In upward air-water churn flow in a vertical tube, the liquid fed at the inlet is entrained upwards by the gas flow in the core, resulting in a wavy film flow. The present work considers the influence of the modelling of the liquid inlet on the wave frequencies, with the final purpose to understand the causes and effects in an actual flow. The perforated wall liquid inlet section, commonly used in experiments, is modelled in simulations as a simple inlet boundary condition on a short section of a pipe wall. At a given liquid mass flow rate in the experiment, the magnitude of the wall normal velocity component is controlled by the inlet area, which is used as a modelling parameter in the present study. The study shows that the calculated wave frequency is proportional to the imposed wall normal velocity at the liquid inlet. The velocity profile at the perforated wall liquid inlet section was not measured in available experiments and presents a major source of uncertainty in simulations. The parametric study revealed that a suitable value for the magnitude of the wall normal velocity can be determined over a range of flow conditions, leading to good agreement of simulated and measured wave frequencies. This finding suggests, that the wave frequency in actual churn flow is not a property depending only on geometric and flow conditions, but on the liquid inlet flow as well. Simulations were performed in three-dimensional tube section using the multiphase solver interFoam from the open-source code OpenFOAM. (C) 2017 Elsevier Ltd. All rights reserved.