Proper Orthogonal Decomposition Based on Vorticity: Application in a Two-Phase Slug Flow

Abstract

Large-scale turbulent flow features in liquid (water) and gas (air) phases in the film region of two-phase slug flow are 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. POD analysis revealed the presence of dominant vortical structures embedded in both liquid and the gas phases. 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%. Linear combination of vorticity modes provided effective qualitative information of the coherent structures in both phases. POD-vorticity modes when compared with POD-velocity modes revealed few similarities among the pair of identified vortical structures. Based on the results, it is concluded that POD vorticity revealed hidden flow features of both phases of slug flow, which eventually provides in-depth and comprehensive description of this complex slug flow phenomenon.