This paper presents a mechanical model, along with a numerical scheme for obtaining approximating solutions for the resulting initial-boundary-value problem, for describing the pig movement in transient two-phase gas pipelines. By taking advantage of the best features of the existing models presented in the literature so far, an idealized general purpose pig model is proposed, contemplating the possibility of representing, within a same context, different types of pigs or pig functions. Both mechanical and hydrodynamic friction forces at the interface of the pig and the pipe wall, as well as by-pass flow rates for the liquid and gaseous phases, are naturally incorporated in the modeling in a coherent mechanical context. The governing equations of the two-phase flow model are intentionally written in a general form, so that different existing models can be used within the framework presented herein. Following this same strategy, a detailed numerical scheme is presented in which the discretization of the flux terms are left open, so that different numerical strategies of first or higher orders can be accommodated without any additional difficulties.

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