A numerical simulation of the churn flow regime of air-water and R134a vapour-liquid mixtures by means of the volume of fluid (VOF) method is presented. The focus of the paper is on the inlet region of a vertical pipe. An axisymmetrical domain is used, reproducing the region next to the porous wall liquid injector of a typical test section for the investigation of vertical gas-liquid flows. A simplified model of the levitation process of the ring-type waves typical in churn flow is proposed. The influence of the gas Froude number on the waves amplitude is shown by means of the simplified model and used to explain the numerical results. A comparison of the numerical results with experimental wave frequency data and visualizations available in the literature is performed. The velocity field in the forming wave region and the pressure and shear stress variations along the interface are shown. Simulations have been performed at different liquid and gas superficial velocities and pipe diameters and the influence of these parameters on the gas-liquid interface is discussed. (C) 2009 Elsevier Ltd. All rights reserved.