Stresses in smooth flows of dense granular mediaM. Depken1, J. B. Lechman2, M. van Hecke3, W. van Saarloos1 and G. S. Grest2
1 Instituut-Lorentz, Universiteit Leiden - Postbus 9506, 2300 RA Leiden, The Netherlands
2 Sandia National Laboratories - Albuquerque, NM 87185, USA
3 Kamerlingh Onnes Lab, Universiteit Leiden - Postbus 9504, 2300 RA Leiden, The Netherlands
received 5 March 2007; accepted in final form 13 April 2007; published June 2007
published online 11 May 2007
The form of the stress tensor is investigated in smooth, dense granular flows which are generated in split-bottom shear geometries. We find that, within a fluctuation fluidized spatial region, the form of the stress tensor is directly dictated by the flow field: The stress and strain-rate tensors are co-linear. The effective friction, defined as the ratio between shear and normal stresses acting on a shearing plane, is found not to be constant but to vary throughout the flowing zone. This variation cannot be explained by inertial effects, but appears to be set by the local geometry of the flow field. This is in agreement with a recent prediction, but in contrast with most models for slow grain flows, and points to there being a subtle mechanism that selects the flow profiles.
83.80.Fg - Rheology: Granular solids.
45.70.Mg - Granular flow: mixing, segregation and stratification.
45.70.-n - Granular systems .
© Europhysics Letters Association 2007