Effect of confinement in wall-bounded non-colloidal suspensions

This paper presents three-dimensional numerical simulations of non-colloidal dense suspensions in a wall-bounded shear flow at zero Reynolds number. Simulations rely on a fictitious domain method with a detailed modelling of particle-particle and wall-particle lubrication forces, as well as contact forces including particle roughness and friction. This study emphasizes the effect of walls on the structure, velocity and rheology of a moderately confined suspension (channel gap to particle radius ratio of 20) for a volume fraction range 0.1 <= phi <= 0.5. The wall region shows particle layers with a hexagonal structure. The size of this layered zone depends on volume fraction and is only weakly affected by friction. This structure implies a wall slip which is in good accordance with empirical models. Simulations show that this wall slip can be mitigated by reducing particle roughness. For phi (sic) 0.4, wall-induced layering has a moderate impact on the viscosity and second normal stress difference N-2. Conversely, it significantly alters the first normal stress difference N-1 and can result in positive N-1, in better agreement with some experiments. Friction enhances this effect, which is shown to be due to a substantial decrease in the contact normal stress vertical bar Sigma(c)(xx)vertical bar (where x is the velocity direction) because of particle layering in the wall region.