On the wall boundary condition for the velocity in concentrated suspensions

Continuum models for concentrated suspension have succeedded in reproducing most features of channel flows, except for volume-fraction profiles in the vicinity of the walls. It has been stipulated that this could be traced to the choice of the no-slip boundary condition for the velocity of the granular phase, which conflicts experiments indicating the existence of a slip layer. In fact, particle migration is inhibited in the absence of sufficient slip and, therefore, numerical simulations tend to overestimate the volume fraction. In this paper, we explore whether apparent slip and true slip boundary conditions can mitigate those discrepancies. Our study suggests that when adequate slip is prescribed at the wall, the numerical predictions can match the experimentally observed values near the walls. However, there is a complex correlation between required slip and the average concentration; this is in line with recent experimental measurements of the near-wall slip in concentrated suspension •The role of the velocity boundary condition and the near-wall volume fraction profile is investigated numerically.•Higher slip at the wall improves the near-wall volume fraction profile predictions.•Existing correlations for the slip coefficient need to be revised.