The results of CFD simulation of a rotating cylindrical gas sparger in a downflowing liquid are presented. The simulations cover one-phase flow (73% aqueous glycerol) and two-phase flow (air-water) with special care taken to modelling the gas cavity that forms under such conditions. The velocity field measured with the aid of Particle Image Velocimetry (PIV) is compared with the simulation results for the case of one-phase flow.