Lagrangian particle tracking and Direct Numerical Simulation of turbulence are used to highlight an intermittent phenomenon which is relevant for particle resuspension dynamics in an unbaffled tank reactor stirred by an eight blade paddle impeller. This phenomenon is associated with the Ekman pumping effect and its frequency scales in terms of the impeller rotation speed. We examine the dispersion of three swarms of heavy particles of different diameter demonstrating qualitative and quantitative features of the resuspension phenomenon. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.