Two-fluid continuum models for bubbly flows need detailed information on interphase momentum interaction terms. In order to be able to study these aspects, a rigorous modelling of the behaviour of individual bubbles is needed. In this work, the behaviour of a single bubble in a periodical domain has been simulated. This method has allowed us to mimic the motion of a swarm of bubbles adopting limited resources of calculation and to obtain an averaged result. In this way the interaction between a swarm of bubbles and the liquid is reduced to the analysis of the interaction between a single bubble and a portion of the liquid phase. The study has been carried out by means of computational fluid dynamics software and adopting the volume of fluid method. Adopting this information it is possible to verify the correlations used to estimate the drag coefficient of the bubbles and to relate the value of C-D to the level of gas hold-up in the system. The values obtained have been compared with an experimental correlation (A.I.Ch.E. J. 25 (1979) 843) and with similar calculations performed adopting the Lattice-Boltzmann scheme for the solution of the equations (J. Fluid Mech. 452 (2002) 61). (C) 2004 Elsevier Ltd. All rights reserved.