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 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.

Numerical Investigation on the Spatiotemporal Correlation between Hydraulic Loss and Vortex at Turbine Mode of a Pump-turbine

“…Vorticity is generally regarded as the most widely used vortex identification method [7].These days, Q-criterion [8] and omega vortex identification method [9] are also adopted to identify the vortex structure in the investigation of hydrofoil and hydraulic machinery. Liu et al [10] proposed the conception of Rortex, which can accurately identify the rigid rotational part in vortex.…”

Dissipation-optimized proper orthogonal decomposition

“…Similar properties with respect to POD convergence may therefore be expected, providing the basis for comparison between results found in the present work and comparable results on enstrophy-based POD found in literature. Enstrophy-based POD was utilized by Huang (1994) and by Kostas, Soria, and Chong (2005) for identifying coherent flow structures from 2D particle image velocimetry (PIV) measurements of a backward-facing step flow, and similarly by Munir et al (2022), based on 2D measurements of two-phase slug flow made by combining PIV and laserinduced fluorescence. Sengupta et al (2015) demonstrated reduced-order modeling of a flow past a cylinder applying enstrophy-based POD to 2D DNS data, using the resulting modes in a vorticity-formulation of the Navier-Stokes equations.…”

On the Usefulness of the Proper Orthogonal Decomposition on the Description of the Highly Concentrated Sediment Release Phenomena Resulting from a Two-Phase Solid-Fluid Simulation: Effect of the Ambient Current

“…As previously observed, POD has been used in two-phase applications. In particular, some authors [16][17][18][19][20] have applied this specific technique to identify the dominant structures in the flow dynamics of horizontal slug flow commonly observed in many industrial processes. Both phases, the liquid (water or oil) and the gaseous, have been explored.…”