Dry granular flow in a vane shear cell: flow characteristics and rheological laws

The flow dynamics of dry granular flows is significantly affected by a prominent feature of a granular mass known as dilatancy. Although their rheological behaviour has been characterised by way of friction and dynamic dilatancy laws, the role of dilatancy in a granular flow has not been much appreciated. In this study, using a vane rheometer, an experimental investigation was conducted on uniform glass beads of d = 3 mm at different initial relative densities (15 ≤ D_r ≤ 66%) and shear rates that spanned over four orders of magnitude. The flow characteristics in terms of effective friction, volume change and velocity field were obtained and evaluated. The effective friction shows a descending–ascending pattern corresponding to a transitional behaviour from a velocity-weakening solid-like behaviour to a velocity-strengthening liquid-like behaviour. The volume-change measurements show that all specimens dilate and reach almost the same density at each shear rate. The velocity field follows a Gaussian pattern characterised by the slipping velocity at the boundary, the interlayer slippage between particles and the interlayer disorder of the particles. A new non-monotonic friction law and a dynamic dilatancy law are presented as governing rheological laws based on the inertial number and by introducing an effective dilation coefficient. This effective dilation coefficient successfully captures the role of dilation-induced secondary vortex flows in the dry granular flows.