Wake dynamics of a square cylinder while moving upward in quiescent water

We experimentally investigate the wake dynamics of a square cylinder rising through quiescent water over a range of Froude numbers (Fr). Time-resolved particle image velocimetry provides velocity and vorticity fields that enable pressure reconstruction and vortex characterization. Diagnostics based on swirl strength (λci), the Okubo–Weiss parameter (W), and a shear–vortex interaction measure (Λ) reveal that the wake is governed by a persistent pair of counter-rotating vortices rather than by periodic shedding. Circulation exhibits a two-regime dependence on Fr, with a sharp increase below Fr≈1 and saturation above this threshold, mirroring entrainment force scaling reported previously. While vortex area remains nearly constant, swirl strength and negative-W regions expand with Fr, indicating that entrainment enhancement arises from intensified rotation rather than an enlarged vortex footprint. These findings provide new physical insight into vortex-free-surface interactions and enrich the understanding of entrainment mechanisms in unsteady wakes, with implications for multiphase flows and the hydrodynamic design of naval and offshore structures.