Effect of particle size dispersion on granular lubrication regimes

This article is devoted to the study of granular flow regimes undergoing shear in a parallel-plate configuration with particular interest in granule dispersion. Both experimental and numerical investigations are carried out. Experimental investigations are conducted in a shear cell consisting of two parallel discs that contain the granular media. One disc is fixed, whereas the other rotates to shear the media. A normal force is imposed, and the gap is measured. Numerical investigations are based on simulations that use discrete element method (DEM), which allows local investigation of the flow across the gap. This article focuses on the effect of a mixture of size to the flow regime observed when shear velocity increases. The imposed loads on the granular media are weak, and thus, it is expected that the granular regime will pass from a quasi-static regime to a kinetic one. It is found that the granule mixture has a major influence on the normal displacement (lift) between the discs, whereas the effect on the measured friction coefficient is less pronounced. Numerical investigations are in agreement with experimental measurements, provided that the important parameters such as the local contact condition, i.e. the coefficient of restitution and the local friction coefficient, are properly specified. For the simulations presented, the coefficient of restitution and local coefficient of friction are 0.9 and 0.3, respectively. The results of an extensive set of simulations are presented to study the effect of change in the local friction coefficient and coefficient of restitution. For a low value of restitution coefficient (0.7), it is shown that all the media thickness is sheared when the proportion of large granules tends to 50 per cent, although only one part of the whole thickness is sheared when the proportion of large granules tends to 0. For a higher value of restitution coefficient (0.9), the transition from dense to kinetic regime occurs for a higher shear velocity when the granule sizes are the same. In this case, a high level of vibration is observed at low velocities both experimentally and numerically. Numerical simulations confirm that these vibrations can be analysed as stick—slip phenomena. Results show that the coefficient of restitution has no effect at low speeds (dense regime), but has a pronounced effect at high speeds (kinetic regime). This result is in accordance with the previous studies. The local coefficient of friction has a high effect when it varies from 0 to 0.3 and low effect when it is above 0.3. This also confirms previous studies.