This paper is concerned with the modeling and analysis of detailed distributions of flow field and phase concentration in slug flows. The computational approach was based on first-principle simulations of transient three-dimensional combined gas/liquid flows. The tracking of moving gas/liquid interfaces was accomplished using a modified Volume-of-Fluid method. The results of such ‘virtual experiments’ have been ensemble-averaged and compared against experimental data. The main focus of the analysis was on a consistent derivation and proper physical interpretation of the various local terms arising from the averaging, which appear in the average multifield conservation equations. It has been demonstrated that for flow conditions in which the geometry of gas/liquid interface (such as the shape of Taylor bubbles in slug flows) is a part of the solution, the process of ensemble-averaging introduces two groups of terms: the ‘bulk terms’ that are determined with respect to phasic average variables, and the interface excess terms that normally combine the average variables and those determined strictly at the interface. In particular, the above applies to the total shear stress, as well as to the interfacial forces in slug flows. A detailed analytical derivation of all such terms has been performed. The consistency of the proposed approach has been demonstrated by comparing the results of model predictions against experimental data for various flow regimes. It has been shown that the distributions of average (but still local, radially dependent) flow parameters such as the: velocity, void fraction, shear-induced turbulent stress, and bubble-induced turbulent stress, are in good agreement with the measurements.