Microscopic origins of shear stress in dense fluid–grain mixtures
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A numerical model is used to simulate rheometer experiments at constant normal stress on dense suspensions of spheres. The complete model includes sphere–sphere contacts using a soft contact approach, short range hydrodynamic interactions defined by frame-invariant expressions of forces and torques in the lubrication approximation, and drag forces resulting from the poromechanical coupling computed with the DEM-PFV technique. Series of simulations in which some of the coupling terms are neglected highlight the role of the poromechanical coupling in the transient regimes. They also reveal that the shear component of the lubrication forces, though frequently neglected in the literature, has a dominant effect in the volume changes. On the other hand, the effects of lubrication torques are much less significant. The bulk shear stress is decomposed into contact stress and hydrodynamic stress terms whose dependency on a dimensionless shear rate—the so called viscous number \(I_v\)—are examined. Both contributions are increasing functions of \(I_v\), contacts contribution dominates at low viscous number (\(I_v<0.15\)) whereas lubrication contributions are dominant for \(I_v> 0.15\), consistently with a phenomenological law infered by other authors. Statistics of microstructural variables highlight a complex interplay between solid contacts and hydrodynamic interactions. In contrast with a popular idea, the results suggest that lubrication may not necessarily reduce the contribution of contact forces to the bulk shear stress. The proposed model is general and applies directly to sheared immersed granular media in which pore pressure feedback plays a key role (triggering of avalanches, liquefaction).
KeywordsGranular suspension Rheology Lubrication Shear flow Discrete element method Hydromechanical coupling
This work is supported by the Ph.D. grant awarded to D. Marzougui by the University of Grenoble-Alpes. This work has been supported by the Ph.D. grant awarded to D. Marzougui by the University of Grenoble-Alpes. The content of the manuscript has not been published in previous publications. The study participants are the first author and co-authors and they consented to publish on June 12th, 2014. No institutional review board was required for this publication. The work as a whole has been approved by the IMEP-2 comitee of doctoral studies at Univ Grenoble Alpes on july 3rd 2011.
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