Wall stresses in granular Couette flows of mono-sized particles and binary mixtures

Granular shear flows are studied in a gravity-free Couette geometry using a two-dimensional discrete element computer simulation. Upper and lower bounding walls are flat and frictional and move in opposing directions, while the right and left boundaries are periodic. Mono-size flows are examined at various concentrations and three different gap widths. Flows of binary mixtures with diameter ratios of 2, 5, and 10 are investigated as well. Mixture solid fraction ratios of small to large particles range from 0.4 to 5, with a constant overall solid fraction of 0.75 in two dimensions. Normal and shear stresses on the bounding walls are measured for various flow conditions. Both normal and shear stresses increase with solid fraction in same-size flows, and show a dependence on the wall spacing at low concentrations. Same-size particle flows show the existence of a critical wall solid fraction at which the granular temperature, strain rate, and stresses increase suddenly. Stresses in mixture flows with low solid fraction ratios of small to large particles are higher than for the mono-size system. For a fixed overall solid fraction of 0.75, mixture flow stresses also increase with diameter ratio of large to small particles. The ratio of shear to normal stress decreases with solid fraction in same-size flows. For mixture flows with constant overall solid fraction, the ratio increases with solid fraction ratio for size ratios of 5 and 10; it remains relatively constant in flows with a size ratio of only 2.