This paper probes solids suspension mechanisms in stirred tanks. The hypothesis is that at the point where solids are just suspended a critical flow condition exists close to the bottom of the tank for a fixed solids content and similar geometries. The critical flow condition may be dominated by the turbulence or by the mean flow. Two Lightnin A310 impellers with diameters of D=T/3 and D=T/2 were tested at varying off-bottom clearances. The mean and the turbulent fluctuating velocities close to the bottom of the tank were determined using both Ply and LES. The results showed that the mechanism of solids suspension is different for the two impeller diameters. Turbulence is the dominant mechanism for the T/3 impeller. For the T/2 impeller some combination of turbulence and mean flow is required. A comparison between the two impeller diameters in terms of power consumption suggests that solids suspension is more efficient when the turbulence mechanism dominates. (C) 2012 Elsevier Ltd. All rights reserved.