Large-scale turbulent flow features in liquid (water) and gas (air) phases in the film region of two-phase slug flow were identified by using proper orthogonal decomposition (POD) of the vorticity fields. Linear combination of POD vorticity modes is used for the qualitative visualization of coherent structures. The vorticity fields are computed from the instantaneous two-dimensional velocity fields measured using a combined particle image velocimetry and laser-induced fluorescence technique (PIV-LIF). Vorticity modes are calculated and compared with the curl of POD velocity mode. For the gas phase, most vortical structures appeared in pairs with positive and negative rotations and POD analysis for the liquid phase identified a group of dominant vortical structures near the lower wall. It is also observed that the gas phase revealed more eddies than the liquid phase. The proportion of enstrophy is higher in the gas phase as first POD vorticity mode contained 7.5% of the total enstrophy, while for the liquid phase, the first mode captured 6.8%. The linear combination of vorticity modes provided effective qualitative information of the coherent structures in both phases. In the gas phase, the identified structures are present near the upper wall and in the middle of the flow, while in the liquid phase, dominant flow patterns appeared near the lower wall. It is therefore beyond doubt that POD based on vorticity can reveal hidden flow features of both phases of slug flow which eventually provides in-depth description of this complex flow phenomenon.

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