Shear stresses developed during rapid shear of concentrated suspensions of large spherical particles between concentric cylinders

Abstract

Experiments were performed on concentrated suspensions of large (0·97–1·78 mm mean diameters) neutrally buoyant spherical particles sheared in a concentric-cylinder Couette-flow apparatus in which the inner cylinder rotated while the outer one was fixed. The variations of shear stress with apparent shear rate, concentration, particle diameter and wall roughness were studied, and the results are compared with related experiments of Bagnold. Generally the shear stresses measured in the present experiments were larger than those of Bagnold. The difference can be attributed to differences in the experimental arrangements; Bagnold's flexible-walled inner cylinder was fixed while the outer cylinder rotated. A strong effect of wall roughness was observed. The higher stresses generated with rough walls imply that particle ‘slip’ may have occurred in the smooth wall tests. The larger stresses might also be due to an increase in strength of the interparticle collisions caused by the roughness. No dependence of stress upon particle diameter d was observed for concentrations of about 0·3, but a strong dependence (> d2) was found at the highest concentrations with the rough walls.