Stresses in smooth flows of dense granular media

The form of the stress tensor is investigated in smooth, dense granular flows whichare generated in split-bottom shear geometries. We find that, within a fluctuation fluidized spatialregion, the form of the stress tensor is directly dictated by the flow field: The stress and strain-ratetensors are co-linear. The effective friction, defined as the ratio between shear and normal stressesacting 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 geometryof the flow field. This is in agreement with a recent prediction, but in contrast with most modelsfor slow grain flows, and points to there being a subtle mechanism that selects the flow profiles.