the effective wall friction, and related scalings, at the cylindrical wall. In this work the flow is at fixed normal stress, i.e. the upper wall is free to move vertically under the action of the imposed normal force and the reaction of the particles contained in the cylinder. The top and bottom walls are bumpy, while the cylindrical wall is smooth but frictional. Simulations were performed with N slightly polydisperse spheres (uniform number distribution in the range 0.9d − 1.1d) interacting through perfectly inelastic collisions and Coulomb friction (μp = 0.5). Interactions of particles with the flat walls were perfectly inelastic and frictional (with a coefficient of friction μpw)
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